Characterizing the Role of Drift-wave Turbulence in a Reversed Field Pinch PDF Download

Author: Takashi Nishizawa
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Languages : en
Pages : 0

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Book Description
Trapped-electron-mode (TEM) microturbulence appears in the edge of MST RFP plas- mas that have reduced tearing instability and tokamak-level confinement using current profile control. High-frequency density fluctuations (k[modifier letter up tack][small 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 cor- relation analysis for spectroscopic data resolves simultaneously the fluctuations in both the turbulent radial velocity (vr ∼ 1km/s) and impurity density. Their correlation reveals an inward flux of C2+ impurities, which is the first direct evidence for trans- port associated with TEM turbulence in the RFP. The C2+ 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 reso- lution. 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.