(Transcript of the lesson commentary.)

Basic Principles of Inertial Fusion

Thermonuclear fusion is a process that takes place all over the universe in the cores of stars. The light nuclei of atoms fuse together, releasing large amounts of energy. Various types of fusion reactions take place in stars.

On Earth, we are trying to achieve the fusion of deuterium with tritium. Both are isotopes of hydrogen, deuterium has one proton and one neutron in its nucleus, while the rare and radioactive tritium has one proton and two neutrons.

To fuse, the atoms need to overcome the repulsive electrostatic forces that act between their positively charged nuclei. This can be achieved, for example, by high temperatures or by external forces pushing them together.

In thermonuclear fusion research, there are two main approaches to force particles to fuse. In magnetic confinement, a cage of magnetic fields is created in which particles are trapped and heated to hundreds of millions of degrees.

In contrast, inertial confinement tries to compress the fusion fuel into the smallest possible volume in a brief moment. Inside, the temperature and pressures become so high that fusion occurs.

The first idea for compressing hydrogen fuel came from fission bomb research. The fusion mixture was compressed by the intense X-rays produced by the fission envelope explosion. The first successful demonstration of this principle was the explosion of the Ivy Mike thermonuclear bomb in 1952. To avoid the need to detonate an atomic bomb for fusion research, John Nuckolls devised in 1960 that a sufficiently small hydrogen target of a few milligrams could be compressed by other means. Various mechanisms, or drivers — such as heavy ion beams, X-rays, or hypervelocity pellet guns — were considered, but the most effective has been found to be the laser. The indirect drive method was developed first, which more closely simulated the compression of the fuel by X-rays during a bomb explosion, and later other methods were explored. The first laser for inertial fusion research was the Russian Kalmar in 1971. The Janus laser of Lawrence Livermore National Laboratory followed in 1974. Other notable lasers include the Long path, Cyclops, Argus or Shiva laser. In 1980, the hot electrons problem was solved by converting from infrared to ultraviolet wavelengths. The most powerful NIF fusion laser has been running since 2010. On December 5, 2022 it reached ignition, after delivering 2.05 MJ to the pellet.

The laser-driven inertial fusion research facility consists of several basic components. The most important is the target, a small ball of hydrogen fuel placed exactly in the middle of the large spherical chamber. It is compressed on all sides by laser beams. They achieve enormous energy and power of several petawatts in repeated passes through the amplifiers. The chamber has walls of several centimetres thick aluminium panels covered with boron infused concrete. Dozens of openings pass through the chamber. In addition to laser beams, various diagnostics monitor through them the progress of target compression and fusion. 

Target

Since we need to compress the fuel target symmetrically into the smallest possible volume, logically the target has the shape of a sphere. The typical size of the target pellet varies from around 0.5 mm to 5 mm in diameter and contains a few milligrams of fuel.

Its surface is a solid membrane called an ablator.  Underneath it is hydrogen fuel. To achieve the best symmetry and homogeneity, it takes the form of a frozen layer with the gas in the center of the target.

The ablator is often made from thin plastic membrane, but glass, beryllium, or even synthetic diamond can be used. Some ablators are composed of multiple layers. During compression of the pellet, lasers or X-rays hit the surface of the ablator and vaporize it, creating a shock wave that compresses the pellet's interior. The surface of the ablator must be perfectly smooth. Even irregularities as small as bacteria could cause uneven compression of the target and prevent fusion from being achieved. It is required that the inhomogeneities in the ablator be no greater than 10—7 metres. In the manufacturing of the target, the ablator is produced first and then filled with deuterium or a mixture of deuterium and tritium.

To ensure homogenous distribution of material, the pellet is then frozen and a thin layer of solid hydrogen is formed on the inner side of the ablator.  On this layer apply the same demands on smoothness. After freezing, the layer is irradiated with a low-power infrared laser to partially melt and smooth out during refreezing. The tritium-containing mixture has a self-smoothing ability. Radiation from the tritium heats the ice slightly, allowing it to partially melt, which then freezes into a smooth layer. The interior of the target is inspected by X-ray microscopy or even X-ray tomography. Any inaccuracy must be corrected.

The finished target is then stored and handled at temperatures of approximately 18 kelvin. The cost of producing one target can be up to a few thousand dollars. 

