Laser nuclear fusion: current status, challenges and prospect

Abstract

In 2009, in Lawrence Livermore National Laboratory, USA, National Ignition Facility (NIF) — the largest thermonuclear fusion device ever made — was launched. Its main part is a multi-beam laser whose energy in a nanosecond pulse exceeds 1 MJ. Its task is to compress deuterium-tritium fuel to a density over a few thousand times higher than that of solid-state DT and heat it to 100 millions of K degrees. In this case, the process of fuel compression and heating is realized in an indirect way — laser radiation, in the ultraviolet range, is converted in the so-called hohlraum, a 1 centimetre cylinder with a spherical DT pellet inside, into very intense soft X radiation symmetrically illuminating the DT pellet. For the first time ever, the fusion device's energetic parameters are sufficient for achieving the ignition and self-sustained burn of thermonuclear fuel on a scale allowing for the generation of energy far bigger than that delivered to the fuel.

The main purpose of the current experimental campaign on NIF is bringing about, within the next two-three years, a controlled thermonuclear "big bang" in which the fusion energy will exceed the energy delivered by the laser at least ten times. The expected "big bang" would be the culmination of fifty years of international efforts aiming at demonstrating both physical and technical feasibility of generating, in a controlled way, the energy from nuclear fusion in inertial confined plasma and would pave the way for practical realization of the laser-driven thermonuclear reactor.

This paper briefly reviews the basic current concepts of laser fusion and main problems and challenges facing the research community dealing with this field. In particular, the conventional, central hot spot ignition approach to laser fusion is discussed together with the more recent ones — fast ignition, shock ignition and impact ignition fusion. The research projects directed towards building an experimental laser-driven thermonuclear reactor are presented as well.

Key words. laser, plasma, inertial fusion, laser acceleration.

1. Introduction

Nuclear fusion is one of the most promising and forward-looking directions in search of new sources of energy. The main advantages of fusion energy, as opposed to the energy sources using fossil resources or the nuclear energy using a heavy nuclei fission reaction, are as follows: (a) practically unlimited resources of raw materials — deuterium from sea water and lithium for the production of tritium, in the crust of the earth; (b) energy produced is "clean" — it does not emit greenhouse gases and there is no long-lived radioactive waste; (c) a fusion power plant is safe — there is no possibility of an uncontrolled "nuclear explosion". As opposed to the so-called renewable energy sources, solar, wind and so on, the energy production using nuclear fusion is incomparably more efficient.

The fusion reaction of nuclei of hydrogen isotopes, namely deuterium and tritium, is the core of fusion energy, although other less efficient reactions are taken into consideration as well. As a result of this reaction, a helium-4 nucleus and a neutron are produced and energy of 17.6 MeV is emitted, including 14.1 MeV of the neutron energy — much higher than the initial energy of deuterium and tritium. In order that the reaction efficiency be high and energy production on a macroscopic scale be possible, the energy of colliding nuclei of deuterium and tritium must be high enough to overcome the electrostatic repulsion between nuclei, and those reactions should take place in sufficiently large amounts and under appropriate conditions. The environment in which those conditions may be close to optimal is deuterium-tritium plasma of temperature about 10 keV, that is about 100 millions of kelvin degrees, and of appropriate density and volume. If such plasma can be generated, the main problem is to confine it in time to let a substantial amount of DT fuel "react". In space conditions, high-temperature hydrogen plasma is confined by gravitational forces, in the Sun and other stars. In terrestrial conditions, the confinement of hot plasma is possible either by a strong magnetic field or inertial forces. In the first case we are talking about magnetic confinement fusion, and in the second one about inertial confinement fusion or inertial fusion.

Inertial fusion with a high energy gain was demonstrated, in the form of a hydrogen bomb, almost 60 years ago. Unfortunately, the solution used in this case — a nuclear fission explosion as a driver of DT fusion — is not suitable to be implemented in the case of controlled and "clean" fusion energy production. The driver in the case of controlled inertial fusion is now a laser, and laser fusion right now is a synonym for inertial fusion.

This paper briefly reviews the basic current concepts of laser fusion and main problems and challenges facing the research community dealing with this field. In particular, the conventional, central hot spot ignition approach to laser fusion is discussed together with the more recent ones — fast ignition, shock ignition and impact ignition fusion. The research projects directed towards building an experimental laser-driven thermonuclear reactor are presented as well.

