Steady magnetic fields degrade plasma created by rotating magnetic fields in the Flinders ST*

The Flinders ST is a spherical tokamak project which has concentrated on the production by induction, of a low resistivity target plasma suitable for rotating magnetic field (RMF) current drive tests. Antennas for the production of the RMF fields were installed on the spherical tokamak device, so that it could operate as a device with pure RMF current drive (Rotamak), or purely as an inductively driven ST. Experiments with combinations of inductive and RMF current drive have been made. Experimental results obtained in this small Flinders ST device indicate areas of concern for the RMF current drive technique. First, the experiments show that the strong steady magnetic fields required for the spherical tokamak configuration degrade, and probably extinguish the RMF created plasma, when these transverse steady fields exceed some critical value, which is small compared to that required for ST operation. Second, a measure of the resistance of the pure RMF current drive device, using the ST solenoid and OH coils to create a small loop voltage purely as a diagnostic, shows that the RMF steady current is driven in a very resistive path.