Inertial confinement fusion

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 science".

Inertial confinement fusion (ICF) is a type of fusion energy research that attempts to initiate nuclear fusion reactions by heating and compressing a fuel target, typically in the form of a pellet that typically contains a mixture of deuterium and tritium. Fuel pellets are about the size of a pinhead and contain around 10 milligrams of fuel.

To compress and heat the fuel, energy is delivered to the outer layer of the target using high-energy beams of photons, electrons or ions, although almost all ICF devices as of 2020 used lasers. The beams heat the outer layer, which explodes outward, which produces a reaction force against the remainder of the target, which accelerates it inwards, which compresses the fuel. This process creates shock waves that travel inward through the target. Sufficiently powerful shock waves can compress and heat the fuel at the center such that fusion occurs.

ICF is one of two major branches of fusion energy research, the other is magnetic confinement fusion. When it was first publicly proposed in the early 1970s, ICF appeared to be a practical approach to power production and the field flourished. Experiments during the 1970s and '80s demonstrated that the efficiency of these devices was much lower than expected, and reaching ignition would not be easy. Throughout the 1980s and '90s, many experiments were conducted in order to understand the complex interaction of high-intensity laser light and plasma. These led to the design of newer machines, much larger, that would finally reach ignition energies.

The largest operational ICF experiment is the National Ignition Facility (NIF) in the US. In 2021, an experiment reached 70% efficiency.^[1]

Description[]

Basic fusion[]

Indirect drive laser ICF uses a hohlraum which is irradiated with laser beam cones from either side on its inner surface to bathe a fusion microcapsule inside with smooth high intensity X-rays. The highest energy X-rays can be seen leaking through the hohlraum, represented here in orange/red.

Fusion reactions combine lighter atoms, such as hydrogen, together to form larger ones. This occurs when two atoms come close enough that the nuclear force pulls them together. Atomic nuclei are positively charged, and thus repel each other due to the electrostatic force. Overcoming this repulsion to bring the nuclei close enough requires energy, known as the Coulomb barrier or fusion barrier energy.

Generally, less energy will be needed to cause lighter nuclei to fuse, as they have less electrical charge and thus a lower barrier energy. Additionally, the nuclear force increases with more nucleons, the total number of protons and neutrons. Thus, the overall energy barrier is minimized for atoms with the greatest number of neutrons compared to protons. This ratio is maximized in the two isotopes of hydrogen, deuterium with one proton and one neutron, and tritium with one proton and two neutrons. This is known simply as D-T, and is the most studied fusion fuel. As the mass of the nuclei increases, at some point the reaction no longer gives off net energy—the energy needed to overcome the energy barrier is greater than the energy released in the resulting reaction.

A mix of D-T at standard conditions does not undergo fusion; the nuclei must be forced together before the nuclear force can pull them together into stable collections. Even in the hot, dense center of the sun, the average proton exists for billions of years before it fuses.^[2] For practical fusion power systems, the rate must be dramatically increased by heating the fuel to tens of millions of degrees, or compressing it to immense densities. The temperature and pressure required for any particular fuel to fuse is known as the Lawson criterion. These conditions have been known since the 1950s when the first hydrogen bombs were built. To meet the Lawson Criterion is extremely difficult on Earth, which explains why fusion research has spanned many years.^[3]

Thermonuclear devices[]

In a hydrogen bomb, the fusion fuel is compressed and heated with a separate fission bomb. A variety of mechanisms transfers the energy of the device's primary (fission) stage into the fusion fuel. A primary mechanism is that the flash of x-rays given off by the primary is trapped within the engineered case of the bomb, causing the volume between the case and the bomb to fill with an x-ray gas. These x-rays evenly illuminate the outside of the secondary fusion stage, rapidly heating it until it explodes outward. This outward blowoff causes the rest of the secondary to be compressed inward until it reaches the temperature and density where fusion reactions begin.

