Cross-Separatrix Simulations of Turbulent Transport in the Field-Reversed Configuration

Experimental progress by TAE Technologies has led to field-reversed configuration (FRC) plasmas with lifetimes of several milliseconds. Doppler Backscattering (DBS) measurements of C-2U show that density fluctuations in the core and scrape-off layer (SOL) regions are distinct: core fluctuations are consistently low in amplitude while SOL fluctuations have large amplitude at ion-scales and decrease towards electron-scales. In qualitative agreement, local linear simulations using the Gyrokinetic Toroidal Code (GTC) show the absence of instability in the core and the presence of instability in the SOL.

Using the cross-separatrix particle-in-cell A New Code (ANC), we address micro-turbulence in a global FRC geometry. Global simulations show fluctuations spreading from SOL to core with saturated core fluctuations an order of magnitude lower than the SOL. Simulations, domain limited to the SOL, show saturation at around eφ/T_e~O(10^-2) with an inverse spectral cascade. These features remain present in more realistic global cross-separatrix simulations and contribute to the fluctuation spectra evolving towards the robust SOL and quiescent core spectra, consistent with experimental DBS measurements.