Runaway electrons and the evolution of the plasma current in tokamak disruptions

After the thermal quench of a tokamak disruption, the plasma current decays and is partly replaced by runaway electrons. A quantitative theory of this process is presented, where the evolution of the toroidal electric field and the plasma current is calculated self-consistently. In large tokamaks most runaways are produced by the secondary (avalanche) mechanism, but the primary (Dreicer) mechanism plays a crucial role in providing a seed for the avalanche. As observed experimentally, up to 50%-60% of the plasma current is converted into runaways in the Joint European Torus [P. H. Rebut , Nucl. Fusion 25, 1011 (1985)], and the conversion is predicted to be somewhat larger in ITER [R. Aymar , Plasma Phys. Controlled Fusion 44, 519 (2002)]. Furthermore, the postdisruption current profile is found to be more peaked than the predisruption current-so much, in fact, that the central current density can increase although the total current falls. It is also found that the runaway current profile easily becomes radially filamented. These results may have implications for the stability of the postdisruption plasma. (c) 2006 American Institute of Physics.