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{{short description|Branch of fusion energy research}}File:NOVA laser.jpg|thumb|right|upright=1.3|Inertial confinement fusion using lasers rapidly progressed in the late 1970s and early 1980s from being able to deliver only a few joules of laser energy (per pulse) to being able to deliver tens of kilojoules to a target. At this point, very large scientific devices were needed for experimentation. Here, a view of the 10 beam LLNL Nova laser, shown shortly after the laser's completion in 1984. Around the time of the construction of its predecessor, the Shiva laser, laser fusion had entered the realm of "big sciencebig scienceInertial confinement fusion (ICF) is a fusion energy process that initiates nuclear fusion reactions by compressing and heating targets filled with fuel. The targets are small pellets, typically containing deuterium (2H) and tritium (3H).Energy is deposited in the target's outer layer, which explodes outward. This produces a reaction force in the form of shock waves that travel through the target. The waves compress and heat it. Sufficiently powerful shock waves generate fusion.ICF is one of two major branches of fusion energy research; the other is magnetic confinement fusion (MCF). When first proposed in the early 1970s, ICF appeared to be a practical approach to power production and the field flourished. Experiments demonstrated that the efficiency of these devices was much lower than expected. Throughout the 1980s and '90s, experiments were conducted in order to understand the interaction of high-intensity laser light and plasma. These led to the design of much larger machines that achieved ignition-generating energies.The largest operational ICF experiment is the National Ignition Facility (NIF) in the US. In 2022, the NIF produced fusion, delivering 2.05 megajoules (MJ) of energy to the target which produced 3.15 MJ, the first time that an ICF device produced more energy than was delivered to the target.WEB, National Ignition Facility achieves fusion ignition,weblink 2022-12-13, Lawrence Livermore National Laboratory, en, WEB, Adrienne Vogt, Mike Hayes, Ella Nilsen, Elise Hammond, 2022-12-13, December 13, 2022 US officials announce nuclear fusion breakthrough,weblink 2022-12-14, CNN, en, {{Toclimit|3}}

Description

Fusion basics

Fusion reactions combine smaller atoms to form larger ones. This occurs when two atoms (or ions, atoms stripped of their electrons) come close enough to each other that the nuclear force dominates the electrostatic force that otherwise keeps them apart. Overcoming electrostatic repulsion requires kinetic energy sufficient to overcome the Coulomb barrier or fusion barrier.WEB,weblink Basic fusion physics, International Atomic Energy Agency, 12 October 2016, Less energy is needed to cause lighter nuclei to fuse, as they have less electrical charge and thus a lower barrier energy. Thus the barrier is lowest for hydrogen. Conversely, the nuclear force increases with the number of nucleons, so isotopes of hydrogen that contain additional neutrons reduce the required energy. The easiest fuel is a mixture of 2H, and 3H, known as D-T.The odds of fusion occurring are a function of the fuel density and temperature and the length of time that the density and temperature are maintained. Even under ideal conditions, the chance that a D and T pair fuse is very small. Higher density and longer times allow more encounters among the atoms. This cross section is further dependent on individual ion energies. This combination, the fusion triple product, must reach the Lawson criterion, to reach ignition.WEB, Mark, Hoffman,weblink What Is The Lawson Criteria, Or How to Make Fusion Power Viable, Scienceworldreport.com, 2013-03-23, 2014-08-23,

Thermonuclear devices

{{see also|Teller-Ulam design}}The first ICF devices were the hydrogen bombs invented in the early 1950s. A hydrogen bomb consists of two bombs in a single case. The first, the primary stage, is a fission-powered device normally using plutonium. When it explodes it gives off a burst of thermal X-rays that fill the interior of the specially designed bomb casing. These X-rays are absorbed by a special material surrounding the secondary stage, which consists mostly of the fusion fuel. The X-rays heat this material and cause it to explode. Due to Newton's Third Law, this causes the fuel inside to be driven inward, compressing and heating it. This causes the fusion fuel to reach the temperature and density where fusion reactions begin.WEB,weblink Section 4.0 Engineering and Design of Nuclear Weapons, Sublette, Carey, 2019-03-19, Nuclear Weapon Archive, en, 2021-02-09, live,weblink" title="web.archive.org/web/20210206233652weblink">weblink 2021-02-06, dmy-all, {{sfn|Nuckolls|1998|p=1}}In the case of D-T fuel, most of the energy is released in the form of alpha particles and neutrons. Under normal conditions, an alpha can travel about 10 mm through the fuel, but in the ultra-dense conditions in the compressed fuel, they can travel about 0.01 mm before their electrical charge, interacting with the surrounding plasma, causes them to lose velocity.{{sfn|Keefe|1982|p=10}} This means the majority of the energy released by the alphas is redeposited in the fuel. This transfer of kinetic energy heats the surrounding particles to the energies they need to undergo fusion. This process causes the fusion fuel to burn outward from the center. The electrically neutral neutrons travel longer distances in the fuel mass and do not contribute to this self-heating process. In a bomb, they are instead used to either breed tritium through reactions in a lithium-deuteride fuel, or are used to split additional fissionable fuel surrounding the secondary stage, often part of the bomb casing.The requirement that the reaction has to be sparked by a fission bomb makes this method impractical for power generation. Not only would the fission triggers be expensive to produce, but the minimum size of such a bomb is large, defined roughly by the critical mass of the plutonium fuel used. Generally, it seems difficult to build efficient nuclear fusion devices much smaller than about 1 kiloton in yield, and the fusion secondary would add to this yield. This makes it a difficult engineering problem to extract power from the resulting explosions. Project PACER studied solutions to the engineering issues,{{sfn|Nuckolls|1998|p=1}} but also demonstrated that it was not economically feasible. The cost of the bombs was far greater than the value of the resulting electricity.JOURNAL, F., Long,weblink Peaceful Nuclear Explosions, Bulletin of the Atomic Scientists, 32, 8, October 1976, 18, 10.1080/00963402.1976.11455642, 1976BuAtS..32h..18L,

