HTPD 2018

A versatile combination Doppler Backscattering (DBS) and Cross-Polarization Scattering (CPS) diagnostic for the C-2W Beam-Driven Field-Reversed Configuration is described. This system is capable of measuring density fluctuations and perpendicular magnetic field fluctuations across a wide wavenumber range, with typical resolution ≤ 0.4. Four tunable frequencies (26 GHz ≤ f ≤ 60 GHz corresponding to plasma densities 0.8x10^19m-3 ≤ n_e ≤ 3x10^19m-3) are launched via quasi-optical beam combiners/polarizers and an adjustable parabolic focusing mirror selecting the beam incidence angle. GENRAY ray tracing shows that the incident X-mode and backscattered CPS O-mode beam trajectories essentially overlap for C-2W plasma parameters, allowing simultaneous detection of ñ and B~ from the same scattering volume. We also discuss DBS measurements of the toroidal wavenumber spectrum of gyro-scale density fluctuations in the previous C-2U FRC (0.5 ≤ k rho_s ≤ 10 with the ion sound gyroradius rho_s). Only high-k (electron-scale) density fluctuations have been detected in the C-2U core, while a broad exponential wavenumber spectrum was observed in the scrape-off layer surrounding the FRC plasma, in agreement with gyrokinetic simulations [1]. [1] L. Schmitz et al., Nat. Comm. 7, 13860 (2016).