2 / 2021-03-16 08:44:17

The Role of Instabilities on Runaway Electron Loss in EAST Disruptions

plasma MHD instabilities,tokamak disruption,runaway electron loss

Runaway electrons (REs) generated by disruptions pose serious threats, including heat loads and sputtering of the armour material to the integrity of fusion device plasma-facing-component surface. Understanding of the mechanism of RE loss during disruptions, in order to propose effective methods to minimize the deleterious effects of REs, is needed. Runaway currents have been detected during unintended disruptions in the circular plasma on EAST. The RE plateau can carry up to 80% of the pre-disruptive plasma current.

Significant RE loss events due to abnormal instabilities are observed during RE plateau phase in unintended EAST disruptions. A fast runaway current decay of up to ~ 700 kA/s during plateau phase is achieved when a loop voltage is below ~-1 V ^[1]. Besides, two distinct types of RE-related relaxation phenomena, distinguished on the basis of the amplitude of magnetic fluctuations, are found during the RE decay phase. Large-amplitude magnetohydrodynamic activity with indications of RE loss covering the entire energy range is observed during the RE plateau when the edge safety factor decreases to less than 3, and the external kink mode is discussed to resolve this anomaly ^[2]. Burst-like relaxations with small-amplitude magnetic fluctuations and ~0.6 kHz frequency are confirmed from the spikes in the hard X-ray array signals under a negative loop voltage, and REs with medium energy are significantly lost at the same time. A possible mechanism for the instability is that due to the negative loop voltage, electric field de-accelerates REs and decrease the energy in the medium energy region, and finally, the modification of RE energy spectrum excites this kinetic instability ^[3]. These results will further deepen the understanding of RE loss in EAST and be an important part of RE mitigation or avoidance research in future.