Three-Dimensional Observation of an Helical Hot Structure during a Sawtooth Crash in the WT-3 Tokamak

Sawtooth crashes in an Ohmically heated plasma in the WT-3 tokamak have been observed by using soft x-ray computer tomography at three different poloidal cross sections around the torus. Initially, collapsing proceeds slowly with keeping the helical structure of an $m=1/n=1$ hot core around the torus. It accelerates as the helical hot structure is strongly deformed and fades away in the manner that the hot core at the high field side becomes obscure and disappears, while that at the low field side is deformed into a thin crescent aligned along the inversion circle, which survives even at the completion of the crash.

Figure 1

(a) Top view of the WT-3 tokamak. (b) Cross-sectional view of the SXCT system.

Figure 2

Time evolutions of the SX images at three ports. In each SX image, the left side is the high field side. The dashed circles denote the inversion circle with $r_{\mathrm{i}\mathrm{n}\mathrm{v}}=63\mathrm{m}\mathrm{m}$.

Figure 3

Schematic presentation of an $m=1/n=1$ helical hot structure around the torus at the various times in Fig. 2. The helical structure toroidally rotates in the reverse direction of the plasma current as time proceeds.

Figure 4

Time evolutions of (a) total SX emissions inside the inversion radius and (b)–(f) the ECE signals emitted at various radial locations on the midplane at the 11th port section.

Figure 5

ECE signals and SXCT images for a crash where the hot crescent happens to come by the ECE port.