Avoidance of impurity-induced current quench using lower hybrid current drive

This work reports observations of a tokamak plasma that experienced a thermal quench due to a large, transient high-Z influx but avoided a current quench. This is argued to be caused by the presence of lower hybrid range of frequency (LHRF) waves that sustain a non-thermal, current-carrying electron population. In Alcator C-Mod L-mode plasmas at I_p = 450 kA, n_e = 0.5 × 10²⁰ m^?3, nearly all of the current can be sustained non-inductively by injecting ≃700 kW of LHRF power at 4.6 GHz and n_∥ = 1.9. A sudden influx of a large amount of tungsten, n_z/n_e ≃ 0.0044, triggers a cooling wave that propagates at 2-3 m s?1 all the way into the core, dropping on-axis Te from 3 keV to temperatures less than measurement floor of 50 eV. An off-axis reheat begins after 100 ms, but Te profiles remain hollow for 300-350 ms after the injection. Throughout this temperature evolution, the plasma density, current and shape remain unchanged to within 10%. Following the expulsion of the tungsten, the plasma returns to its baseline conditions and the plasma ends as planned with a controlled current ramp-down. Energy balance analysis shows the LHRF power continues to be absorbed in the plasma after the thermal quench, as a significant fraction of it is needed to be consistent with radiated power measurements. Examination of current relaxation time, τR, and fast-electron slowing down time, τS, indicate the LHRF must contribute to driving current, despite the low temperatures, as the current remains nominally stationary despite τR < 5 ms and τS < 50 ms for relativistic electrons. These measurements represent an important existence proof of a possible technique for avoidance of disruptions caused by sudden, unplanned influx of impurities in the form of dust or flakes of high-Z wall material. Implications and suggestions for future experimental and modeling and simulation work are summarized.