Gyrokinetic simulation of energetic particle driven toroidal Alfven eigenmodes in micro-turbulence

We present nonlinear simulations of the evolution of energetic particles (EP) driven Reverse Shear Alfven

Eigenmodes (RSAE) in the presence of ITG turbulence, using the gyrokinetic delta-f Particle-in-Cell code

GEM [Chen and Parker, J. Comp. Physics 220, 839 (2007)].

Ions are gyrokinetic and electrons are drift-kinetic. The electron magnetic fluttering nonlinearity plays a crucial role in the saturation of RSAE.

The self-coupling of RSAE and the generation of high-n fluctuations due to magnetic fluttering is found to be the dominant saturation mechanism.

The evolution of a single RSAE is found to depend on the initial condition. If the simulation is initialized with the ITG turbulence,

then the RSAE grows to higher amplitude and transport than if the simulation is initialized with the RSAE. A numerical marker distribution

is constructed and used in the particle weight equation for long-time delta-f PIC simulations. Simulations of multiple RSAEs in the presence

of micro-turbulence will be presented. The delta-f method is not advantageous when a large portion of EPs is redistributed or lost.

A full-f PIC method for EPs will be presented and implemented in GEM for the simulation of strong bursts of EP transport with

arbitrary anisotropic EP distributions. Techniques for reducing noise in a full-f simulation will be discussed.