Lower Hybrid Current Drive Experiments on MST

Lower hybrid current drive has been offered as a means to reduce tearing f\/luctuations and improve confinement in the reversed field pinch. The third generation interdigital-line antenna has been installed in MST and preliminary testing has been completed. Source power to the antenna has been increased to $>$220kW in both feed directions. At high power, the $n_{\|}$ spectrum has been measured and is peaked at $\sim\!\!7.5$ as expected with excellent directionality. Hard x-ray bremsstrahlung emission from rf-generated fast electrons with energies up to and beyond 60 keV has been observed using CdZnTe detectors. Emission in the co-current drive direction is toroidally localized in the plane of the antenna while the counter-current drive direction produces an order of magnitude less flux and peaks off-plane of the antenna. It is surmised that this localization results from diffusion of fast electrons away from a local current structure created by launch into low-confinement plasmas. Collimated x-ray emission profiles near the antenna will also be presented.