On a fusion chain reaction via suprathermal ions in high-density H-$^{11}$B plasma.

The $^{11}$B$(p,\alpha)2\alpha$ fusion reaction is particularly attractive for energy production purposes because of its aneutronic character and the absence of radioactive species among reactants and products. Its exploitation in the thermonuclear regime, however, appears to be prohibitive due to the low reactivity of the $^{11}$B fuel at temperatures up to 100 keV. A fusion chain sustained by elastic collisions between the alpha particles and fuel ions, this way scattered to suprathermal energies, has been proposed as a possible route to overcome this limitation. Based on a simple model, this work investigates the reproduction process in an infinite, non-degenerate $^{11}$B plasma, in a wide range of densities and temperatures which are of interest for laser-driven experiments ($10^{24} \lesssim n_e \lesssim 10^{28} {\rm cm}^{-3}$, $T_e \lesssim 100$ keV, $T_i \sim$ 1 keV). In particular, cross section data for the $\alpha$-$p$ scattering which include the nuclear interaction have been used. The multiplication factor, $k_\infty$, increases markedly with electron temperature and less significantly with plasma density. However, even at the highest temperature and density considered, and despite a more than twofold increase by the inclusion of the nuclear scattering, $k_\infty$ turns out to be of the order of $10^{-2}$ only. In general, values of $k_\infty$ very close to 1 are needed in a confined scheme to enhance the suprathermal-to-thermonuclear energy yield by factors of up to $10^3$-$10^4$.