2. Basic concepts of laser fusion

In the conventional approach to laser fusion, as proposed in the early 1970s, a spherical target containing DT fuel is symmetrically illuminated by many beams of a nanosecond laser, or by X-rays generated by this laser, as a result of which rapidly expanding plasma is produced on the target's surface. The momentum of the expanding plasma is balanced by the momentum of the inner part of the target, which leads to the implosion of the target and the fuel compression. The decrease of volume of the imploding target is accompanied by the increase of the temperature inside it — the process is roughly of isobaric character — and the increase of the fuel density. At sufficient reduction in volume, the temperature in the centre of a properly designed target rises to very high values, about 10 keV. A "hot spot" is formed which allows the self-ignition of the fuel; the situation here is analogous to the diesel engine. Such a kind of fuel ignition is referred to as central ignition or central hot spot ignition.

In order that the energy produced in the fusion reactions be higher than the energy of the laser — that is, that the energy gain, the ratio of fusion energy to laser energy, exceed one — the density of the compressed fuel should be very high, over 1000 times the density of a solid DT, and its mass should exceed a certain critical value of about a milligram. Achieving such extremely high temperatures and densities of the DT fuel with supercritical mass using the central hot spot approach requires a nanosecond laser, of about 10 nanoseconds, with very high energy of at least 1 megajoule, extremely high symmetry of the target's illumination, and meeting several other technically difficult conditions, discussed below. To reduce these requirements, several alternative ways to ignite the fuel have been proposed. They include:

• Fast ignition. In this approach the ignition of the DT fuel, initially compressed with a multi-beam laser to about 1000 times the density of solid DT, is done as a result of its very rapid heating, over some 10 to 20 picoseconds, by a very intense flux of particles — electrons or ions — at an intensity of about ten to the twentieth watts per square centimetre. In this case the situation is analogous to that in a petrol engine, where the ignition of the compressed fuel is initiated with a spark from the spark plug.
• Shock ignition. Here the "ignitor" is a strong, converging shock wave generated in the DT target in the final stage of its compression with the most intense final part of an appropriately shaped laser pulse compressing the fuel. In fact, it is an intermediate variant between fast ignition made by an outer energy source and the central hot spot variant discussed earlier.
• Impact ignition, in which a rapid heating of the compressed fuel is the result of the impact of a micro-projectile of mass around a millionth to a ten-thousandth of a gram accelerated to a velocity above 1000 kilometres per second.

All these advanced concepts promise higher energy gain with laser energy much lower than in the case of central hot spot ignition. However, independently of the ignition scheme, to achieve ignition and energy gain some basic conditions for parameters of the compressed DT fuel have to be fulfilled. They concern the ion temperature, the density and the areal mass density — the "confinement parameter", the fuel density multiplied by the compressed fuel radius. The temperature should be roughly in the range 10 to 100 keV, as below 10 keV the rate of the deuterium-tritium reaction is much lower than that at the optimum of about 30 keV. Minimum values for the density and the areal density can be derived from the assumption that the permissible mass of DT fuel — limited by the permissible explosion energy of about 1 gigajoule — is limited to about 10 milligrams. Equation 1 in the source states that condition: four thirds of pi, times the fuel density, times the cube of the fuel radius, is less than that permissible mass.

Equation 2 in the source defines the burn fraction — the part of the fuel which is actually consumed — as the areal density divided by the sum of the areal density and the burn parameter, and requires that it exceed some practically useful value. Here the burn parameter is a slowly varying function of the ion temperature, equal to about 7 grams per square centimetre at 30 keV.

Assuming a fuel mass of 10 milligrams, a burn parameter of 7 grams per square centimetre and a required burn fraction of 0.2, we arrive at Equation 3 in the source, that the fuel density must exceed 200 grams per cubic centimetre, and Equation 4 in the source, that the areal density must exceed 2 grams per square centimetre. It should be noted, however, that the hot spot areal density can be much lower than that determined by Equation 4 — about 0.2 to 0.4 grams per square centimetre — though the hot spot density should be high and its temperature must be about 10 keV.

3. Central hot spot ignition scheme

As explained in Section 2, in the central hot spot option of laser fusion, self-ignition of nuclear fuel takes place in the centre of a spherical DT pellet due to roughly isobaric compression of the pellet to very high densities, over 1000 times the density of solid DT. The particular stages of the central hot spot approach are: (a) laser beams or laser-produced X-rays rapidly heat the surface of the fusion pellet, forming plasma expanding outwards with a high velocity of about a thousand kilometres per second; (b) due to the rocket-like blow-off of the hot plasma a strong converging shock wave is formed near the plasma ablation surface which compresses DT fuel inside the pellet; (c) during the final stage of the pellet implosion the fuel density reaches a very high value and the temperature in the pellet centre, at the hot spot, attains about 10 keV and the hot spot is ignited; (d) thermonuclear burn spreads rapidly through the compressed high-density fuel, yielding many times the input laser energy.

