Current Quench Avoidance using Lower Hybrid Current Drive in the Alcator C-Mod Tokamak

Experiments from Alcator C-Mod demonstrate that following a full thermal collapse due to a large impurity influx, a current quench (CQ) can be avoided using lower hybrid (LH) current drive. This existence proof shows a CQ does not necessarily follow a thermal quench (TQ) when the helicity is maintained externally via RF, opening new optimization strategies for disruption avoidance. In a 400 kA, n_e,bar~ 0.5x10²⁰ m^-3, 5.4 T L-mode plasma, 700 kW of 4.6 GHz LH power is used to approach fully non-inductive operation, dropping V_loop from 0.8 V to ~0.1 V. A high-Z injection increases the P_RAD above the input power, causing a cooling wave to propagate inward with temperatures dropping over ~100 ms to those resembling a post-TQ plasma. While the I_p and n_e,bar remain fixed within ~10%, on-axis T_e drops from 3 keV to 10’s of eV, and T_e becomes peaked off-axis at r/a ~ 0.7 with values as low as 170 eV. As the impurity density decays, the plasma re-heats, the RF turns off and the plasma reverts to its pre-injection state over ~500 ms. During this event, V_loop reaches up to 1.5 V, corresponding to a max E/E_crit ~ 9. Similar plasmas exhibit the high-Z cooling behavior, but MHD-induced loss of fast electrons and termination of LH power results in a transition to a standard CQ in < 10 ms.