Overview of MST LHRF Experiments

Lower Hybrid current drive has been offered as a means of improving confinement in the reversed field pinch by reducing tearing fluctuations. Modeling suggests that a slow wave launched at 800 MHz and an nparallel of 7.5 will penetrate near the region of maximum magnetic stochasticity and significantly reduce core tearing mode activity. The particular constraints of MST lead to the use of an interdigital-line structure rather than the traditional waveguide grill antenna. While there are several drawbacks to this type of antenna including the lack of phasing control, the launched spectrum displays good directivity, and loading studies indicate that the antenna operates well in a variety of plasma conditions. Toroidally localized hard x-rays in standard plasmas with energies up to 50 keV have been observed. This emission is likely the result of edge interaction with the near field of the antenna. Preliminary measurements in the soft x-ray regime are also consistent with this hypothesis.