Lasers

The word laser is an abbreviation of Light Amplification by Stimulated Emission of Radiation. It is a source of monochromatic, coherent light. This means that, unlike a conventional light source, a laser produces light of only one wavelength, and moreover, these waves are synchronised. As a result, the laser beam remains narrow over long distances and can be focused to a small point.

The laser was invented in 1960. It was based on the principle of a previously developed maser that amplified microwaves, but the laser operated in the visible light spectrum. First functional laser built by Theodore H. Maiman used a flashlamp-pumped synthetic ruby crystal to produce red laser light at 694 nanometers wavelength. The device was only capable of pulsed operation. The gas laser followed, using neon and helium as a medium, which was already capable of continuous operation. In 1962, the gallium-arsenide semiconductor laser was developed. A later improvement, which operated continuously at room temperature, saw the light of day in 1970. This gave rise to laser diodes and enabled the widespread use of lasers in a variety of fields and applications. In 2015, a so-called white laser was developed that simultaneously emits blue, red and green light.

The principle of the laser is based on the properties of electrons. Usually, electrons stay as close as possible to their atomic nuclei. This is called the ground state.

If we give the electrons energy, they move further away from the nuclei. We say the electron is excited.

This state is unstable and the electron will return to the ground state sooner or later. The excess energy it has is emitted as a photon. Its energy, and therefore its wavelength, corresponds exactly to the energy difference between the ground and excited state.

The de-excitation may occur spontaneously or it may be stimulated. This takes place when the excited electron collides with a photon that has the same wavelength as the photon that the electron would have emitted when deexcited. This principle uses the laser.

The core of the laser is the optical medium. This can be a solid, e.g. a ruby crystal or neodymium-doped glass, but also a gas such as carbon dioxide. Excitation of electrons inside the medium is done by a powerful flashlamp that gives them energy. When most of the electrons are excited, the so-called population inversion occurs. During deexcitation, they release photons which pass by other excited electrons and cause them to de-excite, releasing more photons of the same wavelength. The wave of photons bounces off mirrors at the ends of the optical medium and stimulates other electrons to de-excite as they pass through. A beam of monochromatic, coherent radiation is produced.

The National Ignition Facility laser is the most powerful laser for inertial fusion research. The facility spreads over an area equivalent to three football fields on the campus of Lawrence Livermore National Laboratory, USA. With peak energy of 500 trillion watts, it has higher power output than the all reactors in the United States together. Its 192 laser beams deliver about 2 MJ of energy to the surface of the target.

In order to achieve such a high energy, the original relatively weak laser signal must be amplified by passing through a whole array of amplifiers. At their core are neodymium-doped glass and powerful flashlamps. NIF’s amplifiers use 3,072 phosphate glass slabs “doped” with neodymium atoms, which impart a pinkish color to the glass.

The electrons in the glass are excited by strong flashlamps. When a low-energy laser pulse from the injection laser system passes through the slabs, the neodymium atoms release their extra energy into the laser pulse in the form of photons, increasing the pulse’s brightness.

Finally, the originally infrared light is transformed into ultraviolet light. The laser beams pass into the reaction chamber and strike the surface of the target from all sides at once.

All this happens in just five microseconds, during which time the light travels a distance of 1.5 kilometres. 

Compression

Process of target compression can be divided into four stages.

At first laser beams impact the surface of the target from all sides. Their immense energy instantly vaporizes the upper layer of the ablator. Perfect synchronisation of the laser beams, their equal energy, and a perfectly smooth ablator surface are essential to ensure that all parts of the target start to compress at the same time and with the same force.

Second, the vaporized surface od ablator blows away from the target. Since the law of conservation of momentum applies, the rest of the ablator starts hurtling into the center of the target, compressing it.

Third, a shock wave propagates into the core of the target from all sides. The wave is pushing the target material, compressing it and heating it at the same time. There were problems in the past with hot electrons inadvertently preheating the target, so the compression was not as effective. This was avoided by converting from infrared light to ultraviolet light, which does not produce hot electrons. With successful compression the density at the target's centre reaches up to 1,000 g/cm³, or 1,000 times the density of water, and the temperature exceeds 100 million degrees.

Fourthly, thermonuclear fusion is ignited. The alpha particles produced by the fusion strike the surrounding particles in the target, heating them up so that the fusion can continue. In an incredibly short moment, just a few tens of nanoseconds, part of the target undergoes fusion. Much later, the rest of the target is blown apart.

