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Introduction, section snippets, references (73), cited by (9).

Back to the future: Revisiting the perspectives on nuclear fusion and juxtaposition to existing energy sources

• • Nuclear fusion is the “holy grail” of transition to an emission-free energy system.
• • Since 1983 the energy gain factor of nuclear fusion energy has been steadily growing.
• • The breakthrough of 2022 gave an impetus to further commercial interest in nuclear fusion energy research.
• • There are increasing public subsidization schemes, more engagement of established private enterprises, and a growing number of start-ups worldwide.
• • The earliest 2030 and latest 2050 nuclear fusion will be more competitive than conventional energy sources.
• • In the long run, nuclear fusion energy will crowd out conventional fossil fuels and other renewables.

Development of nuclear fusion

Fusion vs fission, credit statement, declaration of competing interest, structure and environmental impact of global energy consumption, renew sustain energy rev, energy supply, its demand and security issues for developed and emerging economies, carbon neutrality: toward a sustainable future, analysis of wind energy conversion system using weibull distribution, procedia eng, sustainable management of lithium and green hydrogen and long-run perspectives of electromobility, technol forecast soc change, the energy futures we want: a research and policy agenda for energy transitions, energy res social sci, decarbonising europe–eu citizens' perception of renewable energy transition amidst the european green deal, review on smart grid control and reliability in presence of renewable energies: challenges and prospects, math comput simulat, re-examining the role of nuclear fusion in a renewables-based energy mix, evaluation of the hydrogen solubility and diffusivity in proton-conducting oxides by converting the psl values of a tritium imaging plate, nuclear materials and energy, a comparative study on the hydrogen dissolution and release behaviors in the zirconate proton conductors by tds and tmap4 analysis, j alloys compd, nuclear design analyses of the helium cooled lithium lead blanket for a fusion power demonstration reactor, fusion eng des, an overview of the eu breeding blanket design strategy as an integral part of the demo design effort, european demo design and maintenance strategy, environmental assessment of multiple “cleaner electricity mix” scenarios within just energy and circular economy transitions, in italy and europe, j clean prod, quantifying management efficiency of energy recovery from waste for the circular economy transition in europe, at the nexus of circular economy, equity crowdfunding and renewable energy sources: are enterprises from green countries more performant, nuclear fusion: the promise of endless energy, renewable energy market update. outlook for 2023 and 2024, can the development of fusion energy be accelerated an introduction to the proceedings, philosophical trans. royal soc. a, energy security pathways in south east europe: diversification of the natural gas supplies, energy transition, and energy futures, pathways to the hydrogen mobility futures in german public transportation: a scenario analysis, energy storage and the renewable energy transition, affordable clean energy transition in developing countries: pathways and technologies, civic engagement and energy transition in the nordic-baltic sea region: parametric and nonparametric inquiries.

• Leruth, L., Mazarei, A., Régibeau, P., Renneboog, L. Green energy depends on critical minerals. Who controls the supply...

Energy transmission metals

The mitigating role of blockchain-enabled supply chains during the covid-19 pandemic, int j oper prod manag, cling together, swing together: the contagious effects of covid-19 on developing countries through global value chains. the world economy, a brief history of nuclear fusion, how many years away is fusion energy a review, j fusion energy, the human rights commitments of private fusion energy companies, j appl econ sci, fusion energy breakthrough by us scientists boosts clean power hopes, european research roadmap to the realisation of fusion energy, china beats the drum for faster fusion energy results, the economic impact of fusion powerin the uk's 2050 energy mix. doctor of phililosophy, design and optimization of power conversion system for a steady state cfetr power plant, fixed-time fractional-order sliding mode controller with disturbance observer for u-tube steam generator, improvement in thermal stability of ods-w alloy through formation of complex oxide dispersoids, hierarchical hypervapotron structure integrated with microchannels for advancement of thermohydraulic performance, global development and readiness of nuclear fusion technology as the alternative source for clean energy supply, green energy technologies: a key driver in carbon emission reduction.

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Science News

Here’s how scientists reached nuclear fusion ‘ignition’ for the first time.

The experiment, performed in 2022, also revealed a never-before-seen phenomenon

In December 2022, scientists at the National Ignition Facility (pictured) achieved nuclear fusion “ignition,” in which the energy produced by the fusing of atomic nuclei exceeds that needed to kick the fusion off.

Jason Laurea/LLNL

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By Emily Conover

February 16, 2024 at 9:30 am

One of nuclear fusion’s biggest advances wouldn’t have happened without some impeccable scientific artistry.

In December 2022, researchers at Lawrence Livermore National Laboratory in California created fusion reactions that produced an excess of energy — a first. In the experiment, 192 lasers blasted a small chamber, setting off fusion reactions — in which smaller atomic nuclei merge to form larger ones — that released more energy than initially kicked them off ( SN: 12/12/22 ). It’s a milestone known as “ignition,” and it has been decades in the making.

Now, researchers have released details of that experiment in five peer-reviewed papers published online February 5 in Physical Review Letters and Physical Review E . The feat demanded an extraordinary level of finesse, tweaking conditions just so to get more energy out of the lasers and create the ideal conditions for fusion.

The work is “exquisitely beautiful,” says physicist Peter Norreys of the University of Oxford. Norreys, who was not involved with the research, compares the achievement to conducting a world-class orchestra: Different elements of the experiment had to be meticulously coordinated and precisely timed.

Scientists also discovered a long-predicted heating effect that could expose the physics of other violent environments, such as exploding stars called supernovas. “People say [physics is] a dry subject,” Norreys says. “But I always think that physics is at the very forefront of creativity,”

The road to nuclear fusion’s big break

Fusion, the same process that takes place in the sun, is an appealing energy source. Fusion power plants wouldn’t emit greenhouse gases. And unlike current nuclear fission power plants, which split atomic nuclei to produce energy, nuclear fusion plants wouldn’t produce dangerous, long-lived radioactive waste. Ignition is the first step toward harnessing such power.

Generating fusion requires extreme pressures and temperatures. In the experiment, the lasers at LLNL’s National Ignition Facility pelted the inside of a hollow cylinder, called a hohlraum, which is about the size of a pencil eraser. The blast heated the hohlraum to a sizzling 3 million degrees Celsius — so hot that it emitted X-rays. Inside this X-ray oven, a diamond capsule contained the fuel: two heavy varieties of hydrogen called deuterium and tritium. The radiation vaporized the capsule’s diamond shell, triggering the fuel to implode at speeds of around 400 kilometers per second, forming the hot, dense conditions that spark fusion.

Previous experiments had gotten tantalizingly close to ignition ( SN: 8/18/21 ). To push further, the researchers increased the energy of the laser pulse from 1.92 million joules to 2.05 million joules. This they accomplished by slightly lengthening the laser pulse, which blasts the target for just a few nanoseconds, extending it by a mere fraction of a nanosecond. (Increasing the laser power directly, rather than lengthening the pulse, risked damage to the facility.)

The team also thickened the capsule’s diamond shell by about 7 percent — a difference of just a few micrometers — which slowed down the capsule’s implosion, allowing the scientists to fully capitalize on the longer laser pulse.  “That was a quite remarkable achievement,” Norreys says.

But these tweaks altered the symmetry of the implosion, which meant other adjustments were needed. It’s like trying to squeeze a basketball down to the size of a pea, says physicist Annie Kritcher of LLNL, “and we’re trying to do that spherically symmetric to within 1 percent.”

That’s particularly challenging because of the mishmash of electrically charged particles, or plasma, that fills the hohlraum during the laser blast. This plasma can absorb the laser beams before they reach the walls of the hohlraum, messing with the implosion’s symmetry.

To even things out, Kritcher and colleagues slightly altered the wavelengths of the laser beams in a way that allowed them to transfer energy from one beam to another. The fix required tweaking the beams’ wavelengths by mere angstroms — tenths of a billionth of a meter.

“Engineering-wise, that’s amazing they could do that,” says physicist Carolyn Kuranz of the University of Michigan in Ann Arbor, who was not involved with the work. What’s more, “these tiny, tiny tweaks make such a phenomenal difference.”

After all the adjustments, the ensuing fusion reactions yielded 3.15 million joules of energy — about 1.5 times the input energy, Kritcher and colleagues reported in Physical Review E . The total energy needed to power NIF’s lasers is much larger, around 350 million joules. While NIF’s lasers are not designed to be energy-efficient, this means that fusion is still far from a practical power source.

Another experiment in July 2023 used a higher-quality diamond capsule and obtained an even larger energy gain of 1.9, meaning it released nearly twice as much energy as went into the reactions ( SN: 10/2/23 ). In the future, NIF researchers hope to be able to increase the laser’s energy from around 2 million joules up to 3 million , which could kick off fusion reactions with a gain as large as 10.

What’s next for fusion

The researchers also discovered a long-predicted phenomenon that could be useful for future experiments: After the lasers heated the hohlraum, it was heated further by effects of the fusion reactions, physicist Mordy Rosen and colleagues report in Physical Review Letters .

Following the implosion, the ignited fuel expanded outward, plowing into the remnants of the diamond shell. That heated the material, which then radiated its heat to the hohlraum. It’s reminiscent of a supernova, in which the shock wave from an exploding star plows through debris the star expelled prior to its explosion ( SN: 2/8/17 ).

