Experimental study of double tearing mode on EAST tokamak

The double tearing mode (DTM) in a high βN (βN > 1.5) discharge with internal transport barrier on EAST was investigated. A 15 kHz tearing mode (TM) (m ≥ 3, n = 3) appears at outer q = 2 surface at first, which is stable in the high βN phase. Then a 2 kHz TM (m = 2, n = 1) occurs at inner q = 2 surface. Soon after, high βN collapsed with the crash of ELM, and DTM formed during the collapse of βN. The positions of the two islands of the DTM are consistent with the q = 2 surface. The temperature fluctuations are the strongest between the two magnetic islands. A statistical analysis of high βN discharges operating with the reversed magnetic shear configuration in the EAST 2015-2018 campaign revealed the existence of the DTM in many discharges. During the DTM phase, all βN are reduced by 10%-30% within 0.1 s. However, there are two different βN behaviors thereafter-with and without βN recovery. Studying the physical mechanism of βN recovery during the DTM phase will benefit steady-state operation with reversed shear configurations in the future.