So far, the best experiments release energy in units of MJ.

There are three basic approaches to target compression, direct drive, indirect drive and fast ignition.

In direct drive, the lasers strike directly on the surface of the sphere. However, the spots where the center of the beam hits are always compressed more than the spots between the two beam impacts. The resulting instabilities mix the denser and thinner parts of the target, cause fuel mixing, cooling, less compression and reaching lower temperatures. It is therefore very difficult to achieve perfect compression. The advantage is that in addition to the target, no other expensive equipment like a hohlraum needs to be manufactured.

A method called indirect drive attempts to disperse the energy of the radiation hitting the surface of the target more evenly. The entire target is suspended on plastic fibres in the middle of a hollow cylinder called a hohlraum. It’s made of lead, gold, or gold-coated uranium. The laser beams are directed through holes inside the cylinder so that they do not hit the target but fall on the inner walls of the chamber.  Upon impact, intense X-rays are produced. As the hohlraum is effectively a resonant cavity, the X-rays will uniformly fill the cavity and symmetrically compress the target suspended in the hohlraum centre on plastic strings until fusion occurs. The indirect drive method leads to better compression of the target, higher temperatures and pressures, and therefore better fusion ignition. However, in addition to the production of the target, it is also demanding for the precise production of the hohlraum, whose inner surface must be extremely smooth with irregularities not exceeding 100 nanometres. The entire hohlraum and the target are single-use devices because they are destroyed during the experiment.

In fast ignition method, a cone is introduced into the target and directed towards its centre. First, the fuel is compressed to a certain density and temperature by lasers hitting the surface of the target, and then an ultrashort pulse sent through the cone to the centre of the target ignites the precompressed fuel. Sometimes a target is placed inside the cone which, after being hit by a laser, generates a shower of protons or neutrons which then compress the fuel and transfer energy to it. 

Inertial Fusion Power Plant

In December 5, 2022, the NIF facility achieved a fusion that released 3.15 MJ of energy, while the lasers that ignited the fusion delivered only 2.05 MJ. This marks the first time in the history of fusion research that the scientific breakeven threshold, where the fusion released more energy than was put into igniting it, has been passed. Ignition was achieved. NIF has since repeated the successful ignition several times.

Despite this achievement, it can only perform on average one experiment per day, and the cost of shot and target construction is very high.

However, a power plant would need the fusion reaction to occur ideally several times per second. To do this, the production of the target would need to be made cheaper and simpler and the target would need to be placed periodically in the centre of the chamber. In addition, heat dissipation and the generation of tritium, which is almost non-existent on Earth and is an essential component of the fuel, would need to be resolved.

There are several concepts that address the possibility of using inertial fusion for power generation.

LIFE, or Laser Inertial Fusion Energy, was a project developed at Lawrence Livermore National Laboratory. It was abandoned in 2013, as ignition was still not achieved at that time. The project analyzed two possible approaches.

The first option addressed the question of how to increase the repetition rate of the laser, which normally has to be cooled and adjusted between shots. Inventing a way to simplify and cheapen the production of the fuel target was also essential. Rapid freezing of liquid hydrogen droplets was considered. These would then be thrown into the chamber and ignited when passing through its centre.

The second version featured a hybrid fussion-fission power plant. The reaction chamber would be surrounded by an envelope of uranium fuel in the form of TRISO particles. The neutrons produced from the fusion would be used to fission natural uranium. The envelope would be cooled by liquid salts containing lithium, which would produce tritium, an important component of fusion fuel.

The HYLIFE II project opted to use a heavy ion beam to compress the target. The surface of the chamber is covered with liquid FLiBe salts, which serve as radiation shielding, to wash the chamber of target debris, and to breed tritium. This project also faced the question of how to produce cheaply enough targets.

An alternative is a combination of different approaches such as magneto-inertial fusion. In this, the plasma is first pre-compressed using magnetic fields, eliminating the need to produce a costly target. Subsequently, it is compressed by laser or ion beams to temperatures and densities at which fusion is ignited. Development of magneto-inertial fusion is being pursued by companies such as General fusion or Helion energy.

Thanks to the success of the NIF, inertial fusion is the first type of fusion that has broken the magic threshold of scientific breakeven and achieved ignition. But it still has a long way to go to be used as a power plant.