Realisation of fusion by laser method

Laser fusion method, another approach to nuclear fusion

Fusion technology, described in ‘Fusion and AI technology’, is attracting attention as a next-generation power generation technology and is the subject of various studies. In the aforementioned blog, we mainly discussed plasma confinement technology in tokamak structures as an ignition process for power generation.

核融合研究より

Here, another ignition method, the inertial method, in which lasers or particle beams are irradiated onto the fuel sphere, is described. The inertial method is also known as laser fusion.

In the laser fusion method, a small spherical pellet with a diameter of a few millimetres containing deuterium (D) and tritium (T) is used as the fusion fuel, and the surface of the pellet is irradiated with a high-power laser beam. The principle is that the inside of the pellet is rapidly compressed, creating a high-temperature (tens of millions of degrees Celsius) and high-pressure (several hundred atmospheres) state in the centre, and nuclear fusion is initiated.

レーザー方式の核融合はどこまで進んでいるか

Compared with the magnetic field confinement method, this method allows for a simpler furnace structure, does not require the equipment to be a high vacuum and the fuel size can be reduced, thus making the equipment smaller. It also has the advantage that the power can be varied by changing the frequency of detonation.

On the other hand, various challenges exist, making it difficult to realise the technology in the ignition phase, and it has been derided as a technology that is ‘30 years away from practical use’.

In 2021, the Lawrence Livermore National Laboratory in the USA succeeded in generating a fusion reaction by focusing 192 lasers on a fuel pellet made of deuterium and tritium, and achieved an output of 3.15 megajoules (1.15 megajoules in difference) for an input power of approximately 2 megajoules. The first time this has been achieved. Although this energy is a very small amount, equivalent to the energy required to boil ten kettles of water, it was a landmark report as it proved the ‘existence’ of ignition in the laboratory.

Phys. Rev. Lett. 129, 075001 (2022) – Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.129.075001

Phys. Rev. E 106, 025201 (2022) – Design of an inertial fusion experiment exceeding the Lawson criterion for ignition
https://journals.aps.org/pre/abstract/10.1103/PhysRevE.106.025201

Phys. Rev. E 106, 025202 (2022) – Experimental achievement and signatures of ignition at the National Ignition Facility
https://journals.aps.org/pre/abstract/10.1103/PhysRevE.106.025202

Three peer-reviewed papers highlight scientific results of National Ignition Facility record yield shot | Lawrence Livermore National Laboratory
https://www.llnl.gov/news/three-peer-reviewed-papers-highlight-scientific-results-national-ignition-facility-record

In contrast to the conventional Central Ignition method (Central Ignition), which creates this high sound pressure at once, the research team using LFEX (Laser for Fast Ignition Experiments) at Osaka University has separated the processes of pellet compression and ignition in order to more efficiently achieve high energy density by separating the processes of pellet compression and ignition.

The method involves applying a laser to the pellet once to create a state where the evaporated outer shell compresses the centre of the pellet by reaction (the main objective at this time is to increase the density of the pellet and not to achieve the high temperature conditions required for ignition), and then applying additional high energy to the centre of the pellet to ignite it. The central ignition system can be compared to a diesel engine, which ignites spontaneously by compression only, whereas the gasoline engine ignites with a spark plug after compression.

In the Osaka University report, heating up to 10 million degrees is achieved by combining a laser diode from Hamamatsu Photonics, which created the photomultiplier tube that makes up the Kamiokande, which has enabled the observation of neutrinos, and a YAG ceramic crystal from Kamijima Chemical Industry as the laser medium.

In particular, Kamijima Chemical Industry’s YAG ceramic crystals have been the talk of the town in recent years for their unique transparency technology in addition to the heat resistance and high thermal conductivity that make nuclear fusion a reality.

Fusion power generation is an environmentally friendly dream energy that uses hydrogen as fuel instead of fossil fuels and emits helium, which is highly valuable in industrial fields, instead of greenhouse gases and radioactive waste.

reference book

Reference books on laser fusion technology are listed below.

1. basic theory and background
– ‘An Introduction to Inertial Confinement Fusion‘

– ‘Plasma Physics and Fusion Energy’ by Jeffrey P. Freidberg

2. aspects of laser technology.
– ‘High-Power Laser-Plasma Interaction’ by C. S. Liu and V. K. Tripathi

– ‘Fundamentals of Photonics’ by Bahaa E. A. Saleh and Malvin Carl Teich

3. experimentation and applications
– ‘Laser-Plasma Interactions and Applications’ edited by Paul McKenna, Dmitri Batani, and Lucca Lancia

– ‘Fusion: Science, Policy, and Politics’ by Robin Herman

4. professional resources
– ‘Introduction to Inertial Confinement Fusion’ by Susanne Pfalzner

– ‘Progress in Inertial Fusion Energy’ edited by Mitsuru Kikuchi and Kenro Miyamoto.