Absence of Ion-scale Core Turbulence and Transport Barrier Formation with Passive/active Divertor Biasing in the C-2/C-2U Field Reversed Configuration

Experimentally measured inverted core density fluctuations spectra show the absence of ion-scale modes in the FRC core, in agreement with linear, local gyrokinetic simulations. The absence of ion-scale core fluctuations is attributed to a combination of Finite Larmor radius effects, short fieldline connection length, and the radially increasing magnetic field gradient. In contrast, ion-scale modes driven unstable by the radial density and electron temperature gradients are observed in the FRC scrape-off layer (SOL) with characteristic wavenumbers 2 $\le \quad k\rho_{s} \quad \le $ 40, consistent with the unstable mode spectrum indicated by linear gyrokinetic calculations. Electostatic passive or active divertor biasing (via a large radius LaB$_{6}$ electron emitter) maintains sufficient \textbf{\textit{E}}\texttimes \textbf{\textit{B}} rotational shear just outside the FRC separatrix to establish an effective radial transport barrier, with a large critical density gradient comparable to or above the linear instability threshold from gyrokinetic simulations. An advanced Doppler Backscattering diagnostic design for C-2W will be shown.