Quasilinear fluxes for parallel magnetic fluctuations

The Trapped Gyro-Landau Fluid (TGLF) equations [1] do not have enough velocity space moments to compute the energy and momentum transport contributions from parallel magnetic field perturbations. A higher moment system requires determining new closure coefficients. The curvature drift resonance closure coefficients in TGLF are computed using the Beer-Hammett method [2]. This method numerically determines the closure coefficients that minimize the error in a fit to linear response functions. Unfortunately, the fitting procedure does not result in gyro-Landau fluid quasi-linear fluxes that preserve the Onsager symmetries. A new system of linear gyrofluid equations is derived with closure coefficients that preserve Onsager symmetries. A system of equations with enough velocity space moments to compute the full set of electromagnetic fluxes is constructed. The importance of the parallel magnetic fluctuations for transport is assessed for high beta spherical tokamak discharges in MAST-U. [1] G. M. Staebler, J. E. Kinsey, and R. E. Waltz, Phys. Plasmas 12 (2005) 102508. [2] M. A. Beer and G. W. Hammett, Phys. Plasmas 3 (1996) 4046.