Mechanism of action{{Anchor|ICF mechanism of action}}

The energy needed to overcome the Coulomb barrier corresponds to the energy of the average particle in a gas heated to 100 million K. The specific heat of hydrogen is about 14 Joule per gram-K, so considering a 1 milligram fuel pellet, the energy needed to raise the mass as a whole to this temperature is 1.4  megajoules (MJ).{{sfn|Emmett|Nuckolls|Wood|1974|p=24}}In the more widely developed magnetic fusion energy (MFE) approach, confinement times are on the order of one second. However, plasmas can be sustained for minutes. In this case the confinement time represents the amount of time it takes for the energy from the reaction to be lost to the environment - through a variety of mechanisms. For a one second confinement, the density needed to meet the Lawson criterion is about 1014 particles per cubic centimetre (cc).{{sfn|Emmett|Nuckolls|Wood|1974|p=24}} For comparison, air at sea level has about 2.7 x 1019 particles/cc, so the MFE approach has been described as "a good vacuum".Considering a 1 milligram drop of D-T fuel in liquid form, the size is about 1 mm and the density is about 4 x 1020/cc. Nothing holds the fuel together. Heat created by fusion events causes it to expand at the Emmett|Nuckolls|Wood|1974|p=25}}The rate of fusion reactions is a function of density, and density can be improved through compression. If the drop is compressed from 1 mm to 0.1 mm in diameter, the confinement time drops by the same factor of 10, because the particles have less distance to travel before they escape. However, the density, which is the cube of the dimensions, increases by 1,000 times. This means the overall rate of fusion increases 1,000 times while the confinement drops by 10 times, a 100-fold improvement. In this case 10% of the fuel undergoes fusion; 10% of 1 mg of fuel produces about 30 MJ of energy, 30 times the amount needed to compress it to that density.{{sfn|Emmett|Nuckolls|Wood|1974|pp=25-26}}The other key concept in ICF is that the entire fuel mass does not have to be raised to 100 million K. In a fusion bomb the reaction continues because the alpha particles released in the interior heat the fuel around it. At liquid density the alphas travel about 10 mm and thus their energy escapes the fuel. In the 0.1 mm compressed fuel, the alphas have a range of about 0.016 mm, meaning that they will stop within the fuel and heat it. In this case a "propagating burn" can be caused by heating only the center of the fuel to the needed temperature. This requires far less energy; calculations suggested 1 kJ is enough to reach the compression goal.{{sfn|Emmett|Nuckolls|Wood|1974|p=26}}Some method is needed to heat the interior to fusion temperatures, and do so while when the fuel is compressed and the density is high enough.{{sfn|Emmett|Nuckolls|Wood|1974|p=26}} In modern ICF devices, the density of the compressed fuel mixture is as much as one-thousand times the density of water, or one-hundred times that of lead, around 1000 g/cm3.{{sfn|Malik|2021|p=284}} Much of the work since the 1970s has been on ways to create the central hot-spot that starts off the burning, and dealing with the many practical problems in reaching the desired density. The initial solution to the heating problem involved deliberate "shaping" of the energy delivery. The idea was to use an initial lower-energy pulse to vaporize the capsule and cause compression, and then a very short, very powerful pulse near the end of the compression cycle. The goal is to launch shock waves into the compressed fuel that travel inward to the center. When they reach the center they meet the waves coming in from other sides. This causes a brief period where the density in the center reaches much higher values, over 800 g/cm3.{{sfn|Pfalzner|2006|p=15}}The central hot spot ignition concept was the first to suggest ICF was not only a practical route to fusion, but relatively simple. This led to numerous efforts to build working systems in the early 1970s. These experiments revealed unexpected loss mechanisms. Early calculations suggested about 4.5x107 J/g would be needed, but modern calculations place it closer to 108 J/g. Greater understanding led to complex shaping of the pulse into multiple time intervals.WEB,weblink Pulse Shaping, LLNL,

Fast ignition

The fast ignition approach employs a separate laser to supply additional energy directly to the center of the fuel. This can be done mechanically, often using a small metal cone to puncture the outer fuel pellet wall to inject the energy into the center. In tests, this approach failed{{Citation needed|reason= Please add source|date=November 2023}} because the laser pulse had to reach the center at a precise moment, while the center is obscured by debris and free electrons from the compression pulse. It also has the disadvantage of requiring a second laser pulse, which generally involves a completely separate laser.

Shock ignition

Shock ignition is similar in concept to the hot-spot technique, but instead of achieving ignition via compression heating, a powerful shock wave is sent into the fuel at a later time through a combination of compression and shock heating. This increases the efficiency of the process while lowering the overall amount of power required.

Direct vs. indirect drive

In the simplest method of inertial confinement, the fuel is arranged as a sphere. This allows it to be compressed uniformly from all sides. To produce the inward force, the fuel is placed within a thin capsule that absorbs energy from the driver beams, causing the capsule shell to explode outward. The capsule shell is usually made of a lightweight plastic, and the fuel is deposited as a layer on the inside by injecting and freezing the gaseous fuel into the shell.Shining the driver beams directly onto the fuel capsule is known as "direct drive". The implosion process must be extremely uniform in order to avoid asymmetry due to Rayleigh–Taylor instability and similar effects. For a beam energy of 1 MJ, the fuel capsule cannot be larger than about 2 mm before these effects disrupt the implosion symmetry. This limits the size of the laser beams to a diameter so narrow that it is difficult to achieve in practice.Alternatively "indirect drive" illuminates a small cylinder of heavy metal, often gold or lead, known as a hohlraum. The beam energy heats the hohlraum until it emits X-rays. These X-rays fill the interior of the hohlraum and heat the capsule. The advantage of indirect drive is that the beams can be larger and less accurate. The disadvantage is that much of the delivered energy is used to heat the hohlraum until it is "X-ray hot", so the end-to-end energy efficiency is much lower than the direct drive method.

