Lower hybrid experiments using an interdigital line antenna on the reversed field pinch

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 n∥ of 7.8 will penetrate to the region of maximum magnetic stochasticity and significantly reduce core tearing mode activity.;The particular constraints of the Madison Symmetric Torus lead to the use of a novel interdigital-line traveling wave antenna structure rather than the traditional waveguide grill antenna. Several generations of this antenna type have been constructed and installed in MST. Scattering parameters have been treasured and with the addition of external tuning, the antenna suffers from less than -15 dB of reflection in most plasma conditions. The latest generation antenna has achieved ≳ 220 kW of applied power. Measurements of the launch spectrum show a lower peak n∥ than was designed. Subsequent modeling of the antenna geometry provides the reason and offers a method to compensate without fabricating another antenna.;The launch spectrum displays good directivity, and the antenna operates well in a variety of plasma conditions. Coupling is compared to theory and simulation and shows good qualitative agreement, though lack of good edge density profile measurements limits the prospects for predictive capability. The use of a plasma limiter has been shown to reduce the dependence of coupling on the plasma density, and local gas puffing has been shown to maintain the amount of loading even in low density or high confinement plasmas.;A hard x-ray survey of rf in standard MST plasmas shows a toroidal asymmetry in the hard x-ray flux. Modeling indicates that this flux is consistent with electrons being accelerated to high energies in the near-field of the antenna. Analysis indicates that power losses to these electrons may be on the order of several percent of the input power.