Application of langmuir-probes to an IC/EC-heated scrape-off layer

Abstract

Langmuir probes of different type (single, double, triple) are widely used to monitor essential physical parameters of the scrape-off layer (SOL) during ohmic as well as heated phases of the tokamak discharge, despite the fact that the interpretation of probe measurements in strong magnetic fields and under the presence of HF is fairly complicated. The results of probe measurements carried out in the SOL of T-10 during ICRH are summarized and compared to results obtained during ECRH, with the aim to help judge proposed mechanisms or models for the influences of HF-heating on the SOL. The two types of additional heating were found to cause basically different reactions of the plasma density and the electron temperature measured by Langmuir probes. In a second part the previously reported existence of a poloidal electric field during ICRH at JET is discussed in more detail. Unfortunately, no progress was made in understanding the build-up of such a field in terms of the wave processes accompanying the heating. But some proposals for experiments to settle this point can be made. In case the measured potential differences are not a very complex artefact of an electric probe under difficult plasma conditions, but produced by a genuine electric dc-field, the consequences for the SOL are described. A closer look at the experimental results demonstrates that the field strength as well as the direction don't remain constant during the heating pulse and there seems to be even a partial breakdown of the field close to the end of the heating pulse in some cases. The enhanced radial particle transport due to the additional E × B-drift as well as the heating from a possible small longitudinal component of the hypothetical field are included in simple particle and heat balance equations leading to expressions for the e-folding lengths for plasma density and electron temperature in the SOL, which can be compared with experiment.