When the reactions begin, the energy released from them in the form of particles keeps the reaction going. In the case of D-T fuel, most of the energy is released in the form of alpha particles and neutrons. In the incredibly high-density fuel mass, the alpha particles cannot travel far before their electrical charge interacting with the surrounding plasma causes them to lose velocity. This transfer of kinetic energy heats the surrounding particles to the energies they need to undergo fusion as well. 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 more tritium through reactions in a lithium-deuteride fuel, or are used to fission additional fissionable fuel surrounding the secondary stage.

The requirement that the reaction has to be sparked by a fission bomb makes the method impractical for power generation. Not only would the fission triggers be expensive to produce, but the minimum size of such a bomb is too large, defined roughly by the critical mass of the plutonium fuel used. Generally, it seems difficult to build nuclear 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, but also demonstrated that it was not economically feasible. The cost of the bombs was far greater than the value of the resulting electricity.

ICF mechanism of action[]

One of the PACER participants, John Nuckolls, began to explore what happened to the size of the primary required to start the fusion reaction as the size of the secondary was scaled down. He discovered that as the secondary reaches the milligram size, the amount of energy needed to spark it fell into the megajoule range. Below this mass, the fuel became so small after compression that the alphas would escape.

A megajoule was far below even the smallest fission triggers, which were in the terajoule range. The question became whether another method could deliver those megajoules. This led to the idea of a "driver", a device that would beam the energy at the fuel from a distance. That way the resulting fusion explosion did not damage it, so that it could be used repeatedly.

By the mid-1960s, it appeared that the laser could evolve to provide the required energy. Generally, ICF systems use a single laser whose beam is split up multiple beams that are subsequently individually amplified by a trillion times or more. These are sent into the reaction chamber, called the target chamber, by mirrors positioned in order to illuminate the target evenly over its whole surface. The heat applied by the driver causes the outer layer of the target to explode, just as the outer layers of an H-bomb's fuel cylinder do when illuminated by the X-rays of the fission device. The explosion velocity is on the order of 10⁸ meters per second.^[4]

The material exploding off the surface causes the remaining material on the inside to be driven inwards, eventually collapsing into a tiny near-spherical ball.^[5] In modern ICF devices, the density of the resulting fuel mixture is as much as one-thousand times the density of water, or one-hundred that of lead, around 1000 g/cm³. This density is not high enough to create useful fusion on its own. However, during the collapse of the fuel, shock waves also form and travel into the center of the fuel at high speed. When they meet their counterparts moving in from the other sides of the fuel in the center, the density of that spot is raised.

Given the correct conditions, the fusion rate in the region highly compressed by the shock wave can give off significant amounts of highly energetic alpha particles. Due to the high density of the surrounding fuel, they move only a short distance before "thermalising", losing their energy to the fuel as heat. This additional energy causes additional reactions, giving off more high-energy particles. This process spreads outward from the centre, leading to a kind of self-sustaining burn known as ignition.

Schematic of the stages of inertial confinement fusion using lasers. The blue arrows represent radiation; orange is blowoff; purple is inwardly transported thermal energy.

1. Laser beams or laser-produced X-rays rapidly heat the surface of the fusion target, forming a surrounding plasma envelope.
2. Fuel is compressed by the rocket-like blowoff of the hot surface material.
3. During the final part of the capsule implosion, the fuel core reaches 20 times the density of lead and ignites at 100,000,000 ˚C.
4. Thermonuclear burn spreads rapidly through the compressed fuel, yielding many times the input energy.

Compression vs. ignition[]

Increased compression improves alpha capture without limit; in theory, a fuel with infinite density is best. Compression has a number of practical limitations, especially once it begins to become electron degenerate, which occurs at about 1000 times the density of water (or 100 times that of lead). To produce this level of compression in D-T fuel, it must be imploded at about 140 km/second, which requires about 10⁷ Joules per gram of fuel (J/g). For milligram-sized fuels, this is not a particularly large amount of energy and can be provided by modest devices.

