Global gyrokinetic stability of collisionless microtearing modes in large aspect ratio tokamaks

Abstract

Linear full radius gyrokinetic calculations show the existence of unstable microtearing modes (MTMs) in purely collisionless, high temperature, large aspect ratio tokamak plasmas. The present study takes into account fully gyrokinetic highly passing ions and electrons. The global 2-D structures of the collisionless mode with full radius coupling of the poloidal modes is obtained and compared with another electromagnetic mode, namely, the Alfvén Ion Temperature Gradient (AITG) mode (or Kinetic Ballooning Mode, KBM) for the same equilibrium profile. Several important characteristics of the modes are brought out and compared, such as a clear signature in the symmetry properties of the two modes, the plasma–β dependence, and radial and poloidal length scales of the electrostatic and magnetic vector potential fluctuations. Extensive parameter scans for this collisionless microtearing mode reveal the scaling of the growth rate with β and the electron temperature gradient ηe. Scans at different β values show an inverse relationship between the ηe threshold and β, leading to a stability diagram, and implying that the mode might exist at moderate to strong temperature gradients for finite β plasmas in large aspect ratio tokamaks. In contrast to small aspect ratio tokamaks where the trapped electron magnetic drift resonance is found to be important, in large aspect ratio tokamaks, a strong destabilization due to the magnetic drift resonance of passing electrons is observed and is identified as a possible collisionless drive mechanism for the collisionless MTM.

Role of Trapped Electrons on Global Gyrokinetic Linear Stability of Collisionless Microtearing Modes

“…[14] pertaining to a spherical tokamak with high-β plasma, the magnetic drift resonance of trapped electrons is found to drive the collisionless MTM instability. At larger aspect ratios, the fraction of trapped particles is relatively small, and it has been found that collisionless MTM is driven unstable by the magnetic drift resonance of highly passing electrons [15]. Ions dynamics, on the other hand, do not affect the mode significantly.…”

“…In this work, many of the recent results described in Ref. [15] will be elaborated, where it is shown that passing electron alone suffice to drive MTM unstable in large aspect ratio tokamaks. In the later part, the effect of trapped electrons is addressed.…”

“…Recent investigations have found that MTM is unstable in the absence of collisions as well [12,14,15], triggering a need to understand collisionless drive mechanisms. In the work of Ref.…”

“…For the ρ * (ρ/a, normalized ion radius) value considered, the global mode structure is found to be sensitive to profile variations in Ref. [15]. The trapped fraction was ignored with a view to gain insight into the nature of MTMs.…”

“…The model assumptions and formulation are detailed in Refs. [15,16,17,18,19,20] and references thereof. In this work, many of the recent results described in Ref.…”