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

The conventional approach for thermal quench mitigation in a tokamak disruption is through a high-Z impurity injection that radiates away the plasma's thermal energy before it reaches the wall. The price to pay is a robust Ohmic-to-runaway current conversion. An alternative approach is to deploy a low-Z (either deuterium or hydrogen) injection that aims to slow down the thermal quench, and ideally aligns it with the current quench. 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 a thin plasma sheath at the wall, in addition to Bremsstrahlung bulk losses. 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 thermal quench rate to the order of 10 milliseconds, enough to be comparable to the current quench.