Gyrokinetic particle-in-cell simulations of electromagnetic turbulence in the presence of fast particles and global modes*

Global nonlinear gyrokinetic simulations in the electromagnetic regime have recently become possible on a large scale due to the mixed-variable formulation [1] of the equations solved as well as various improvements in the computing power available which include the GPU-enabling of gyrokinetic codes. We perform global gyrokinetic simulations of electromagnetic turbulence using ORB5 [2] and EUTERPE [3] codes in presence of energetic particles and global MHD-like (e.g. tearing or internal kink) modes at different beta values in realistically shaped tokamak and stellarator geometries. Electromagnetic turbulence is considered in the low-beta Ion-Temperature-Gradient driven (ITG) regime as well as the high-beta Kinetic-Ballooning-Mode (KBM) regime. Saturation of KBM instabilities is demonstrated in global gyrokinetic simulations for tokamak (using ORB5) and stellarator (using EUTERPE) plasmas. The physics of the saturation is addressed for single-n and broad-spectrum KBM instabilities. Zonal flow generation via a beat-driven or a modulational mechanism is considered in the electromagnetic regime. Turbulence suppression is studied in presence of the energetic particles. Numerical studies have also been performed for the global gyrokinetic dynamics of tearing and kink instabilities in tokamak plasmas. In the nonlinear regime, the tearing mode has been simulated both without and in presence of the ambient turbulence. Even for a flat background pressure, perturbations of density and electrostatic potential in strongly driven islands produce Kelvin-Helmholtz unstable flows, leading to strong decay of the magnetic island. For finite background gradients, first evidence of turbulence-driven magnetic islands is obtained in toroidal gyrokinetic simulations for different types of the turbulence. Interaction of internal kink instabilities with the fast ions has been addressed including destabilization of the fishbones. Non-adiabatic chirping dynamics of Alfvenic modes has been considered.

[1] A. Mishchenko et al, Phys. Plasmas 21, 092110 (2014)

[2] E. Lanti et al, Comp. Phys. Comm. 251, 107072 (2020)

[3] R. Kleiber et al, Comp. Phys. Comm. 295, 109013 (2024)