Gyrokinetic microinstabilities in ASDEX Upgrade edge plasmas

Results of linear gyrokinetic simulations of ASDEX Upgrade [O. Gruber et al, Nucl. Fusion 39, 1321 (1999)] edge plasmas, with experimentally determined geometry and input parameters, are presented. It is found that in the near-edge region, microtearing modes can exist under conditions found in conventional tokamaks. As one enters the steep-gradient region, the growth rate spectrum is dominated—down to very low wavenumbers—by electron temperature gradient modes. The latter tend to peak near the X-point(s) and possess properties which may explain the ratios of the density and temperature gradient scale lengths that have been observed in various experiments over the last decade.