New regime of runaway discharges in tokamaks

A new regime of runaway discharges has been found in TCABR (Tokamak Chauffage Alfvén Brésilien). This regime is obtained by initiating the discharge with low filling pressure and, after the initial current rise, maintaining a large filling rate. The line density reaches a maximum value around 2×10¹⁹ m^-3, during the current ramp-up phase, and then drops by a factor of around four in the quasi-stationary phase of the discharge, when a new regime is achieved. The most distinctive features of this regime, as compared to `conventional' runaway discharges reported in the literature, are (i) maintenance of the runaway discharge, with the plasma current almost entirely provided by the runaway beam, in a cold background plasma and with strong neutral gas injection; (ii) enhancement of the relaxation instability with strong spikes in the Hα emission and loop voltage correlated with sawtooth relaxation of the line density; and (iii) plasma detachment from the limiter. A simple phenomenological model, based upon straightforward particle and energy balance calculations, is proposed to explain the experimental observations. According to this model, the plasma is rather cold and the short pulses of gas ionization and the related density spikes are due to sudden plasma heating caused by the relaxation instability. Furthermore, it seems that the runaway generation for the conditions of the experiments can be explained only if the secondary generation process is invoked.