Soft-X ray imaging diagnostics of helical structures in the low-aspect-ratio RFP RELAX

Soft-X ray (SXR) imaging diagnostic techniques using tangential pinhole camera have been developed and applied to the RELAX ($R$/$a$=0.5m/0.25m, $A$=2) for the study of three-dimensional (3-D) magnetic structures in a low-aspect-ratio RFP configuration. An experimental 2-D image has been compared with calculated tangential images using model profiles for emissivity, to identify the most plausible SXR emissivity profile for the experimental 2-D image. It has been found that in shallow-reversal RFP plasmas with relatively high fill pressure, a helically deformed core has been formed in RELAX. The SXR emissivity in the helical core is estimated to be 2-3 times higher than in the background. According to time variation measured by AXUV array, the SXR emissivity is the highest in the core region with quasi-periodic oscillation of the peak position, which is an indication of toroidal rotation of the helical hot core.