Slide-away distributions and relevant collective modes in high-temperature plasmas

Abstract

The evolution of the electron distribution function, when an electric field that is not too small in comparison with the critical electron runaway field is applied along the confining magnetic field of a high temperature plasma, is analysed. In the regimes considered, a finite fraction of the electron population has magnetically trapped orbits, and is not appreciably affected by the applied electric field, while the distribution of circulating electrons tends to “slide away” as a whole. Then the Spitzer-Härm model for the current-carrying electron distribution is inadequate, and the role that collective modes, in particular current-driven microinstabilities, and collisions can play in producing a stationary electron distribution is analysed. Modes at the ion plasma frequency, ωpi, that are driven by the positive slope of the current-carrying electron distribution, can be excited, when the average electron drift velocity is a finite fraction of the electron thermal velocity, and transfer transverse energy to the main body of the electron distribution. These features are consistent with the experimental observations performed on the Alcator device. Modes at the “reduced” electron plasma frequency (k‖/k)ωpe can also be excited both in connection with the modes at ωpi and independently. Modes at the electron gyrofrequency Ωe associated with the loss-cone feature that the electron distribution tends to develop are considered, among others, as a factor for the strongly enhanced electron cyclotron emission experimentally observed in regimes where non-thermal electron distributions have been realized.