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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 most often contains a mixture of deuterium and tritium. Typical 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 laser light, electrons or ions, although for a variety of reasons, almost all ICF devices as of 2020 have used lasers. The heated outer layer explodes outward, producing a reaction force against the remainder of the target, accelerating it inwards, compressing the fuel. This process is also designed to create shock waves that travel inward through the target. A sufficiently powerful set of shock waves can compress and heat the fuel at the center so much that fusion reactions occur.
ICF is one of two major branches of fusion energy research, the other being 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, designed using the decades-long experience of earlier experiments. Like those earlier experiments, however, NIF has failed to reach ignition and is, as of 2015, generating about 1⁄3 of the required energy levels.
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 costs a considerable amount of energy, which is 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 is increased 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 increase, there is a point where 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 fusion 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 will exist for billions of years before it fuses. 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 pressures. 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 H-bombs were built. To meet the Lawson Criterion is extremely difficult on Earth, which explains why fusion research has taken many years to reach the current high state of technical prowess.
In a hydrogen bomb, the fusion fuel is compressed and heated with a separate fission bomb (see Teller-Ulam design). A variety of mechanisms transfers the energy of the fission "primary" explosion 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 fusion section, the "secondary", 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 very far before their electrical charge interacting with the surrounding plasma causes them to slow down. 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 much 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 spark off fission events in a surrounding shell.
The requirement that the reaction has to be sparked off by a fission bomb makes the method impractical for power generation. Not only would the fission triggers be expensive to produce, but there is a minimum size that such a bomb can be built, 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 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 there was some other way to 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 develop to the point where the required energy levels would be available. Generally, ICF systems use a single laser whose beam is split up into a number of beams which are subsequently individually amplified by a trillion times or more. These are sent into the reaction chamber, called the target chamber, by a number of 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.
The material exploding off the surface causes the remaining material on the inside to be driven inwards with great force, eventually collapsing into a tiny near-spherical ball. 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 any useful rate of 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 much further.
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 being "thermalised", losing their energy to the fuel as heat. This additional energy will cause additional fusion reactions in the heated fuel, giving off more high-energy particles. This process spreads outward from the centre, leading to a kind of self-sustaining burn known as ignition.
Compression vs. ignition
There is no limit to the advantages to compression in terms of alpha capture, in theory, a fuel with infinite density is best. Actually compressing the fuel 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 is not alone enough to heat the fuel to the required temperatures where fusion reactions will start, 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. There have been a number of suggestions on ways to reduce this.
In the most-used system to date, "central hot spot ignition", the pulse of energy from the driver is shaped such that a number of shocks (3 or 4) 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 off 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.
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 almost always demands a completely separate laser.
A relatively new technique, as of 2007, is "shock ignition". This 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 in order 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
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 collapse in the center. To produce the inward force, the fuel is placed within a thin shell that will capture 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 inserting a small amount of gas into the shell and then freezing it.
The idea of having the driver shine directly on the fuel is known as the "direct drive" approach. 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 in total, the fuel capsule cannot be larger than about 2 mm before these effects will destroy the implosion symmetry. This produces a very tight limit on 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 they perform the heating of the capsule. The advantage of this approach is that the beams can be larger and less accurate, which greatly eases the design of the driver. 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.
Issues with successful achievement
The primary problems with increasing ICF performance since the early experiments in the 1970s have been of energy delivery to the target, controlling symmetry of the imploding fuel, preventing premature heating of the fuel before maximum density is achieved, preventing premature mixing of hot and cool fuel by hydrodynamic instabilities, and the formation of a 'tight' shockwave convergence at the compressed fuel center.
In order to focus the shock wave on the center of the target, the target must be made with extremely high precision and sphericity with aberrations of no more than a few micrometres over its surface (inner and outer). Likewise the aiming of the laser beams must be extremely precise and the beams must arrive at the same time at all points on the target. Beam timing is a relatively simple issue though and is solved by using delay lines in the beams' optical path to achieve picosecond levels of 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 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 being formed.
All of these problems have been substantially mitigated to varying degrees in the past two decades of research by using various beam smoothing techniques and beam energy diagnostics to balance beam to beam energy; however, RT instability remains a major issue. Target design has also improved tremendously over the years. Modern cryogenic hydrogen ice targets tend to freeze a thin layer of deuterium just on the inside of a plastic sphere while irradiating it with a low power IR laser to smooth its inner surface while monitoring it with a microscope equipped camera, thereby allowing the layer to be closely monitored ensuring its "smoothness". Cryogenic targets filled with a deuterium tritium (D-T) mixture are "self-smoothing" due to the small amount of heat created by the decay of the radioactive tritium isotope. This is often referred to as "beta-layering".