“This is exactly the collision that’s happening in this hohlraum,” says Rosen, of LLNL, a coauthor of the study. In addition to explaining supernovas, the effect could help scientists study the physics of nuclear weapons and other extreme situations.

NIF is not the only fusion game in town. Other researchers aim to kick off fusion by confining plasma into a torus, or donut shape, using a device called a tokamak. In a new record, the Joint European Torus in Abingdon, England, generated 69 million joules , a record for total fusion energy production, researchers reported February 8.

After decades of slow progress on fusion, scientists are beginning to get their atomic orchestras in sync.

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Nuclear Fusion is the acknowledged world-leading journal specializing in fusion. The journal covers all aspects of research, theoretical and practical, relevant to controlled thermonuclear fusion.

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E. Joffrin et al 2024 Nucl. Fusion 64 112019

Within the 9th European Framework programme, since 2021 EUROfusion is operating five tokamaks under the auspices of a single Task Force called 'Tokamak Exploitation'. The goal is to benefit from the complementary capabilities of each machine in a coordinated way and help in developing a scientific output scalable to future largre machines. The programme of this Task Force ensures that ASDEX Upgrade, MAST-U, TCV, WEST and JET (since 2022) work together to achieve the objectives of Missions 1 and 2 of the EUROfusion Roadmap: i) demonstrate plasma scenarios that increase the success margin of ITER and satisfy the requirements of DEMO and, ii) demonstrate an integrated approach that can handle the large power leaving ITER and DEMO plasmas. The Tokamak Exploitation task force has therefore organized experiments on these two missions with the goal to strengthen the physics and operational basis for the ITER baseline scenario and for exploiting the recent plasma exhaust enhancements in all four devices (PEX: Plasma EXhaust) for exploring the solution for handling heat and particle exhaust in ITER and develop the conceptual solutions for DEMO. The ITER Baseline scenario has been developed in a similar way in ASDEX Upgrade, TCV and JET. Key risks for ITER such as disruptions and run-aways have been also investigated in TCV, ASDEX Upgrade and JET. Experiments have explored successfully different divertor configurations (standard, super-X, snowflakes) in MAST-U and TCV and studied tungsten melting in WEST and ASDEX Upgrade. The input from the smaller devices to JET has also been proven successful to set-up novel control schemes on disruption avoidance and detachment.

A. V. Timofeev 1966 Nucl. Fusion 6 93

The flute oscillations of a hot-ion plasma in a non-uniform magnetic field are discussed. It is shown that resonances, at which the phase velocity of the waves is the same as the velocity of the ion Larmor drift, can have a stabilizing effect. In particular, with careful selection of the resonance characteristics, one can control the most dangerous "mode 1" of the flute oscillations. Also examined are resonances of the second type, which appear only in an electric field; at them the phase velocity of the wave is the same as the velocity of the plasma electric drift. In the presence of such resonances, the natural frequencies of the flute oscillations become complex of necessity and, accordingly, the oscillations themselves either grow or are damped.

I.F. Kvartskhava et al 1963 Nucl. Fusion 3 285

We discuss the results of experiments carried out with a z pinch of plane configuration and a theta pinch of round cross section. It is shown that under certain discharge conditions the behaviour of the plasma in these pinches is characterized by a far-reaching analogy. On this basis an attempt has been made to use the results of investigation of the plane z pinch for a better understanding of the processes under the complicated conditions of discharge in a theta pinch with a trapped reverse field. In particular we consider a possible mechanism of plasma rotation that has been observed in the theta pinch.under these discharge conditions.

H. Gota et al 2024 Nucl. Fusion 64 112014

TAE Technologies' fifth-generation fusion device, C-2W (also called 'Norman'), is the world's largest compact-toroid device and has made significant progress in field-reversed configuration (FRC) plasma performance. C-2W produces record breaking, macroscopically stable, high-temperature advanced beam-driven FRC plasmas, dominated by injected fast particles and sustained in steady state, which is primarily limited by neutral-beam (NB) pulse duration. The NB power supply system has recently been upgraded to extend the pulse length from 30 ms to 40 ms, which allows for a longer plasma lifetime and thus better characterization and further enhancement of FRC performance. An active plasma control system is routinely used in C-2W to produce consistent FRC performance as well as for reliable machine operations using magnet coils, edge-biasing electrodes, gas injection and tunable-energy NBs. Google's machine learning framework for experimental optimization has also been routinely used to enhance plasma performance. Dedicated plasma optimization experimental campaigns, particularly focused on the external magnetic field profile and NB injection (NBI) optimizations, have produced a superior FRC plasma performance; for instance, achieving a total plasma energy of ∼13 kJ, a trapped poloidal magnetic flux of ∼16 mWb (based on the rigid-rotor model) and plasma sustainment in steady state up to ∼40 ms. Furthermore, under some operating conditions, the electron temperature of FRC plasmas at a quiescent phase has successfully reached up to ∼1 keV at the peak inside the FRC separatrix for the first time. The overall FRC performance is well correlated with the NB and edge-biasing systems, where higher total plasma energy is obtained with higher NBI power and applied voltage on biasing electrodes. C-2W operations have now reached a mature level where the machine can produce hot, stable, long-lived, and repeatable plasmas in a well-controlled manner.

G. Staebler et al 2024 Nucl. Fusion 64 103001

The theory, development, and validation of reduced quasilinear models of gyrokinetic turbulent transport in the closed flux surface core of tokamaks is reviewed. In combination with neoclassical collisional transport, these models are successful in accurately predicting core tokamak plasma temperature, density, rotation, and impurity profiles in a variety of confinement regimes. Refined experimental tests have been performed to validate the predictions of the quasilinear models, probing changes in the dominant gyrokinetic instabilities, as reflected in fluctuation measurements, cross-phases, and transport properties. These tests continue to produce a deeper understanding of the complex mix of instabilities at both electron and ion gyroradius scales.

C.T. Holcomb et al 2024 Nucl. Fusion 64 112003

S.A.M. McNamara et al 2024 Nucl. Fusion 64 112020

C.F. Maggi et al 2024 Nucl. Fusion 64 112012

In 2021 JET exploited its unique capabilities to operate with T and D–T fuel with an ITER-like Be/W wall (JET-ILW). This second major JET D–T campaign (DTE2), after DTE1 in 1997, represented the culmination of a series of JET enhancements—new fusion diagnostics, new T injection capabilities, refurbishment of the T plant, increased auxiliary heating, in-vessel calibration of 14 MeV neutron yield monitors—as well as significant advances in plasma theory and modelling in the fusion community. DTE2 was complemented by a sequence of isotope physics campaigns encompassing operation in pure tritium at high T-NBI power. Carefully conducted for safe operation with tritium, the new T and D–T experiments used 1 kg of T (vs 100 g in DTE1), yielding the most fusion reactor relevant D–T plasmas to date and expanding our understanding of isotopes and D–T mixture physics. Furthermore, since the JET T and DTE2 campaigns occurred almost 25 years after the last major D–T tokamak experiment, it was also a strategic goal of the European fusion programme to refresh operational experience of a nuclear tokamak to prepare staff for ITER operation. The key physics results of the JET T and DTE2 experiments, carried out within the EUROfusion JET1 work package, are reported in this paper. Progress in the technological exploitation of JET D–T operations, development and validation of nuclear codes, neutronic tools and techniques for ITER operations carried out by EUROfusion (started within the Horizon 2020 Framework Programme and continuing under the Horizon Europe FP) are reported in (Litaudon et al Nucl. Fusion accepted), while JET experience on T and D–T operations is presented in (King et al Nucl. Fusion submitted).

J.R. Harrison et al 2024 Nucl. Fusion 64 112017

Recent results from MAST Upgrade are presented, emphasising understanding the capabilities of this new device and deepening understanding of key physics issues for the operation of ITER and the design of future fusion power plants. The impact of MHD instabilities on fast ion confinement have been studied, including the first observation of fast ion losses correlated with Compressional and Global Alfvén Eigenmodes. High-performance plasma scenarios have been developed by tailoring the early plasma current ramp phase to avoid internal reconnection events, resulting in a more monotonic q profile with low central shear. The impact of m / n = 3/2, 2/1 and 1/1 modes on thermal plasma confinement and rotation profiles has been quantified, and scenarios optimised to avoid them have transiently reached values of normalised beta approaching 4.2. In pedestal and ELM physics, a maximum pedestal top temperature of ∼350 eV has been achieved, exceeding the value achieved on MAST at similar heating power. Mitigation of type-I ELMs with n = 1 RMPs has been observed. Studies of plasma exhaust have concentrated on comparing conventional and Super-X divertor configurations, while X-point target, X-divertor and snowflake configurations have been developed and studied in parallel. In L-mode discharges, the separatrix density required to detach the outer divertors is approximately a factor 2 lower in the Super-X than the conventional configuration, in agreement with simulations. Detailed analysis of spectroscopy data from studies of the Super-X configuration reveal the importance of including plasma-molecule interactions and D 2 Fulcher band emission to properly quantify the rates of ionisation, plasma-molecule interactions and volumetric recombination processes governing divertor detachment. In H-mode with conventional and Super-X configurations, the outer divertors are attached in the former and detached in the latter with no impact on core or pedestal confinement.

