MEASUREMENT AND CHARACTERIZATION OF FAST ELECTRON CREATION, TRAPPING, AND ACCELERATION IN AN RF-COUPLED HIGH-MIRROR-RATIO MAGNETIC MIRROR

Abstract

The PFRC-II in seed plasma mode is a tandem magnetic mirror. In one end cell is a low-power double-saddle antenna which produces cold (5 eV), tenuous (10^11 /cm^3 ) plasma. Using x-ray pulse-height detectors to probe a previously unmeasured energy range of electrons, I measure a component with temperatures up to 3 keV. I characterize their life cycle, including a Fermi-Ulam-like acceleration process which allows them to attain energies in excess of 30 keV.

The fast electrons are born at 300 - 600 eV temperature in one end via secondary electron emission through an RF sheath. These electrons consist of < 1% of the plasma density, yet receive a large portion of the power. The phenomenon pushes models of a similar system, materials processing reactors, into lower-pressure and more-magnetized regimes, with implications on power balance and surface charging.

The electrons enter the center cell in the loss cone. There, even though the commonly used adiabatic parameter is small, ρ e ∇B/B << 1, they accumulate and persist for hundreds of transits due to the non-adiabaticity of magnetic moment. The same dynamics also lead to de-trapping in magnetic mirror-based fusion reactors.

Under low-pressure conditions, ∼ 10% of these electrons are accelerated still further, up to 3 keV temperature, some electrons above 30 keV, by a form of Fermi-Ulam acceleration. I measure a voltage oscillation consistent with two-stream instability caused by electrons from the last end cell re-entering as a beam. Non-adiabaticity of magnetic moment is essential to destroy resonances between mirror transit time and oscillation period, destroying barriers in phase space.

I compare the proposed mechanisms to approximate models. The proposed mechanism for creation is compared to an approximate kinetic model which includes confinement by a plasma-terminating plate with a fluctuating potential. The proposed mechanism for accumulation in the center cell is compared to the nonlinear-resonance overlap model of Chirikov, and ground-truthed with a Boris algorithm simulation. The proposed mechanism for their acceleration is compared to an energy diffusion model. Their mechanism for Fermi-Ulam voltage fluctuation is compared to a nonlinear saturation model. The mechanism for resonance breaking is compared to a 2D numerical map model.