Closer to Laser-Driven Suns: A Milestone in Laboratory-Scale Fusion Science

Closer to Laser-​​Driven Suns: A Milestone in Laboratory-​​Scale Fusion Science

Using the world’s most powerful assembly of lasers, a team of American physicists and engineers have, for the first time, extracted more energy from laboratory-​​scale nuclear fusion than was absorbed by the fuel to trigger it. This crosses an important symbolic threshold on the path towards useful industrial exploitation of boundless energy from nuclear fusion and the laboratory accessibility of an extreme physics regime only previously accessible on Earth in the experimental detonations of nuclear weapons or encountered in stars and other astrophysical systems.

These latest developments, from the US National Nuclear Security Administration’s National Ignition Facility in California, are still well short of the long-​​sought goal of lab-​​scale thermonuclear “ignition”, the point where the fusible fuel keeps burning in a self-​​sustained way, releasing enormous amounts of energy after the initial energy source is removed until all the fuel is consumed. However, these latest results represent a substantial milestone on the path to thermonuclear ignition in a laser-​​driven inertial confinement fusion system, and the most advanced progress yet reported in the world towards fusion ignition in a system of this type.

Whilst nuclear fission, the process that powers today’s familiar nuclear power reactors, extracts energy from the fission (or splitting) of very heavy nuclei such as uranium into lighter products, nuclear fusion — the process responsible for energy generation in stars and most of the energy generation in thermonuclear weapons — produces energy from the fusion of light nuclei, usually heavy isotopes of hydrogen, producing a heavier nucleus such as helium-​​4 as the end product, along with prodigal amounts of energy relative to the mass of the fuel consumed.

Self-sustaining nuclear fusion is at the heart of the Sun - life on Earth would probably not be possible without it.

Self-​​sustaining nuclear fusion is at the heart of the Sun — life on Earth would probably not be possible without it. Creative Commons licensed Flickr photo by pedromourapinheiro.

Useful power generation from nuclear fusion has been eagerly investigated and pursued by physicists since nuclear fusion was first achieved and observed in the laboratory in 1933. Australian physicist Mark Oliphant, then working in Cambridge, discovered the acceleration of deuterons into a target of heavy water released a large amount of energy. The possibility of nuclear fusion in an “ignited” self-​​propagating system generating a useful energy output garnered renewed interest in 1952 when manmade thermonuclear ignition, with a large energy output from the self-​​sustaining “burning” of fusible fuel, was first achieved in the “Ivy Mike” test — the world’s first thermonuclear weapon. However, producing thermonuclear ignition on an industrial scale, taming the enormous power of the hydrogen bomb in a controlled way, is immensely difficult because of the extremely high confinement densities and plasma temperatures that have to be maintained.

Confinement of a plasma of fusible nuclei in strong magnetic fields in a toroidal “Tokamak” reactor is one popular approach towards achieving this. Laser-​​driven inertial confinement fusion, similar to the inertial confinement fusion technology of hydrogen bombs, where the energy supplied in the form of intense light from multiple powerful lasers is another approach of interest, and this is the technology that is developed and studied at the National Ignition Facility.

A lot of energy must be pumped into the fuel to drive the nuclei together, overcoming their electrostatic repulsion, to achieve nuclear fusion. At NIF, 192 powerful lasers direct their beams together at a tiny gold chamber called a hohlraum. The hohlraum in a laser-​​driven ICF system is named from the German for “hollow space”, in the ever-​​imaginative naming tradition of physicists. This hollow cavity is essentially a good approximation to a perfect blackbody radiator, absorbing the ultraviolet light from the laser system which strikes it, heating up to a very high temperature and re-​​radiating some of that energy as X-​​rays.

Inside the hohlraum is a tiny spherical plastic capsule about two millimetres wide containing the fusible fuel itself. The plastic target pellet is filled with gaseous deuterium and tritium and cooled to an extremely low temperature, resulting in the inside surface of the ablator (the outer plastic shell) being coated with a thin, smooth layer of frozen deuterium and tritium about 100 microns thick with the internal space containing deuterium-​​tritium mixture in the gaseous phase.

Some of the thermal X-​​rays radiated from the hohlraum are absorbed by the fuel capsule, and the resulting rapid heating causes the outer plastic shell to explode. The reaction exerted on the fuel inside, essentially imploding it, provides the inertial confinement in inertial confinement fusion, raising the confinement density high enough to ignite fusion. Deuterium and tritium are used as the fusion fuels here, since D-​​T fusion is the easiest fusion reaction to ignite under the most (relatively) gentle conditions, making it the fusion reaction of choice for manmade fusion reactor designs and fusion experiments. The deuterium-​​tritium fusion reaction generates a stable helium-​​4 nucleus as its fusion product, along with an extra neutron.

These neutrons are important in practical fusion engineering as they carry away part of the energy generated outside the plasma, and outside the magnetic field in a magnetic system, into a blanket of liquid lithium that surrounds the reactor vessel. Here the neutrons dissipate their energy as heat, where it can be transferred to a heat engine for useful work. The neutrons convert lithium (which is abundant in nature) into naturally-​​scarce tritium which is the ultimate fuel for the reaction, along with deuterium. When the shot takes place, the fuel capsule’s diameter is ultimately compressed down to 1/​35 of its original size, the fusion reaction is completed in 150 picoseconds, with a pressure in the core of the plasma of 150 billion atmospheres and a temperature and density about three times those of the core of the Sun.

A diagram of the NIF hohlraum target assembly, from Hurricane et al.

A diagram of the NIF target assembly, hohlraum and fuel pellet. Image from Hurricane et al., Fuel gain exceeding unity in an inertially confined fusion implosion, courtesy of the openly-​​accessible Nature article preview page.

However, achieving a net energy gain in the fuel capsule itself is only a step on the path to useful energy generation from laser-​​driven ICF. There are significant energy losses at every other step in the chain, such as storing energy and pumping optical energy into the enormous lasers, the nonlinear optics that convert the frequency of the light, and the fraction of energy absorbed by the hohlraum that is not radiated into the fuel capsule. Although fusion in the fuel capsule generates as much energy as is put into it, generating an energy output from fusion equal to the overall energy input into the laser system is still a long away from reality.

These significant results, recently published in Nature by physicist Omar Hurricane and his colleagues at LLNL, have been achieved in NIF experiments over the last six months or so, and rely on shaping the laser pulses to deliver more power early in the pulse. This creates a relatively high temperature in the hohlraum that “fluffs up” the plastic shell of the fuel capsule, reducing the abrupt transition in density at the capsule’s edge which helps significantly to slow down the growth of hydrodynamic Rayleigh-​​Taylor instabilities in the exploding capsule that disrupt the uniform heating and confinement of the fusion fuel by mixing cold material with hot fuel. These results of recent experiments are repeatable and have achieved a fuel energy gain of between 1.2 and 1.9, meaning that the energy released from fusion of the fuel is 1.2 to 1.9 times the energy absorbed by the fuel; a world-​​record result in laser-​​driven ICF studies.

Furthermore, it is known that much of the fusion energy release was contributed by heating of the fuel by the energetic alpha particles formed as the product of the fusion reaction, which stop in a very short range in matter, heating the fusion fuel as they pass through and stop within it. This is an important requirement for self-​​sustaining fusion burning. This demonstration of significant “alpha heating”, as it is known, is a significant advance over previous results in the field of laser-​​driven ICF research. The fuel energy gain has been increased by about an order of magnitude over past experiments thanks to these relatively recent improvements, with one of the experiments producing a degree of implosion and confinement density sufficiently high that they reached more than half of the Lawson criterion — a figure of merit for determining the point of heating and confinement at which ignition can be achieved.

That said, however, the total energy gain — the ratio of fusion energy out to laser energy in — is only about 1 percent at present. Although NIF is producing important, promising new results, it is not only far short of breaking even at present, but also far short of the expected performance of the system and far short of living up to the hype that has been coming out of Livermore surrounding the facility for many years. Although Livermore physicists were predicting ignition by the end of the year in 2010, the expectations of a fast, easy path to breakeven with the powerful new facility have not been realised — in fact, we have learned something about the previously unexpected flaws and limitations of the physics models that were previously used to predict relatively fast, easy ignition at NIF. Although the National Ignition Facility was built with the primary purpose of modelling the physics of nuclear weapons, helping to understand processes in the relevant extreme density and temperature regimes that are otherwise unreachable without nuclear explosive tests and experiments which the United States (along with most other nations) presently agree not to undertake, some of the system’s laser time is dedicated to basic ICF fusion research.

Although we are getting closer to both breakeven in laser-​​driven ICF systems and to sustained fusion burning in magnetic confinement reactors, new magnetic-​​confinement Tokamak reactors employing modern, powerful superconducting magnets are probably likely to achieve useful, fully-​​breakeven energy generation from fusion before ICF systems do. For example, the International Thermonuclear Experimental Reactor currently being constructed in France is expected to be the world’s first fusion plant that actually produces more energy overall than it consumes. As another example of an active magnetic confinement program, the Joint European Torus tokamak in Oxfordshire in 1997 — nearly 20 years ago — generated 16 megawatts of power from a total of 24 MW put into the system. However, there are different measures of success and figures of merit used in the study of inertial confinement and magnetic confinement fusion systems, meaning that it is difficult to draw comparisons between systems like NIF and magnetic systems such as JET or ITER in terms of how far advanced or “how close” they are in progress towards ignition (in the case of ICF) or sustained burning and confinement (in magnetic systems).

For further background reading on NIF and its target design, LLNL’s July/​August 1999 Science and Technology Review may be of interest — although somewhat dated, most of the content is still relevant.

[Feature image: Creative Commons licensed Wikimedia Commons photo.]