Linear gyrokinetic simulations of microinstabilities within the pedestal region of H-mode NSTX discharges in a highly shaped geometry
Authors: M. Coury, W. Guttenfelder, D.R.
Mikkelsen, J.M. Canik, G.P. Canal, A. Diallo, S. Kaye, G. J.
Kramer, R. Maingi, and the NSTX-U team
Abstract: Linear (local) gyrokinetic predictions of
edge microinstabilities in highly shaped, lithiated and
non-lithiated NSTX discharges are reported using the gyrokinetic
code GS2. Microtearing modes dominate the non-lithiated pedestal
top. The stabilization of these modes at the lithiated pedestal
top enables the electron temperature pedestal to extend further
inwards, as observed experimentally. Kinetic ballooning modes are
found to be unstable mainly at the mid-pedestal of both types of
discharges, with un- stable trapped electron modes nearer the
separatrix region. At electron wavelengths, ETG modes are found to
be unstable from mid-pedestal outwards for ne,exp ~2.2, with higher growth
rates for the lithiated discharge. Near the separatrix, the
critical temperature gradient for driving ETG modes is reduced in
the presence of lithium, reflecting the reduction of the lithiated
density gradients observed experimentally. A preliminary linear
study in the edge of non-lithiated discharges shows that the
equilibrium shaping alters the electrostatic modes stability,
found more unstable at high plasma shaping.
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