The effect of energetic electrons on the collisionless fusion plasma sheath in ion cyclotron range of frequencies heating

Abstract

A one-dimensional model, consisting of a hydrodynamic radio-frequency (RF) sheath model for the ion cyclotron range of frequencies heating and an equivalent circuit model, is used to study the structure of the collisionless RF sheath of a fusion plasma containing a beam of energetic electrons. For various energetic electron concentrations and velocities at the plasma-sheath edge, a set of equations describing the model are solved numerically to obtain the potential drop across the RF sheath and the sheath thickness, as well as the spatiotemporal variations of the potential, the ion density, and the background electron and energetic electron densities inside the sheath. Under the current EAST ion cyclotron range of heating conditions, it is observed that even at small beam fluxes, the potential drop across the sheath is enhanced at any time in an RF cycle for different bulk plasma densities, ion temperatures, and frequencies and amplitudes of the disturbance current. When the energetic electron component is included, the physical sputtering yields of the RF sheath wall materials such as titanium and iron become significant as a result of the enhancement of the drop in the sheath potential.