Francesco Romanelli et al 2024 Nucl. Fusion 64 112015

An overview is presented of the progress since 2021 in the construction and scientific programme preparation of the Divertor Tokamak Test (DTT) facility. Licensing for building construction has been granted at the end of 2021. Licensing for Cat. A radiologic source has been also granted in 2022. The construction of the toroidal field magnet system is progressing. The prototype of the 170 GHz gyrotron has been produced and it is now under test on the FALCON facility. The design of the vacuum vessel, the poloidal field coils and the civil infrastructures has been completed. The shape of the first DTT divertor has been agreed with EUROfusion to test different plasma and exhaust scenarios: single null, double null, X-divertor and negative triangularity plasmas. A detailed research plan is being elaborated with the involvement of the EUROfusion laboratories.

Latest articles

G. Suárez López et al 2024 Nucl. Fusion 64 126012

We study numerically the feasibility of achieving the L-H transition in the current EU-DEMO tokamak baseline using uniquely direct electron heating. The ASTRA code coupled to the TGLF turbulent transport model is used to predict steady-state kinetic plasma profiles for diverse numerical scans. Among them, we have varied the separatrix electron density, the total amount of ECRH power, the microwave beam deposition profile and the plasma impurity content. The solutions are then compared to L-H transition scaling laws to assess whether the found plasma state would enter into H-mode. We find the plasma density and impurity content to be the key variables setting the boundaries in parameter space where the L-H transition is feasible. When impurities can be controlled under a certain threshold, given here for a fully shaped DEMO plasma, the L-H transition is achieved in all the studied conditions.

L.F. Lu et al 2024 Nucl. Fusion 64 126013

Ion cyclotron resonant frequency (ICRF) induced impurity production has raised many concerns since ITER proposed to change the first wall material from beryllium to tungsten. Enhanced DC plasma potential ( V DC ) due to radio frequency (RF) sheath rectification is well known as one of the most important mechanisms behind the RF induced impurities. Our previous work (Lu et al 2018 Plasma Phys. Control. Fusion 60 035003) considered the impact of both the slow wave and the fast wave on the RF sheath rectification in a 2D geometry. It can barely recover the double-hump structure of the V DC poloidal distribution observed in various machines when only the slow wave is modelled using the multi-2D approach which intrinsically assumes the poloidal wavenumber k z is zero. The fast wave on the other hand is found to be more sensitive to a finite k z and may need to be tackled in 3D. This work reports our recent progress on the 3D RF sheath modelling. In this new code, the latest RF sheath boundary conditions (Myra 2021 J. Plasma Phys . 87 905870504) and the realistic 3D ICRF antennas are implemented. Compared to the 2D results, the 3D code could well recover the double-hump poloidal distribution of V DC even with the fast wave included, which confirms our speculation on the necessity of treating the fast wave in 3D. While the double-hump pattern is robust in the simulation, the amplitude of V DC is found to be affected by the magnetic tilt angle and the antenna geometry. This emphasizes the importance of adopting a realistic antenna geometry in the RF sheath modelling. The double-hump V DC poloidal structure breaks as the magnetic tilt angle increases. This is explained by the gyrotropic property of the cold plasma dielectric tensor. The spatial proximity effect we identified in the previous 2D simulations is still valid in 3D. Finally, simulation shows the slow wave dominates the RF sheath excitation in the private scrape-off layer (SOL), while the fast wave gradually takes over when moving to the far SOL region. This code could be a new tool to provide numerical support for ITER impurity assessment and ICRF antenna design.

H. Betar et al 2024 Nucl. Fusion 64 126014

J. Bucalossi et al 2024 Nucl. Fusion 64 112022

The mission of WEST (tungsten-W Environment in Steady-state Tokamak) is to explore long pulse operation in a full tungsten (W) environment for preparing next-step fusion devices (ITER and DEMO) with a focus on testing the ITER actively cooled W divertor in tokamak conditions. Following the successful completion of phase 1 (2016-2021), phase 2 started in December 2022 with the lower divertor made entirely of actively cooled ITER-grade tungsten mono-blocks. A boronization prior the first plasma attempt allowed for a smooth startup with the new divertor. Despite the reduced operating window due to tungsten, rapid progress has been made in long pulse operation, resulting in discharges with a pulse length of 100 s and an injected energy of around 300 MJ per discharge. Plasma startup studies were carried out with equatorial boron nitride limiters to compare them with tungsten limiters, while Ion Cyclotron Resonance Heating assisted startup was attempted. High fluence operation in attached regime, which was the main thrust of the first campaigns, already showed the progressive build up of deposits and appearance of dust, impacting the plasma operation as the plasma fluence increased. In total, the cumulated injected energy during the first campaigns reached 43 GJ and the cumulated plasma time exceeded 5 h. Demonstration of controlled X-Point Radiator regime is also reported, opening a promising route for investigating plasma exhaust and plasma-wall interaction issues in more detached regime. This paper summarises the lessons learned from the manufacturing and the first operation of the ITER-grade divertor, describing the progress achieved in optimising operation in a full W environment with a focus on long pulse operation and plasma wall interaction.

H. Saitoh et al 2024 Nucl. Fusion 64 126011

We report the electrostatic and electromagnetic behaviors of low-frequency fluctuations and their spatial structures observed in the RT-1 (Ring Trap 1) levitated dipole experiment. By using movable Langmuir probes capable of operating under the high-heat flux conditions, we investigated the spatial structures of electrostatic fluctuations in the plasma and compared them with magnetic fluctuation properties. Low-frequency electrostatic fluctuations in low-beta plasma transact into electromagnetic modes in high-beta operation, the latter of which has been found with edge magnetic probes in previous studies. Multi-point measurements with the Langmuir probes revealed that, in low-beta plasma, the fluctuations propagate in the electron diamagnetic direction and exhibit a toroidal mode number of 3 or 4 in a broad region across different magnetic surfaces. In the high-beta plasma, the phase velocity of the fluctuations has a clear dependence on the magnetic surfaces and reverses its toroidal propagation direction according to plasma conditions. These observations are consistent with the interpretation that density fluctuations transported by the drift motion of plasma generate magnetic fluctuations in high-beta conditions, suggesting a similarity with the so-called entropy mode.

Review articles

G.D. Conway et al 2022 Nucl. Fusion 62 013001

Geodesic acoustic modes (GAMs) are ubiquitous oscillatory flow phenomena observed in toroidal magnetic confinement fusion plasmas, such as tokamaks and stellarators. They are recognized as the non-stationary branch of the turbulence driven zonal flows which play a critical regulatory role in cross-field turbulent transport. GAMs are supported by the plasma compressibility due to magnetic geodesic curvature—an intrinsic feature of any toroidal confinement device. GAMs impact the plasma confinement via velocity shearing of turbulent eddies, modulation of transport, and by providing additional routes for energy dissipation. GAMs can also be driven by energetic particles (so-called EGAMs) or even pumped by a variety of other mechanisms, both internal and external to the plasma, opening-up possibilities for plasma diagnosis and turbulence control. In recent years there have been major advances in all areas of GAM research: measurements, theory, and numerical simulations. This review assesses the status of these developments and the progress made towards a unified understanding of the GAM behaviour and its role in plasma confinement. The review begins with tutorial-like reviews of the basic concepts and theory, followed by a series of topic orientated sections covering different aspects of the GAM. The approach adopted here is to present and contrast experimental observations alongside the predictions from theory and numerical simulations. The review concludes with a comprehensive summary of the field, highlighting outstanding issues and prospects for future developments.

L. Marrelli et al 2021 Nucl. Fusion 61 023001

This paper reviews the research on the reversed field pinch (RFP) in the last three decades. Substantial experimental and theoretical progress and transformational changes have been achieved since the last review (Bodin 1990 Nucl. Fusion 30 1717–37). The experiments have been performed in devices with different sizes and capabilities. The largest are RFX-mod in Padova (Italy) and MST in Madison (USA). The experimental community includes also EXTRAP-T2R in Sweden, RELAX in Japan and KTX in China. Impressive improvements in the performance are the result of exploration of two lines: the high current operation (up to 2 MA) with the spontaneous occurrence of helical equilibria with good magnetic flux surfaces and the active control of the current profile. A crucial ingredient for the advancements obtained in the experiments has been the development of state-of-art active feedback control systems allowing the control of MHD instabilities in presence of a thin shell. The balance between achievements and still open issues leads us to the conclusion that the RFP can be a valuable and diverse contributor in the quest for fusion electricity.

