Firewall effect on electron acceleration by R-waves and parallel electric fields

TL;DR

This study reveals a novel electron acceleration mechanism involving resonance trapping and reversal of acceleration, with implications for plasma physics and fusion device control.

Contribution

It uncovers a new electron dynamics phenomenon in the presence of R-waves and electric fields, demonstrating a firewall effect that suppresses runaway electrons.

Findings

Electrons can become resonantly trapped and reverse acceleration despite constant electric fields.

R-waves can induce a firewall effect, preventing further runaway-electron acceleration in fusion plasmas.

Counterintuitive electron energization occurs during resonance trapping.

Abstract

We report an unanticipated electron dynamics in a classical setting of a uniform magnetic field, a parallel electric field, and a right-handed circularly polarized wave (R-wave). The setting admits a natural trajectory that a particle accelerated by the electric field reaches a Doppler-shifted cyclotron resonance and becomes trapped in the resonance space. Remarkably, once it becomes resonantly trapped, the electron undergoes reversal of parallel acceleration together with perpendicular energization, despite the parallel electric field remaining constant. This counterintuitive behavior has important implications for particle scattering in various laboratory and space plasmas. Applied to fusion devices, particle-in-cell simulations show that an externally injected R-wave can act as a firewall suppressing further runaway-electron acceleration.