Sensitivity of electron temperature measurements with the tunnel probe to a fast electron component

Abstract

The tunnel probe is a new kind of Langmuir probe for fast dc measurements of ion flux and electron temperature in the tokamak scrape-off layer. The probe is calibrated using two-dimensional kinetic analysis of the ion current distribution on the concave conductors. Though qualitative agreement with classical Langmuir probe measurements was found, the electron temperature given by the tunnel probe is several times lower. One possible explanation might be an overestimation of the electron temperature by the Langmuir probe, due to a non-Maxwellian electron velocity distribution which can be modelled in a first approach as a two-temperature distribution. Hence the possible influence on the tunnel probe of a small population of nonthermal electrons is investigated by means of the two-dimensional kinetic code XOOPIC. It is found that this influence can be seen as the result of two combined physical effects: nonthermal electrons will reach the back plate (BP) and the ion current distribution over tunnel and the BP will change. The resulting dependence on probe bias and parallel ion current density of the TP sensitivity to nonthermal electrons is not reflected in CASTOR measurement results. Thus nonthermal electrons on their own cannot fully explain the discrepancy between Langmuir and tunnel probe measurements.