Debye screening and injection of positrons across the magnetic surfaces of a pure electron plasma in a stellarator

Abstract

Just as quasineutral plasmas, pure electron plasmas display collective behavior and screen externally imposed electrostatic potentials. However, for pure electron plasmas, the screening of large negative potentials is fully nonlinear and deviates significantly from the textbook Debye screening. Such potentials create an evacuated region that may penetrate a long distance away from the source of the potential perturbation. An analytic model for this case is presented and compared to numerical solutions of the pure electron plasma equilibrium equation in a stellarator. Debye screening in non-neutral plasmas is asymmetric; in a pure electron plasma, negative potentials are not screened as effectively as positive potentials. Because external potentials can penetrate deep into the plasma, they can have a strong effect on particle orbit quality. Importantly, the assessment of transport driven by internal objects, e.g., diagnostic rods, depends on this. Numerical calculations are presented, showing that the poor shielding of negative potentials can be used not only to perturb and worsen confinement of a pure electron plasma, but can also be used to improve confinement modestly and can be used for injection of positrons across the magnetic surfaces in an initially pure electron plasma.