Central hot spot ignition can be realized both in the direct-drive and indirect-drive schemes. In the first one, many — tens of — ultraviolet nanosecond laser beams symmetrically irradiate the fusion pellet directly. In the second one, the pellet is placed in a small, about 1 centimetre, cylinder called a hohlraum, made of a high atomic number metal such as gold, uranium or some high-Z metal mix. The laser beams irradiate the inner side of the hohlraum, heating it to produce hot plasma which radiates mostly thermal soft 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 laser beams directly. The absorption of thermal X-rays by the target is more efficient than the direct absorption of laser light; however, the hohlraum also takes up considerable energy to heat on its own, thus significantly reducing the overall efficiency of laser-to-target energy transfer.

The main challenges for the direct-drive and indirect-drive central hot spot schemes are basically the same. They include: achieving high laser-to-fuel energy transfer efficiency, controlling the symmetry of the imploding fuel, preventing pre-heating of the fuel by hot electrons and X-rays, preventing premature mixing of hot and cool fuel by hydrodynamic — mostly Rayleigh-Taylor — instabilities, and the formation of a "tight" shock-wave convergence at the compressed fuel centre. To meet these requirements, short-wavelength radiation must be used to compress the fuel, the pellet must be made with extremely high precision and sphericity with aberrations of no more than a few micrometres over its surface, both inner and outer, the laser or X-ray beams must be extremely precise, and the beams must arrive at the same time at all points on the pellet.

The advantage of the indirect-drive scheme is more stable and more symmetric implosion — irradiation of the pellet by thermal radiation in the hohlraum is more homogeneous than by many laser beams — as well as higher ablation pressure, due to the shorter wavelength of X-rays; therefore lower energy absorbed in the pellet is required to compress the fuel. However, due to the lower overall energetic efficiency of this scheme, usually higher input laser energy is needed here for fusion ignition than in the case of the direct-drive scheme. Anyway, for both schemes this energy is of order 1 megajoule.

At present, only one laser facility produces laser energy above 1 megajoule and is able to meet the requirements for fusion ignition and energy gain. This is the National Ignition Facility launched at Lawrence Livermore National Laboratory in the USA in 2009. In an advanced stage of construction is also the megajoule laser facility LMJ in France, which is predicted to be launched in 2014. Both NIF and LMJ are designed to achieve ignition and an energy gain of about ten using the indirect-drive central hot spot scheme.

National Ignition Facility

The principal goal of NIF is to achieve ignition of a DT fuel pellet and provide access to high-energy-density physics regimes needed for experiments related to national security, fusion energy, and frontier scientific exploration in such fields as astrophysics, nuclear physics, and material science. To reach this goal, the National Ignition Campaign was established with the aim of performing credible experimental campaigns on NIF and demonstrating, within a few years, a reliable and repeatable fusion ignition source. The first stage of the campaign is focused on refining detailed requirements on targets, laser and diagnostics to optimise the compression and ignition process and to balance risk.

The laser beams in NIF begin with nanojoule pulses from a solid-state laser. The precisely shaped pulses, about 15 nanoseconds long, are divided into 48 beams and passed through preamplifiers. Those beams are subsequently divided further into a total of 192 beams, each about 40 centimetres square in profile, that pass six times through neodymium-glass amplifiers, ultimately achieving a total of 4 megajoules of 1051 nanometre laser light. These 192 beams are transported in sets of four "quads" to a target chamber, at which potassium dihydrogen phosphate crystal sheets convert the 1051 nanometre light first to 526 nanometre light, then in a second crystal to 1.8 megajoules of 351 nanometre light. This ultraviolet light then enters a 10 metre diameter target chamber, where the beams are focused onto the inner wall of a hohlraum that houses the spherical DT pellet. The laser light produces X-rays inside the hohlraum that ablate the shell of the pellet, which then heats and compresses the DT fuel to a temperature of about 10 keV and a density of several hundred grams per cubic centimetre, which should produce ignition. Under these conditions, the DT fuel in the pellet should burn to produce more than 10 megajoules of energy within 10 to 100 picoseconds.

