Historical research guide · 1101 · 2012
Laser nuclear fusion: current status, challenges and prospect
J. Badziak
In one minute
Jan Badziak, of Poland’s Institute of Plasma Physics and Laser Microfusion, wrote this review at the moment inertial fusion first had a megajoule laser and had not yet made it light. He walks through the whole approach: squeeze a peppercorn of deuterium and tritium with laser light until it is a thousand times denser than a solid, get the middle of it hot enough to ignite, and let a burn wave run outward through the rest. He sets out the numbers the compressed fuel has to hit, then compares four ways of lighting it — the conventional central hot spot, fast ignition with a picosecond particle beam, shock ignition with a converging shock wave, and impact ignition with a microscopic projectile flying at thousands of kilometres a second — with the energy each route needs and the problem each one still has. He closes with the arithmetic of a power station, which reduces to one clean line: target gain multiplied by driver efficiency must equal one.
Connection
Chapter 12 treats fusion as the energy substrate the rest of the programme runs on, and this is the clearest short map of the laser route to it: what the fuel must reach, which four ignition schemes are on the table, and exactly which number a reactor turns on. Chapter 9 gains the extreme-plasma end of the same subject — gigaampere electron currents, relativistic laser-plasma interaction, and matter driven to states no other terrestrial machine reaches.
From the source
For laser-driven deuterium-tritium fuel with a permissible mass around ten milligrams and a useful burn fraction, the compressed fuel must reach a density above 200 grams per cubic centimetre and an areal density above 2 grams per square centimetre, at an ion temperature in the range of ten to a hundred kiloelectronvolts; the hot spot itself needs only about 0.2 to 0.4 grams per square centimetre but must reach about ten kiloelectronvolts.Section 2, Equations 1 to 4
The National Ignition Facility delivers 1.8 megajoules of ultraviolet light in 192 beams into a centimetre-scale gold hohlraum; in the first cryogenic implosion with 1.6 megajoules on 0.17 milligrams of fuel the compressed fuel reached about 600 grams per cubic centimetre — more than two thousand times solid density — an areal density near 1 gram per square centimetre, an average ion temperature of 3.5 kiloelectronvolts, and a thousand million million fusion neutrons.Section 3, National Ignition Facility
Fast ignition separates the driver that compresses the fuel from the one that lights it. Numerical simulation fixes the optimum ignitor delivered to fuel at 300 grams per cubic centimetre as about 17 kilojoules in about 20 picoseconds into a 20 micrometre spot at some seven times ten to the nineteenth watts per square centimetre; the first integrated experiment, on Gekko XII with a petawatt heating pulse, raised the neutron yield a thousandfold at 20 to 30 per cent coupling efficiency.Section 4.1, Equations 5 to 9 and the integrated Gekko XII experiment
Shock ignition lights the fuel with a converging shock wave of about 300 megabar initial pressure, driven by a sub-nanosecond laser spike at the end of a slower implosion and amplified as it converges and collides with the shock rebounding from the centre; it has been confirmed in advanced simulation and in a small-scale proof-of-principle experiment, and needs only 200 to 300 kilojoules and 200 to 300 terawatts.Section 4.2
Impact ignition drives a microscopic projectile into the compressed fuel at around five thousand kilometres per second, converting its kinetic energy directly into a hot spot; a Gekko XII experiment at Osaka raised the fusion neutron yield a hundredfold over the conventional scheme, and gains near a hundred appear feasible, with the open problem being how to accelerate a multi-microgram, high-density projectile past a thousand kilometres per second.Section 4.3 and the Osaka Gekko XII result
The power-plant condition reduces to one line: with about 40 per cent thermal conversion and a quarter of the electricity recirculated to run the driver, the target gain multiplied by the driver efficiency must equal one. A gain of 100 therefore needs a 10 per cent efficient driver firing five to ten times a second — an order of magnitude beyond both the gain of ten designed for NIF and LMJ and the roughly one per cent efficiency of their lasers.Section 5, Equations 11 and 12
The way in
https://doi.org/10.2478/v10175-012-0084-8How to cite it
J. Badziak (2012) Laser nuclear fusion: current status, challenges and prospect doi:10.2478/v10175-012-0084-8