Gyro-kinetic theory and global simulations of the collisionless tearing instability: The impact of trapped particles through the magnetic field curvature

Abstract

The linear instability of the tearing mode is analyzed using a gyrokinetic approach within a Hamiltonian formalism, where the interaction between particles and the tearing mode through the wave-particle resonance is retained. On the one hand, the curvature of the magnetic field is shown to play no role in the linear instability when only passing particles are present in the plasma. On the other hand, the presence of trapped particles leads to an overall increase in the growth rate. Gyrokinetic simulations using the state-of-the-art Gkw code confirm these findings and are further used to investigate the impact of the magnetic field curvature and the temperature gradient on tearing modes including the effect of trapped particles. Without the temperature gradient, wave-particle resonance with the trapped electrons tends to stabilize the tearing mode, while with the finite temperature gradient, the magnetic curvature tends to destabilize the tearing mode, suggesting an interchange mechanism. The balance of these two stabilizing/destabilizing effects leads to a threshold in the temperature gradient beyond which the magnetic curvature plays a destabilizing role. This opens the way for a deeper understanding and control of the tearing instability in fusion plasmas.