Certain targets are surrounded by a small metal cylinder which is irradiated by the laser beams instead of the target itself, an approach known as "indirect drive". In this approach the lasers are focused on the inner side of the cylinder, heating it to a superhot plasma which radiates mostly in X-rays. The X-rays from this plasma are then absorbed by the target surface, imploding it in the same way as if it had been hit with the lasers directly. The absorption of thermal x-rays by the target is more efficient than the direct absorption of laser light, however these hohlraums or "burning chambers" also take up considerable energy to heat on their own thus significantly reducing the overall efficiency of laser-to-target energy transfer. They are thus a debated feature even today; the equally numerous "direct-drive" design does not use them. 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 being explored. Lasers have improved dramatically since the 1970s, scaling up in energy and power from a few joules and kilowatts to megajoules (see NIF laser) 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. On the downside, it is very difficult to achieve the very high 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, reducing the overall efficiency of the coupling of the ion beam's energy to that of the imploding target further.
In the US
Inertial confinement fusion history can be traced back to the "Atoms For Peace" conference held in 1957 in Geneva. This was a large, international UN sponsored conference between the superpowers of the US and Russia. Among the many topics covered during the event, 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.
This meeting led to the Operation Plowshare efforts, given this name in 1961. Three primary concepts were studied as part of Plowshare; energy generation under Project PACER, the use of large nuclear explosions for excavation, and as a sort of nuclear fracking for the natural gas industry. PACER was directly tested in December 1961 when the 3 kt Project Gnome device was emplaced in bedded salt in New Mexico. In spite of all theorizing and attempts to stop it, radioactive steam was released from the drill shaft, some distance from the test site. Further studies as part of PACER led to a number of engineered cavities replacing natural ones, but through this period the entire Plowshare efforts turned from bad to worse, especially after the failure of 1962's Sedan which released huge quantities of fallout. PACER nevertheless continued to receive some funding until 1975, when a 3rd party study demonstrated that the cost of electricity from PACER would be the equivalent to conventional nuclear plants with fuel costs over ten times as great as they were.
Another outcome of the "Atoms For Peace" conference was to prompt John Nuckolls to start considering 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 output. 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. He proposed building, in effect, tiny all-fusion explosives using a tiny drop of D-T fuel suspended in the center of a metal shell, today known as a hohlraum. The shell provided the same effect as the bomb casing in an H-bomb, trapping x-rays inside so they irradiate the fuel. The main difference is that the X-rays would not be supplied by a fission bomb, but some sort of external device that heated the shell from the outside until it was glowing in the x-ray region (see thermal radiation). The power would be delivered by a then-unidentified pulsed power source he referred to using bomb terminology, the "primary".
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 would be dramatically more efficient in terms of gain. 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 is heated. In the ICF case, the entire hohlraum is filled with high-temperature radiation, limiting losses.
Around the same time (in 1956) a meeting was organized at the Max Planck Institute in Germany by the 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. 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).
In 1964 Winterberg proposed that ignition could be achieved by an intense beam of microparticles accelerated to a velocity of 1000 km/s. And in 1968, he proposed to use intense electron and ion beams, generated by Marx generators, for the same purpose. 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 also 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.
In the USSR
Through the late 1950s, Nuckolls and collaborators at the Lawrence Livermore National Laboratory (LLNL) ran a number of computer simulations of the ICF concept. In early 1960 this produced 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 10. At the time the laser had not yet been invented, and a wide variety of possible drivers were considered, including pulsed power machines, charged particle accelerators, plasma guns, and hypervelocity pellet guns.
Through the year two key theoretical advances were made. New simulations 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 the efficiency of the implosion, and thereby greatly lowered the energy required to compress it. Using these improvements, it was calculated that a driver of about 1 MJ would be needed, a five-fold improvement. Over the next two years several 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 very small systems with as little as 1 μg of D-T fuel.
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-scale laser study effort directed toward the ICF approach. Even at this early stage the suitability of the ICF system for weapons research was well understood, and the primary reason for its ability to gain funding. Over the next decade, LLNL made several small experimental devices for basic laser-plasma interaction studies.
In 1967 Kip Siegel started KMS Industries using the proceeds of the sale of his share of an earlier company, Conductron, a pioneer in holography. In the early 1970s he formed KMS Fusion to begin development of a laser-based ICF system. 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, who demanded funding for their own efforts. 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 1970s energy crisis which added impetus to many energy projects.
In 1972 Nuckolls wrote an influential public paper in Nature 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.
In spite of limited resources and numerous business problems, KMS Fusion successfully demonstrated fusion from the ICF process on 1 May 1974. However, this success was followed not long after by Siegel's death, and the end of KMS fusion about a year later, having run the company on Siegel's life insurance policy. 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.
Although KMS's success led to a major development effort, the advances that followed were, and still are, hampered by the seemingly intractable problems that characterize fusion research in general.
