Generation and transport of runaway electrons during sawteeth crash in the ADITYA tokamak

Abstract

Sawteeth oscillations are commonly observed in temporal profiles of temperatures in almost all tokamaks including ADITYA tokamak. In typical discharges of ADITYA, bursts of hard x-ray (HXR) due to interaction of runaway electrons (RE) with limiter, during sawteeth crash are regularly observed in initial period of plasma current flat-top. These HXR bursts are highly correlated with the sawteeth-crash in time, which suggests that sawtooth crash generates these runaway electrons, which then yield the HXR bursts due to their interaction with the limiter. The electric field induced in the toroidal direction due to change in poloidal magnetic field during the sawtooth crash is found to be higher than the critical electric field required for runaway generation. This induced toroidal electric field during each sawtooth crash generates the REs, which then travel to the limiter to give a HXR burst. Furthermore, it is observed that in the later period of the plasma current flat-top of the discharge, no HXR bursts accompany the sawtooth crash. This indicates different transport mechanisms of runaway electrons in the initial and later parts of plasma current flat-top. Further investigation revealed that the overlapping inflated magnetic islands seem to be responsible for loss of sawtooth-crash generated REs and subsequent HXR bursts in the initial phase. Whereas no overlapping of magnetic islands and presence of good magnetic surfaces in-between the islands restricts the transportation of REs leading to absence of correlated HXR bursts in the later phase of plasma current flat-top.