A new regime of runaway discharges has been found in TCABR (Tokamak ChauffageAlfvén Brésilien). This regime is obtained by initiating the discharge withlow filling pressure and, after the initial current rise, maintaining a largefilling rate. The line density reaches a maximum value around2×1019 m-3, during the current ramp-up phase, and then drops bya factor of around four in the quasi-stationary phase of the discharge, when anew regime is achieved. The most distinctive features of this regime, ascompared to `conventional' runaway discharges reported in the literature, are(i) maintenance of the runaway discharge, with the plasma current almostentirely provided by the runaway beam, in a cold background plasma and withstrong neutral gas injection; (ii) enhancement of the relaxation instabilitywith strong spikes in the Hα emission and loop voltage correlated withsawtooth relaxation of the line density; and (iii) plasma detachment from thelimiter. A simple phenomenological model, based upon straightforward particleand energy balance calculations, is proposed to explain the experimentalobservations. According to this model, the plasma is rather cold and the shortpulses of gas ionization and the related density spikes are due to suddenplasma heating caused by the relaxation instability. Furthermore, it seemsthat the runaway generation for the conditions of the experiments can beexplained only if the secondary generation process is invoked.
Runaway discharges in TCABR