A feedback control system for the stabilization of resistive wall mode (RWM) was applied to a low-aspectratio reversed field pinch (RFP) with minimum power supply capabilities to control the single mode. The system consists of 64 saddle coils (4 and 16 in poloidal and toroidal direction, respectively) in the actuator covering the whole torus on the outer surface of the vacuum vessel. The sensor coils also have the same structure. The saddle coils are connected in series to control the single m/n = 1/2 mode, which has the largest growth rate in RELAX. The radial component of the magnetic field from the sensor coils was suppressed to the preset level and the m/n = 1/2 magnetic mode, which otherwise grows with field penetration time of the vessel, was reduced to 0.1% of the edge poloidal field throughout the discharge. The RFP discharge duration has been extended to ∼3.5 ms, the upper bound determined by the saturation of the iron core. Finally, the MHD control issues in a low-A machine are discussed.

We have developed a two-dimensional (2D) electron temperature (T(e)) diagnostic system for thermal structure studies in a low-aspect-ratio reversed field pinch (RFP). The system consists of a soft x-ray (SXR) camera with two pin holes for two-kinds of absorber foils, combined with a high-speed camera. Two SXR images with almost the same viewing area are formed through different absorber foils on a single micro-channel plate (MCP). A 2D Te image can then be obtained by calculating the intensity ratio for each element of the images. We have succeeded in distinguishing T(e) image in quasi-single helicity (QSH) from that in multi-helicity (MH) RFP states, where the former is characterized by concentrated magnetic fluctuation spectrum and the latter, by broad spectrum of edge magnetic fluctuations.

A Thomson scattering diagnostic system has been developed for measuring electron temperatures in lowaspect-ratio reversed-field pinch (RFP) plasmas in REversed-field pinch of Low Aspect ratio eXperiment (RE-LAX). In the range of plasma currents I p = 50 -80 kA, the central electron temperature was around 100 eV and, showed a weakly increasing trend as I p increases. To estimate the central electron pressure p e0 , a density calibration was performed from simultaneous measurements with a 104-GHz microwave interferometer. The maximum p e0 increased with I p up to ∼ 70 kA to preserve the approximate relation p e0 /B θa 2 ∼ constant, where B θa is the edge poloidal field. In the higher current region, p e0 tended to saturate, which may be improved by optimizing minute control of equilibrium in the higher current region.