Ignition threshold for non-Maxwellian plasmas

TL;DR

This paper investigates the ignition thresholds for non-Maxwellian plasmas, especially $p$-$^{11}$B, showing that nonthermal ion distributions like monoenergetic beams can significantly lower the energy and density requirements for ignition.

Contribution

It introduces a method to estimate the upper bounds of ignition threshold reductions using nonthermal ion distributions, specifically monoenergetic beams, in $p$-$^{11}$B and DT plasmas.

Findings

Monoenergetic beams can lower ignition thresholds.

Nonthermal distributions improve fusion reactivity.

Upper bounds for threshold reductions are established.

Abstract

An optically thin $p$-$^{11}$B plasma loses more energy to bremsstrahlung than it gains from fusion reactions, unless the ion temperature can be elevated above the electron temperature. In thermal plasmas, the temperature differences required are possible in small Coulomb logarithm regimes, characterized by high density and low temperature. The minimum Lawson criterion for thermal $p$-$^{11}$B plasmas and the minimum $\rho R$ required for ICF volume ignition are calculated. Ignition could be reached more easily if the fusion reactivity can be improved with nonthermal ion distributions. To establish an upper bound for this utility, we consider a monoenergetic beam with particle energy selected to maximize the beam- thermal reactivity. Channeling fusion alpha energy to maintain such a beam facilitates ignition at lower densities and $\rho R$, improves reactivity at constant pressure, and could be used to remove helium ash. The gains realized with a beam thus establish an upper bound for the reductions in ignition threshold that can be realized with any nonthermal distribution; these are evaluated for $p$-$^{11}$B and DT plasmas.