Unfortunately, while this level of compression efficiently traps the alphas, it alone is not enough to heat the fuel to the required temperatures, at least 50 million Kelvin. To reach these conditions, the velocity has to be about 300 km/second, requiring 10⁹ J, which is significantly more difficult to achieve.

Central hot spot ignition[]

In the most-used system to date, "central hot spot ignition", the pulse of energy from the driver is shaped such that 3 or 4 shocks are launched into the capsule. This compresses the capsule shell and accelerates it inwards forming a spherically-imploding mass, which travels at about 300 km/sec. The shell compresses and heats the inner gas until its pressure increases enough to resist the converging shell. This launches a reverse shock wave which decelerates the shell, and briefly increases the density to enormous values. The goal of this concept is to spark enough reactions that alpha self-heating takes place in the rest of the still inrushing fuel. This requires about 4.5x10⁷ J/g, but a series of practical losses raise this to about 10⁸ J.

Fast ignition[]

In the "fast ignition" approach, a separate laser is used to provide the additional energy directly to the center of the fuel. This can be arranged through mechanical means, often using a small metal cone that punctures the outer fuel pellet wall to allow the laser light access to the center. In tests, this approach has failed as the pulse of light has to reach the center at a precise time, when it is obscured by the debris and especially free electrons from the compression pulse. It also has the disadvantage of requiring a second laser pulse, which generally demands a completely separate laser.

Shock ignition[]

"Shock ignition" is similar in concept to the hot-spot technique, but instead of ignition being achieved via compression heating of the hotspot, a final powerful, shock is sent into the fuel at a late time to trigger ignition through a combination of compression and shock heating. This increases the efficiency of the process with an eye to lowering the overall amount of power required.

Direct vs. indirect drive[]

Mockup of a gold plated National Ignition Facility (NIF) hohlraum.

In the simplest conception of the ICF approach, the fuel is arranged as a sphere. This allows it to be pushed inward from all sides. To produce the inward force, the fuel is placed within a thin shell that captures the energy from the driver and explode outward. In practice, the capsules are normally made of a lightweight plastic and the fuel is deposited as a layer on the inside by injecting a gas into the shell and then freezing it.

The idea of having the driver shine directly on the fuel is known as "direct drive". In order for the fusion fuel to reach the required conditions, the implosion process must be extremely uniform in order to avoid significant asymmetry due to Rayleigh–Taylor instability and similar effects. For beam energy of 1 MJ, the fuel capsule cannot be larger than about 2 mm before these effects destroy the implosion symmetry. This limits the size of the beams, which may be difficult to achieve in practice.

This has led to an alternative concept, "indirect drive", where the beam does not shine on the fuel capsule directly. Instead, it shines into a small cylinder of heavy metal, often gold or lead, known as a "hohlraum". The beams are arranged so they do not hit the fuel capsule suspended in the center. The energy heats the hohlraum until it begins to give off X-rays. These X-rays fill the interior of the hohlraum and heat the capsule. The advantage of this approach is that the beams can be larger and less accurate, which greatly eases driver design. 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 efficiency is much lower than the direct drive concept.

Challenges[]

The primary problems with increasing ICF performance are energy delivery to the target, controlling symmetry of the imploding fuel, preventing premature heating of the fuel before sufficient density is achieved, preventing premature mixing of hot and cool fuel by hydrodynamic instabilities, and the formation of a 'tight' 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 aberrations of no more than a few micrometres over its (inner and outer) surface. Likewise the aiming of the laser beams must be precise in space and time. Beam timing is relatively simple and is solved by using delay lines in the beams' optical path to achieve picosecond timing accuracy. The other major problem plaguing the achievement of high symmetry and high temperatures/densities of the imploding target are 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 on the target and of "hot spots" within a beam diameter hitting a target which induces uneven compression on the target surface, thereby forming Rayleigh-Taylor instabilities^[6] in the fuel, prematurely mixing it and reducing heating efficacy at the time of maximum compression. The Richtmyer-Meshkov instability is also formed during the process due to shock waves.

