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

• Published in: Physics of plasmas 2015-01, Vol.22 (1)
• Main Authors: Carmody, D., Pueschel, M. J., Anderson, J. K., Terry, P. W.
• Format: Article
• Language: English
• Subjects: Aspect ratio; Computer simulation; Discharge; Heat flux; Ion temperature; Perturbation methods; Plasma currents; Plasma physics; Plasmas (physics); Stability; Tearing modes (plasmas); Temperature gradients; Tokamak devices; Toruses; Transport; Variation
• ISSN: 1070-664X; 1089-7674
• DOI: 10.1063/1.4905709

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.