3-D soft-X ray imaging diagnostics for the study of MHD mode dynamics in RELAX

In a low-$A$ RFP machine RELAX ($R$ = 0.51 m/$a$ = 0.25 m ($A$ = 2)), a quasi-periodic transition to quasi-single helicity (QSH) state has been observed in shallow-reversal discharge regions. During the QSH state, the fluctuation power is concentrated to the dominant $m$ = 1/$n$ = 4 mode. We have applied a SXR pin-hole camera and a ICCD camera to take tangential soft-X ray (SXR) images (snapshots) of the RFP during the QSH state, identifying characteristic helical SXR structures suggesting hot or dense helical core. As a next step, we have been developing a SXR imaging diagnostic system for 3-D structural studies. The system uses multiple SXR cameras together with high-speed cameras to take time-evolution of SXR images from tangential and vertical directions simultaneously for the study of dynamic structures of 3-D SXR emissivity, through which we expect to discuss 3-D dynamics of MHD instabilities associated with the QSH state in RELAX. Initial results will be reported, together with some discussion on 3-D reconstruction techniques.