Edge microstability of NSTX plasmas without and with lithium-coated plasma-facing components

Abstract

The pedestal structure in NSTX is strongly affected by lithium coatings applied to the PFCs. In discharges with lithium, the density pedestal widens, and the electron temperature (Te) gradient increases inside a radius of ψN ∼ 0.95, but is unchanged for ψN > 0.95. The inferred effective electron thermal and particle profiles reflect the profile changes: is slightly increased in the near-separatrix region, and is reduced in the region ψN < 0.95 in the with-lithium case. The profile shows a broadening of the region with low diffusivity with lithium, while the minimum value within the steep-gradient region is comparable in the two cases. The linear microstability properties of the edge plasma without and with lithium have been analysed. At the pedestal top microtearing modes are unstable without lithium. These are stabilized by the stronger density gradient with lithium, becoming TEM-like with growth rates reduced and comparable to E × B shearing rates. In the region ψN > 0.95, both the pre- and with-lithium cases are calculated to be unstable to ETG modes, with higher growth rates with lithium. Both cases are also found to lie near the onset for kinetic ballooning modes, but in the second-stable region where growth rates decrease with increasing pressure gradient.

Linear gyrokinetic simulations of microinstabilities within the pedestal region of H-mode NSTX discharges in a highly shaped geometry

“…The difference between the study reported herein and the work of Ref. 16 is the change in the plasma shaping and the lithium dose that altered the pedestal structure. Here also, the suppression of ELMs and the flattening of the edge density profiles are observed from the lithiated experimental data.…”

“…It was also concluded in Ref. 16 that the decrease in the pedestal top energy transport and the widening of the pedestal region in lithiated discharges were related to the stabilization of MT modes by density gradients. The suppression of ELMs in lithiated discharges may be explained by the stiffness of the electron temperature profile due to unstable ETG modes coupled with the strong reduction in the electron density, when compared to a non-lithiated discharge.…”

“…15 The comparative linear gyrokinetic analysis of Ref. 16, performed in an "intermediate" triangularity (d $ 0:46), low elongation (j $ 1:8) confinement regime, investigated the effect of lithium coating on the pedestal structure. It was found that at the top of the pedestal region without lithium coating, microtearing (MT) modes 17,18 are dominant, replaced by trapped electron (TE) modes [19][20][21] with lithium-coated PCFs.…”

“…11 The data were obtained from a highly shaped (d $ 0:6 À 0:7; j $ 2:2) reference case without lithium coating for comparison to a highly shaped high-dose lithium-coated PFC case. GS2 was successfully used to study, for example, the plasma edge microstability 16,27 and electron-scale instabilities (i.e., highk ? , with k ?…”

Microtearing turbulence limiting the JET-ILW pedestal

“…Experimentally, fluctuations have been diagnosed that are consistent with MTM [22,23,25], KBM [22,[24][25][26], and trapped electron modes (TEM) [22], while linear gyrokinetic modeling has identified MTM [19,[27][28][29][30], TEM [30,31], ETG [29,30,32,33], KBM [19,26,27,29,30,34,35], and unidentified drift waves [36]. Due to the large number of possible instabilities, and the complexity of the nonlinear turbulent state (that reflects the underlying instabilities in not so obvious ways), no clear picture has emerged regarding the relative importance of these modes for pedestal transport.…”