RF potential oscillations in a magnetized capacitive discharge

Radiofrequency (RF) oscillations of the electric potential in a magnetized plasma can be the source of bad measurements with Langmuir probes or can enhance instabilities and turbulent transport across the magnetic field lines. Low RF oscillations lead to a quiet plasma ideal for comparison with steady state models, while high RF oscillations are much more relevant to study plasmas at the edge of tokamaks, especially in the vicinity of Ion Cyclotron RF (ICRF) antennas. Our plasma device, Aline, is a linear magnetized chamber equipped with a RF capacitive cathode designed to study RF sheath and potential biasing of the plasma close to a RF source. The RF system can be driven in 2 modes: without DC current by adding a capacitor between the RF amplifier and the cathode or with DC current. Plasma and floating potentials have been measured by a RF compensated Langmuir probe mounted on a 3D manipulator, providing maps of plasma characteristics in the machine. Potential maps have been obtained for the two RF modes and compared to an equivalent electric model taking into account the RF sheath physics. The main result is that plasma potential oscillations are very small (12 V) with the capacitor. Without the capacitor, the near field RF coupling is similar to the case of ICRF antennas, but potential oscillations in the plasma still remain small. This is due to the saturation of the DC current on the RF surface, which is generally much smaller than the grounded wall of the device. The DC biasing is also shifted toward the positive values, which in turn induces strong convective cells. These cells can be analysed thanks to the electric potential maps.