Comparative measurements of the plasma potential with the ball-pen and emissive probes on the CASTOR tokamak

A novel approach to the direct measurement of the plasma potential in magnetized plasmas, using the so-called "ball-pen probe", was recently tested in the CASTOR tokamak. Comparison with the standard technique of plasma potential measurement using the emissive probe is reported. It is found that the plasma potential determined by the emissive probe is systematically lower than that measured by the ball-pen probe. The difference is of the of the of kT e/e. A possible reason of this difference is the space charge occurring in the proximity of the emissive probe.