Study of confined runaway electrons in ADITYA-U Tokamak*

Studying confined runaway electrons is essential to suppress or mitigate their population. Hard X-ray spectrometry is an established diagnostic to detect thin bremsstrahlung emission generated by confined runaway electrons. A diagnostic has been developed to measure and characterize the thin target bremsstrahlung [1] from the core plasma. A special setup, which is an arrangement of lead collimators and a shielded detector has been deployed in Aditya-U Tokamak to look at the selective region of the bulk plasma. It aims to measure solid angle integrated spectra of bremsstrahlung photons. Collimators with different pinhole diameters look at the different solid angles. By this experimental setup, the spatial energy distribution of runaway electrons is obtained. Also, there is evidence that sawtooth crashes due to internal kink mode instability in the core accompanying the HXR bursts [2]. So this special diagnostic is used to interpret the role of sawtooth-like core instability in the generation or the transport of runaway electrons. The thick target bremsstrahlung from the limiter is also recorded at the same time. Correlation analyses of the two types of signals demonstrate the stochasticity of the magnetic fluctuations in the core region and the consequent radial transport coefficient [3] of the non-thermal electrons.