Probe Diagnostic of Electron Temperature with Steep Gradient

Abstract

In a plasma with a strong electron temperature gradient along magnetic field lines the electron velocity distribution near a target may deviate appreciably from a Maxwellian: due to their long mean free path electrons from the hot region transport their energy non locally into the colder region thus forming a non Maxwellian tail in the velocity distribution. The sheath potential (well below the electron saturation current) is determined by this high energy tail of the electron distribution. The probe characteristic of an electric probe near or below the floating point thus measures the “temperature” of the high energy tail, not of the bulk electron distribution. Therefore an overestimation of the electron temperatures by the probe measurements may occur. This effect is studied numerically using a 1d kinetic particle code. Plasma streams through the computation area to an absorbing target representing the pin of a probe. The electron temperature gradient is produced by the energy transfer condition of the target sheath. From the floating sheath potential drop an effective temperature of transmitted electrons may be derived. This effective electron temperature is different from the temperature at the sheath edge. It renders the temperature of electrons upstream of the sheath by about a mean free path length λ = 10λes where λes is the mean free path length for thermal electrons at the sheath edge.