The critical role of helium ash on proton-boron 11 reactor breakeven

Proton-Boron 11 (pB11) fusion is safe and clean, but its low cross sections at high temperatures make it extremely difficult to harness for breakeven power production. This problem is dramatically exacerbated by fusion-born helium ash. As we show here, if the helium is allowed to linger in the plasma for an energy confinement time, it massively increases both the plasma pressure and bremsstrahlung losses, making a breakeven power plant impossible even under fairly optimistic assumptions. This leads to two primary strategies for achieving a pB11 fusion power plant. First, in an inertial confinement fusion (ICF) approach, one can ensure that the plasma is opaque to bremsstrahlung; however, breakeven at tolerable yields requires going to unprecedentedly high densities [1]. Alternatively, in Magnetic Confinement Fusion (MCF), one can use waves or other plasma manipulations to remove helium ash from the plasma on a timescale much shorter than the energy confinement time, while keeping the bulk of the helium energy in the plasma [2]. We argue that any pB11 fusion concept must fundamentally incorporate one of these two strategies.