An Inertial confinement fusion target, which was a foam filled cylindrical target with machined perturbations, being compressed by the Nova Laser. This shot was done in 1995. The image shows the compression of the target, as well as the growth of the Rayleigh-Taylor instabilities.^[7]

All of these problems have been significantly mitigated by beam smoothing techniques and beam energy diagnostics to balance beam to beam energy; however, RT instability remains a major issue. Target design has improved tremendously. 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 IR laser to smooth its inner surface and monitoring it with a microscope equipped camera, thereby allowing the layer to be closely monitored ensuring its "smoothness".^[8] Cryogenic targets filled with D-T are "self-smoothing" due to the small amount of heat created by tritium decay. This is often referred to as "beta-layering".^[9]

An inertial confinement fusion fuel microcapsule (sometimes called a "microballoon") of the size used on the NIF which can be filled with either deuterium and tritium gas or DT ice. The capsule can be either inserted in a hohlraum (as above) and imploded in the indirect drive mode or irradiated directly with laser energy in the direct drive configuration. Microcapsules used on previous laser systems were significantly smaller owing to the less powerful irradiation earlier lasers were capable of delivering to the target.

In the indirect drive approach^[10] 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 the 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 them is also imploded mainly by X-ray radiation.

A variety of ICF drivers are evolving. Lasers have improved dramatically, scaling up from a few joules and kilowatts to megajoules and hundreds of terawatts, using mostly frequency doubled or tripled light from neodymium glass amplifiers.

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.

History[]

Conception[]

United States[]

IFC history began as part of the "Atoms For Peace" conference. This was a large, international UN sponsored conference between the superpowers of the US and the Soviet Union. Some thought was given to using a hydrogen bomb to heat a water-filled cavern. The resulting steam would then be used to power conventional generators, and thereby provide electrical power.^[11]

This meeting led to the Operation Plowshare efforts, named in 1961. Three primary concepts were part of Plowshare; energy generation under Project PACER, the use of nuclear explosions for excavation, and for 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. Radioactive steam was released from the drill shaft, at some distance from the test site. Further studies led to engineered cavities replacing natural ones, but the Plowshare efforts 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 conventional nuclear plants.^[12]

Another outcome of the "Atoms For Peace" conference was to prompt Nuckolls to consider what happens on the fusion side of the bomb. A thermonuclear bomb has two parts, a fission-based "primary" and a fusion-based "secondary". When primary explodes, it releases X-rays which implode the secondary. Nuckolls' earliest work concerned the study of how small the secondary could be made while still having a large gain to provide net energy. This work suggested that at very small sizes, on the order of milligrams, very little energy would be needed to ignite it, much less than a fission primary.^[11] 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 not be supplied by a fission bomb, but by some sort of 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, the "primary".^[13]

The main advantage to this scheme is the efficiency of the fusion process 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.^[13] 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.^[14]

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.^[15] 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).^[16]

In 1964 Winterberg proposed that ignition could be achieved by an intense beam of microparticles accelerated to a velocity of 1000 km/s.^[17] And in 1968, he proposed to use intense electron and ion beams, generated by Marx generators, for the same purpose.^[18] The advantage of this proposal is that the generation of charged particle beams is not only less expensive than the generation of 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.^[19]

Early research[]

Through the late 1950s, Nuckolls and collaborators at Lawrence Livermore National Laboratory (LLNL) completed computer simulations of the ICF concept. In early 1960 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 wide variety of possible drivers were considered, including pulsed power machines, charged particle accelerators, plasma guns, and hypervelocity pellet guns.^[20]

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. Additionally, the shell was made 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,^[21] 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.^[22]