Challenges

The primary challenges with increasing ICF performance are:

• Improving the energy delivered to the target
• Controlling symmetry of the imploding fuel
• Delaying fuel heating until sufficient density is achieved
• Preventing premature mixing of hot and cool fuel by hydrodynamic instabilities
• Achieving shockwave convergence at the fuel center

In order to focus the shock wave on the center of the target, the target must be made with great precision and sphericity with tolerances of no more than a few micrometres over its (inner and outer) surface. The lasers must be precisely targeted in space and time. Beam timing is relatively simple and is solved by using delay lines in the beams' optical path to achieve picosecond accuracy. {{Anchor|instability2016-01-29}}The other major issue is so-called "beam-beam" imbalance and beam anisotropy. These problems are, respectively, where the energy delivered by one beam may be higher or lower than other beams impinging and of "hot spots" within a beam diameter hitting a target which induces uneven compression on the target surface, thereby forming Rayleigh-Taylor instabilitiesJOURNAL, Hayes, A. C., Jungman, G., Solem, J. C., Bradley, P. A., Rundberg, R. S., 2006, Prompt beta spectroscopy as a diagnostic for mix in ignited NIF capsules, Modern Physics Letters A, 21, 13, 1029, physics/0408057, 2006MPLA...21.1029H, 10.1142/S0217732306020317, 119339212, in the fuel, prematurely mixing it and reducing heating efficacy at the instant of maximum compression. The Richtmyer-Meshkov instability is also formed during the process due to shock waves. These problems have been mitigated by beam smoothing techniques and beam energy diagnostics; however, RT instability remains a major issue. Modern cryogenic hydrogen ice targets tend to freeze a thin layer of deuterium on the inside of the shell while irradiating it with a low power infrared laser to smooth its inner surface and monitoring it with a microscope equipped camera, thereby allowing the layer to be closely monitored.WEB,weblinkweblink" title="web.archive.org/web/20090511175948weblink">weblink dead, Inertial Confinement Fusion Program Activities, April 2002, May 11, 2009, Cryogenic targets filled with D-T are "self-smoothing" due to the small amount of heat created by tritium decay. This is referred to as "beta-layering".WEB,weblinkweblink" title="web.archive.org/web/20090511180008weblink">weblink dead, Inertial Confinement Fusion Program Activities, March 2006, May 11, 2009, In the indirect drive approach,{{Citation|last1=Lindl |first1=John |last2=Hammel |first2=Bruce |contribution=Recent Advances in Indirect Drive ICF Target Physics |year=2004 |title=20th IAEA Fusion Energy Conference |publisher=Lawrence Livermore National Laboratory |url=http://fire.pppl.gov/iaea04_lindl.pdf |archive-url=https://ghostarchive.org/archive/20221009weblink |archive-date=2022-10-09 |url-status=live |access-date=August 23, 2014}} the absorption of thermal x-rays by the target is more efficient than the direct absorption of laser light. However, the hohlraums take up considerable energy to heat, significantly reducing energy transfer efficiency. Most often, indirect drive hohlraum targets are used to simulate thermonuclear weapons tests due to the fact that the fusion fuel in weapons is also imploded mainly by X-ray radiation.ICF drivers are evolving. Lasers have scaled up from a few joules and kilowatts to megajoules and hundreds of terawatts, using mostly frequency doubled or tripled light from neodymium glass amplifiers.{{Citation needed|date=February 2023}}Heavy ion beams are particularly interesting for commercial generation, as they are easy to create, control, and focus. However, it is difficult to achieve the energy densities required to implode a target efficiently, and most ion-beam systems require the use of a hohlraum surrounding the target to smooth out the irradiation.{{Citation needed|date=February 2023}}

History

{{see also|Timeline of nuclear fusion}}

Conception

United States

ICF history began as part of the "Atoms For Peace" conference in 1957. This was an international, UN-sponsored conference between the US and the Soviet Union. Some thought was given to using a hydrogen bomb to heat a water-filled cavern. The resulting steam could then be used to power conventional generators, and thereby provide electrical power.{{sfn|Nuckolls|1998|p=1}}This meeting led to Operation Plowshare, formed in June 1957 and formally named in 1961. It included three primary concepts; energy generation under Project PACER, the use of nuclear explosions for excavation, and for fracking in the natural gas industry. PACER was directly tested in December 1961 when the 3 kt Project Gnome device was detonated in bedded salt in New Mexico. While the press looked on, radioactive steam was released from the drill shaft, at some distance from the test site. Further studies designed engineered cavities to replace natural ones, but Plowshare turned from bad to worse, especially after the failure of 1962's Sedan which produced significant fallout. PACER continued to receive funding until 1975, when a 3rd party study demonstrated that the cost of electricity from PACER would be ten times the cost of conventional nuclear plants.F.A. Long, "Peaceful nuclear explosions", Bulletin of the Atomic Scientists, October 1976, pp. 24-25.Another outcome of Atoms For Peace was to prompt John Nuckolls to consider what happens on the fusion side of the bomb as fuel mass is reduced. This work suggested that at sizes on the order of milligrams, little energy would be needed to ignite the fuel, much less than a fission primary.{{sfn|Nuckolls|1998|p=1}} He proposed building, in effect, tiny all-fusion explosives using a tiny drop of D-T fuel suspended in the center of a hohlraum. The shell provided the same effect as the bomb casing in an H-bomb, trapping x-rays inside to irradiate the fuel. The main difference is that the X-rays would be supplied by an external device that heated the shell from the outside until it was glowing in the x-ray region. The power would be delivered by a then-unidentified pulsed power source he referred to, using bomb terminology, as the "primary".{{sfn|Nuckolls|1998|p=2}}The main advantage to this scheme is the fusion efficiency at high densities. According to the Lawson criterion, the amount of energy needed to heat the D-T fuel to break-even conditions at ambient pressure is perhaps 100 times greater than the energy needed to compress it to a pressure that would deliver the same rate of fusion. So, in theory, the ICF approach could offer dramatically more gain.{{sfn|Nuckolls|1998|p=2}} This can be understood by considering the energy losses in a conventional scenario where the fuel is slowly heated, as in the case of magnetic fusion energy; the rate of energy loss to the environment is based on the temperature difference between the fuel and its surroundings, which continues to increase as the fuel temperature increases. In the ICF case, the entire hohlraum is filled with high-temperature radiation, limiting losses.{{sfn|Nuckolls|1998|p=3}}

