Instability excitation and modification due to compressional magnetic fluctuation in global gyrokinetics

The compressional magnetic field fluctuation B_// is commonly ignored in gyrokinetic simulations, and only the electric potential Φ and the parallel magnetic potential A_// are considered. B_// tends to have a small effect on turbulence in core plasmas but can strongly affect electromagnetic modes in the tokamak pedestal, reconnection turbulence in the solar corona, and drift-wave turbulence in LAPD high-β experiments [Pueschel et al., PoP 22, 062105 (2015)].

We derive and implement a set of radially global gyrokinetic equations including compressional magnetic fluctuations into the turbulence code GENE, and benchmark against local flux-tube scenarios of a standard tokamak case [Görler et al., PoP 23, 072503 (2016)]. The new code is then compared with local simulations at the LAPD high-β experimental parameters and magnetic reconnection in the solar corona with realistic β and hydrogen mass ratio. 

Due to the use of finite-element radial base functions, the perpendicular magnetic potentials A_x,y need to be computed separately. This decouples B_// from its gyroaveraged quantity 〈B_//〉, thus a new gyroaverage procedure for the compressional magnetic field is implemented.