Overview of gyrokinetic studies of finite-βmicroturbulence

Abstract

Recent results on electromagnetic turbulence from gyrokinetic studies in different magnetic configurations are overviewed, detailing the physics of electromagnetic turbulence and transport, and the effect of equilibrium magnetic field scale lengths. Ion temperature gradient (ITG) turbulence is shown to produce magnetic stochasticity through nonlinear excitation of linearly stable tearing-parity modes. The excitation, which is catalyzed by the zonal flow, produces an electron heat flux proportional to β 2 that deviates markedly from quasilinear theory. Above a critical beta known as the non-zonal transition (NZT), the magnetic fluctuations disable zonal flows by allowing electron streaming that shorts zonal potential between flux surfaces. This leads to a regime of very high transport levels. Kinetic ballooning mode (KBM) saturation is described. For tokamaks saturation involves twisted structures arising from magnetic shear; for helical plasmas oppositely inclined convection cells interact by mutual shearing. Microtearing modes are unstable in the magnetic geometry of tokamaks and the reversed field pinch (RFP). In NSTX instability requires finite collisionality, large beta, and is favored by increasing magnetic shear and decreasing safety factor. In the RFP, a new branch of microtearing with finite growth rate at vanishing collisionality is shown from analytic theory to require the electron grad-B/curvature drift resonance. However, gyrokinetic modeling of experimental MST RFP discharges at finite beta reveals turbulence that is electrostatic, has large zonal flows, and a large Dimits shift. Analysis shows that the shorter equilibrium magnetic field scale lengths increase the critical gradients associated with the instability of trapped electron modes, ITG and microtearing, while increasing beta thresholds for KBM instability and the NZT.

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.…”

Linear gyrokinetic simulations of microinstabilities within the pedestal region of H-mode NSTX discharges in a highly shaped geometry

“…For example, scans over the electron b / n e T e =B 2 T (B T is the toroidal magnetic field) parameter may also be required to characterize pedestal instabilities for the NSTX high-b plasmas. 37 The extensive work of Ref. 18 on MT modes in NSTX plasmas shows that microtearing modes, producing electron thermal transport, are driven by electron temperature gradients above critical b values.…”

Tractable flux-driven temperature, density, and rotation profile evolution with the quasilinear gyrokinetic transport model QuaLiKiz

“…In JETTO-SANCO 1 s of JET plasma simulation with QuaLikiz costs ∼ 10 hours walltime using 10 CPUs. All simulations covered both ion and electron scales with a wavenumber spectrum of k θ ρ s = [0.1, 0.175, 0.25, 0.325, 0.4, 0.5, 0.7, 1, 1.8, 3,9,15,21,27,36,45].…”