The Effect of Superthermal Electrons on Mach Probe Diagnostics

For the analysis of plasma flow velocity, the Mach probe is widely used. Depending on the existence of superthermal electrons particularly at energies higher than the probe biasing voltage, collected ion saturation current can be reduced. This phenomenon can cause considerable errors in flow velocity estimation. The effect of superthermal electrons has been investigated in the steady-state linear divertor plasma simulator MAP-II. The directional measurement of electron energy distribution function (EEDF) suggests the energy of superthermal electrons and their generation mechanisms. The directional dependence of superthermal electrons is clearly observed from the angular distribution of ion saturation current measured using the Mach probe and a directional Langmuir probe (DLP). The error in flow velocity estimation under the conditions of a weakly magnetized plasma ( a p /ρ i ∼1) is calculated.