Recently, stationary plasma with a world-record pulse length of 1056 s was realised on the Experimental Advanced Superconducting Tokamak (EAST). In this work, the core magnetohydrodynamics events and the mode coupling process were investigated during EAST long pulse operation with dominant electron heating, pure radio frequency wave heating, and low collisionality by using several diagnostics and the nonlinear numerical code M3D. A saturated m/n=1/1 kink mode was observed in the core region, where a stable internal transport barrier was found in the electron temperature channel. The frequencies and 2D structures of these modes were studied using a combination of soft X-ray imaging and electron cyclotron emission diagnostics. The m/n=1/1 mode frequency exhibited a chirping down feature with time, and its chirping rate corresponded to the electron diamagnetic drift frequency change rate. A twisted pattern (resembling a tai-chi structure in shape) was reconstructed by soft X-ray tomography for the m/n=1/1 mode. The electron temperature and density perturbations caused by m/n=1/1 differ in size, where the latter is much smaller. The destabilization of m/n=1/1 was due to strong central heating caused by a combination of electron cyclotron resonance heating and the lower hybrid current drive. In the presence of the m/n=1/1 mode, a negative current was generated on the magnetic axis that broadened the core current profile anomalously. An m/n=3/2 tearing mode triggered by the m/n=1/1 mode was also observed. The m/n=3/2 mode has a lower frequency than the m/n=1/1 mode and carries an m/n=3/2 island with a detectable size. A novel 3D magnetohydrodynamics model that evolves the plasma density and temperature separately is applied to analyse the m/n=3/2 mode triggered by m/n=1/1. It is found that a toroidal current density at the q=1.5 surface caused by non-axisymmetric density perturbation during the m/n=1/1 nonlinear growth phase was generated and thus destabilized the m/n=3/2 tearing mode. The modelled electron temperature and density perturbations both agree well with the experimental observations. Finally, the interactions between the m/n=1/1 mode and fast electrons and the active control of this mode are also presented.

Hard x-ray (HXR) burst is found during internal crash in the flat top current stage of experimental advanced superconducting tokamak (EAST) discharges and it is caused by fast electrons. The generated electrons during internal crashes may be an operational safety issue in advanced tokamaks. During an internal crash, locations of fast electron generation from HXR evolution agree with areas of magnetic reconnection from soft x-ray (SXR) tomographic reconstruction. Further statistical analyses show a 27 μs time difference between SXR crashes and HXR bursts, and the agreement between time broadening of HXR bursts and estimated characteristic time of magnetic reconnection in EAST. The magnetic reconnections during internal crash are proved to generate fast electrons, by both spatial and temporal agreements.

Avoiding the plasma instability, especially the disruption instability, is an important problem for the stable operation of tokamak. Large-scale instabilities driven by free energy evolve nonlinearly and lead to the disruption. The microscale turbulence is highly sensitive to the change of free energy. The paper show the electron-scale turbulence evolution in the pre-precursor phase of TMs included disruption with the CO2 laser coherent scattering system in EAST. In the pre-precursor phase of disruption, it is observed that the characteristics of turbulence (e.g. intensity, spatial correlation) have obviously changed for more than 30 ms. In addition, before TM (n = 1) included major disruption, the spatial-correlation of turbulence in two regions (ρ = 0 − 0.4 and ρ = 0.4 − 0.8) increase obviously, while opposite turbulence spatial-correlation evolution was observed before TM (n = 1) included minor disruption. The warning time for disruption with microscale turbulence is competitive while 30 ms for ITER[1]. According to the experimental results in EAST, it may provide a new experimental evidence for the method improvement of predicting disruption.