Germany

In 1956 a meeting was organized at the Max Planck Institute in Germany by fusion pioneer Carl Friedrich von Weizsäcker. At this meeting Friedwardt Winterberg proposed the non-fission ignition of a thermonuclear micro-explosion by a convergent shock wave driven with high explosives.Archives of Library University of Stuttgart, Konvolut 7, Estate of Professor Dr. Hoecker, 1956 von Weizsäcker, Meeting in Göttingen Further reference to Winterberg's work in Germany on nuclear micro explosions (mininukes) is contained in a declassified report of the former East German Stasi (Staatsicherheitsdienst).Stasi Report of the former East German Democratic Republic, MfS-AGM by "Der Bundesbeauftragte für die Unterlagen des Staatsicherheitsdienstes der ehemaligen Deutschen Demokratischen Republik," Zentralarchiv, Berlin, 1987In 1964 Winterberg proposed that ignition could be achieved by an intense beam of microparticles accelerated to a velocity of 1000 km/s.F. Winterberg, Z. f. Naturforsch. 19a, 231 (1964) In 1968, he proposed to use intense electron and ion beams generated by Marx generators for the same purpose.F. Winterberg, Phys. Rev. 174, 212 (1968) The advantage of this proposal is that charged particle beams are not only less expensive than laser beams, but can entrap the charged fusion reaction products due to the strong self-magnetic beam field, drastically reducing the compression requirements for beam ignited cylindrical targets.

USSR

In 1967, research fellow Gurgen Askaryan published an article proposing the use of focused laser beams in the fusion of lithium deuteride or deuterium.JOURNAL,weblink Gurgen Askaryan, ru:Новые физические эффекты, New Physical Effects, ru, Nauka i Zhizn, 11, 1967, 105, Gurgen Askaryan,

Early research

Through the late 1950s, and collaborators at Lawrence Livermore National Laboratory (LLNL) completed computer simulations of the ICF concept. In early 1960, they performed a full simulation of the implosion of 1 mg of D-T fuel inside a dense shell. The simulation suggested that a 5 MJ power input to the hohlraum would produce 50 MJ of fusion output, a gain of 10x. This was before the laser and a variety of other possible drivers were considered, including pulsed power machines, charged particle accelerators, plasma guns, and hypervelocity pellet guns.{{sfn|Nuckolls|1998|p=4}}Two theoretical advances advanced the field. One came from new simulations that considered the timing of the energy delivered in the pulse, known as "pulse shaping", leading to better implosion. The second was to make the shell much larger and thinner, forming a thin shell as opposed to an almost solid ball. These two changes dramatically increased implosion efficiency and thereby greatly lowered the required compression energy. Using these improvements, it was calculated that a driver of about 1 MJ would be needed,{{sfn|Nuckolls|1998|p=5}} a five-fold reduction. Over the next two years, other theoretical advancements were proposed, notably Ray Kidder's development of an implosion system without a hohlraum, the so-called "direct drive" approach, and Stirling Colgate and Ron Zabawski's work on systems with as little as 1 μg of D-T fuel.{{sfn|Nuckolls|1998|pp=4-5}}The introduction of the laser in 1960 at Hughes Research Laboratories in California appeared to present a perfect driver mechanism. Starting in 1962, Livermore's director John S. Foster, Jr. and Edward Teller began a small ICF laser study. Even at this early stage the suitability of ICF for weapons research was well understood and was the primary reason for its funding.{{sfn|Nuckolls|1998|p=6}} Over the next decade, LLNL made small experimental devices for basic laser-plasma interaction studies.

Development begins

In 1967 Kip Siegel started KMS Industries. In the early 1970s he formed KMS Fusion to begin development of a laser-based ICF system.Sean Johnston, "Interview with Dr. Larry Siebert" {{Webarchive|url=https://web.archive.org/web/20121012113800weblink |date=2012-10-12 }}, American Institute of Physics, 4 September 2004 This development led to considerable opposition from the weapons labs, including LLNL, who put forth a variety of reasons that KMS should not be allowed to develop ICF in public. This opposition was funnelled through the Atomic Energy Commission, which demanded funding. Adding to the background noise were rumours of an aggressive Soviet ICF program, new higher-powered CO2 and glass lasers, the electron beam driver concept, and the energy crisis which added impetus to many energy projects.{{sfn|Nuckolls|1998|p=6}}In 1972 John Nuckolls wrote a paper introducing ICF and suggesting that testbed systems could be made to generate fusion with drivers in the kJ range, and high-gain systems with MJ drivers.{{Citation |last1=Nuckolls |first1=John |last2=Wood |first2=Lowell |last3=Thiessen |first3=Albert |last4=Zimmerman |first4=George |title=Laser Compression of Matter to Super-High Densities: Thermonuclear (CTR) Applications |journal=Nature |volume=239 |issue=5368 |year=1972 |pages=139–142 |doi=10.1038/239139a0 |bibcode = 1972Natur.239..139N |s2cid=45684425 }}{{Citation |last=Lindl |first=J.D. |contribution=The Edward Teller medal lecture: The evolution toward Indirect Drive and two decades of progress toward ICF ignition and burn |year=1993 |title=International workshop on laser interaction and related plasma phenomena |publisher=Department of Energy (DOE)'s Office of Scientific and Technical Information (OSTI) |url=http://www.osti.gov/bridge/servlets/purl/10126383-6NAuBK/native/10126383.pdf |archive-url=https://ghostarchive.org/archive/20221009weblink |archive-date=2022-10-09 |url-status=live |access-date=August 23, 2014}}In spite of limited resources and business problems, KMS Fusion successfully demonstrated IFC fusion on 1 May 1974.WEB,weblink Philip, Wyatt, The Back Page, Aps.org, December 2009, 2014-08-23, This success was soon followed by Siegel's death and the end of KMS Fusion a year later. By this point several weapons labs and universities had started their own programs, notably the solid-state lasers ((List of laser types#Solid-state lasers|Nd:glass laser)s) at LLNL and the University of Rochester, and krypton fluoride excimer lasers systems at Los Alamos and the Naval Research Laboratory.

