Electromagnetic gyro-fluid simulations with BOUT++

Transport at the pedestal top is important for the inter-ELM pedestal dynamics. Linear gyrokinetic analysis of the pedestal during an ELM cycle on MAST has shown kinetic ballooning modes to be unstable at the knee of the pressure profile and in the steep pedestal region whilst microtearing modes (MTMs) and electron temperature gradient modes (ETGs) dominate in the shallow gradient region inboard of the pedestal top [1,2]. The transition between these instabilities at the pedestal knee has been observed in low and high collisionality MAST pedestals, and is expected to play a role in the broadening of the pedestal [3]. Nonlinear simulations are needed in this region to understand the microturbulence, the corresponding transport fluxes, and to gain further insight into the processes underlying the pedestal evolution. Such gyrokinetic simulations are, however, numerically challenging. To further improve the prospect of large scale microinstability analysis in the pedestal region it is desirable to introduce reduced models which capture the relevant physics. Using the plasma simulation framework BOUT++ [4] an electromagnetic gyro-fluid model [5] has recently been implemented. Whilst this offers the potential for significantly reduced computational cost compared to the gyrokinetic simulations against which it has been benchmarked, several challenges must be overcome to realise this potential and these will be discussed.