Runaway to Ohmic Conversion in Tokamak Disruptions

The termination of an existing runaway electron (RE) beam has emerged as a central thrust in disruption research. An intriguing scenario for achieving this result is by inducing a RE to Ohmic conversion, whereby REs are either decayed in energy or expelled from the plasma leaving near bulk electrons to carry the plasma current. We have investigated this scenario in two limits. The first is for the case of an axisymmetric plasma such that the RE to Ohmic conversion can only be realized by slowing down the existing REs. A hybrid fluid-kinetic model of the coupled system is employed whereby fluid equations are used to describe the bulk plasma, whereas REs are described kinetically. This framework is utilized to impose constraints on the bulk plasma that must be achieved in order to induce the RE to Ohmic conversion in an axisymmetric plasma. The second limit considered corresponds to the case where a large fraction of REs are lost via an MHD event. In this limit, a critical question is whether remnant REs are able to reform by the avalanche amplification mechanism. A sharp transition is identified in the phase space defined by the amount of injected material and the strength of the MHD event that delineates regimes where a RE to Ohmic conversion can be achieved.