High-energy ICF experiments (multi-hundred joules per shot and greater experiments) began in earnest in the early-1970s, when lasers of the required energy and power were first designed. This was some time after the successful design of magnetic confinement fusion systems, and around the time of the particularly successful tokamak design that was introduced in the early '70s. Nevertheless, high funding for fusion research stimulated by the multiple energy crises during the mid to late 1970s 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, very well funded and started a major laser fusion development program. Their Janus laser started operation in 1974, and validated the approach of using Nd:glass lasers to generate very high power devices. Focusing problems were explored in the Long path laser and Cyclops laser, which led to the larger Argus laser. None of these were intended to be practical ICF devices, but each one advanced the state of the art to the point where there was some confidence the basic approach was valid. At the time it was believed that making a much larger device of the Cyclops type could both compress and heat the ICF targets, leading to ignition in the "short term". This was a misconception based on extrapolation of the fusion yields seen from experiments utilizing the so-called "exploding pusher" type of fuel capsules. During the period spanning the years of the late '70s and early '80s the estimates for laser energy on target needed to achieve ignition doubled almost yearly as the various plasma instabilities and laser-plasma energy coupling loss modes were gradually understood. The realization that the simple exploding pusher target designs and mere few kilojoule (kJ) laser irradiation intensities would never scale to high gain fusion yields led to the effort to increase laser energies to the 100 kJ level in the UV and to the production of advanced ablator and cryogenic DT ice target designs.
Shiva and Nova
One of the earliest serious and large scale attempts at an ICF driver design was the Shiva laser, a 20-beam neodymium doped glass laser system built at the 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 which would frequency triple the infrared light from the laser into the ultraviolet at 351 nm. Newly discovered schemes to efficiently frequency triple 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 in its goal of achieving ignition, this time due to severe variation in laser intensity in its beams (and differences in intensity between beams) caused by filamentation which resulted in large non-uniformity in irradiation smoothness at the target and asymmetric implosion. The techniques pioneered earlier could not address these new issues. But again 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 80's, but Nova nevertheless successfully gathered enough information for a next generation machine.
National Ignition Facility
The resulting design, now known as the National Ignition Facility, started construction at LLNL in 1997. NIF's main objective will be to operate as the flagship experimental device of the so-called nuclear stewardship program, supporting LLNLs traditional bomb-making role. Completed in March 2009, NIF has now conducted experiments using all 192 beams, including experiments that set new records for power delivery by a laser. The first credible attempts at ignition were initially scheduled for 2010, but ignition was not achieved as of September 30, 2012. As of October 7, 2013, the facility is understood to have achieved an important milestone towards commercialization of fusion, namely, for the first time a fuel capsule gave off more energy than was applied to it. This is still a long way from satisfying the Lawson criterion, but is a major step forward. In June, 2018 the NIF announced attainment of a record production of 54kJ of fusion energy output.
A more recent development is the concept of "," which may offer a way to directly heat the high density fuel after compression, thus decoupling the heating and compression phases of the implosion. In this approach the target is first compressed "normally" using a driver laser system, and then 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 hopefully starting fusion ignition. The two types of fast ignition are the "plasma bore-through" method and the "cone-in-shell" method. In the first method the petawatt laser is simply expected to bore straight through the outer plasma of an imploding capsule and to impinge on and heat the dense core, whereas 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 petawatt 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, and an entirely new £500 million facility, known as HiPER, proposed for construction in the European Union. If successful, the fast ignition approach could dramatically lower the total amount of energy needed to be delivered to the target; whereas NIF uses UV beams of 2 MJ, HiPER's driver is 200 kJ and heater 70 kJ, yet the predicted fusion gains are nevertheless even higher than on NIF.
Laser Mégajoule, the French project, has seen its first experimental line achieved in 2002, and its first target shots were finally conducted in 2014. The machine was roughly 75% complete as of 2016.
Using a different approach entirely is the z-pinch device. Z-pinch uses massive amounts of electric current which is 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 and thereby generating a high-power x-ray pulse that can be used to drive the implosion of 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.
As an energy source
Practical power plants built using ICF have been studied since the late 1970s when ICF experiments were beginning to ramp up to higher powers; they are known as inertial fusion energy, or IFE plants. These devices would deliver a successive stream of targets to the reaction chamber, several a second typically, and capture the resulting heat and neutron radiation from their implosion and fusion to drive a conventional steam turbine.
IFE faces continued technical challenges in reaching the conditions needed for ignition. But even if these were all to be solved, there are a significant number of practical problems that 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 '80s, 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.
With the recent introduction of fast ignition and similar approaches, things have changed dramatically. 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 the 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 a real possibility with some additional development.
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 the maximum credible yield of 45 MJ. Using the same sorts of numbers in a reactor combining fast ignition with newer lasers would offer dramatically improved performance. 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. Even a very 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.
