Predictions on runaway current and energy during disruptions in tokamak plasmas

Abstract

A simple model for a tokamak disruption, taking into account the replacement of the plasma current by the runaway current, is used to evaluate the generation and energy of the runaway population during the current quench phase of a fast disruptive event. The potential efficiency of the ripple resonance and the magnetic fluctuations for runaway current mitigation during plasma disruptions, as well as their dependence on the runaway generation mechanism, are discussed. Predictions are made for the Joint European Torus (JET) [Nucl. Fusion 25, 1011 (1985)] and the projected International Thermonuclear Experimental Reactor (ITER) [ITER EDA Agreement and Protocol 2, International Atomic Energy Agency, Vienna, 1994]. It is shown that the ripple resonance leads to a reduction in the runaway beam energy if the runaway production is dominated by the Dreicer generation process; however, the effect will be negligible if the secondary generation mechanism is included. The effect of anomalous radial runaway losses induced by enhanced magnetic fluctuations is stronger. Large enough levels of magnetic fluctuations, leading to runaway electron loss rates in excess of 103 s−1, can efficiently limit the number and energy of the runaway electrons.