Mohamed Abdou et al 2021 Nucl. Fusion 61 013001

The tritium aspects of the DT fuel cycle embody some of the most challenging feasibility and attractiveness issues in the development of fusion systems. The review and analyses in this paper provide important information to understand and quantify these challenges and to define the phase space of plasma physics and fusion technology parameters and features that must guide a serious R&D in the world fusion program. We focus in particular on components, issues and R&D necessary to satisfy three 'principal requirements': (1) achieving tritium self-sufficiency within the fusion system, (2) providing a tritium inventory for the initial start-up of a fusion facility, and (3) managing the safety and biological hazards of tritium. A primary conclusion is that the physics and technology state-of-the-art will not enable DEMO and future power plants to satisfy these principal requirements. We quantify goals and define specific areas and ideas for physics and technology R&D to meet these requirements. A powerful fuel cycle dynamics model was developed to calculate time-dependent tritium inventories and flow rates in all parts and components of the fuel cycle for different ranges of parameters and physics and technology conditions. Dynamics modeling analyses show that the key parameters affecting tritium inventories, tritium start-up inventory, and tritium self-sufficiency are the tritium burn fraction in the plasma ( f b ), fueling efficiency ( η f ), processing time of plasma exhaust in the inner fuel cycle ( t p ), reactor availability factor (AF), reserve time ( t r ) which determines the reserve tritium inventory needed in the storage system in order to keep the plant operational for time t r in case of any malfunction of any part of the tritium processing system, and the doubling time ( t d ). Results show that η f f b > 2% and processing time of 1–4 h are required to achieve tritium self-sufficiency with reasonable confidence. For η f f b = 2% and processing time of 4 h, the tritium start-up inventory required for a 3 GW fusion reactor is ∼11 kg, while it is <5 kg if η f f b = 5% and the processing time is 1 h. To achieve these stringent requirements, a serious R&D program in physics and technology is necessary. The EU-DEMO direct internal recycling concept that carries fuel directly from the plasma exhaust gas to the fueling systems without going through the isotope separation system reduces the overall processing time and tritium inventories and has positive effects on the required tritium breeding ratio (TBR R ). A significant finding is the strong dependence of tritium self-sufficiency on the reactor availability factor. Simulations show that tritium self-sufficiency is: impossible if AF < 10% for any η f f b , possible if AF > 30% and 1% ⩽ η f f b ⩽ 2%, and achievable with reasonable confidence if AF > 50% and η f f b > 2%. These results are of particular concern in light of the low availability factor predicted for the near-term plasma-based experimental facilities (e.g. FNSF, VNS, CTF), and can have repercussions on tritium economy in DEMO reactors as well, unless significant advancements in RAMI are made. There is a linear dependency between the tritium start-up inventory and the fusion power. The required tritium start-up inventory for a fusion facility of 100 MW fusion power is as small as 1 kg. Since fusion power plants will have large powers for better economics, it is important to maintain a 'reserve' tritium inventory in the tritium storage system to continue to fuel the plasma and avoid plant shutdown in case of malfunctions of some parts of the tritium processing lines. But our results show that a reserve time as short as 24 h leads to unacceptable reserve and start-up inventory requirements. Therefore, high reliability and fast maintainability of all components in the fuel cycle are necessary in order to avoid the need for storing reserve tritium inventory sufficient for continued fusion facility operation for more than a few hours. The physics aspects of plasma fueling, tritium burn fraction, and particle and power exhaust are highly interrelated and complex, and predictions for DEMO and power reactors are highly uncertain because of lack of experiments with burning plasma. Fueling by pellet injection on the high field side of tokamak has evolved to be the preferred method to fuel a burning plasma. Extrapolation from the DIII-D penetration scaling shows fueling efficiency expected in DEMO to be <25%, but such extrapolations are highly uncertain. The fueling efficiency of gas in a reactor relevant regime is expected to be extremely poor and not very useful for getting tritium into the core plasma efficiently. Gas fueling will nonetheless be useful for feedback control of the divertor operating parameters. Extensive modeling has been carried out to predict burn fraction, fueling requirements, and fueling efficiency for ITER, DEMO, and beyond. The fueling rate required to operate Q = 10 ITER plasmas in order to provide the required core fueling, helium exhaust and radiative divertor plasma conditions for acceptable divertor power loads was calculated. If this fueling is performed with a 50–50 DT mix, the tritium burn fraction in ITER would be ∼0.36%, which is too low to satisfy the self-sufficiency conditions derived from the dynamics modeling for fusion reactors. Extrapolation to DEMO using this approach would also yield similarly low burn fraction. Extensive analysis presented shows that specific features of edge neutral dynamics in ITER and fusion reactors, which are different from present experiments, open possibilities for optimization of tritium fueling and thus to improve the burn fraction. Using only tritium in pellet fueling of the plasma core, and only deuterium for edge density, divertor power load and ELM control results in significant increase of the burn fraction to 1.8–3.6%. These estimates are performed with physics models whose results cannot be fully validated for ITER and DEMO plasma conditions since these cannot be achieved in present tokamak experiments. Thus, several uncertainties remain regarding particle transport and scenario requirements in ITER and DEMO. The safety standard requirements for protection of the public and release guidelines for tritium have been reviewed. General safety approaches including minimizing tritium inventories, reducing tritium permeation through materials, and decontaminating material for waste disposal have been suggested.

Boris N. Breizman et al 2019 Nucl. Fusion 59 083001

Of all electrons, runaway electrons have long been recognized in the fusion community as a distinctive population. They now attract special attention as a part of ITER mission considerations. This review covers basic physics ingredients of the runaway phenomenon and the ongoing efforts (experimental and theoretical) aimed at runaway electron (RE) taming in the next generation tokamaks. We emphasize the prevailing physics themes of the last 20 years: the hot-tail mechanism of runaway production, RE interaction with impurity ions, the role of synchrotron radiation in runaway kinetics, RE transport in presence of magnetic fluctuations, micro-instabilities driven by REs in magnetized plasmas, and vertical stability of the plasma with REs. The review also discusses implications of the runaway phenomenon for ITER and the current strategy of RE mitigation.

Accepted manuscripts

Zhong et al 

I-mode is a promising regime for future fusion reactors due to the high energy confinement and the moderate particle confinement. However, the effect of lithium, which has been widely applied for particle recycling and impurity control, on I-mode plasma is still unclear. Recently, experiments of real-time lithium powder injection on I-mode plasma have been carried out in EAST Tokamak. It was found that the confinement performance of the I-mode can be improved by the lithium powder injection, which can strongly reduce electron turbulence (ET) and then trigger ion turbulence (IT). Four different regimes of I-mode have been identified in EAST. The Type I I-mode plasma is characterized by the weakly coherent mode (WCM) and the geodesic-acoustic mode (GAM). The Type II I-mode is featured as the WCM and the edge temperature ring oscillation (ETRO). The Type III I-mode corresponds to the plasma with the co-existence of ETRO, GAM, and WCM. The Type IV I-mode denotes the plasma with only WCM but without ETRO and GAM. It has been observed that WCM and ETRO are increased with lithium powder injection due to the reduction of ion and electron turbulence, and the enhancement of the pedestal electron temperature gradient. EAST experiments demonstrate that lithium powder injection is an effective tool for real-time control and confinement improvement of I-mode plasma.

Tierens et al 

Radiofrequency sheath rectification is a phenomenon relevant to the operation of Ion Cyclotron Range of Frequencies (ICRF) actuators in tokamaks. Techniques to model the sheath rectification on 3D ICRF antenna geometries have only recently become available. In this work, we apply the "sheath-equivalent dielectric layer" technique, used previously only on linear devices, in tokamak geometry, computing rectified sheath potentials on the the WEST ICRF antenna. Advancing the state of the art in sheath rectification modeling, we compute the sheath potentials not just on the limiters, but also on the Faraday Screen bars. The calculations show a peak rectified DC potential of 300 V on the limiters and 500 V on the Faraday screen. Assuming a typical sputtering yield curve, the RF sheath rectification increases the sputtering yield from the limiters by a factor of 2.6 w.r.t. the sputtering due to the non-rectified thermal sheath.

Meng et al 

Divertor detachment operation compatible with the core plasma is an effective method to alleviate the steady-state heat flux approaching the divertor target; this scheme will be adopted by high-performance tokamaks in the future, such as ITER. Currently, two ITER-like tungsten divertors with different geometries have been installed at the top and bottom of Experimental Advanced Superconducting Tokamak (EAST), providing conditions for the present study to investigate the effect of divertor geometry on the compatibility between detachment and core plasma. Recent H-mode experiments show that the electron temperature and heat flux of the outer target of the lower divertor decrease lower than those of the upper divertor after similar amounts of impurity are truly injected into the plasma. SOLPS simulations further prove that the closed lower divertor with a 'corner slot' structure is beneficial for trapping impurity and deuterium particles, thereby increasing momentum and energy losses. For the upper single-null discharges, the divertor electron temperature can be relatively reduced by increasing impurity seeding, but it is also easy to decrease the plasma stored energy and even lead to an H-L back transition. In addition, statistical data on discharges with impurity seeding show that the proportion of detachment discharges achieved by using the closed divertor is higher than that achieved by using the open divertor. The discharges with the closed divertor after detachment have less damage to the core plasma performance than those with the open divertor, irrespective of whether neon or argon seeding is employed. This research will support long-pulse detachment operation in EAST and provide a reference for other tokamaks in divertor design.

Zhao et al 

The next generation of large tokamaks, including ITER, will be equipped with a disruption mitigation system (DMS) that can be activated if a disruption is deemed to be imminent. Introducing impurities by pellet (large or shattered) or massive gas injection has been shown to be an effective mitigation mechanism on many tokamaks. The goal of the mitigation is to lessen the thermal and electromagnetic loads from the disruption without generating enough high-energy (runaway) electrons to damage the device. Variations of this mitigation process with impurity injection are presently being tested on many experiments. We have modeled one such impurity injection experiment on DIII-D using the M3D-C1 nonlinear 3D extended MHD code (Jardin et al 2012 J. Comput. Sci. Discovery), The model includes an argon large pellet injection and ablation model, impurity ionization, recombination, and radiation, and runaway electron formation and subsequent evolution, including both Dreicer and avalanche sources. We obtain reasonable agreement with the experimental results for the timescale of the thermal and current quench and for the magnitude of the runaway electron plateau formed during the mitigation. This is the first 3D full MHD simulation with pellets and REs to simulate the disruption process and it also provides a partial validation of the M3D-C1 DMS model.

