Evaluation of Some Possible Scenarios for Validating the Theory of RF Current Condensation in the DIII-D Tokamak*

Electron cyclotron current drive can stabilize large islands and is planned for stabilization of tearing modes on ITER. The theory of RF current condensation [1] predicts a nonlinear amplification of power deposition due to the temperature sensitivity of ECCD, which improves stabilization efficiency by self-focusing power deposition at island O-points. Alternatively, premature nonlinear damping can occur at island peripheral, "shadow" island center and reduce stabilization efficiency. We are pursuing numerical simulations to evaluate possible scenarios for validating the theory using the top launch capability of the DIII-D tokamak. Our simulations employ a numerical code OCCAMI (Of Current Condensation Amid Magnetic Islands) [2] which iterates between the GENRAY geometrical optics ray tracer for trajectory and power deposition, and a heat diffusion equation solver for evolving island temperature profile. References [1] Reiman, A. H., & Fisch, N. J. (2018). Suppression of Tearing Modes by Radio Frequency Current Condensation. Physical Review Letters, 121(22), 225001.  [2] Nies, R., Reiman, A. H., Rodriguez, E., Bertelli, N., & Fisch, N. J. (2020). Calculating RF Current Condensation with Consistent Ray-tracing and Island Heating. Physics of Plasmas, 27(9), 092503.