Microinstability in the pedestal

The pedestal is a region of increased equilibrium pressure gradients near the edge of high performance toroidal plasmas. While the MHD properties of this region have been fairly well-characterized, pedestal microinstabilities and turbulence remain rela- tively unexplored, particularly in steep gradient regions. In this thesis, we describe a new microinstability caused by the steep equilibrium temperature gradients and com- plex magnetic geometry. This instability has a critical temperature gradient that is much higher than core temperature gradients, and hence likely exists only in pedestals. Basic analytic arguments show that in the presence of magnetic shear and steep temperature gradients, this mode must be one of the fastest growing modes. In realistic magnetic equilibria that we study, it is the fastest growing mode at almost all scales comparable to ion and electron gyroradii. Therefore, it is a robust feature of pedestal microinstability. We also investigate nonlinear pedestal microturbulence. We find the turbulent satu- rated state to be inhomogeneous in the poloidal angle, in strong contrast to core micro- turbulence that is typically well-correlated for long distances along magnetic field lines. Turbulence is particularly strong in poloidal regions of (i) weaker local magnetic shear and (ii) shorter distances between flux surfaces. These two effects cause the perpendicular wavenumber of the turbulence to have a particularly strong poloidal angle dependence, which results in the turbulence being strongly damped by finite Larmor radius effects in certain poloidal regions, hence causing the turbulent poloidal inhomogeneity. While we find that the linear instabilities caused by magnetic drifts are the fastest growing modes, nonlinear simulations appear to reach a quasi-steady state insensitive to these drifts and dominated by the branch of electron temperature gradient turbulence caused by parallel streaming. The heat flux in this quasi-steady state is roughly constant in time, but some modes that appear to be driven by magnetic drifts have not yet saturated.