Zhang et al 

In the 2023 experiment campaign, we measured ion cyclotron emission (ICE) signals on the Experimental Advanced Superconducting Tokamak (EAST), edge ICE excited by tritium ions. A fusion product derived from the deuterium–deuterium (D–D) fusion reaction, whose spectral peak matches the fundamental cyclotron frequency of the tritium ions (ω CT ) in the plasma edge near the last closed flux surface, was observed using the ion cyclotron range of frequency (ICRF) antenna-based diagnostic system at the plasma boundary on the low field side in the EAST. In this study, we present the first observation of ICE with frequency matching at the plasma boundary. The excitation position of ICE is approximately R = 2.29 m on EAST, and we find that ICE is easier to excite below a certain threshold of plasma radiation. To investigate the excitation mechanism of ICE, we obtained the tritium ion distribution via the TRANSP/Fusion Products Model code and used it to explain the excitation mechanism of ICE. The given distribution has a bump-on tail structure in the energy direction and anisotropy in the pitch angle direction. In addition, we explain why high-energy tritium ions can reach and accumulate at the plasma boundary. It is important to study ICE because ICE can help distinguish the species of fusion-product ions, which can also help monitor the fusion alpha ions in large fusion devices, such as CFETR, DEMO, and ITER.

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• 1960-present Nuclear Fusion doi: 10.1088/issn.0029-5515 Online ISSN: 1741-4326 Print ISSN: 0029-5515

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Gaining Ground in Nuclear Fusion

Researchers at the National Ignition Facility (NIF) are celebrating the end of 2022 with a long-awaited result. The team reported yesterday at a press briefing that one of its laser-powered fusion reaction trials produced 1.5 times more energy than it consumed. The result is the second major advance reported this year by the NIF team (see Viewpoint: Fusion Turns Up the Heat ). That demonstration produced a gain (the ratio of energy out to energy in) of 0.72, while this experiment achieves a gain greater than one—a first for any fusion experiment. The breakthrough signifies the culmination of over 60 years of work on fusion research, motivated by the dream of clean, abundant energy. However, researchers note that a commercial laser-based fusion reactor remains “decades away.”

The record-breaking fusion experiment occurred at 1:03 a.m. PST on December 5, 2022, when NIF researchers fired 2 MJ of energy at a fuel target and recorded a fusion energy release of just over 3 MJ. At the press briefing, US Secretary of Energy Jennifer Granholm offered a big congratulations to the NIF team for a result that she said “will go down in the history books.” Lawrence Livermore National Laboratory (LLNL) director Kim Budil said that the result is a huge step for fusion research. She added, “I cannot wait to see where it takes us.”

Based at LLNL in California, the $3.5 billion NIF facility studies future energy production as well as nuclear arms capabilities without the need of actual weapons testing. The technique used to generate nuclear reactions is called inertial confinement fusion (ICF). In a typical experiment, researchers fire 192 high-powered laser beams at a millimeter-wide metal cylinder that holds a peppercorn-sized capsule filled with nuclear fuel. The beams heat the cylinder to a few million degrees Celsius, causing it to release a torrent of x rays that heat and crush the capsule. As a result, the fuel—a mixture of two forms of heavy hydrogen, deuterium and tritium—can briefly become hotter and denser than the Sun. Under these conditions, the hydrogen nuclei fuse together to form helium, producing a few-billionths-of-a-second-long burst of energy.

The NIF experiments faced several setbacks after they started in 2009. In the last few years, however, the team has made steady progress toward yesterday’s goal. One of the major steps occurred in August 2021 when researchers measured an output energy of 1.3 MJ from one of their trials (see Research News: Ignition First in a Fusion Reaction ). After a thorough analysis, the team demonstrated that the burn was self-sustaining, a situation that is called “ignition.” This achievement was roundly applauded, but the gain was only 0.72—putting it below the fabled gain-of-one threshold that much of the fusion community was waiting for.

Since then, NIF researchers have performed several follow-up experiments, implementing improvements to various components to reach a gain of one or more. At the press briefing, Michael Stadermann, who is responsible for NIF’s target fabrication, mentioned the implementation of machine-learning tools in selecting capsules with fewer defects—which can disrupt fuel implosion despite being as small as bacteria. NIF implosion-modeler Annie Kritcher described other changes, including an 8% boost in the laser energy and new techniques for balancing the laser energy that strikes the target.

Researchers outside of NIF have expressed admiration for the breakthrough. “It proves that the long-sought-after goal, the ‘holy grail’ of fusion, can indeed be achieved,” said Jeremy Chittenden from Imperial College London. Matthew Zepf from the Helmholtz Institute Jena in Germany credited the development to improved target design and the use of magnetic fields (see Focus: Magnetic Field Heats Up Fusion ). “From the previous result my judgement was that reaching the gain-of-one threshold was simply a matter of time—although this has come somewhat faster than I expected,” Zepf said. “Exciting times.”

The excitement stems in part from the potential that the new result could have for clean energy. Compared to fission reactions, fusion reactions should produce far less radioactive waste. Fusion fuel is also abundant, as hydrogen can be retrieved from ocean water. However, those at the press briefing stressed that there is much to do to turn laser-based ICF technology from a scientific wonder to a practical energy source.

One of the main obstacles to commercialization is the overall efficiency of the process. Each firing of the lasers requires 300 MJ of electricity, meaning that the fusion reactions are operating at a net loss of 99% of the initial energy input. Another drawback is the repetition rate, which is currently limited to roughly one laser shot per day. A commercial fusion reactor would need to perform a few shots per minute, according to Budil. Overcoming these challenges will take time. “Not six decades, I don’t think. Not five decades, which is what we used to say,” Budil said. Instead, she thinks “a few decades of research on the underlying technologies could put us in a position to build a power plant.” She also remarked that another fusion technique, called magnetically confined fusion, is in some ways closer to commercialization (see Q&A: A 4D Fusion Puzzle ).

Despite the hurdles, fusion researchers and supporters have reason to be optimistic with the ongoing progress. “This milestone moves us one significant step closer to the possibility of zero-carbon, abundant fusion energy powering our society,” Granholm said.

–Michael Schirber

Michael Schirber is a Corresponding Editor for  Physics Magazine based in Lyon, France.

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Achievement of Target Gain Larger than Unity in an Inertial Fusion Experiment

H. abu-shawareb et al. (the indirect drive icf collaboration), phys. rev. lett. 132 , 065102 – published 5 february 2024, see viewpoint: nuclear-fusion reaction beats breakeven.

• Citing Articles (31)
• ACKNOWLEDGMENTS

On December 5, 2022, an indirect drive fusion implosion on the National Ignition Facility (NIF) achieved a target gain G target of 1.5. This is the first laboratory demonstration of exceeding “scientific breakeven” (or G target > 1 ) where 2.05 MJ of 351 nm laser light produced 3.1 MJ of total fusion yield, a result which significantly exceeds the Lawson criterion for fusion ignition as reported in a previous NIF implosion [H. Abu-Shawareb et al. (Indirect Drive ICF Collaboration), Phys. Rev. Lett. 129 , 075001 (2022) ]. This achievement is the culmination of more than five decades of research and gives proof that laboratory fusion, based on fundamental physics principles, is possible. This Letter reports on the target, laser, design, and experimental advancements that led to this result.

• Received 27 October 2023
• Accepted 3 January 2024

DOI: https://doi.org/10.1103/PhysRevLett.132.065102

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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Nuclear-Fusion Reaction Beats Breakeven

Published 5 february 2024.

Scientists have now vetted details of the 2022 laser-powered fusion reaction that produced more energy than it consumed.

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Authors & Affiliations

Energy principles of scientific breakeven in an inertial fusion experiment, o. a. hurricane, d. a. callahan, d. t. casey, a. r. christopherson, a. l. kritcher, o. l. landen, s. a. maclaren, r. nora, p. k. patel, j. ralph, d. schlossberg, p. t. springer, c. v. young, and a. b. zylstra, phys. rev. lett. 132 , 065103 (2024), hohlraum reheating from burning nif implosions, m. s. rubery, m. d. rosen, n. aybar, o. l. landen, l. divol, c. v. young, c. weber, j. hammer, j. d. moody, a. s. moore, a. l. kritcher, a. b. zylstra, o. hurricane, a. e. pak, s. maclaren, g. zimmerman, j. harte, and t. woods, phys. rev. lett. 132 , 065104 (2024), observations and properties of the first laboratory fusion experiment to exceed a target gain of unity, a. pak et al., phys. rev. e 109 , 025203 (2024), design of the first fusion experiment to achieve target energy gain g > 1, a. l. kritcher et al., phys. rev. e 109 , 025204 (2024), article text.

Vol. 132, Iss. 6 — 9 February 2024

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Left: schematic of the target and laser configuration including the hohlraum. Bottom right: the HDC capsule and DT fuel configuration. Top right: total laser power vs time and radiation temperature T r as a function of time. The HDC capsule thickness was increased by ∼ 7 % and the laser energy was increased by ∼ 7.9 % for N221204 (red line) compared to N210808 (black line).