"High-energy" ICF

High-energy ICF experiments (multi-hundred joules per shot) began in the early 1970s, when better lasers appeared. Funding for fusion research was stimulated by energy crises produced rapid gains in performance, and inertial designs were soon reaching the same sort of "below break-even" conditions of the best MCF systems.LLNL was, in particular, well funded and started a laser fusion development program. Their Janus laser started operation in 1974, and validated the approach of using Nd:glass lasers for high power devices. Focusing problems were explored in the Long path and Cyclops lasers, which led to the larger Argus laser. None of these were intended to be practical devices, but they increased confidence that the approach was valid. It was then believed that a much larger device of the Cyclops type could both compress and heat targets, leading to ignition. This misconception was based on extrapolation of the fusion yields seen from experiments utilizing the so-called "exploding pusher" fuel capsule. During the late 1970s and early 1980s the estimates for laser energy on target needed to achieve ignition doubled almost yearly as plasma instabilities and laser-plasma energy coupling loss modes were increasingly understood. The realization that exploding pusher target designs and single-digit kilojoule (kJ) laser irradiation intensities would never scale to high yields led to the effort to increase laser energies to the 100 kJ level in the ultraviolet band and to the production of advanced ablator and cryogenic DT ice target designs.

Shiva and Nova

One of the earliest large scale attempts at an ICF driver design was the Shiva laser, a 20-beam neodymium doped glass laser system at LLNL that started operation in 1978. Shiva was a "proof of concept" design intended to demonstrate compression of fusion fuel capsules to many times the liquid density of hydrogen. In this, Shiva succeeded, reaching 100 times the liquid density of deuterium. However, due to the laser's coupling with hot electrons, premature heating of the dense plasma was problematic and fusion yields were low. This failure to efficiently heat the compressed plasma pointed to the use of optical frequency multipliers as a solution that would frequency triple the infrared light from the laser into the ultraviolet at 351 nm. Schemes to efficiently triple the frequency of laser light discovered at the Laboratory for Laser Energetics in 1980 was experimented with in the 24 beam OMEGA laser and the NOVETTE laser, which was followed by the Nova laser design with 10 times Shiva's energy, the first design with the specific goal of reaching ignition.Nova also failed, this time due to severe variation in laser intensity in its beams (and differences in intensity between beams) caused by filamentation that resulted in large non-uniformity in irradiation smoothness at the target and asymmetric implosion. The techniques pioneered earlier could not address these new issues. This failure led to a much greater understanding of the process of implosion, and the way forward again seemed clear, namely to increase the uniformity of irradiation, reduce hot-spots in the laser beams through beam smoothing techniques to reduce Rayleigh–Taylor instabilities and increase laser energy on target by at least an order of magnitude. Funding was constrained in the 1980s.

National Ignition Facility

(File:U.S. Department of Energy - Science - 115 057 004 (17974887118).jpg|thumb|National Ignition Facility target chamber)The resulting 192-beam design, dubbed the National Ignition Facility, started construction at LLNL in 1997. NIF's main objective is to operate as the flagship experimental device of the so-called nuclear stewardship program, supporting LLNLs traditional bomb-making role. Completed in March 2009,WEB, Bob, Hirschfeld,weblink DOE announces completion of world's largest laser, Publicaffairs.llnl.gov, March 31, 2009, 2014-08-23, dead,weblink May 27, 2010, NIF experiments set new records for power delivery by a laser.NEWS,weblink Laser fusion test results raise energy hopes, BBC News, Jason Palmer, 2010-01-28, 2010-01-28, WEB,weblink Initial NIF experiments meet requirements for fusion ignition, Lawrence Livermore National Laboratory, 2010-01-28, 2010-01-28, dead,weblink 2010-05-27, As of September 27, 2013, for the first time fusion energy generated was greater than the energy absorbed into deuterium–tritium fuel.JOURNAL, Philip Ball, Laser fusion experiment extracts net energy from fuel, 12–27, Nature (journal), Nature, 12 February 2014,weblink 10.1038/nature.2014.14710, 138079001, 2014-02-13, Philip Ball, NEWS,weblink Nuclear fusion milestone passed at US lab, 7 October 2013, BBC News, fusion reaction exceeded the amount of energy being absorbed by the fuel, 8 October 2013, JOURNAL, Hurricane, O. A., Callahan, D. A., Casey, D. T., Celliers, P. M., Cerjan, C., Dewald, E. L., Dittrich, T. R., Döppner, T., Hinkel, D. E., Hopkins, L. F. Berzak, Kline, J. L., Le Pape, S., Ma, T., MacPhee, A. G., Milovich, J. L., 2014-02-20, Fuel gain exceeding unity in an inertially confined fusion implosion,weblink Nature, en, 506, 7488, 343–348, 10.1038/nature13008, 24522535, 2014Natur.506..343H, 4466026, 0028-0836, In June, 2018 NIF announced record production of 54kJ of fusion energy output.WEB,weblink NIF achieves record double fusion yield, Lawrence Livermore National Laboratory, 2018-06-13, 2019-11-11, On August 8, 2021WEB,weblink Nuclear Fusion Energy Breakthrough: Ignition Confirmed in Record 1.3 Megajoule Shot, August 14, 2022, Lawrence, Livermore, the NIF produced 1.3MJ of output, 25x higher than the 2018 result, generating 70% of the break-even definition of ignition - when energy out equals energy in.JOURNAL, 2021-09-28, Fusion news ignites optimism, Nature Photonics, en, 15, 10, 713, 10.1038/s41566-021-00890-z, 2021NaPho..15..713., 1749-4893, free, As of December 2022, the NIF claimsWEB, Fusion Energy Breakthrough Scam {{!, New Energy Times |url=https://news.newenergytimes.net/2022/12/11/fusion-energy-breakthrough-scam/ |access-date=2022-12-30 |website=news.newenergytimes.net}} to have become the first fusion experiment to achieve scientific breakeven on December 5, 2022, with an experiment producing 3.15 megajoules of energy from a 2.05 megajoule input of laser light (somewhat less than the energy needed to boil 1 kg of water) for an energy gain of about 1.5.WEB, National Ignition Facility achieves fusion ignition,weblink December 13, 2022, www.llnl.gov, en, WEB, DOE National Laboratory Makes History by Achieving Fusion Ignition,weblink December 13, 2022, Energy.gov, en, NEWS, The New York Times, Scientists Achieve Nuclear Fusion Breakthrough With Blast of 192 Lasers, Kenneth Chang, December 13, 2022,weblink WEB, Bush, Evan, Lederman, Josh, December 13, 2022, We have 'ignition': Fusion breakthrough draws energy gain,weblink December 13, 2022, NBC News,

