Role of Negative Potential Barrier Formed by Electrons Confined in the Stochastic Magnetic Region of Helical Nonneutral Plasmas

Abstract

Nonneutral plasmas confined in the helical magnetic surface (HMS) region can be produced by injecting electrons from outside the last closed flux surface (LCFS). Recently, we numerically calculated outward electron orbits that extend to the inner HMS region. Once it penetrates the HMS region, the injected electron is never lost to the chamber wall, because the negative self-electric potential φ_s in the stochastic magnetic region (SMR) acts as a potential barrier. Remarkably, during the reflection process at the potential barrier, the electron is still trapped in the foot of the negative φ_s region, although it completely overcomes the SMR. The electron then resumes inward movement across the HMS.