Target gain vs calendar date. The horizontal labels mark the beginning of each year. The color of the narrow target gain bars represents different implosion designs, and the dashed horizontal line represents the target gain = 1     per the NAS ignition criteria [ 26 ].

3D reconstruction of the time-integrated emission-weighted neutron emissivity from two neutron images taken on each shot from orthogonal lines of sight (image projections), for NIF shots (a) N210307, (b) N210808, and (c) N221204. The left color bar corresponds to the 3D represented volume; the right color bar is for the 2D projections of this volume.

NIF DT shot data are plotted in the space of inferred hot spot Lawson parameter which corresponds to hot spot peak pressure times burn duration, p τ , and DD ion temperature, which is the temperature T determined by the neutron-time-of-flight spread of 2.5 MeV neutrons that come from deuterium-deuterium fusion. The curves denote the ignition boundary per the GLC equation described in the text assuming no mix (solid) and with a multiplier of 1.5 and 2 × on the radiative loss to account for increased levels of higher atomic number mix (dashed curves). Experiments N210808 and N221204 exhibit little mixing, Z 2 ¯ ∼ 1.03 ± 0.02 , while other experiments can exhibit higher values depending upon capsule quality, fill-tube size, and hydrodynamic instability control. Data from the Hybrid-E series are highlighted as red points, while earlier campaigns are shown in other colors.

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More durable metals for fusion power reactors

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For many decades, nuclear fusion power has been viewed as the ultimate energy source. A fusion power plant could generate carbon-free energy at a scale needed to address climate change. And it could be fueled by deuterium recovered from an essentially endless source — seawater.

Decades of work and billions of dollars in research funding have yielded many advances, but challenges remain. To Ju Li, the TEPCO Professor in Nuclear Science and Engineering and a professor of materials science and engineering at MIT, there are still two big challenges. The first is to build a fusion power plant that generates more energy than is put into it; in other words, it produces a net output of power. Researchers worldwide are making progress toward meeting that goal.

The second challenge that Li cites sounds straightforward: “How do we get the heat out?” But understanding the problem and finding a solution are both far from obvious.

Research in the MIT Energy Initiative (MITEI) includes development and testing of advanced materials that may help address those challenges, as well as many other challenges of the energy transition. MITEI has multiple corporate members that have been supporting MIT’s efforts to advance technologies required to harness fusion energy.

The problem: An abundance of helium, a destructive force

Key to a fusion reactor is a superheated plasma — an ionized gas — that’s reacting inside a vacuum vessel. As light atoms in the plasma combine to form heavier ones, they release fast neutrons with high kinetic energy that shoot through the surrounding vacuum vessel into a coolant. During this process, those fast neutrons gradually lose their energy by causing radiation damage and generating heat. The heat that’s transferred to the coolant is eventually used to raise steam that drives an electricity-generating turbine.

The problem is finding a material for the vacuum vessel that remains strong enough to keep the reacting plasma and the coolant apart, while allowing the fast neutrons to pass through to the coolant. If one considers only the damage due to neutrons knocking atoms out of position in the metal structure, the vacuum vessel should last a full decade. However, depending on what materials are used in the fabrication of the vacuum vessel, some projections indicate that the vacuum vessel will last only six to 12 months. Why is that? Today’s nuclear fission reactors also generate neutrons, and those reactors last far longer than a year.

The difference is that fusion neutrons possess much higher kinetic energy than fission neutrons do, and as they penetrate the vacuum vessel walls, some of them interact with the nuclei of atoms in the structural material, giving off particles that rapidly turn into helium atoms. The result is hundreds of times more helium atoms than are present in a fission reactor. Those helium atoms look for somewhere to land — a place with low “embedding energy,” a measure that indicates how much energy it takes for a helium atom to be absorbed. As Li explains, “The helium atoms like to go to places with low helium embedding energy.” And in the metals used in fusion vacuum vessels, there are places with relatively low helium embedding energy — namely, naturally occurring openings called grain boundaries.

Metals are made up of individual grains inside which atoms are lined up in an orderly fashion. Where the grains come together there are gaps where the atoms don’t line up as well. That open space has relatively low helium embedding energy, so the helium atoms congregate there. Worse still, helium atoms have a repellent interaction with other atoms, so the helium atoms basically push open the grain boundary. Over time, the opening grows into a continuous crack, and the vacuum vessel breaks.

That congregation of helium atoms explains why the structure fails much sooner than expected based just on the number of helium atoms that are present. Li offers an analogy to illustrate. “Babylon is a city of a million people. But the claim is that 100 bad persons can destroy the whole city — if all those bad persons work at the city hall.” The solution? Give those bad persons other, more attractive places to go, ideally in their own villages.

To Li, the problem and possible solution are the same in a fusion reactor. If many helium atoms go to the grain boundary at once, they can destroy the metal wall. The solution? Add a small amount of a material that has a helium embedding energy even lower than that of the grain boundary. And over the past two years, Li and his team have demonstrated — both theoretically and experimentally — that their diversionary tactic works. By adding nanoscale particles of a carefully selected second material to the metal wall, they’ve found they can keep the helium atoms that form from congregating in the structurally vulnerable grain boundaries in the metal.

Looking for helium-absorbing compounds

To test their idea, So Yeon Kim ScD ’23 of the Department of Materials Science and Engineering and Haowei Xu PhD ’23 of the Department of Nuclear Science and Engineering acquired a sample composed of two materials, or “phases,” one with a lower helium embedding energy than the other. They and their collaborators then implanted helium ions into the sample at a temperature similar to that in a fusion reactor and watched as bubbles of helium formed. Transmission electron microscope images confirmed that the helium bubbles occurred predominantly in the phase with the lower helium embedding energy. As Li notes, “All the damage is in that phase — evidence that it protected the phase with the higher embedding energy.”

Having confirmed their approach, the researchers were ready to  search for helium-absorbing compounds that would work well with iron, which is often the principal metal in vacuum vessel walls. “But calculating helium embedding energy for all sorts of different materials would be computationally demanding and expensive,” says Kim. “We wanted to find a metric that is easy to compute and a reliable indicator of helium embedding energy.”

They found such a metric: the “atomic-scale free volume,” which is basically the maximum size of the internal vacant space available for helium atoms to potentially settle. “This is just the radius of the largest sphere that can fit into a given crystal structure,” explains Kim. “It is a simple calculation.” Examination of a series of possible helium-absorbing ceramic materials confirmed that atomic free volume correlates well with helium embedding energy. Moreover, many of the ceramics they investigated have higher free volume, thus lower embedding energy, than the grain boundaries do.

However, in order to identify options for the nuclear fusion application, the screening needed to include some other factors. For example, in addition to the atomic free volume, a good second phase must be mechanically robust (able to sustain a load); it must not get very radioactive with neutron exposure; and it must be compatible — but not too cozy — with the surrounding metal, so it disperses well but does not dissolve into the metal. “We want to disperse the ceramic phase uniformly in the bulk metal to ensure that all grain boundary regions are close to the dispersed ceramic phase so it can provide protection to those regions,” says Li. “The two phases need to coexist, so the ceramic won’t either clump together or totally dissolve in the iron.”

Using their analytical tools, Kim and Xu examined about 50,000 compounds and identified 750 potential candidates. Of those, a good option for inclusion in a vacuum vessel wall made mainly of iron was iron silicate.

Experimental testing

The researchers were ready to examine samples in the lab. To make the composite material for proof-of-concept  demonstrations , Kim and collaborators dispersed nanoscale particles of iron silicate into iron and implanted helium into that composite material. She took X-ray diffraction (XRD) images before and after implanting the helium and also computed the XRD patterns. The ratio between the implanted helium and the dispersed iron silicate was carefully controlled to allow a direct comparison between the experimental and computed XRD patterns. The measured XRD intensity changed with the helium implantation exactly as the calculations had predicted. “That agreement confirms that atomic helium is being stored within the bulk lattice of the iron silicate,” says Kim.

To follow up, Kim directly counted the number of helium bubbles in the composite. In iron samples without the iron silicate added, grain boundaries were flanked by many helium bubbles. In contrast, in the iron samples with the iron silicate ceramic phase added, helium bubbles were spread throughout the material, with many fewer occurring along the grain boundaries. Thus, the iron silicate had provided sites with low helium-embedding energy that lured the helium atoms away from the grain boundaries, protecting those vulnerable openings and preventing cracks from opening up and causing the vacuum vessel to fail catastrophically.

The researchers conclude that adding just 1 percent (by volume) of iron silicate to the iron walls of the vacuum vessel will cut the number of helium bubbles in half and also reduce their diameter by 20 percent — “and having a lot of small bubbles is OK if they’re not in the grain boundaries,” explains Li.

Thus far, Li and his team have gone from computational studies of the problem and a possible solution to experimental demonstrations that confirm their approach. And they’re well on their way to commercial fabrication of components. “We’ve made powders that are compatible with existing commercial 3D printers and are preloaded with helium-absorbing ceramics,” says Li. The helium-absorbing nanoparticles are well dispersed and should provide sufficient helium uptake to protect the vulnerable grain boundaries in the structural metals of the vessel walls. While Li confirms that there’s more scientific and engineering work to be done, he, along with Alexander O'Brien PhD ’23 of the Department of Nuclear Science and Engineering and Kang Pyo So, a former postdoc in the same department, have already developed a startup company that’s ready to 3D print structural materials that can meet all the challenges faced by the vacuum vessel inside a fusion reactor.

