Experimental study of core MHD events in thousand-second improved confinement plasma on the EAST tokamak

Abstract

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.

Observation of fast electron redistribution during saturated kink mode in high β

“…This study concentrates on the formation and long-sustention of ITB in EAST discharges with lower hybrid wave (LHW) and electron cyclotron radio heating (ECRH). Characterized by dominant electron heating and minimal torque input, these discharges closely mimic the conditions anticipated in future reactors [2,3]. Recent experiments in the AUG tokamak have illustrated that the presence of fast particles can induce robust, axisymmetric E × B flows, playing a pivotal role in ITB formation [4].…”

“…Milder m/n = 1/1 modes with higher frequencies and smaller amplitudes are better to confine fast electrons (energy < 50 keV). Frequency chirping in these m/n = 1/1 modes can reach up to 80%, primarily due to variations in local diamagnetic drift velocity [2]. The generation of super-fast electrons has the capability to drive non-inductive current and alter V loop , with a maximal V loop shift recorded at approximately 20 mV at t = 24.8 s. The contribution of fast electrons to plasma pressure aligns with the predictions made by CQL3D, as delineated in figure 9.…”

“…It is imperative to mitigate the continuous growth of the m/n = 1/1 mode for two primary reasons. First, the robust presence of the m/n = 1/1 mode could potentially incite tearing modes due to the forced magnetic reconnection process [2]. And second, too strong m/n = 1/1 mode can led to a significant expulsion of fast electrons (energy < 50 keV), resulting in a reduction in T e0 and overall plasma stored energy, as illustrated in figure 12.…”

Identification of MHD modes on EAST using a deep learning framework

“…The causes for the generation of TMs in EAST are diverse and complex. For example, it has been observed that m/n = 2/1 TM [27] and 3/2 TM [28] are triggered by the strongly coupled m/n = 1/1 internal mode, and m/n = 2/2 sawtooth mode and its higher-order harmonic mode excite m/n = 3/2 TM. Generally, the m/n = 3/2 TM and m/n = 2/1 TM are characterized by the formation of magnetic islands on q = 1.5 and q = 2 rational magnetic surfaces, respectively.…”