ICF systems face some of the same secondary power extraction problems as magnetic systems in generating useful power from their reactions. One of the primary concerns is how to successfully remove heat from the reaction chamber without interfering with the targets and driver beams. Another serious concern is that the huge number of neutrons released in the fusion reactions react with the plant, causing them to become intensely radioactive themselves, 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 will have to be replaced frequently.
One current concept in dealing with both of these problems, as shown in the HYLIFE-II baseline 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. The tritium produced by fissioning lithium nuclei can also be extracted in order to close the power plant's thermonuclear fuel cycle, a necessity for perpetual operation because tritium does not exist in quantity naturally and must be manufactured. Another concept, Sombrero, uses a reaction chamber built of Carbon-fiber-reinforced polymer which has a very low neutron cross section. Cooling is provided by a molten ceramic, chosen because of its ability to stop the neutrons from traveling any further, while at the same time being an efficient heat transfer agent.
Even if these technical advances solve the considerable problems in IFE, another factor working against IFE is the cost of the fuel. Even as Nuckolls was developing his earliest detailed calculations on the idea, co-workers pointed this out: 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.
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. At the time this objection was first noted, Nuckolls suggested using liquid droplets sprayed into the hohlraum from an eye-dropper-like apparatus. Given the ever-increasing demands for higher uniformity of the targets, this approach does not appear practical, as even the inner ablator and fuel itself currently costs several orders of magnitude more than this. Moreover, Nuckolls' solution had the fuel dropped into a fixed hohlraum that would be re-used in a continual cycle, but at current energy levels the hohlraum is destroyed with every shot.
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 even more important. They are also far less developed than the indirect-drive systems, and face considerably more technical problems in terms of implosion physics. Currently there is no strong consensus whether a direct-drive system would actually be less expensive to operate.
The various phases of such a project are the following, the sequence of inertial confinement fusion development follows much the same outline:
- Burning demonstration
- Reproducible achievement of some fusion energy release (not necessarily a Q factor of >1).
- High gain demonstration
- Experimental demonstration of the feasibility of a reactor with a sufficient energy gain.
- Industrial demonstration
- Validation of the various technical options, and of the whole data needed to define a commercial reactor.
- Commercial demonstration
- Demonstration of the reactor ability to work over a long period, while respecting all the requirements for safety, liability and cost.
At the moment, according to the available data, inertial confinement fusion experiments have not gone beyond the first phase, although Nova and others have repeatedly demonstrated operation within this realm. In the short term a number of new systems are expected to reach the second stage.
For a true industrial demonstration, further work is required. In particular, the laser systems need to be able to run at high operating frequencies, perhaps one to ten times a second. Most of the laser systems mentioned in this article have trouble operating even as much as once a day. Parts of the HiPER budget are dedicated to research in this direction as well. Because they convert electricity into laser light with much higher efficiency, diode lasers also run cooler, which in turn allows them to be operated at much higher frequencies. HiPER is currently studying devices that operate at 1 MJ at 1 Hz, or alternately 100 kJ at 10 Hz.
R&D continued on inertial fusion energy in the European Union and in Japan. The High Power laser Energy Research (HiPER) facility is a proposed experimental fusion device undergoing preliminary design for possible construction in the European Union to continue the development of laser-driven inertial confinement approach. HiPER is the first experiment designed specifically to study the fast ignition approach to generating nuclear fusion. Using much smaller lasers than conventional designs, yet produces fusion power outputs of about the same magnitude would offer a much higher Q with a reduction in construction costs of about ten times. Theoretical research since the design of HiPER in the early 2000s has cast doubt on fast ignition but a new approach known as shock ignition has been proposed to address some of these problems. Japan developed the KOYO-F fusion reactor design and laser inertial fusion test (LIFT) experimental reactor. In April 2017, Bloomberg News reported that Mike Cassidy, former Google vice-president and director of Project Loon with Google[x], started a clean energy startup, Apollo Fusion, to develop a hybrid fusion-fission reactor technology.
Nuclear weapons program
The very hot and dense conditions encountered during an Inertial Confinement Fusion experiment are similar to those created in a thermonuclear weapon, and have applications to the nuclear weapons program. 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 corporate expertise in the nuclear weapons program is another motivation for pursuing ICF. 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. It has been argued that some aspects of ICF research may violate the Comprehensive Test Ban Treaty or the Nuclear Non-Proliferation Treaty. In the long term, despite the formidable technical hurdles, ICF research might potentially lead to the creation of a "pure fusion weapon".
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. In combination with fissionable materials, neutrons produced by ICF can potentially be used in Hybrid Nuclear Fusion designs to produce electric power.
- 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.
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- National Ignition Facility Project
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- 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