Over two years, experimental and numerical efforts within the campaign have resulted in significant improvements in the NIF laser parameters — for example an increase in ultraviolet laser energy from 1.1 to 1.7 megajoules — in the hohlraum X-ray parameters, in the symmetry and velocity of the pellet implosion, as well as in the parameters of the compressed fuel. In particular, in the first implosion experiment with cryogenic DT fuel of 0.17 milligrams, performed with 192 ultraviolet laser beams of total energy 1.6 megajoules, extremely high compressed fuel parameters were achieved: a fuel density of about 600 grams per cubic centimetre — more than 2000 times the solid DT density — a fuel areal density of about 1 gram per square centimetre, and an average ion temperature of about 3.5 keV, and, as a result, a thousand million million fusion neutrons were produced. Although these parameters are still lower than required for ignition and significant energy gain, it is believed that this goal is to be reached within the next two years. If achieved, ignition and energy gain on NIF would be a major step towards demonstrating the feasibility of energy production from fusion and would likely open the door for building an experimental laser-driven fusion reactor. It would also be highly stimulating for the development of the advanced laser fusion concepts described in the next section.

4. Advanced laser fusion concepts

4.1. Fast ignition

Fast ignition is a novel approach to laser fusion which differs from the conventional central hot spot approach in using separate drivers for compression and ignition of the hydrogen fuel. In this approach, the fuel pre-compressed by a long-pulse, nanosecond driver — laser beams or X-rays — is ignited by a short-pulse, picosecond, ultra-intense particle beam at about ten to the twentieth watts per square centimetre. Fast ignition has some significant potential advantages over conventional laser fusion: higher gain, lower overall driver energy, the reduction in symmetry requirements, and flexibility in compression drivers. The price to be paid is the need for efficient production of, and coupling to the fuel of, a particle beam of extreme parameters.

Fast ignition requires that a small part of the DT fuel, about ten micrograms, compressed to about 1000 times the solid density — that is, above 200 grams per cubic centimetre — is heated by an external ignitor to a temperature of about 10 keV. If the volume of the compressed fuel is sufficiently large, with a confinement parameter above 2 grams per square centimetre, a thermonuclear burn wave ignited in the hot spot propagates through the fuel and thus energy is produced. To compress the fuel, both direct-drive and indirect-drive approaches can be used. In the second approach, the X-rays can be produced by a laser, by a heavy ion beam from an accelerator, or by a Z-pinch. As an ignitor, fast electron, proton, or light ion beams driven by a short-pulse petawatt laser can be used, though conventionally accelerated heavy ion beams are considered as well.

In the original fast ignition scheme of Tabak and colleagues, the ultra-intense, roughly 10 picosecond laser pulse penetrates close to the dense fuel through a channel bored in the surrounding plasma by the light pressure of a preceding, roughly 100 picosecond laser pulse. The relativistic, megaelectronvolt electron beam produced at the interaction of the ultra-intense pulse with the critical surface of the dense plasma core ignites the fuel. A crucial issue for this scheme is the effective formation of the channel and the transport of the ultra-intense pulse through it.

More recently proposed, an alternative fast ignition concept uses a hollow high atomic number cone inserted into a standard spherical shell target. The cone provides an open path for the ultra-intense laser beam and allows the beam to be focused inside the cone and to generate fast electrons at its tip, very close to the dense plasma produced by the cone-guided implosion. A variant of the cone scheme uses a thin foil target, placed in the cone at some distance from the cone tip, to generate a proton or ion beam igniting the compressed fuel. The cone concept makes transport of the ignition laser beam toward the dense fuel core easier, but on the other hand it complicates the target structure and disturbs the spherical symmetry of implosion.

The first integrated compression-plus-heating fast-ignition experiment was performed by a Japan and United Kingdom team. They used the Gekko XII nanosecond laser — nine beams, 2.5 kilojoules, 0.53 micrometres — for a deuterated-plastic shell implosion in cone-guided geometry, and a petawatt laser of 300 joules in 0.5 picoseconds for fast plasma heating. It was found that the neutron yield increased by 1000 times, from ten thousand to ten million, due to the petawatt laser heating, and the coupling efficiency from the petawatt laser to the thermal plasma approached 20 to 30 per cent. It was also inferred that the cone does not substantially degrade the target implosion. This breakthrough experiment provided strong support for the fast ignition concept and stimulated worldwide research on it.

General requirements for the fast ignitor were determined by a series of two-dimensional numerical hydrodynamic simulations. The optimal values of the ignitor beam energy and intensity to be delivered to the fuel, and the corresponding optimal pulse duration and beam radius, were parameterised as a function of the fuel density. Equation 5 in the source gives the optimal ignitor energy as 18 kilojoules multiplied by the fuel density in units of 300 grams per cubic centimetre raised to the power minus 1.85. Equation 6 in the source gives the optimal intensity as 6.8 times ten to the nineteenth watts per square centimetre multiplied by the same density ratio raised to the power 0.95. Equation 7 in the source gives the optimal pulse duration as 21 picoseconds multiplied by the density ratio raised to the power minus 0.85. Equation 8 in the source gives the optimal beam radius as 20 micrometres multiplied by the density ratio raised to the power minus 0.97.

