Peak density detection and fluctuation properties near the null-field radius in C-2W Field-Reversed Configuration (FRC) plasmas

In the C-2W Field Reversed Configuration (FRC), density peaking near the null-field radius is expected from simple rigid rotor FRC equilibria but is in reality strongly modified by energetic ion orbit dynamics [1]. Due to the limited spacing of FIR interferometer chords it has been difficult to reliably quantify the peak density near the null-field radius for FRC equilibrium reconstruction. As the plasma density increases during FRC ramp-up and decreases during ramp-down, the peak density (1.9-3.7x10¹⁹ m^-3) in the radial profile is determined, within error limits set by beam refraction, from the loss of microwave (O-mode) probing beam reflection, using a range of frequencies (40-55 GHz). Doppler Backscattering measurements have also provided transport-relevant fluctuation properties, including near the null-field radius. We present here initial data that point to significant differences in radial correlation length and skewness of the amplitude distribution in hydrogen and deuterium plasmas. Radial transport barrier formation in both hydrogen and deuterium plasmas maintains a core plasma state characterized by very low ion-scale fluctuation levels [2] in agreement with global gyrokinetic simulations [3].