A circular equilibrium model for local gyrokinetic simulations of ion temperature gradient fluctuations in reversed field pinches

Abstract

A simple large-aspect-ratio (R 0 =r) circular equilibrium model is developed for low-beta reversed field pinch (RFP) geometry. The model is suitable for treating small scale instability and turbulent transport driven by ion temperature gradient (ITG) and related electron drift modes in gyrokinetic simulations. The equilibrium model is an RFP generalization of the common tokamak s-a model to small safety factor (q), where the poloidal field dominates the toroidal field. The model accommodates the RFP toroidal field reversal (where q vanishes) by generalizing the cylindrical force-free Bessel function model (BFM) [J. B. Taylor, Phys. Rev. Lett. 33, 1139 (1974)] to toroidal geometry. The global equilibrium can be described in terms of the RFP field reversal and pinch parameters ½F; H. This new toroidal Bessel function model (TBFM) has been incorporated into the gyrokinetic code GYRO [J. Candy and R. E. Waltz, J.Comput. Phys. 186, 545 (2003)] and used here to explore local electrostatic ITG adiabatic electron instability rates for typical low-q RFP parameters. V

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

“…Gyrokinetics, the primary tool used to evaluate the nature of microinstabilities, has shown that modes familiar from tokamaks-such as ion temperature gradient (ITG) modes, trapped electron modes (TEM), or microtearing (MT) modes-can also arise in the RFP, albeit with modified characteristics. [2][3][4][5][6] The RFP versions of these modes are generally marked by critical gradients that differ from the tokamak thresholds by a factor of the aspect ratio.…”

Gyrokinetic studies of microinstabilities in the reversed field pinch

“…Moreover, in adapting gyrokinetic codes originally developed for the tokamak, we are careful to capture all of the effects arising from an equilibrium magnetic field with comparable poloidal and toroidal field components. This can be done using the toroidal Bessel function model in the GYRO code [21], with RFP-appropriate representations of the curvature drift and parallel derivative [10]. We demonstrate here through benchmarking studies that quantitatively idential results can be obtained with corresponding modifications…”

“…A more detailed explanation of the changes and their effect on linear growth rates is given in Ref. [10], and only an outline will be given here.…”

“…After years of some uncertainty about what type of microinstability is most compatible with the high shear and low safety factor conditions of the RFP, numerical solutions of gyrokinetic models in RFP geometry have shown that many modes familiar from the tokamak may also arise in the RFP [8][9][10]. For example, ion temperature gradient (ITG) driven modes can dominate instability at zero β, above its critical temperature gradient, albeit with a somewhat different character from their tokamak counterpart.…”

Density fluctuation measurements by far-forward collective scattering in the MST reversed-field pinch

“…These high-performance plasmas are achieved either through externally added pulsed poloidal current drive (PPCD), 2 or through the spontaneous evolution of RFP plasmas at high plasma current. 3,4 Numerical simulations of these improved confinement plasmas indicate that unstable ion temperature gradient (ITG) 5,6 and micro-tearing modes 7 may dominate the transport. In addition, RFP plasmas heated by neutral beam injection (NBI) show the evidence of narrowband fast-particle instabilities that can effect energetic ion confinement.…”