Edge pedestal control in quiescent H-mode discharges in DIII-D using co-plus counter-neutral beam injection

Abstract

Abstract. We have made two significant discoveries in our recent studies of quiescent H-mode (QH-mode) plasmas in DIII-D. First, we have found that we can control the edge pedestal density and pressure by altering the edge particle transport through changes in the edge toroidal rotation. This allows us to adjust the edge operating point to be close to, but below the ELM stability boundary, maintaining the ELM-free state while allowing up to a factor of two increase in edge pressure. The ELM boundary is significantly higher in more strongly shaped plasmas, which broadens the operating space available for QH-mode and leads to improved core performance. Second, for the first time on any tokamak, we have created QH-mode plasmas with strong edge co-rotation; previous QH-modes in all tokamaks had edge counter rotation. This result demonstrates that counter NBI and edge counter rotation are not essential conditions for QH-mode. Both these investigations benefited from the edge stability predictions based on peeling-ballooning mode theory. The broadening of the ELM-stable region with plasma shaping is predicted by that theory. The theory has also been extended to provide a model for the edge harmonic oscillation (EHO) that regulates edge transport in the QH-mode. Many of the features of that theory agree with the experimental results reported either previously or in the present paper. One notable example is the prediction that co-rotating QH-mode is possible provided sufficient shear in the edge rotation can be created.

Correlations between quasi-coherent fluctuations and the pedestal evolution during the inter-edge localized modes phase on DIII-Da)

“…One of these modes (QCM) has been shown to be localized in the bottom of the pedestal steep gradient using the mirror Langmuir probes on C-Mod. 21 The EHO is thought to be a saturated kink-peeling mode of low-n, 41 localized in the edge pedestal region. Quasi-coherent fluctuations are observed in the region of the pedestal in the ELMy discharges discussed here and in a number of ELM-free regimes.…”

Inboard and outboard radial electric field wells in the H- and I-mode pedestal of Alcator C-Mod and poloidal variations of impurity temperature

“…This shows that Eqs. (8) and (9) remain valid as long as lB/B θ is replaced by the puff width 2W . The reason for this is that the extent of the heat source/sink along the magnetic field is now 2W , while it is lB/B θ in the axisymmetric case.…”

Observation of pedestal turbulence in edge localized mode-free H-mode on experimental advanced superconducting tokamak

“…Turbulence structures in ELM-free H-mode have been observed in magnetic and density fluctuations in other devices. [10][11][12][13][14][15][16][17][18][19][20][21] Low frequency outer mode (OM) at the top of the pedestal in JET indicates a mechanism to delay the appearance of the first ELM. 10 Quasi-coherent (QC) oscillation in Alcator C-Mod is interpreted as a possible mechanism to provide radial transport through the pedestal region in enhanced D a (EDA) Hmode.…”

“…10 Quasi-coherent (QC) oscillation in Alcator C-Mod is interpreted as a possible mechanism to provide radial transport through the pedestal region in enhanced D a (EDA) Hmode. 11 More recently, the high frequency coherent modes (CMs) observed in DIII-D 18 show an ability to saturate the pedestal pressure and have characteristics of kinetic ballooning mode (KBM). 19,20 Using the beam emission spectroscopy and the correlation reflectometry on NSTX, an ion-scale micro-turbulence in ion temperature gradient (ITG) and/or KBMs has been measured.…”