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.^[23] 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.^[24] 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.^[23]

In 1972 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.^[25][26]

In spite of limited resources and business problems, KMS Fusion successfully demonstrated fusion from the ICF process on 1 May 1974.^[27] However, this success was soon followed by Siegel's death, and the end of KMS fusion about a year later.^[24] By this point several weapons labs and universities had started their own programs, notably the solid-state lasers (Nd:glass lasers) 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. Nevertheless, funding for fusion research 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 magnetic 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. At the time it was believed that making 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 UV 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 and compressed its pellets to 100 times the liquid density of deuterium. However, due to the laser's strong coupling with hot electrons, premature heating of the dense plasma (ions) was problematic and fusion yields were low. This failure by Shiva 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. Newly discovered schemes to efficiently triple the frequency of high intensity laser light discovered at the Laboratory for Laser Energetics in 1980 enabled this method of target irradiation to be experimented with in the 24 beam OMEGA laser and the NOVETTE laser, which was followed by the Nova laser design with 10 times the energy of Shiva, the first design with the specific goal of reaching ignition conditions.

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 the increase in uniformity of irradiation, the reduction of hot-spots in the laser beams through beam smoothing techniques to reduce Rayleigh–Taylor instabilities imprinting on the target and increased laser energy on target by at least an order of magnitude. Funding for fusion research was severely constrained in the 1980's.

National Ignition Facility[]

National Ignition Facility target chamber

The resulting 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,^[28] NIF has now conducted experiments using all 192 beams, including experiments that set new records for power delivery by a laser.^[29][30] As of October 7, 2013, for the first time a fuel capsule gave off more energy than was applied to it.^[31][32] In June, 2018 the NIF announced attainment of a record production of 54kJ of fusion energy output.^[33]

Fast ignition[]

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The concept of "fast ignition" may offer a way to directly heat fuel after compression, thus decoupling the heating and compression phases of the implosion. 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 ultra-short pulse ultra-high power petawatt (PW) laser delivers a single pulse focused on one side of the core, dramatically heating it and starting ignition.

The two types of fast ignition are the "plasma bore-through" method and the "cone-in-shell" method. In plasma bore-through, the second laser is expected to bore straight through the outer plasma of an imploding capsule and to impinge on and heat the dense 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 of the capsule. In this second method, when the capsule is imploded, the laser has a clear view straight to the high density core and does not have to waste energy boring 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 the fast ignition approach, including upgrades to the OMEGA laser at the University of Rochester, 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 be operated at much higher frequencies. HiPER proposed to operate at 1 MJ at 1 Hz, or alternately 100 kJ at 10 Hz. The project last produced an update in 2014.^[34]

It was expected to offer a higher Q with a 10x reduction in construction costs times.

Other projects[]

The French Laser Mégajoule achieved its first experimental line in 2002, and its first target shots were conducted in 2014.^[35] 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.^[36][37]

Shock ignition was proposed to address problems with fast ignition.^[38][39][40] Japan developed the KOYO-F design and laser inertial fusion test (LIFT) experimental reactor.^[41][42][43] In April 2017, clean energy startup Apollo Fusion began to develop a hybrid fusion-fission reactor technology.^[44][45]

In Germany, technology company Marvel Fusion develops a novel quantum-enhanced approach to laser-initiated inertial confinement fusion.^[46] The startup adopted a short-pulsed high energy laser and the aneutronic fuel pB11.^[47][48][49] Founded in Munich 2019, Marvel Fusion aims to build and operate commercial fusion power plants in the range of 1 to 3 GW by 2030.^[50][51]

As an energy source[]

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Practical power plants built using ICF have been studied since the late 1970s; they are known as inertial fusion energy (IFE) plants. These devices would deliver several targets/second to the reaction chamber, and capture the resulting heat and neutron radiation from their implosion and fusion to drive a conventional steam turbine.