This research was supported by Eni S.p.A. through the MIT Energy Initiative. Additional support was provided by a Kwajeong Scholarship; the U.S. Department of Energy (DOE) Laboratory Directed Research and Development program at Idaho National Laboratory; U.S. DOE Lawrence Livermore National Laboratory; and Creative Materials Discovery Program through the National Research Foundation of Korea.

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LLNL’s Breakthrough Ignition Experiment Highlighted in Physical Review Letters

A 3D reconstruction of the time-integrated emission-weighted neutron emissivity (total neutrons emitted per unit volume) from two neutron images taken on each shot from orthogonal lines of sight (image projections) for NIF shots on (a) March 7, 2021, (b) August 8, 2021, and (c) December 5, 2022. The neutron yield increased by 20-fold, culminating in the 2022 shot when 2.05 megajoules (MJ) of ultraviolet laser light produced a total fusion energy yield of 3.15 MJ, meeting the National Academy of Science definition of ignition. Credit: Mark Meamber

The details of Lawrence Livermore National Laboratory (LLNL)’s historic December 5, 2022, experiment that achieved fusion ignition in a laboratory for the first time are presented in the cover article in the February 5 issue of Physical Review Letters (PRL).

In the paper, researchers explain how, after a decade of overcoming challenges and working to improve the laser drive and optics, experimental design, computer modeling, and target quality in dozens of experiments on LLNL’s National Ignition Facility (NIF), they finally succeeded in achieving “target gain”—producing more energy (3.15 megajoules) than the amount of laser energy delivered to the fusion target (2.05 MJ). That is the definition of ignition used by the National Academy of Science in a 1997 review of NIF (see “Achieving Fusion Ignition” ).

The PRL paper, one of the first peer-reviewed publications describing the ignition experiment, was authored by more than 1,370 researchers from 44 international institutions who contributed over the decades to the remarkable scientific breakthrough. It is accompanied by companion papers in PRL and Physical Review E (PRE) providing further details about the physics theory, design, experimental advancements, and target improvements that enabled the December 5 shot to exceed the extreme conditions that exist in the center of stars and achieve ignition.

“The demonstrated level of target gain of 1.5 times was the first time that fusion target gain was unambiguously achieved in the laboratory in any fusion scheme,” the researchers said. “This achievement is the culmination of more than five decades of research and gives proof that controlled laboratory fusion energy based on fundamental physics principles is possible.”

The PRL paper traces the history of research into fusion, including the innovative 1972 proposal by former LLNL Director John Nuckolls and colleagues for an inertial confinement fusion (ICF) scheme using the radiative power of intense lasers to compress, heat, and confine a reacting plasma (see “A Legacy of Lasers and Laser Fusion Pioneers” ).

The paper also enumerates the challenges the researchers faced in attempting to achieve the self-sustaining nuclear fusion that powers the sun and the stars, where gravitational forces provide the natural confinement, compression, and heating required for fusion. Other than at NIF, the world’s largest and highest-energy laser, these conditions occur on Earth only in an exploding thermonuclear weapon.

A primary goal of ignition is to help ensure the reliability and safety of the nation’s nuclear deterrent in the absence of underground weapons testing. A related LLNL mission is to enhance understanding of the requirements for developing inertial fusion energy as a safe, clean, and virtually unlimited source of energy for the world.

Overcoming the Challenges

“A fundamental obstacle to realizing a fusion energy source,” the researchers said, “has been the ability to control and heat a plasma (a mixture of ions and free electrons) to the conditions required for ignition and to confine the plasma at these conditions over long enough time scales such that more fusion energy is produced than was supplied to initiate the reaction.”

The research that led to LLNL’s success in overcoming this and many related challenges comprises the bulk of the PRL paper; a summary of that work can be found here and in this paper reviewing the history of the ICF program.

While the PRL paper focuses on the December 5, 2022, ignition experiment, the researchers note that three subsequent experiments in 2023 also achieved target gain greater than one. An experiment on July 30 using 2.05 MJ of laser energy resulted in even more energy than the December experiment—3.88 MJ, the highest yield achieved to date. On October 8, NIF achieved fusion ignition for the third time with 1.9 MJ of laser energy resulting in 2.4 MJ of fusion energy yield.

And on October 30, NIF set a new record for laser energy delivered, firing 2.2 MJ of energy for the first time on an ignition target, building on a capability first tested in 2018. The experiment resulted in 3.4 MJ of fusion energy yield. These results demonstrate that NIF can repeatedly conduct fusion experiments at multi-megajoule levels.

The researchers envisage additional improvements in NIF’s yield by further increasing the laser energy and improving hohlraum efficiency to drive larger targets and by optimizing drive and capsule dopant profiles.

They also acknowledged that the energy produced by the fusion reaction is far less than the energy required to fire the facility’s 192 powerful laser beams. Target gain greater than unity “does not imply net energy gain from a practical fusion energy perspective,” they said, “because the energy consumed by the NIF laser facility is typically 100 times larger” than the laser energy delivered to the target.

“The NIF laser architecture and target configuration was chosen to give the highest probability for fusion ignition for research purposes and was not optimized to produce net energy for fusion energy applications,” they said. “Inertial fusion energy applications requiring advancements to the underlying scheme require further development, such as laser energy usage, shot rate, target robustness, higher fuel compression levels, and cost.”

In the wake of the repeated ignition success at NIF, LLNL is now developing an IFE Institutional Initiative to explore those requirements. In December the Laboratory was named by the U.S. Department of Energy to lead the IFE Science and Technology Accelerated Research for Fusion Innovation and Reactor Engineering (STARFIRE) Hub, a four-year, $16-million project to accelerate IFE science and technology.

Here are summaries of the other PRL and PRE papers, all led by Laboratory authors:

• “Energy principles of scientific breakeven in an inertial fusion experiment,” by ICF Chief Scientist Omar Hurricane and colleagues. The paper, which was selected as a PRL Editors’ Suggestion, reports on the physics principles of the design changes that led to the first NIF controlled fusion experiment to produce target gain greater than unity, or “scientific breakeven.” Key elements of the success came from reducing “coast-time” (the time duration between the end of the laser pulse and implosion peak compression) and maximizing the internal energy delivered to the hot spot (the yield-producing part of the fusion fuel), according to the paper. The relationship between implosion asymmetries and hydrodynamic-induced mixing of target capsule material with the fuel, which shifts the threshold for ignition to higher implosion kinetic energy, is also discussed.
• “Design of the first fusion experiment to achieve target energy gain > 1” by Annie Kritcher, the December 5 experiment’s lead designer and team lead for integrated modeling, and colleagues. This PRE paper describes how early NIF experiments were impacted by increased target defects that seeded hydrodynamic instabilities or unintentional low-mode asymmetries from non-uniformities in the target or laser delivery, which led to reduced fusion yields of less than one MJ. The paper details the design changes , including using an extended higher-energy laser pulse to drive a thicker diamond capsule, that led to increased fusion energy output as well as improved robustness for achieving high fusion energies in the presence of significant low-mode asymmetries.
• “Observations and properties of the first laboratory fusion experiment to exceed target gain of 1,” by Art Pak, team lead for stagnation science, Alex Zylstra, the December 5 shot’s principal experimentalist, and colleagues. This PRE paper describes the experimental evidence for the increased drive on the capsule using additional laser energy and control over known degradation mechanisms, which are critical to achieving high performance. Improved fuel compression relative to previous MJ-yield experiments were observed, enabling the study of novel signatures of the ignition and burn propagation to high yield in the laboratory for the first time.
• “Hohlraum reheating from burning NIF implosions,” by Michael Rubery, lead experimentalist, and Mordy Rosen, lead designer/analyst, and colleagues. This paper, also a PRL Editor’s Suggestion, describes, for the first time, reheating of ICF drive hohlraums by the rapidly expanding burning capsule stagnating upon its own previously ablated material and radiating x rays. The hohlraum then reaches temperatures that exceed the peak achieved by the 2 MJ NIF laser system, as measured by the NIF Dante calorimetry systems. The reheating has long been predicted by simulation, but it is only now, with NIF historically exceeding target gain greater than unity, that researchers can observe this phenomenon in the laboratory and understand it more fully, which will allow future applications of this enhanced hohlraum heating.

More Information:

• The Age of Ignition

“What Is Fusion Ignition” (Video)

“Tracing the Steps to LLNL’s Fusion Ignition Breakthrough,” NIF & Photon Science News , February 5, 2024

“Nuclear-Fusion Reaction Beats Breakeven,” Physics , February 5, 2024

“Dan Casey: Applying Ignition’s Lessons to Inertial Fusion Energy,” NIF & Photon Science News , January 10, 2024

“LLNL-Led Team Receives DOE Award to Establish Inertial Fusion Energy Hub,” NIF & Photon Science News , December 7, 2023

“LLNL’s NIF Delivers Record Laser Energy,” NIF & Photon Science News , November 16, 2023

“Nuclear Fusion and the Future of Energy,” NIF & Photon Science News , October 12, 2023

“Physics principles of inertial confinement fusion and U.S. program overview,” Reviews of Modern Physics , June 27, 2023

“Ignition Experiment Advances Stockpile Stewardship Mission,” NIF & Photon Science News , March 9, 2023

“IAEA Webinar Explores LLNL’s Ignition and Energy Gain Breakthroughs,” NIF & Photon Science News , March 6, 2023

“Ignition Gives U.S. ‘Unique Opportunity’ to Lead World’s IFE Research,” NIF & Photon Science News , February 2, 2023

“Milestone Shot Enhances Future of Stockpile Stewardship and Fusion Energy Science,” NIF & Photon Science News , February 15, 2022

“Building to a Solution: The Elements of a Fusion Breakthrough,” NIF & Photon Science News , November 2, 2021

“Fusion Supports the Stockpile,” Science & Technology Review , March, 2021

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A series of research papers renews hope that the long-elusive goal of mimicking the way the sun produces energy might be achievable.

