Advanced Diagnostics for the Wavenumber Spectrum of Density and Magnetic Field Fluctuations in the C-2U and C-2W FRCs

An advanced multi-channel combined Doppler Backscattering (DBS)/Cross-Polarization (CPS)[1] diagnostic for the C-2W Field-Reversed Configuration (FRC) will be discussed, allowing simultaneous measurements of the wavenumber spectrum of density fluctuation and magnetic fluctuations. Cross-correlation analysis on ñ and B_r should be possible as the backscattered O-mode DBS return and the backscattered X-mode CPS signal originate essentially from the same scattering volume. GENRAY ray tracing predicts that magnetic fluctuations with 2 ≤ k_torrho_s ≤ 50 can be accessed in the FRC core and scrape-off layer (SOL). DBS data from the C-2U FRC already show absence of ion-scale density turbulence in the FRC core. Global gyrokinetic simulations attribute core stability to Finite Larmor radius effects, short fieldline connection length, and favorable magnetic field gradient. Transport analysis indicates near-classical core ion thermal diffusivity. In contrast, multi-scale turbulence including short-scale electron modes is observed via DBS in the mirror-confined SOL plasma, in agreement with global gyrokinetic simulations predicting unstable drift-interchange modes for k_torrho_s > 1.5. [1] X.L. Zou et al., Phys. Rev. Lett. 75 1090-93 (1991); [2] L. Schmitz et al., Nature Comm. 7 13860 (2016).