Equation 5 shows that the required ignitor energy decreases fairly rapidly when the fuel density increases. However, the higher the density, the higher the demands for the compression driver. As a compromise, a value of about 300 grams per cubic centimetre is usually accepted. For such a density, the ignitor parameters are, as Equation 9 in the source records: about 17 kilojoules of energy, about 7 times ten to the nineteenth watts per square centimetre of intensity, about 20 picoseconds of pulse duration, and a beam radius of about 20 micrometres.

These parameters are extremely demanding and cannot be achieved with a conventional particle accelerator. The only way to produce such ultra-intense particle beams seems to be the use of laser acceleration.

Equations 5 to 9 specify the required parameters of the ignitor — a particle beam — delivered to the fuel, that is to the hot spot. To determine the required parameters of the ignitor driver, for example the laser, we must introduce some coupling factors. The most important one, determining the practical feasibility of fast ignition, is the ratio of the energy deposited to the fuel by the ignitor particle beam to the energy of the ignitor driver. Equation 10 in the source writes this "total" energy conversion efficiency as the product of three factors reflecting three main stages of ignitor-driver-to-fuel interaction: the energetic efficiency of particle beam production at the source, the efficiency of the beam transport from the source to the dense fuel, and the efficiency of the beam energy deposition into the dense fuel, that is into the hot spot. Both the total efficiency and the particular efficiencies defined above substantially depend on the kind of particles produced by the driver. In the case of a laser driver, the production efficiency is generally higher for electrons than for protons or ions but, on the other hand, the heavier particles are easier to transport and their energy deposition can be higher and better localized.

In recent years, significant progress has been made in defining and solving key problems related to both electron fast ignition and proton or ion fast ignition. In the first option, efforts have been focused on controlling the energy spectrum and the angular divergence of laser-produced electron beams, as well as on ultrahigh-current, gigaampere-scale electron beam transport in dense plasma and the beam energy deposition to the compressed fuel. Fairly impressive progress has also been made in the development of laser-driven ultraintense proton and ion sources for fast ignition. In particular, methods of highly efficient generation — above 10 to 20 per cent production efficiency — of proton and ion beams of the parameters required for fast ignition have been proposed and investigated experimentally or with the use of advanced computer codes.

To summarize, fast ignition is an innovative approach to laser fusion which promises high energy gain at driver energy and cost much lower than in the case of the conventional central hot spot scheme. The first integrated fast-ignition-related experiments strongly support the idea, but great worldwide efforts are necessary to validate the concept at full scale and to mitigate risk. So-far numerical modelling suggests that ignition and energy gain are feasible with a total driver energy of at most 300 kilojoules and an ignition laser energy of at most 100 kilojoules. The laser facilities with multi-kilojoule petawatt lasers just being launched or constructed make proof-of-principle and benchmark experiments at sub-ignition scale possible. Full-scale fast ignition experiments seem to be technologically feasible in the next decade. They would push fast ignition research into a key stage in which the feasibility of fast ignition as a route to inertial fusion energy would be determined.

4.2. Shock ignition

Shock ignition is the newest concept of laser fusion, proposed by Betti and colleagues several years ago, and is actually an intermediate variant between central hot spot ignition and fast ignition. The fuel is first irradiated symmetrically, but driven at a lower velocity than for central hot spot ignition. The temperature of the hot spot generated at the end of the implosion is below the ignition threshold. At an appropriate time, towards the end of the implosion, the fuel pellet is irradiated by an intense sub-nanosecond laser spike, which drives a strong converging shock wave with an initial pressure of about 300 megabar. This pressure is amplified by convergence, and further amplified as the converging shock collides with the outgoing shock bouncing from the centre. The hot spot then undergoes additional heating and ignites.

The shock ignition concept was proved by advanced computer simulations and was also confirmed by a proof-of-principle experiment on a small scale, at multi-kilojoule laser energy. The main advantages of this option are as follows: (a) relatively low laser power, 200 to 300 terawatts, and energy, 200 to 300 kilojoules, needed for ignition; (b) high energy gain, like that for fast ignition; (c) established physics with relatively simple hydrodynamics; (d) employing conventional laser technology and a rather simple target. However, there are several serious problems unsolved so far, in particular: efficient generation of a highly symmetric strong shock with a small number of laser beams, precise timing of the shock, the interaction of the shock with the compressed shell and the bouncing divergent shock, and possible degradation of laser-plasma coupling at the required high intensity of the spike due to parametric plasma instabilities.

