The impact of toroidal mode coupling on high-density discharges in J-TEXT

Density limit has long been a widely studied issue influencing the operating range of tokamaks. The rapid growth of the m/n = 2/1 (where m and n are poloidal and toroidal mode numbers, respectively) tearing mode is generally regarded as a primary precursor to the density limit disruption. In this experiment, the coupling of the m/n = 1/1 mode and the m/n = 2/1 mode in high-density plasma was observed. During a sawtooth cycle, the frequencies of the two modes gradually converge until they become equal. After that, toroidal coupling occurs between the 1/1 and 2/1 modes, resulting in a mutually fixed phase relationship. With the occurrence of toroidal coupling, the 2/1 mode is stabilized. Prior to the disruption, the cessation of the 1/1 and 2/1 mode coupling, along with the rapid growth in the amplitude of the 2/1 mode, can be observed. Additionally, under the same parameters, comparing discharges with or without the 1/1 mode, it is found that the presence of the 1/1 mode leads to higher plasma density and temperature parameters.