Lower Hybrid Experiments on MST

Current drive using RF waves has been proposed as a means to reduce the tearing fluctuations responsible for anomalous energy transport in the RFP. A traveling wave antenna operating at 800 MHz is being used to launch lower hybrid waves into MST to assess the feasibility of this approach. Parameter studies show that edge density is a major factor in antenna/plasma coupling. Gas puffing near the antenna is shown to alter coupling without changing plasma conditions. Hard x-ray emission has been correlated to RF power and is seen to vary strongly with direction of power flow through the antenna. INTRODUCTION Experimental and theoretical work indicate that anomalous energy and particle transport observed in the reversed field pinch (RFP) is due to tearing fluctuations. These fluctuations can be reduced when parallel current is added to the edge of standard RFP plasmas [1]. Inductive parallel current drive (PPCD) experiments on MST [2] have demonstrated nine-fold improvement in energy confinement and a tripling of electron temperature. This technique, however, is inherently transient and non-local. RF current drive is an obvious candidate for steady control, and the lower hybrid slow wave has been proposed as a possible choice for the RFP [3]. An antenna with a 20 cm aperture and 4.8 cm wavelength has been designed and built to launch lower hybrid slow waves into MST in order to address the goal of improving transport in an RFP [4]. With the stringent restraints of the MST vacuum vessel, a traveling wave antenna based on an interdigital line [5] was chosen. RF power enters the structure at one end and then propagates to the other end; along the way some power is radiated as a lower hybrid slow wave. Input power can be fed from either end (port direction) with the output end connected to a dummy load. EXPERIMENTAL OBSERVATIONS The present antenna operates at 80 kW for 10ms. Anticipated RF power needed for fluctuation stabilization is ∼1-2 MW. At present levels of power, coupling and loading issues are the main focus. Power is measured at either feed end and at loops placed along the backplane of the antenna. Figure 1(a) shows RF during a typical discharge. The coupling can be gauged by the power damping length relative to the length of the antenna. Plasma conditions have a strong effect on coupling. The power damping length decreases for high densities and high currents as shown in Figure 1(b). Previous physical models [6] of the coupling in response to plasma conditions do not fully explain the behavior on MST, and this is now an active area of research. 319 Downloaded 11 Oct 2010 to 128.104.166.233. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions