Soft X-ray measurements on the REPUTE-1 reversed field pinch

Soft X-ray measurements have been made with surface barrier diode (SBD) arrays in the REPUTE-1 RFP device (R/a = 82/22 cm). Arrays of seven and eleven SBDs are arranged so that they view a minor cross-section of the plasma from the vertical and horizontal diagnostic ports, respectively. Applying the tomography technique to the soft X-ray data, a two-dimensional emissivity profile can be reconstructed. In the sustainment phase of the RFP discharge, the soft X-ray emissivity profile exhibits a periodic peaking and a subsequent large crash across the minor cross-section. The fluctuation period is in the range of 60–150 μs, whereas the duration of the large crash is always about 30 μs (corresponding to ~ 100 × τA, where τA is the Alfven transit time). The soft X-ray emission is due to two modes: one is a dominantly m = 0 mode, which corresponds to the axisymmetric increase or decrease of the soft X-ray emissivity profile, and the other is an m = 1 mode. The toroidal correlation length of the m = 0 mode ranges from 20° to 80°, while the m = 1 mode has a high toroidal mode number. In the crash process, an m = 1 distortion of the emissivity profile, which indicates reconnection of the flux surfaces, can be observed. The magnetic fluctuations observed simultaneously are well correlated with the soft X-ray fluctuations. It is found that the soft X-ray fluctuations reflect the relaxation mechanism of the RFP plasma. This phenomenon is considered to be driven by global MHD activity.