Shock ignition is considered to be tested at full scale both at NIF and LMJ. However, to accomplish such experiments, significant changes in the geometry of target irradiation are necessary in both facilities, as shock ignition can be realized basically only in the direct-drive scheme.

4.3. Impact ignition

Impact ignition fusion is an old idea, suggested already in the 1960s; however only impact ignition schemes proposed in the last two decades seem to be feasible to be accomplished with current or emerging technologies. In the scheme proposed by Caruso and Pais in 1996, a micro-projectile of about one microgram made of high atomic number material, for example gold, and accelerated to about five thousand kilometres per second collides with the compressed DT fuel at 200 grams per cubic centimetre or more. Due to the collision, the projectile rapidly collapses to high densities, above 1000 grams per cubic centimetre, and a large fraction of its energy is transferred to the fuel in a very short time of about ten picoseconds. As a result, a hot spot is created which ignites the fuel. In the considered case, the minimum kinetic energy of the projectile is only about 10 to 20 kilojoules — much lower than the laser energy needed to compress the fuel, a few hundred kilojoules.

Another, more advanced impact ignition scheme was proposed by Murakami and Nagatomo in 2005. In this scheme, the compressed DT main fuel is ignited by impact collision of another fraction of separately imploded DT fuel, which is accelerated in a hollow conical target to hyper-velocities of about a thousand kilometres per second. Its kinetic energy is directly converted into thermal energy corresponding to temperatures above 5 keV on collision with the main fuel, and this self-heated portion plays the role of the ignitor. The ignitor shell is irradiated typically by a nanosecond laser pulse at intensities above ten to the fifteenth watts per square centimetre and short laser wavelength, for example 0.35 micrometres, to exert ablation pressures accelerating the DT projectile above 100 megabar. It was estimated that the total laser energy needed for compression and ignition of DT fuel in this scheme is about 200 to 300 kilojoules — much lower than in the central hot spot scheme.

The potential of impact ignition for fusion energy production was demonstrated in an experiment performed in Osaka with the multi-kilojoule Gekko XII laser. In the Murakami scheme with a deuterated-plastic shell target, a two-order-of-magnitude increase in the fusion neutron yield was achieved as compared to the conventional central hot spot scheme.

Important advantages of impact ignition in comparison with central hot spot ignition are much lower laser driver energy and higher energy gain — a gain near one hundred seems to be feasible. As compared with fast ignition, its significant merit is simpler physics and no need for a short-pulse multi-petawatt laser, which means a lower cost and less demanding laser technology. The main challenge for this scheme is, however, accelerating a multi-microgram, high-density projectile — at 50 grams per cubic centimetre or more — to the extremely high velocity of over a thousand kilometres per second.

5. Towards an inertial fusion power plant

The possibility of using inertial confinement fusion to build a power plant has been studied since the late 1970s, and such plants are known as inertial fusion energy plants or reactors. In this device, the driver — a laser or particle accelerator — converts electrical power into short pulses of energy, of light, particles or micro-projectiles, and delivers them to the DT fuel pellet to cause implosion, ignition and thermonuclear burn. In a pellet factory, fuel pellets are manufactured, filled with DT fuel and sent to the reaction chamber. In the chamber, the driver beams are directed to the pellet to implode it and to produce thermonuclear energy with a repetition rate of a few times per second. Products of the thermonuclear explosion, in particular neutrons, are captured in a surrounding structure called a blanket, and their energy is converted into thermal energy. In the rest of the reactor, two major processes for material and energy are performed. Tritium and some other target materials are extracted from the re-circulating blanket fluid material and from the reaction chamber exhaust gases. Then these extracted materials are recycled to the target factory. The thermal energy in the blanket fluid is converted into electricity, a portion of which is re-circulated to power the driver.

In terms of the energetic efficiencies of the basic subsystems of the reactor, this cycle gives the requirements for the energetic efficiency of the driver and the fusion target energy gain. For the energy balance, the condition of Equation 11 in the source has to be fulfilled: the recirculated fraction of the electric power, multiplied by the driver efficiency, the thermal conversion efficiency and the target gain, equals one. Assuming a thermal efficiency of 40 per cent, a typical value, and a recirculated fraction of 25 per cent, we arrive at Equation 12 in the source: the target gain multiplied by the driver efficiency equals one.

