Runaway Electron Amplification in Tokamak Disruptions

Runaway electron (RE) generation during a tokamak disruption poses a significant concern to the longevity of plasma facing components. More specifically, amplification of REs by the avalanche mechanism provides a means of converting current carried by near bulk electrons into RE current. The present work focuses on evaluating the efficiency of the avalanche process in tokamak geometry for plasmas with large electric fields and significant impurity content, conditions typical of a tokamak disruption. It is found that the efficiency of the avalanche process depends sensitively on the collisionality of electrons near the critical energy to run away. The presence of impurities plays a multifaceted role in this process. While high-Z impurities increase the collisionality of the overall plasma, they also significantly increase the critical energy to run away, thus decreasing the collisionality near the critical energy to runaway. In contrast, larger quantities of impurities lead to a larger electric field, due to the increase in radiation. The impact of these partially offsetting processes on the net efficiency of the avalanche mechanism will be addressed by integrated drift kinetic simulations of tokamak disruptions.