Impurity transport and zonal flows in improved-­confinement reversed field pinch plasmas

Trapped­-electron-mode (TEM) microturbulence appears in the edge of MST RFP plasmas that have reduced tearing instability and tokamak-level confinement using current profile control. High­-frequency density fluctuations (k_perp*rho_s=0.2­-0.4) emerge with a critical­ gradient threshold as the density profile steepens. These features are consistent with gyrokinetic simulations using GENE that include a small magnetic fluctuation mimicking residual tearing activity, which tends to disrupt zonal flow formation. Here we present direct measurements of impurity transport and zonal flows to investigate TEM turbulence saturation and transport. A new method of linearized spectrum correlation analysis for spectroscopic data resolves simultaneously the fluctuations in both the turbulent radial velocity and impurity density. Their correlation reveals an inward flux of C III impurities, which is the first direct evidence for transport associated with TEM turbulence in the RFP. The C III ions are edge­-localized and evolve from graphite limiters. The profile of the plasma potential is measured in the edge using two multi­-channel capacitive probes, each having 7 mm radial spatial resolution. An edge­-localized flow is observed, and with the probes separated 180 degrees toroidally, the flow has a long­-range correlation characteristic of zonal structure. The amplitude of the flow is modulated by the turbulence, as occurs in predator­-prey-like dynamics. These measurements, together with the gyrokinetic modeling, suggest that transport in RFP plasmas will ultimately be regulated by microturbulence as occurs in tokamak and stellarator plasmas.