Experimental study of double tearing mode on EAST tokamak

The double tearing mode (DTM) in a high β_N (β_N>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 β_N phase. Then a 2 kHz TM (m = 2, n = 1) occurs at inner q = 2 surface. Soon after, high β_N collapsed with the crash of ELM, and DTM formed during the collapse of β_N. 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 β_N are reduced by 10%–30% within 0.1 s. However, there are two different β_N behaviors thereafter—with and without β_N recovery. Studying the physical mechanism of β_N recovery during the DTM phase will benefit steady-state operation with reversed shear configurations in the future.