Direct Plasma Potential and Ion Temperature Measurement in the Plasma Edge of the STOR-M Tokamak

Abstract

Reliable measurements of plasma parameters in the plasma edge are essential to give insight on the plasma state and to ensure that the plasma-wall interactions are below damage thresholds. Using electrostatic probes, the ion temperature is hard to estimate due to the higher mobility of electrons which comprise most of the signal. It is possible to use a Ball-Pen probe (BPP) which can partially shield electrons thereby reducing the electron flux. By applying a varying voltage on the Ball-Pen probe, a symmetrical I-V curve can be obtained, allowing direct plasma potential measurement and measurement of the ion temperature by curve fitting the exponential trend due to ionic thermal motion near the electron saturation region of the I-V curve. Other parameters such as the electron temperature, the electron density, the floating potential, and the plasma potential measured by the Ball-Pen probe in the STOR-M tokamak are compared with those obtained both from a single Langmuir probe and from a triple probe which are already established diagnostics in the STOR-M. It was found that the ion temperature in the plasma edge of the STOR-M ranges from 1 to 15 eV, which is comparable to what has been reported previously. Direct plasma potential measurements were found to be comparable to the Langmuir and triple probe values. More efforts are needed to further validate ion temperature measurements with electrostatic probes in the STOR-M. Solutions include increasing the sweeping frequency of the power supply for better statistics as well as increasing the dynamic range of the measuring circuit.