Microturbulence studies of pulsed poloidal current drive discharges in the reversed field pinch

Abstract

Experimental discharges with pulsed poloidal current drive (PPCD) in the Madison Symmetric Torus reversed field pinch are investigated using a semi-analytic equilibrium model in the gyrokinetic turbulence code Gene. PPCD cases, with plasma currents of 500 kA and 200 kA, exhibit a density-gradient-driven trapped electron mode (TEM) and an ion temperature gradient mode, respectively. Relative to expectations of tokamak core plasmas, the critical gradients for the onset of these instabilities are found to be greater by roughly a factor of the aspect ratio. A significant upshift in the nonlinear TEM transport threshold, previously found for tokamaks, is confirmed in nonlinear reversed field pinch simulations and is roughly three times the threshold for linear instability. The simulated heat fluxes can be brought in agreement with measured diffusivities by introducing a small, resonant magnetic perturbation, thus modeling the residual fluctuations from tearing modes. These fluctuations significantly enhance transport.

Evidence for drift waves in the turbulence of reversed field pinch plasmas

“…In particular, the stability of ion-temperature-gradient (ITG), trapped-electron mode (TEM), and microtearing modes have been investigated analytically and numerically. [14][15][16][17][18][19] Comparisons of experimental measurements with comprehensive gyrokinetic models that use experimental equilibria for several of these improved-confinement cases have confirmed the presence of microtearing modes, 20 TEM, and ITG turbulence. 21 It should also be noted that many of the observed features presented in this paper are similar to features seen in the space turbulence.…”

“…Gyrokinetic studies have been performed for improvedconfinement MST plasma equilibria, motivated by the likelihood that drift waves are important for transport when tearing is greatly reduced. 18,19 Many of the standard drift wave modes can be unstable. Fluctuation measurements in these improved-confinement MST plasmas show clear evidence for an unstable density-gradient-driven TEM that has a density-gradient threshold and peaks in the strong gradient region of the plasma.…”

Upgrading a high-throughput spectrometer for high-frequency (<400 kHz) measurements

“…The Ion-Doppler Spectrometer II (IDSII) used for Charge Exchange Recombination Spectroscopy (CHERS) 1 was originally designed to measure ion dynamics associated with tearing modes with frequencies up to 50 kHz. We could also address the physics of plasma fluctuations at higher frequencies, e.g., micro-instabilities 2,3 and Alfvénic fluctuations associated with energetic particle modes, 4 by increasing the bandwidth of the spectrometer electronics to 400 kHz. We developed a model that generates synthetic digitized signals to facilitate the uncertainty estimation in the spectroscopic measurements and the bandwidth optimization.…”

Direct Measurement of a Toroidally Directed Zonal Flow in a Toroidal Plasma

“…Tearing modes in MST can be significantly reduced by applying an inductive current profile control technique [24]. With the tearing mode amplitudes reduced, the density gradient becomes large in the edge, and density-gradient-driven trapped electron modes (TEMs) are unstable [21,25,26]. Importantly, gyrokinetic modeling indicates that electrostatic particle and electron heat transport peak at a relatively large scale for micro-instabilities (k y ρ s ≈ 0.2−0.4), and are regulated by zonal flows [21,25].…”

“…With the tearing mode amplitudes reduced, the density gradient becomes large in the edge, and density-gradient-driven trapped electron modes (TEMs) are unstable [21,25,26]. Importantly, gyrokinetic modeling indicates that electrostatic particle and electron heat transport peak at a relatively large scale for micro-instabilities (k y ρ s ≈ 0.2−0.4), and are regulated by zonal flows [21,25]. Observations here of limit cycle behavior between the zonal flows and the turbulence provide the first experimental investigation of the regulation process in the RFP where zonal flows are also subject to degradation by magnetic fluctuations.…”