Stability of a slab collisional microtearing mode

Abstract

The stability of a collisional microtearing mode (MTM) is investigated by means of linear gyro‐kinetic simulations and a theoretical linear dispersion relation. The electric potential fluctuations and the magnetic curvature are neglected in the simulations to match the theoretical model assumptions. In these conditions, the stability domain, the mode growth rate, and the mode frequency predicted by the theory and the simulations are in satisfactory agreement. The radial structure of the mode has been also investigated, and the localization of the parallel current around the resonant surface predicted by the theory is recovered by the simulations.

Impact of electric potential and magnetic drift on microtearing modes stability

“…3 . In the absence of magnetic drift Ω d = 0, the previous result obtained for a slab collisional tearing mode is qualitatively recovered 32 . The magnetic drift is found to be destabilising, but only in conjunction with a finite collisionality 33 .…”

“…For intermediate collisionality, typical of value in the pedestal region, MTMs are unstable, destabilized by the electron temperature gradient. A fair agreement between analytical calculation and numerical simulations was found, details can be found in 32 . The magnetic drift is then added in the code (green plus sign curve).…”

“…Current sheets are strongly localized around the resonant surface. Linear gyrokinetic simulations 32 show that the mode structure is peaked at the low field side mid-plane and rotates in the electron diamagnetic direction. The width of the current layer, typically a few ρ e , and the sensitivity of magnetic reconnection to dissipation make the numerical calculations of MTMs challenging: it requires a high numerical resolution and a very weak numerical dissipation, especially at low collisionality.…”

“…MTMs are sensitive to β (the Table I: Revelant input parameters used for the theoretical calculations and the linear simulations with GKW. ratio of plasma kinetic to magnetic pressure) and to the electron temperature gradient R/L Te 9,10,32 . Their values are chosen to have a sufficiently high MTM growth rate at high collisionality.…”

“…In previous studies, a linear dispersion relation of a slab microtearing mode using a kinetic approach was established and compared with linear gyrokinetic simulations 32 . The linear stability of the collisionless MTMs predicted by the theory [9][10][11][12][13][14]16 is found consistent with numerical simulations using the gyrokinetic code GKW 31 .…”