It means that for a gain of 100, the driver efficiency has to be 10 per cent. In turn, it means that both the target gain predicted for the NIF and LMJ facilities, about ten, and the efficiency of the lasers used in these devices, about one per cent, have to be increased by an order of magnitude to meet the conditions required for an inertial fusion energy reactor. Moreover, the reactor driver has to work with a repetition rate of about 5 to 10 hertz. Potentially, these requirements are within reach of diode-pumped solid-state lasers and krypton fluoride excimer lasers, as well as heavy ion accelerators. As for the target gain, advanced fusion concepts — fast ignition, shock ignition, impact ignition, or others — seem to be necessary to be employed to reach the reactor requirements.

General reactor concepts have been developed for more than three decades and several advanced designs, like KOYO in Japan and HYLIFE in the United States, have been proposed. At present, three big long-term reactor-oriented projects, which detail all the issues of driver, target physics and production, reactor chamber and diagnostics, are under development. These are the LIFE project in the United States, the FIREX project in Japan and the HiPER project in Europe.

The main goal of the LIFE, Laser Inertial Fusion Energy, project is to build a prototype plant and to develop the technology of inertial fusion energy reactors enabling commercial production of energy from fusion. The plant would use a NIF-like architecture and the indirect-drive fusion scheme with a target energy gain of at least 60. The laser driver of 2.4 megajoules energy in the ultraviolet would work with a repetition rate of 16 hertz. It would be based on currently available materials and market-based technology, using in particular semiconductor diodes for laser pumping. The project is now in the design phase and a turning point for it will be a demonstration of ignition and energy gain on NIF. It is believed that NIF's success will push the project into the next phase, the phase of construction of a prototype fusion plant.

The FIREX, Fast Ignition Realization Experiment, project is aimed at demonstrating fusion ignition and burn using the fast ignition approach. The project is divided into two phases. The goal of the first phase is to demonstrate fast heating of fusion fuel up to the ignition temperature of 5 to 10 keV by a high-energy short-pulse laser of 10 kilojoules in 10 picoseconds, combined with the long-pulse 10 kilojoule, 1 nanosecond Gekko XII implosion laser. In the second phase, the implosion laser is planned to be a 50 kilojoule, 3 nanosecond blue laser, whereas the heating laser is to be a 50 kilojoule, 10 picosecond infrared laser. The goal of this phase is to demonstrate fusion ignition and burn. At present, the fast ignition experiments within the first phase are underway. The European HiPER project is described in the next section.

6. The HiPER project and current fusion-related research of the Polish team

HiPER, the High Power Laser Energy Research Facility, is a project for a European laser infrastructure for nuclear fusion and studies of extreme states of matter. The main long-term goal of this project is to demonstrate the effective production of energy from nuclear fusion driven by lasers of high power and energy working with a high repetition rate, above 1 hertz. The research programme of the project is focused on laser fusion but also includes basic research in other fields, as well as work associated with the development of technologies and techniques necessary to build the HiPER device, especially laser technology.

The laser fusion physics studies include theoretical work, numerical simulations and experiments which roughly can be divided into two groups: studies of DT fuel compression, and research on various advanced schemes of fuel ignition — electron and proton or ion fast ignition, shock ignition and impact ignition. The main technical objective of the former is to optimize the structure of the DT target and specify the parameters of the multi-beam laser compressing the target which would provide the maximum energy gain.

The research in the second group is focused on various physical and technical aspects of the advanced ignition schemes, including entirely new issues related, for example, to relativistic interaction of laser light with matter at intensities of ten to the nineteenth up to ten to the twenty-second watts per square centimetre, generation and transport of electron beams of very high current densities of about ten to the fourteenth amperes per square centimetre, generation of picosecond ion beams of intensities exceeding many times the intensity of beams produced in the biggest accelerators, and the acceleration of macroscopic amounts of matter to velocities never before attainable. The studies also include a range of issues related to the technology of production and transportation of targets, including cryogenic DT targets, and reactor technology, as well as to the development of the methods and diagnostic equipment necessary to conduct this research.

It is anticipated that the HiPER device will be equipped with two coupled lasers of very high energy and power, namely a multi-beam nanosecond laser of about 200 kilojoules and a picosecond laser of about 100 kilojoules and power of several to several dozen petawatts and, possibly, a femtosecond laser of power above 100 petawatts. Thus it would be a unique research infrastructure on a global scale, allowing fundamental research in various fields, including studies of matter at extreme conditions so far unattainable in terrestrial conditions. Currently the following fields of research are being considered:

• laboratory astrophysics — simulation in laboratory conditions of various astrophysical phenomena;
• research on so-called warm dense matter, occurring inside planets, including the Earth's interior;
• testing of materials under very high pressures;
• atomic physics at very high pressures and temperatures;
• ultraintense interaction of laser with plasma and relativistic plasma;
• generation of very intense electron and ion beams, for the purposes of nuclear physics, particle physics and nuclear medicine;
• basic physics in super-strong fields.

