Confinement Time and Ambipolar Potential in a Relativistic Mirror-Confined Plasma

TL;DR

This paper extends the understanding of confinement time in relativistic mirror-confined plasmas, accounting for relativistic electron effects, and verifies the models with simulations, which is crucial for advanced aneutronic fusion fuels like proton-Boron11.

Contribution

It develops a new scaling law for confinement time in relativistic regimes and validates it through simulations, improving predictions for high-temperature plasma confinement.

Findings

Relativistic effects reduce confinement time by a factor S.

Derived accurate forms for confinement time and ambipolar potential.

Validated models with finite-element Fokker-Planck simulations.

Abstract

Advanced aneutronic fusion fuels such as proton-Boron$^{11}$ tend to require much higher temperatures than conventional fuels like deuterium-tritium. For electrons, the bulk plasma temperature can approach a substantial fraction of the rest mass. In a mirror confinement system, where the electrons are confined by an ambipolar potential of at least five electron temperatures, the tail electrons which can escape the potential are fully relativistic, which must be taken into account in calculating their confinement. In this paper, simple estimates are employed to extend the scaling of the confinement time into the relativistic regime. By asymptotically matching this scaling to known solutions in the non-relativistic limit, accurate forms for the confinement time (and thus the the ambipolar potential) are obtained. These forms are verified using finite-element-based Fokker-Planck simulations over a wide range of parameters. Comparing relativistic and nonrelativistic mirror-confined plasmas with the same ratio of confining potential $|e\phi|$ to electron temperature $T_e$ and the same mirror ratio $R$, the net result is a decrease in the confinement time due to relativistic effects by a factor of $S \equiv (1+15T_e/8m_ec^2)/(1+2|e\phi|/m_ec^2)$.