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.

A five-channel microwave interferometer with high temporal resolution and high phase resolution has been developed for measuring electron density profiles and fluctuations on the Sino-UNIted Spherical Tokamak. A correction algorithm, based on the low signal amplitude detection, is proposed to eliminate the fringe jump errors. The correction algorithm achieves an accuracy of 92%. The density profile is reconstructed through the Park-matrix method, with the geometry of magnetic surfaces calculated by the equilibrium fitting. The reconstructed density profiles for discharges with supersonic molecular beam injection are in good agreement with the results of another 94 GHz single-channel horizontal interferometer and the Langmuir probes. The temporal resolution of the system is 0.5 µs and the line-integrated electron density resolution is 1 × 1015 m−2, which benefits from the single sideband modulation technique. Therefore, the multichannel interferometer system is capable of studying the details of the high-frequency (up to 200 kHz) density fluctuation such as that in the minor disruption event.

A quasi-coherent (QC) mode was observed in the core region of low-density ohmic plasmas in Sino-UNIted Spherical Tokamak (SUNIST). In experiments on the QC mode, two sets of moveable Langmuir probes (LPs) were used to measure the local parameters including floating potential, electron temperature, electron density, and so on, as well as their profiles. To monitor the magnetohydrodynamic (MHD) activities, a Mirnov probe (MP) was used to measure the poloidal magnetic fluctuation. The QC mode can be seen in the spectra of floating potential, but there is no similar peak in the spectra of magnetic fluctuation. Thus, the QC mode is probably electrostatic. By analyzing the electrostatic potential fluctuations from the LPs, the features of the QC mode including frequency, wavenumber, propagation direction, and dependence on collisionality are identified, which are consistent with the characteristics of dissipative trapped electron mode (DTEM).