Runaway-loss induced negative and positive loop voltage spikes in the Aditya Tokamak

Negative spikes followed by positive ones in the loop voltage signal during the discharge are observed in the Aditya Tokamak [S. B. Bhatt, Indian J. Pure Appl. Phys. 27, 710 (1989)]. These spikes are always accompanied by hard x-ray bursts caused by sudden loss of runaway electrons. The observed growth of m=3 mode seemed responsible for the losses of localized beams of runaway electrons (E_γ ∼1-5 MeV) from the plasma region around q=3 magnetic surface. The movement of these runaway electrons during their extraction from inside the plasma induces both positive and negative electric fields at those locations. In this paper, a one-dimensional toroidal electric field diffusion model is used to estimate the induced electric field at the plasma boundary, which matches quite well with the observed spikes in loop voltage in both magnitude as well as its temporal evolution.