Fokker-Planck calculations of hot-electron formation by electron cyclotron resonance heating in the TMX-Upgrade tandem mirror

Abstract

The paper studies cold plasma trapping and heating of hot electrons in mirror geometry using a time-dependent, bounce-averaged Fokker-Planck code with quasi-linear diffusion due to RF heating at fundamental and second harmonic frequencies. With the restriction k‖ = 0, the code models the beam-controlled heating (spatially restricted electric fields) that will be used to create thermal barriers in the TMX-Upgrade tandem-mirror experiment. By spatially localizing the microwave beams, which are strongly absorbed in a single pass, the mean hot-electron energy may be controlled. Heating is away from the midplane to control anisotropy (P⊥/P‖ ). For a given magnetic field geometry and cold-plasma source temperature Ts, the parameters of the hot electrons scale with the quantity χ ≡ ϵ2/nsω, where ϵ is the electric field, ns is the cold-plasma density, and ω is the frequency.