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

Abstract

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On the validity of the guiding-center approximation in the presence of strong magnetic gradients

“…These results can, thus, be used to justify the applications of gyrokinetic theory for magnetized plasmas with strong gradients [5]. For example, in the pedestal region of advanced tokamak plasmas [6], where ǫ ≃ 0.2, the guiding-center predictions (8) and ( 9) are 1.3% and 0.5% below the orbit-averaged particle displacement (14) and the orbit-averaged particle drift velocity (21), respectively.…”

“…Because of its important extensions to the gyrokinetic self-consistent treatment of low-frequency fluctuations in magnetized plasmas [4], it is necessary to gain a full understanding of the validity of the guiding-center approximation, especially when plasma gradients are strong (see, e.g., Refs. [5] and [6]). For example, in the pedestal region of advanced tokamak plasmas [6], the gradient length scale can be as small as L ≃ 1 − 2 cm, which means that a 10 keV proton confined by a 5 T magnetic field (with a thermal gyroadius ρ = 2 mm) is represented by ǫ ≃ 0.1 − 0.2, which falls well outside the standard guiding-center limit ǫ ≪ 1.…”

“…[5] and [6]). For example, in the pedestal region of advanced tokamak plasmas [6], the gradient length scale can be as small as L ≃ 1 − 2 cm, which means that a 10 keV proton confined by a 5 T magnetic field (with a thermal gyroadius ρ = 2 mm) is represented by ǫ ≃ 0.1 − 0.2, which falls well outside the standard guiding-center limit ǫ ≪ 1.…”

Finite $β$ Effects on Short Wavelength Ion Temperature Gradient Modes

“…The ion temperature gradient (ITG) [1][2][3][4][5] mode, trapped electron mode (TEM) [6][7][8][9][10][11], and universal drift instabilities [12][13][14], are some of the examples of such unstable modes at the ion scale while the electron temperature gradient mode (ETG) [15][16][17][18] is another class of instabilities at the electron scale. Finite β plasmas also give rise to electromagnetic instabilities such as microtearing modes (MTM) [19][20][21][22][23][24][25][26][27] and kinetic ballooning modes (KBM) [28][29][30][31]. Intermediate to these scales, there exists a class of instabilities driven by a strong ion temperature gradient.…”

“…The role of electromagnetic perturbations on drift modes has been studied extensively. While the electromagnetic perturbation is observed to give rise to instabilities such as the kinetic ballooning mode (KBM) [28][29][30][31], tearing and microtearing modes [19][20][21][22][23][24][25][26], etc., the same is found to stabilize some other drift modes such as the ITG mode, trapped electron mode, universal drift modes, etc [13,[45][46][47][48]. The stabilizing effect of the electromagnetic perturbation on the ITG mode can be attributed to the field line bending induced by the electromagnetic perturbations.…”