Suppression of Coherent and Turbulent Instabilities in the PFRC-2

Despite producing field-reversed configurations (FRCs) surviving up to ~300 ms, the second-generation Princeton Field-Reversed Configuration (PFRC-2) exhibits several macroscopic instabilities. In the electron-heating regime of odd-parity rotating magnetic field (RMF_O) operation, radial line-integrated electron density measurements show both narrow-band and broad-band features in time-series power spectra between ~1 kHz-1 MHz, consistent with both coherent modes and turbulent processes. The coherent instabilities are the previously-identified m = 1 and m = 2 flute-like centrifugal interchange modes (rotating at ~2-30 kHz), which are endemic to FRCs. A new spectrally-filtered, axial fast-camera diagnostic has been installed to observe the entire center-cell diameter at up to 140,000 fps, revealing the behavior of radial and azimuthal structures in both fill-gas and impurity species. Observations suggest that chaotic high-m interchange, drift-interchange, or hot-electron instabilities are likely responsible for the measured turbulent fluctuations, which typically manifest as thin axial plasma columns or curved radial spokes. Herein we report on the behavior of these instabilities as a function of gas species, neutral fill pressure, RMF_O power, and axial magnetic field strength. Various stabilization techniques are assessed including gas puffing, application of magnetic quadrupoles (Ioffe bars), and end-biasing.