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Scientists developing a compact version of a nuclear fusion reactor have shown in a series of research papers that it should work, renewing hopes that the long-elusive goal of mimicking the way the sun produces energy might be achieved and eventually contribute to the fight against climate change.

Construction of a reactor, called Sparc , which is being developed by researchers at the Massachusetts Institute of Technology and a spinoff company, Commonwealth Fusion Systems , is expected to begin next spring and take three or four years, the researchers and company officials said.

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This ambitious timetable is far faster than that of the world’s largest fusion-power project, a multinational effort in Southern France called ITER, for International Thermonuclear Experimental Reactor. That reactor has been under construction since 2013 and, although it is not designed to generate electricity, is expected to produce a fusion reaction by 2035.

Bob Mumgaard, Commonwealth Fusion’s chief executive and one of the company’s founders, said a goal of the Sparc project was to develop fusion in time for it to play a role in mitigating global warming. “We’re really focused on how you can get to fusion power as quickly as possible,” he said.

Fusion, in which lightweight atoms are brought together at temperatures of tens of millions of degrees to release energy, has been held out as a way for the world to address the climate-change implications of electricity production.

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For First Time, Researchers Produce More Energy from Fusion Than Was Used to Drive It, Promising Further Discovery in Clean Power  and Nuclear Weapons Stewardship

WASHINGTON, D.C. — The U.S. Department of Energy (DOE) and DOE’s National Nuclear Security Administration (NNSA) today announced the achievement of fusion ignition at Lawrence Livermore National Laboratory (LLNL)—a major scientific breakthrough decades in the making that will pave the way for advancements in national defense and the future of clean power. On December 5, a team at LLNL’s National Ignition Facility (NIF) conducted the first controlled fusion experiment in history to reach this milestone, also known as scientific energy breakeven, meaning it produced more energy from fusion than the laser energy used to drive it. This historic, first-of-its kind achievement will provide unprecedented capability to support NNSA’s Stockpile Stewardship Program and will provide invaluable insights into the prospects of clean fusion energy, which would be a game-changer for efforts to achieve President Biden’s goal of a net-zero carbon economy.

“This is a landmark achievement for the researchers and staff at the National Ignition Facility who have dedicated their careers to seeing fusion ignition become a reality, and this milestone will undoubtedly spark even more discovery,” said U.S. Secretary of Energy Jennifer M. Granholm . “The Biden-Harris Administration is committed to supporting our world-class scientists—like the team at NIF—whose work will help us solve humanity’s most complex and pressing problems, like providing clean power to combat climate change and maintaining a nuclear deterrent without nuclear testing.”

“We have had a theoretical understanding of fusion for over a century, but the journey from knowing to doing can be long and arduous. Today’s milestone shows what we can do with perseverance,” said Dr. Arati Prabhakar, the President’s Chief Advisor for Science and Technology and Director of the White House Office of Science and Technology Policy .

“Monday, December 5, 2022, was a historic day in science thanks to the incredible people at Livermore Lab and the National Ignition Facility. In making this breakthrough, they have opened a new chapter in NNSA’s Stockpile Stewardship Program,” said NNSA Administrator Jill Hruby . “I would like to thank the members of Congress who have supported the National Ignition Facility because their belief in the promise of visionary science has been critical for our mission. Our team from around the DOE national laboratories and our international partners have shown us the power of collaboration.”

“The pursuit of fusion ignition in the laboratory is one of the most significant scientific challenges ever tackled by humanity, and achieving it is a triumph of science, engineering, and most of all, people,” LLNL Director Dr. Kim Budil said. “Crossing this threshold is the vision that has driven 60 years of dedicated pursuit—a continual process of learning, building, expanding knowledge and capability, and then finding ways to overcome the new challenges that emerged. These are the problems that the U.S. national laboratories were created to solve.”

“This astonishing scientific advance puts us on the precipice of a future no longer reliant on fossil fuels but instead powered by new clean fusion energy,” U.S. Senate Majority Leader Charles Schumer said. I commend Lawrence Livermore National Labs and its partners in our nation’s Inertial Confinement Fusion (ICF) program, including the University of Rochester’s Lab for Laser Energetics in New York, for achieving this breakthrough. Making this future clean energy world a reality will require our physicists, innovative workers, and brightest minds at our DOE-funded institutions, including the Rochester Laser Lab, to double down on their cutting-edge work. That’s why I’m also proud to announce today that I’ve helped to secure the highest ever authorization of over $624 million this year in the National Defense Authorization Act for the ICF program to build on this amazing breakthrough.”

“After more than a decade of scientific and technical innovation, I congratulate the team at Lawrence Livermore National Laboratory and the National Ignition Facility for their historic accomplishment,” said U.S. Senator Dianne Feinstein (CA) . “This is an exciting step in fusion and everyone at Lawrence Livermore and NIF should be proud of this milestone achievement.”

“This is an historic, innovative achievement that builds on the contributions of generations of Livermore scientists. Today, our nation stands on their collective shoulders. We still have a long way to go, but this is a critical step and I commend the U.S. Department of Energy and all who contributed toward this promising breakthrough, which could help fuel a brighter clean energy future for the United States and humanity,” said U.S. Senator Jack Reed (RI) , the Chairman of the Senate Armed Services Committee.

“This monumental scientific breakthrough is a milestone for the future of clean energy,” said U.S. Senator Alex Padilla (CA) . “While there is more work ahead to harness the potential of fusion energy, I am proud that California scientists continue to lead the way in developing clean energy technologies. I congratulate the scientists at Lawrence Livermore National Laboratory for their dedication to a clean energy future, and I am committed to ensuring they have all of the tools and funding they need to continue this important work.”

“This is a very big deal. We can celebrate another performance record by the National Ignition Facility. This latest achievement is particularly remarkable because NIF used a less spherically symmetrical target than in the August 2021 experiment,” said U.S. Representative Zoe Lofgren (CA-19) . “This significant advancement showcases the future possibilities for the commercialization of fusion energy. Congress and the Administration need to fully fund and properly implement the fusion research provisions in the recent CHIPS and Science Act and likely more. During World War II, we crafted the Manhattan Project for a timely result. The challenges facing the world today are even greater than at that time. We must double down and accelerate the research to explore new pathways for the clean, limitless energy that fusion promises.”

“I am thrilled that NIF—the United States’ most cutting-edge nuclear research facility—has achieved fusion ignition, potentially providing for a new clean and sustainable energy source in the future. This breakthrough will ensure the safety and reliability of our nuclear stockpile, open new frontiers in science, and enable progress toward new ways to power our homes and offices in future decades,” said U.S. Representative Eric Swalwell (CA-15) . “I commend the scientists and researchers for their hard work and dedication that led to this monumental scientific achievement, and I will continue to push for robust funding for NIF to support advancements in fusion research.”

LLNL’s experiment surpassed the fusion threshold by delivering 2.05 megajoules (MJ) of energy to the target, resulting in 3.15 MJ of fusion energy output, demonstrating for the first time a most fundamental science basis for inertial fusion energy (IFE). Many advanced science and technology developments are still needed to achieve simple, affordable IFE to power homes and businesses, and DOE is currently restarting a broad-based, coordinated IFE program in the United States. Combined with private-sector investment, there is a lot of momentum to drive rapid progress toward fusion commercialization.

Fusion is the process by which two light nuclei combine to form a single heavier nucleus, releasing a large amount of energy. In the 1960s, a group of pioneering scientists at LLNL hypothesized that lasers could be used to induce fusion in a laboratory setting. Led by physicist John Nuckolls, who later served as LLNL director from 1988 to 1994, this revolutionary idea became inertial confinement fusion, kicking off more than 60 years of research and development in lasers, optics, diagnostics, target fabrication, computer modeling and simulation, and experimental design.

To pursue this concept, LLNL built a series of increasingly powerful laser systems, leading to the creation of NIF, the world’s largest and most energetic laser system. NIF—located at LLNL in Livermore, Calif.—is the size of a sports stadium and uses powerful laser beams to create temperatures and pressures like those in the cores of stars and giant planets, and inside exploding nuclear weapons.

Achieving ignition was made possible by dedication from LLNL employees as well as countless collaborators at DOE’s Los Alamos National Laboratory, Sandia National Laboratories, and Nevada National Security Site; General Atomics; academic institutions, including the University of Rochester’s Laboratory for Laser Energetics, the Massachusetts Institute of Technology, the University of California, Berkeley, and Princeton University; international partners, including the United Kingdom’s Atomic Weapons Establishment and the French Alternative Energies and Atomic Energy Commission; and stakeholders at DOE and NNSA and in Congress.