Fast ignition

Fast ignition may offer a way to directly heat fuel after compression, thus decoupling the heating and compression phases. In this approach, the target is first compressed "normally" using a laser system. When the implosion reaches maximum density (at the stagnation point or "bang time"), a second short, high-power petawatt (PW) laser delivers a single pulse to one side of the core, dramatically heating it and starting ignition.JOURNAL,weblink Ignition and high gain with ultrapowerful lasers, Phys. Plasmas, 1, 5, 1626–1634, 1994, 10.1063/1.870664, 2023-11-20, Max Tabak, James Hammer, Michael E. Glinsky, William L. Kruer, Scott C. Wilks, John Woodworth, E. Michael Campbell, Michael D. Perry, Rodney J. Mason, The two types of fast ignition are the "plasma bore-through" method and the "cone-in-shell" method.JOURNAL,weblink 2023-11-20, Experimental studies of the advanced fast ignitor scheme, Phys. Plasmas, 7, 9, 3721–3726, 2000, 10.1063/1.1287419, P. A. Norreys, R. Allott, R. J. Clarke, J. Collier, D. Neely, S. J. Rose, M. Zepf, M. Santala, A. R. Bell, K. Krushelnick, A. E. Dangor, N. C. Woolsey, R. G. Evans, H. Habara, T. Norimatsu, R. Kodama, In plasma bore-through, the second laser bores through the outer plasma of an imploding capsule, impinges on and heats the core. In the cone-in-shell method, the capsule is mounted on the end of a small high-z (high atomic number) cone such that the tip of the cone projects into the core. In this second method, when the capsule is imploded, the laser has a clear view of the core and does not use energy to bore through a 'corona' plasma. However, the presence of the cone affects the implosion process in significant ways that are not fully understood. Several projects are currently underway to explore fast ignition, including upgrades to the OMEGA laser at the University of Rochester and the GEKKO XII device in Japan.HiPer is a proposed £500 million facility in the European Union. Compared to NIF's 2 MJ UV beams, HiPER's driver was planned to be 200 kJ and heater 70 kJ, although the predicted fusion gains are higher than NIF. It was to employ diode lasers, which convert electricity into laser light with much higher efficiency and run cooler. This allows them to operate at much higher frequencies. HiPER proposed to operate at 1 MJ at 1 Hz, or alternately 100 kJ at 10 Hz. The project's final update was in 2014. It was expected to offer a higher Q with a 10x reduction in construction costs times.WEB, 60 Project News,weblink 2021-08-21, Hiper Laser,

Other projects

The French Laser Mégajoule achieved its first experimental line in 2002, and its first target shots were conducted in 2014.WEB,weblink Le Laser Mégajoule, 2016-10-08, 2016-08-11,weblink" title="web.archive.org/web/20160811004348weblink">weblink dead, The machine was roughly 75% complete as of 2016.Using a different approach entirely is the z-pinch device. Z-pinch uses massive electric currents switched into a cylinder comprising extremely fine wires. The wires vaporize to form an electrically conductive, high current plasma. The resulting circumferential magnetic field squeezes the plasma cylinder, imploding it, generating a high-power x-ray pulse that can be used to implode a fuel capsule. Challenges to this approach include relatively low drive temperatures, resulting in slow implosion velocities and potentially large instability growth, and preheat caused by high-energy x-rays.WEB,weblinkweblink" title="web.archive.org/web/20090117115440weblink">weblink dead, Z-Pinch Power Plant a Pulsed Power Driven System for Fusion Energy, January 17, 2009, CONFERENCE, 2002AIPC..651....3G, 10.1063/1.1531270, AIP Conference Proceedings, DENSE Z-PINCHES: 5th International Conference on Dense Z-Pinches., 2002, Grabovskii, E. V., 651, 3–8, Fast Z - Pinch Study in Russia and Related Problems, Shock ignition was proposed to address problems with fast ignition.JOURNAL, Perkins, L. J., Betti, R., LaFortune, K. N., Williams, W. H., 2009, Shock Ignition: A New Approach to High Gain Inertial Confinement Fusion on the National Ignition Facility,weblinkweblink" title="ghostarchive.org/archive/20221009weblink">weblink 2022-10-09, live, Physical Review Letters, 103, 4, 045004, 2009PhRvL.103d5004P, 10.1103/PhysRevLett.103.045004, 19659364, REPORT,weblinkweblink" title="ghostarchive.org/archive/20221009weblink">weblink 2022-10-09, live, HiPER Preparatory Phase Completion Report, HiPER Project Team, 1 December 2013, 1 May 2017, JOURNAL, Ribeyre, X., Schurtz, G., Lafon, M., Galera, S., Weber, S., 2009, Shock ignition: an alternative scheme for HiPER, Plasma Physics and Controlled Fusion, en, 51, 1, 015013, 2009PPCF...51a5013R, 10.1088/0741-3335/51/1/015013, 120858786, 0741-3335, Japan developed the KOYO-F design and laser inertial fusion test (LIFT) experimental reactor.JOURNAL, Norimatsu, Takayoshi, Kozaki, Yasuji, Shiraga, Hiroshi, Fujita, Hisanori, Okano, Kunihiko, Azech, Hiroshi, 2013, Laser Fusion Experimental Reactor LIFT Based on Fast Ignition and the Issue,weblink CLEO: 2013 (2013), Paper ATh4O.3, EN, Optical Society of America, ATh4O.3, 10.1364/CLEO_AT.2013.ATh4O.3, 978-1-55752-972-5, 10285683, JOURNAL, Norimatsu, T., Kawanaka, J., Miyanaga, M., Azechi, H., 2007, Conceptual Design of Fast Ignition Power Plant KOYO-F Driven by Cooled Yb:YAG Ceramic Laser,weblink Fusion Science and Technology, 52, 4, 893–900, 10.13182/fst52-893, 2007FuST...52..893N, 117974702, WEB, Norimatsu, T., 2006, US-Japan workshop on Power Plant Studies and related Advanced Technologies with EU participation (24-25 January 2006, San Diego, CA), Fast ignition Laser Fusion Reactor KOYO-F - Summary from design committee of FI laser fusion reactor,weblinkweblink" title="ghostarchive.org/archive/20221009weblink">weblink 2022-10-09, live, In April 2017, clean energy startup Apollo Fusion began to develop a hybrid fusion-fission reactor technology.NEWS, Stone, Brad, 3 April 2017, Former Google Vice President Starts a Company Promising Clean and Safe Nuclear Energy, Bloomberg.com,weblink 2017-05-01, NEWS, Thompson, Avery, 3 April 2017, Can a Googler's Fusion Startup Kickstart Nuclear Power?, en, Popular Mechanics,weblink 2017-05-01, In Germany, technology company Marvel Fusion is working on (:de:Trägheitsfusion#Lasergetriebene Proton-Bor-Fusion|laser-initiated inertial confinement fusion).WEB, A revolutionary solution for carbon-free energy,weblink 2021-08-10, Marvel Fusion, The startup adopted a short-pulsed high energy laser and the aneutronic fuel pB11.WEB, The case for funding fusion,weblink 2021-08-10, TechCrunch, 10 July 2021, en-US, NEWS, Wengenmayr, Roland, Alternative Kernfusion: Mit Superlasern und einem Quantentrick, de, FAZ.NET,weblink 2021-08-10, 0174-4909, WEB, Marvel Fusion attracts Leading Scientific Talent to Munich,weblink 2021-08-10, Marvel Fusion, It was founded in Munich 2019.WEB, Vecchiato, Alexandra, Erneuerbare Energien: Milliardenprojekt in Penzberg,weblink 2021-08-10, Süddeutsche.de, 28 October 2020, de, WEB, Bär, Markus, Ein Münchner Start-up forscht mit Kernfusion am Feuer der Zukunft,weblink 2021-08-10, Augsburger Allgemeine, de, It works with Siemens Energy, TRUMPF, and Thales.NEWS, 2022-02-03, European industrial giants join nuclear fusion race, Financial Times,weblinkweblink 2022-12-10, subscription, 2022-09-21, The company partnered with Ludwig Maximilian University of Munich in July 2022.WEB, Laserforschung: LMU und Marvel Fusion vereinbaren Kooperation zur Erforschung der laserbasierten Kernfusion,weblink 2022-09-21, www.lmu.de, de, In March 2022, Australian company HB11 announced fusion using non-thermal laser pB11, at a higher than predicted rate of alpha particle creation.WEB,weblink HB11's hydrogen-boron laser fusion test yields groundbreaking results, 29 March 2022, Other companies include NIF-like Longview Fusion and fast-ignition origned Focused Energy.WEB, Startups try to turn laser fusion success into clean power plants,weblink 2023-02-17, www.science.org, en,

