Measurement of electron temperature fluctuations with tunnel probe in the Castor tokamak

The tunnel probe is a new kind of Langmuir probe for use in the tokamak scrape-off layer. 2 dimensional kinetic analysis of the ion current distribution on the concave conductors is used to calibrate the probe for measuring ion flux and electron temperature. Qualitative agreement with classical Langmuir probe measurements is found, but the electron temperature given by the tunnel probe is several times lower. This discrepancy can be caused by secondary electron emission, or the presence of suprathermal electrons. Strong reduction of ion flux and electron temperature fluctuations is observed during electrode biasing. (authors)