Aligning Thermal and Current Quenches with a High Density Low-Z Injection*

The conventional approach for thermal quench (TQ) mitigation in a

  tokamak disruption is through high-Z impurity injection that

  radiates away the plasma thermal energy before it reaches the wall.

  The price to pay is robust Ohmic-to-runaway current conversion.  An

  alternative approach is to deploy low-Z (mostly deuterium or

  hydrogen) injection that aims to slow down the TQ, and ideally

  aligns it with current quench (CQ).  We have investigated this

  approach via 3D MHD simulations using the PIXIE3D code.  By boosting

  the hydrogen density, a fusion-grade plasma is dilutionally cooled at

  approximately the original pressure.  Energy loss to the

  wall is controlled by a Bohm outflow condition at the boundary where

  the magnetic field intercepts the wall.  Robust MHD instabilities proceed as

  usual, while the collisionality of the plasma has been greatly

  increased and parallel transport is now in the Braginskii regime.

  The result is that the decreased transport loss along open field

  lines slows down the TQ enough to be comparable to the CQ.