Applications

Electricity generation

Inertial fusion energy (IFE) power plants have been studied since the late 1970s. These devices were to deliver multiple targets per second into the reaction chamber, using the resulting energy to drive a conventional steam turbine.

Technical challenges

(File:Electra Laser Generates 90K Shots.webm|thumb|alt=The Electra Laser at the Naval Research Laboratory demonstrated more than 90,000 shots over 10 hours at 700 joules. This kind of repetition rate would be needed for an IFC power plant. |The Electra Laser at the Naval Research Laboratory demonstrated more than 90,000 shots over 10 hours at 700 joules.Obenschain, Stephen, et al. "High-energy krypton fluoride lasers for inertial fusion." Applied optics 54.31 (2015): F103-F122.)Even if the many technical challenges in reaching ignition were all to be solved, practical problems abound. Given the 1 to 1.5% efficiency of the laser amplification process and that steam-driven turbine systems are typically about 35% efficient, fusion gains would have to be on the order of 125-fold just to energetically break even.{{sfn|Brueckner|1977|p=31}}An order of magnitude improvement in laser efficiency may be possible through the use of designs that replace flash lamps with laser diodes that are tuned to produce most of their energy in a frequency range that is strongly absorbed. Initial experimental devices offer efficiencies of about 10%, and it is suggested that 20% is possible.{{Citation needed|date=February 2023}}NIF uses about 330 MJ to produce the driver beams, producing an expected yield of about 20 MJ, with maximum credible yield of 45 MJ.

Power extraction

ICF systems face some of the secondary power extraction problems as MCF systems. One of the primary concerns is how to successfully remove heat from the reaction chamber without interfering with the targets and driver beams. Another concern is that the released neutrons react with the reactor structure, mechanically weakening it, and turning it intensely radioactive. Conventional metals such as steel would have a short lifetime and require frequent replacement of the core containment walls. Another concern is fusion afterdamp (debris left in the reaction chamber), which could interfere with subsequent shots, including helium ash produced by fusion, along with unburned hydrogen and other elements used in the fuel pellet. This problem is most troublesome with indirect drive systems. If the driver energy misses the fuel pellet completely and strikes the containment chamber, material could foul the interaction region, or the lenses or focusing elements.One concept, as shown in the HYLIFE-II design, is to use a "waterfall" of FLiBe, a molten mix of fluoride salts of lithium and beryllium, which both protect the chamber from neutrons and carry away heat. The FLiBe is passed into a heat exchanger where it heats water for the turbines.{{Citation |first1=Craig |last1=Olson |first2=Max |last2=Tabak |first3=Jill |last3=Dahlburg |first4=Rick |last4=Olson |first5=Steve |last5=Payne |first6=John |last6=Sethian |first7=John |last7=Barnard |first8=Rick |last8=Spielman |first9=Ken |last9=Schultz |first10=Robert |last10=Peterson |first11=Per |last11=Peterson |first12=Wayne |last12=Meier |first13=John |last13=Perkins |contribution=Inertial Fusion Concepts Working Group, Final Reports of the Subgroups |year=1999 |title=1999 Fusion Summer Study |publisher=Columbia University |url=weblink |archive-url=https://ghostarchive.org/archive/20221009weblink |archive-date=2022-10-09 |url-status=live |access-date=August 23, 2014}} The tritium produced by splitting lithium nuclei can be extracted in order to close the power plant's thermonuclear fuel cycle, a necessity for perpetual operation because tritium is rare and otherwise must be manufactured. Another concept, Sombrero, uses a reaction chamber built of carbon-fiber-reinforced polymer which has a low neutron cross section. Cooling is provided by a molten ceramic, chosen because of its ability to absorb the neutrons and its efficiency as a heat transfer agent.{{Citation |first1=I.N. |last1=Sviatoslavsky |first2=M.E. |last2=Sawan |first3=R.R. |last3=Peterson |first4=G.L. |last4=Kulcinski |first5=J.J. |last5=MacFarlane |first6=L.J. |last6=Wittenberg |first7=E.A. |last7=Mogahed |first8=S.C. |last8=Rutledge |first9=S. |last9=Ghose |first10=R. |last10=Bourque |contribution=SOMBRERO - A Solid Breeder Moving Bed KrF Laser Driven IFE Power Reactor |year=1991 |title=14th IEEE/NPSS Symposium on Fusion Engineering |publisher=Fusion Technology Institute, University of Wisconsin |url=http://fti.neep.wisc.edu/pdf/fdm862.pdf |archive-url=https://ghostarchive.org/archive/20221009weblink |archive-date=2022-10-09 |url-status=live |access-date=August 23, 2014}}

