The determination of sheath potential from retarding field analyser measurements in tokamak edge plasmas

Abstract

Experimental measurements of the energy distributions of ions and electrons arriving at a surface in the boundary layer of a tokamak have been made with a retarding field analyser. The analyser uses a 9 μm entrance slit which is less than the Debye length for the plasma conditions investigated. The electron energy distributions are Maxwellian and the ion energy distributions are displaced Maxwellians, as expected. The sheath potential is derived from the ion energy distribution and found to be ∼2 kT e , slowly decreasing with electron temperature. Measurements of secondary electron emission yield have been made on materials exposed in a tokamak as functions of incident electron energy and angle. This data have been integrated to obtain the effective yield of a full three-dimensional Maxwellian distribution. Using these results the theoretical sheath potentials have been calculated as functions of electron temperature and have been found in good agreement with the values that were measured directly.