Historical research guide · 0792 · 2017
Road map to clean energy using laser beam ignition of boron-hydrogen fusion
H. Hora · S. Eliezer · G.J. Kirchhoff · N. Nissim · J.X. Wang · P. Lalousis · Y.X. Xu · G.H. Miley · J.M. Martinez-Val · W. McKenzie · J. Kirchhoff
In one minute
Heinrich Hora and ten colleagues set out the engineering road map for the fusion fuel that returns charged particles instead of neutrons. Hydrogen fused with boron-11 yields three helium nuclei and nothing radioactive, which is why the site keeps coming back to it — but at ordinary thermal-equilibrium conditions it is about a hundred thousand times harder to ignite than deuterium and tritium. Hora’s route around that is to stop heating the fuel at all. A picosecond laser pulse of petawatt power pushes a solid-density plasma block forward by the light’s own pressure, reaching accelerations above ten to the twentieth centimetres per second squared, and the ignition threshold computed that way comes out about the same for boron as for deuterium–tritium. A second, longer laser pulse wraps the cylindrical fuel in a magnetic field of several kilotesla to hold the reaction together. Their design: fourteen milligrams of fuel, one gigajoule a shot, and the alpha particles converted straight to electricity.
Connection
Chapter twelve’s argument is that aneutronic fusion is the fuel that matters, because it hands you charged particles you can convert directly rather than neutrons you have to shield and that make the reactor radioactive. This is the paper that turns that preference into a reactor with numbers on it — laser powers, field strengths, a fuel mass and a shot rate — and names the measurement that tells you whether your laser is good enough to try.
From the source
Hydrogen fused with boron-11 produces three alpha particles and no neutrons, which is what makes it the one fusion fuel free of radioactive waste; the reaction has been known since Oliphant and Rutherford in 1933, and stays neutron-free as long as the reacting proton energies remain below the MeV range.Sect. 2, opening paragraphs; Eq. (1) and Fig. 8
The ignition is non-thermal. A picosecond laser pulse at ten to the eighteenth watts per square centimetre drives plasma blocks by the nonlinear ponderomotive force rather than by heating, reaching accelerations above ten to the twentieth centimetres per second squared — about a hundred thousand times higher than any acceleration previously measured in a laboratory. Predicted by computation in 1978, it was measured by Sauerbrey in 1996 from the blue Doppler shift of the reflected light and repeated by Földes and colleagues in 2000.Sect. 3, Figs. 4 and 5
When the block-ignition calculation is applied to boron-hydrogen using its measured cross-sections, the threshold comes out close to the deuterium–tritium value — five orders of magnitude better than boron-hydrogen ignition at thermal equilibrium, which is the result the whole road map rests on.Sect. 3, paragraph beginning ‘It was then a surprise’
The reaction multiplies itself. Each event’s three alphas hand their energy on by elastic nuclear collisions to cold boron and hydrogen nuclei in the strongly non-equilibrium plasma, producing an avalanche; the effect was worked out against the measured high-gain boron experiments of Picciotto and of Margarone, later increased by a further factor of ten by Giuffrida and colleagues.Sect. 3, closing paragraphs; Fig. 8
The proposed reactor is fully specified: cylindrical fuel inside a laser-driven capacitor-coil target, where a kilojoule nanosecond pulse raises a magnetic field of several kilotesla to trap the plasma at about one millimetre radius, and a second pulse of 30 kilojoules in a picosecond — 30 petawatts — ignites it end-on. Fourteen milligrams of fuel yields more than one gigajoule, about 300 kilowatt-hours, per shot, inside a steel sphere of at least one metre radius, with a mechanical shock equivalent to roughly 50 grams of chemical explosive. Because the products are charged, holding the Faraday-screened unit at a negative potential just below the alphas’ stopping voltage decelerates them electrostatically and the recovered energy is converted with established high-voltage direct-current technique, avoiding a thermal cycle — provided discharge losses to the reactor wall stay small.Sect. 2, Figs. 2 and 3; Sect. 3, final paragraph
The named go or no-go test for any laboratory attempting this is the pulse contrast ratio: light reflected from the target must show a blue Doppler shift of its spectral lines. Repeat attempts that used lasers of insufficient contrast returned a red shift instead and did not reproduce the block acceleration, so the shift is the check to run before anything else.Sect. 5
The way in
https://doi.org/10.1017/S0263034617000799How to cite it
H. Hora, S. Eliezer, G.J. Kirchhoff, N. Nissim, J.X. Wang, P. Lalousis, Y.X. Xu, G.H. Miley, J.M. Martinez-Val, W. McKenzie, J. Kirchhoff (2017) Road map to clean energy using laser beam ignition of boron-hydrogen fusion doi:10.1017/S0263034617000799