The project was divided into three phases: a preparatory phase from 2008 to 2013, a design and technical phase, and a construction phase. The five-year preparatory phase began in April 2008 with the participation of more than 30 scientific institutions including 26 from 9 European Union countries. Poland in this project is represented by the Institute of Plasma Physics and Laser Microfusion. The aim of the preparatory phase includes, among others: development — based on experimental studies and numerical simulations — of the physical basis for advanced fusion schemes and the specification of the physical and technical conditions of their implementation; development of the conceptual design of the HiPER device; and proposals for long-term programmes for fusion and basic non-fusion research with the use of this device. The work was divided into 15 work packages, of which the Institute contributes to five, its work relating to both fusion and non-fusion research.

Activities of the Institute under the fusion-related work are focused on three main issues:

• development of a proton and ion source of ultrahigh intensity, about ten to the twentieth watts per square centimetre, for proton and ion fast ignition;
• laser acceleration of micro-projectiles for the needs of impact ignition;
• studies of shock wave generation and non-linear interaction of laser with plasma at conditions relevant to shock ignition.

In the package regarding non-fusion research, the Institute performs studies relating to laboratory astrophysics — such as the generation of supersonic plasma jets and their interaction with gases — laser acceleration of ions at relativistic laser intensities, and laser-induced nuclear reactions. All this work, including theoretical analysis, advanced numerical simulations and experiments, is conducted within broad international cooperation, mainly with the research centres participating in the HiPER project.

7. Summary

The fifty-year research on laser fusion resulted in:

• understanding and solving the basic physical issues of the conventional inertial confinement fusion scheme;
• preparation of alternative, potentially more effective, inertial confinement fusion schemes;
• development of technology enabling the building of megajoule fusion devices such as NIF and LMJ;
• development of international inertial confinement fusion programmes — LIFE, HiPER, FIREX — directed towards the design of a fusion reactor.

Currently we are facing a turning point: in the course of the next two-three years NIF will likely prove the physical and technical feasibility of producing energy from fusion, which would open the door for building the first prototype nuclear power plant.

The main challenges for physicists are presently the physical problems occurring in the advanced inertial confinement fusion schemes, such as: generation of ultra-intense particle beams, relativistic laser-plasma interaction, transport and interaction of gigaampere-current beams of particles with plasma, acceleration of micro-projectiles to hyper velocities, and various kinds of instabilities in laser plasma.

For commercial energy production out of laser fusion, further very considerable progress in the technology is necessary, especially in terms of drivers — lasers — production and handling of DT targets, and reactor technology. In the case of the latter there are many problems common to inertial and magnetic confinement fusion — blanket, tritium production, materials, energy conversion systems — and cooperation between the communities representing both directions in solving these problems is highly desirable.

The research on laser fusion, regardless of its main objective of creating an efficient, safe and potentially unlimited source of energy, is also a driving horse of the development of many branches of science, in particular:

• high energy density physics,
• laboratory astrophysics,
• ultra-intense interactions of light with matter and relativistic plasma physics,
• material research,
• particle and dense matter acceleration,
• laser technology,
• and probably many others.

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J. Badziak, Institute of Plasma Physics and Laser Microfusion, 23 Hery Street, Warsaw, Poland. Published as Bulletin of the Polish Academy of Sciences: Technical Sciences 60, number 4, pages 729 to 738 (2012), doi.org/10.2478/v10175-012-0084-8. The publisher states that all content of this journal is available under CC BY 4.0.

(Running heads, page numbers and reference-number markers have been dropped, cited authors named in the prose where the source names them, display equations rendered in words keyed to their source equation numbers, and inequalities written out; the eight figures are not reproduced and the forty-three references are at the source.)

(On this site: NIF’s own account of the facility is at /library/stm-c68b26e4c0, the broader status of inertial confinement fusion at /library/stm-b23ae31c98, and the light-ion driver route at /library/stm-102f44df48. The relativistic laser-matter physics behind fast ignition is at /library/stm-3a51ad6c6a, the magnetised-liner variant of inertial fusion at /library/stm-d11937cf2a, and inertial fusion read as a propulsion system at /library/stm-b87332054a. The aneutronic proton-boron target this site cares most about is at /library/stm-8dfcd2d56d and /library/stm-05100e66da; the plant-level economics are at /library/stm-4fed2cae63 and /library/stm-2d18170fa6; and the lattice-confinement route that reaches the same reactions by another road is at /library/stm-e25595eb3b.)