Economic viability

Another factor working against IFE is the cost of the fuel. Even as Nuckolls was developing his earliest calculations, co-workers pointed out that if an IFE machine produces 50 MJ of fusion energy, a shot could produce perhaps 10 MJ (2.8 kWh) of energy. Wholesale rates for electrical power on the grid were about 0.3 cents/kWh at the time, which meant the monetary value of the shot was perhaps one cent. In the intervening 50 years the real price of power has remained about even, and the rate in 2012 in Ontario, Canada was about 2.8 cents/kWh.WEB,weblink IESO Power Data, Ieso.ca, 2014-08-23, dead,weblink" title="web.archive.org/web/20141002115248weblink">weblink 2014-10-02, Thus, in order for an IFE plant to be economically viable, fuel shots would have to cost considerably less than ten cents in 2012 dollars.Direct-drive systems avoid the use of a hohlraum and thereby may be less expensive in fuel terms. However, these systems still require an ablator, and the accuracy and geometrical considerations are critical. The direct-drive approach still may not be less expensive to operate.

Nuclear weapons

The hot and dense conditions encountered during an ICF experiment are similar to those in a thermonuclear weapon, and have applications to nuclear weapons programs. ICF experiments might be used, for example, to help determine how warhead performance degrades as it ages, or as part of a weapons design program. Retaining knowledge and expertise inside the nuclear weapons program is another motivation for pursuing ICF.Richard Garwin, Arms Control Today, 1997WEB,weblink Science, Lasers.llnl.gov, 2014-08-24, Funding for the NIF in the United States is sourced from the Nuclear Weapons Stockpile Stewardship program, whose goals are oriented accordingly.WEB,weblink Stockpile Stewardship, Lasers.llnl.gov, 2014-08-24, It has been argued that some aspects of ICF research violate the Comprehensive Test Ban Treaty or the Nuclear Non-Proliferation Treaty.WEB, Makhijani, Arjun, Hisham, Zerriffi,weblink Dangerous Thermonuclear Quest, Ieer.org, 1998-07-15, 2014-08-23, In the long term, despite the formidable technical hurdles, ICF research could lead to the creation of a "pure fusion weapon".WEB,weblinkweblink" title="web.archive.org/web/20080309102658weblink">weblink dead, Jones and von Hippel, Science and Global security, 1998, Volume 7 p129-150, March 9, 2008,

Neutron source

ICF has the potential to produce orders of magnitude more neutrons than spallation. Neutrons are capable of locating hydrogen atoms in molecules, resolving atomic thermal motion and studying collective excitations of photons more effectively than X-rays. Neutron scattering studies of molecular structures could resolve problems associated with protein folding, diffusion through membranes, proton transfer mechanisms, dynamics of molecular motors, etc. by modulating thermal neutrons into beams of slow neutrons.JOURNAL, Taylor, Andrew, A Route to the Brightest Possible Neutron Source?, Science (journal), Science, 315, February 2007, 1092–1095, 17322053, 10.1126/science.1127185, Dunne, M, Bennington, S, Ansell, S, Gardner, I, Norreys, P, Broome, T, Findlay, D, Nelmes, R, 5815, 2007Sci...315.1092T, 42506679, In combination with fissile materials, neutrons produced by ICF can potentially be used in Hybrid Nuclear Fusion designs to produce electric power.

See also

• Antimatter catalyzed nuclear pulse propulsion
• Bubble fusion, a phenomenon claimed â€“ controversially â€“ to provide an acoustic form of inertial confinement fusion.
• Dense plasma focus
• Laboratory for Laser Energetics
• Laser Mégajoule
• Leonardo Mascheroni, who proposed using hydrogen fluoride lasers to achieve fusion.
• List of laser articles
• Magnetic confinement fusion
• Magnetized target fusion (MTF)
• Magneto-inertial fusion
• Proton-boron fusion
• Pulsed power

Notes

{{notelist}}

References

{{Reflist|30em}}

Bibliography

• REPORT

• MAGAZINE

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• BOOK

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• JOURNAL

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• BOOK

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• TECH REPORT

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External links

• National Ignition Facility Project
• weblink" title="web.archive.org/web/20040329191800weblink">Zpinch Home Page
• Europe plans laser-fusion facility (Physicsweb)
• Lasers point the way to clean energy (The Guardian)
• weblink" title="web.archive.org/web/20051109102133weblink">National Laser Fusion Energy Development Plan
• weblink" title="web.archive.org/web/20080417072930weblink">Institute of Laser Engineering Osaka University
• weblink" title="web.archive.org/web/20100527174508weblink">Laser Inertial-Confinement Fusion-Fission Energy
• Heavy Ion Fusion

{{Lawrence Livermore National Laboratory|state=autocollapse}}{{Fusion power}}{{Nuclear fusion reactors}}