Particle-in-cell δf gyrokinetic simulations of the microtearing mode

Abstract

The linear stability properties of the microtearing mode are investigated in the edge and core regimes of the National Spherical Torus Experiment (NSTX) using the particle-in-cell method based gyrokinetic code GEM. The dependence of the mode on various equilibrium quantities in both regions is compared. While the microtearing mode in the core depends upon the electron-ion collisions, in the edge region, it is found to be weakly dependent on the collisions and exists even when the collision frequency is zero. The electrostatic potential is non-negligible in each of the cases. It plays opposite roles in the core and edge of NSTX. While the microtearing mode is partially stabilized by the electrostatic potential in the core, it has substantial destabilizing effect in the edge. In addition to the spherical tokamak, we also study the microtearing mode for parameters relevant to the core of a standard tokamak. The fundamental characteristics of the mode remain the same; however, the electrostatic potential in this case is destabilizing as opposed to the core of NSTX. The velocity dependence of the collision frequency, which is crucial for the mode to grow in slab calculations, is not required to destabilize the mode in toroidal devices.

Kinetic ballooning mode under steep gradient: High order eigenstates and mode structure parity transition

“…Due to non-zero β (the ratio of thermal pressure to magnetic pressure), electromagnetic instabilities, including the ideal ballooning mode (IBM) [5] and microscale kinetic ballooning mode (KBM) [6] play central role in the EPED model [7] to predict the width and height of the fusion plasma pedestal. Both KBM [8,9] and microtearing mode (MTM) [10,11] are identified to be important at the plasma edge from experiments and gyrokinetic theory [8,[12][13][14][15][16][17], and thus it is crucial to understand them in order to reveal the complicated nonlinear physics at the transport barrier. These mirco-instabilities also have fundamental importantance in space physics [18].…”

“…Due to its effect on microscopic magnetic island formation and the consequent strong electron transport, MTM has attracted significant interest recently [8, 12-17, 22, 23]. In contrast to the pressure gradient driven KBM, the destabilizing mechanism for MTM has not been fully understood and even can be confusing in literature [15,16]. While tearing or microtearing instability is affected by collision [10,11], kinetic electron effect such as trapped particle effects [11,23], and electron gradient drive [10,11], later experimental observations showed other effects such as magnetic drifts and electrostatic potential can be destabilizing [12] and the collision dependence can be significantly different [15] in spherical tokamak.…”

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.…”

Perturbative Determination of Plasma Microinstabilities in Tokamaks

“…Notably, recent theoretical work suggests that the MTM 13,14 , a small-scale resistive magnetohydrodynamic (MHD) mode not yet included in leading predictive models 15 , might play a critical role in limiting electron thermal transport through the pedestal 16,17 . The presence of pedestal MTMs has been suggested through analysis of so-called "transport fingerprints" 18 and through comparisons of measured magnetic fluctuations with sensitive theory-based (gyrokinetic) simulations [16][17][18][19][20][21][22][23][24][25][26] . However, a conclusive experimental identification of these modes has not yet been presented and is needed to validate the theoretical results.…”