Observation of the current driving due to electron temperature redistribution by anomalous Doppler instability on EAST

The density sweeping-down experiments have been performed in order to investigate the electron dynamics and the effects on instabilities such as magnetohydrodynamics (MHD) in the low collisionality regime on Experimental Advanced Superconducting Tokamak (EAST). It is found that an electrostatic instability is always excited which is recognized as the anomalous Doppler instability (ADI). The ADI is characterized by the stair-stepping electron cyclotron emission (ECE), converting the parallel energy to the perpendicular component of energetic electrons in the direction to the magnetic field. The background electron temperature profile is redistributed which results in the gradient change in the core. Non-negligible bootstrap current causes the loop voltage decrease. The bootstrap current is dissipated by the bursting MHD instability which is explained as the Alfven-acoustic mode. Analysis indicates that bulk electrons are easily trapped in the steepened temperature gradient region, intensifying the asymmetry of electron velocity distribution in the parallel direction and creating the bootstrap current in the low-collisionality regime. In particular, it is shown that the global Alfven-acoustic mode dramatically removes trapped electrons and quenches the bootstrap current, which is the main limit for the bootstrap current life.