Observation of sawtooth crashes by a multi-toroidally positioned soft x-ray computer tomography system in the WT-3 tokamak

Abstract

Sawtooth crashes in an Ohmically heated plasma with a low safety factor of qa ≃ 2.7 in the WT-3 tokamak have been observed by using soft x-ray computer tomography (SXCT) systems attached at three different poloidal cross sections around the torus. With this multi-toroidally positioned (MTP) SXCT system we have examined how the core plasmas collapse at the three different toroidal locations in terms of the shift and the shape of hot cores in order to visualize the whole helical hot structure around the torus during the crash. A variety of crashes have been observed. In any case collapsing proceeds in two steps, called the first and second phases. In the first phase collapsing proceeds slowly keeping the helical hot structure of an m = 1/n = 1 mode and/or an m = 2/n = 2 mode around the torus. In the second phase, collapsing proceeds rapidly as the helical hot structure is strongly deformed, and reaches the end in such a manner that the hot core shifted to the low field side is deformed into a thin crescent shape aligned along the inversion circle while that shifted to the high field side fades away or is deformed into a broad, warm structure depending on the types of crashes. Thus, present MTP SXCT has revealed for the first time a remarkable deformation of the helical hot structure around the torus during the sawtooth crashes.