Dedicated experiments have been carried out at the J-TEXT tokamak for investigating plasma responses to the externally applied resonant magnetic perturbation (RMP) field with various RMP currents under four different toroidal phases. After its penetration, the dominant component of the RMP resonated at q  =  2 surface and excited an m/n  =  2/1 magnetic island, which rotated poloidally when changing the toroidal phase of the RMP. The experimental results show significant influence of the RMP on plasma toroidal and perpendicular rotations, density fluctuations and geodesic acoustic modes (GAMs) as well as nonlinear interaction of ambient turbulence. Whereas the plasma rotations do not show big difference at the RMP-induced island O- and X-point, the turbulence level is substantially lower at the island O-point due to reduced local temperature gradient. The GAM zonal flows outside the island area are profoundly affected by the RMP in various RMP phases. Moreover, it was found that under certain RMP currents the nonlinear coupling of ambient turbulence was considerably enhanced, no matter of at the island O- or X-point. The results suggest a possible controlling role of the RMP in regulating the nonlinear interplay of electrostatic turbulence.

The injection of a large amount of impurities is one of the possible ways of mitigating disruption in large-scale tokamaks. The deposition of impurities at the center of the plasma is the key to the radiation of plasma energy and suppression of runaway. The interaction of the gas jet with the rational surfaces has been studied by scanning the plasma current. The experimental results show that the injection of a massive amount of argon can cool the plasma from the edge to the core region, and the cooling process is accompanied by different magnetohydrodynamic (MHD) modes when the gas jet reaches the corresponding rational surfaces. It is observed that with different edge safety factors and electron density, gas injection can induce different poloidal modes at first. Then, the poloidal mode traverses to lower m (where m is the poloidal mode number) MHD activities until a 2/1 mode is initiated and a thermal quench is started. The experimental results show that the penetration of a gas jet across the rational surfaces is faster in the plasmas with pre-existing large 2/1 tearing modes, which indicates that the 2/1 mode plays an important role in the penetration process. Disruptions triggered by supersonic molecular beam injection display a slower cooling process compared with massive gas injection, which can be divided into four stages. The dominant poloidal mode transition from m  =  3 to m  =  2 is associated with electron temperature recovery.

In cold pulse experiments in J-TEXT, the ion transport shows similar non-local response as the electron transport channel. Very fast ion temperatures decreases are observed in the edge, while the ion temperature in core promptly begin to rise after the injection of cold pulse. Moreover, the cutoff density is also found for the ion non-local effect. The experimental observed density fluctuation in a high frequency ranging from 500 kHz to 2 MHz is obviously reduced in the whole plasma region during non-local transport (NLT) phase.

The capabilities of the joint-Texas experimental tokamak correlation electron cyclotron emission (CECE) diagnostic have recently been extended with an upgrade. Four new yttrium iron garnet (YIG) filters from 4 GHz to 18 GHz with a bandwidth of 90 ∼ 230 MHz are added to the previous 4 channels. Optical optimization of the transmission line has improved the poloidal resolution, which allows kθ < 3.08 cm−1. The improvement of video amplifiers allows the frequency and amplitude gain to be adjusted discretely from 200 kHz to 1 MHz and from 200 to 1000, respectively, for different situations. A controller is designed to remotely adjust the center frequency of the YIG filters. Based on the CECE, the distribution and the effect of magnetohydrodynamic instabilities on electron temperature fluctuations have been observed. The experiment results show good performance of the upgraded CECE diagnostic.

Disruption mitigation experiments with supersonic molecular beam injection of argon gas on J-TEXT were conducted to explore the evolution process during multistage minor disruption. The corresponding numerical simulation was performed using a 3D magnetohydrodynamic (MHD) code, namely, NIMROD (Sovinec et al 2004 J. Comput. Phys. 195 355). The experimental and numerical simulation results are shown in the following. The central plasma temperature decreases to less than tens of electovolts after a relatively long period of multistage thermal collapse. Different MHD instability modes appear when the impurity cold front propagates toward the q = 2 surface. During the burst of MHD activity and central plasma thermal collapse, the heat flux is transferred from the core region to the plasma boundary. Further numerical simulation results show that the MHD process is accompanied by a significant increase in the core-oriented spread of impurities through the q = 2 surface. During the transition stage from the minor disruption to the major disruption, the high order modes play an important role and the magnitude of the n = 3 growth rate is even larger than that of the n = 1 or n = 2 mode. What seems to separate major from minor disruptions is that in a minor disruption, a single MHD crash is not sufficient to cause a full radiative collapse.