Technical challenges[]

Even if the many technical challenges in reaching ignition were all to be solved, practical problems seem just as difficult to overcome. Laser-driven systems were initially believed to be able to generate commercially useful amounts of energy. However, as estimates of the energy required to reach ignition grew dramatically during the 1970s and 1980s, these hopes were abandoned. Given the low efficiency of the laser amplification process (about 1 to 1.5%), and the losses in generation (steam-driven turbine systems are typically about 35% efficient), fusion gains would have to be on the order of 350 just to energetically break even. These sorts of gains appeared to be impossible to generate, and ICF work turned primarily to weapons research.^{[citation needed]}

Fast ignition and similar approaches changed the situation. In this approach gains of 100 are predicted in the first experimental device, HiPER. Given a gain of about 100 and a laser efficiency of about 1%, HiPER produces about the same amount of fusion energy as electrical energy was needed to create it. It also appears that an order of magnitude improvement in laser efficiency may be possible through the use of newer 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.

With "classical" devices like NIF about 330 MJ of electrical power are used to produce the driver beams, producing an expected yield of about 20 MJ, with maximum credible yield of 45 MJ. HiPER requires about 270 kJ of laser energy, so assuming a first-generation diode laser driver at 10% the reactor would require about 3 MJ of electrical power. This is expected to produce about 30 MJ of fusion power.^[52] Even a poor conversion to electrical energy appears to offer real-world power output, and incremental improvements in yield and laser efficiency appear to be able to offer a commercially useful output.

Practical problems[]

ICF systems face some of the same secondary power extraction problems as magnetic systems in generating useful power. 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 plant, causing them to become intensely radioactive, as well as mechanically weakening metals. Fusion plants built of conventional metals like steel would have a fairly short lifetime and the core containment vessels would have to be replaced frequently.

One current concept in dealing with both of these problems, 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 then passed into a heat exchanger where it heats water for use in the turbines.^[53] 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 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.^[54]

An inertial confinement fusion implosion in Nova, creating "micro sun" conditions of tremendously high density and temperature rivalling even those found at the core of our Sun.

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, one might expect that a shot could produce perhaps 10 MJ of power for export. Converted to better known units, this is the equivalent of 2.8 kWh of electrical power. 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 price of power has remained about even with the rate of inflation, and the rate in 2012 in Ontario, Canada was about 2.8 cents/kWh.^[55]

Thus, in order for an IFE plant to be economically viable, fuel shots would have to cost considerably less than ten cents in year 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 program[]

The very hot and dense conditions encountered during an ICF experiment are similar to those created in a thermonuclear weapon, and have applications to nuclear weapons programs. ICF experiments might be used, for example, to help determine how warhead performance will degrade as it ages, or as part of a program of designing new weapons. Retaining knowledge and expertise inside the nuclear weapons program is another motivation for pursuing ICF.^[56][57] Funding for the NIF in the United States is sourced from the 'Nuclear Weapons Stockpile Stewardship' program, and the goals of the program are oriented accordingly.^[58] It has been argued that some aspects of ICF research may violate the Comprehensive Test Ban Treaty or the Nuclear Non-Proliferation Treaty.^[59] In the long term, despite the formidable technical hurdles, ICF research could lead to the creation of a "pure fusion weapon".^[60]

Neutron source[]

Inertial confinement fusion 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.^[61] In combination with fissile materials, neutrons produced by ICF can potentially be used in Hybrid Nuclear Fusion designs to produce electric power.

See also[]

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

References[]

Bibliography[]

External links[]

• National Ignition Facility Project
• Zpinch Home Page
• Europe plans laser-fusion facility (Physicsweb)
• Lasers point the way to clean energy (The Guardian)
• National Laser Fusion Energy Development Plan
• Institute of Laser Engineering Osaka University
• Laser Inertial-Confinement Fusion-Fission Energy
• Heavy Ion Fusion