A new flexible gyro-fluid linear eigensolver

Gyro-fluid equations are velocity space moments of the gyrokinetic equation. Special gyro-Landau-fluid closures have been developed to include the damping due to kinetic resonances fit to the collisionless local response functions. This damping allows for accurate linear eigenmodes to be computed with a relatively low number of velocity space moments compared to gyrokinetic codes. An analysis of the published gyro-Landau-fluid closure schemes finds that the Onsager symmetries of the resulting quasilinear fluxes are not preserved. Onsager symmetry guarantees that the matrix of diffusivities is positive definite, an important property for a transport model. A new, simpler scheme for regularizing the gyro-fluid equations that preserves the Onsager symmetry and is scalable to higher velocity space moments has been developed. Linear eigenmodes from the new system of equations are verified with gyrokinetic results. The new linear gyro-fluid eigensolver (GFS) will be used to extend the TGLF quasilinear transport model so that it can compute the energy and momentum fluxes due to parallel magnetic fluctuations, completing the transport matrix. The GFS equations do not use a bounce average approximation. The GFS equations are fully electromagnetic, with general flux surface magnetic geometry, pitch angle scattering for electron collisions and subsonic equilibrium rotation. The Onsager symmetries enable output of the off-diagonal contributions to the fluxes separating diffusion and convection terms. This will be particularly helpful to multi-ion species plasmas transport studies.