Langmuir Probe Measurements in the WEGA Stellarator

Langmuir probe measurements have been performed in weakly magnetized (0.06 T < B 0 ≤ 0.1 T ), lowpressure (6.× 10 –4 Pa ≤ p ≤ 1.0 × 10 –2 Pa ) helium, hydrogen and argon plasmas of the WEGA stellarator. A simple cylindrical graphite probe as well as a double probe were used. The observed characteristics do not show saturation of the ion current. The reason for this anomaly can be the presence of a fast electron component in the plasma caused by the heating method ( ECRH at 2.45 GHz ). Using the model from [5] for derivation of the current to the probe for a two‐temperature Maxwellian electron distribution the characteristics were successfully fitted. Radial profiles of the electron temperature, density and plasma potential as a function of heating power, neutral gas pressure, magnetic field and rotational transform have been obtained. The plasma potential values were compared to results from emissive probe measurements. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)