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# 27th IAEA Fusion Energy Conference - IAEA CN-258

22-27 October 2018
Mahatma Mandir Conference Centre
Asia/Kolkata timezone
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## Access Requirements for Stationary ELM-suppressed Pedestals in DIII-D and C-Mod Plasmas

25 Oct 2018, 14:00
4h 45m
Mahatma Mandir Conference Centre

Poster

### Speaker

Dr Theresa Wilks (UsMIT)

### Description

Analysis of pedestal characteristics for Quiescent H-mode (QH) and I-mode plasmas from recent experiments on DIII-D and C-Mod exhibit a growing understanding of the access requirements necessary to obtain edge fluctuations or MHD that drive edge particle transport needed to remain ELM-free. In DIII-D QH-mode plasmas, critical values of ExB shear are required in experiment in order to suppress the transition from QH-mode to ELMy H-mode. The experimental shear values for QH-modes ranging between $q_{95}$~3-5 and $\delta$~0.36-0.68 are compared with theory [1] to show good agreement with the predicted scaling parameters of $c_s/\sqrt{L_p \Delta x}$, where $c_s$ is the ion acoustic velocity, $L_p$ the pressure gradient scale length, and $\Delta x$ is the radial width of the mode. The scaling of the critical shearing rate agrees with experiment, but the absolute magnitude of the limit is over-predicted by theory by two orders of magnitude. Through a normalized predictive scaling, the model demonstrates dynamic transitions into and out of QH-mode qualitatively within a single plasma discharge. C-Mod I-mode plasmas, which lack an edge particle barrier and exhibit characteristic edge fluctuations over a broad range of $B_\phi$ [2], meet upper limits in performance determined by H-mode access. The maximum radial electric field well in I-mode increases with magnetic field strength, suggesting the expanded window for I-mode at high field is linked to a critical value of $E_r$/B required to induce an H-mode transition. C-Mod I-mode pedestals are analyzed over varied magnetic fields (2.8-5.8T) and auxiliary power (1.5-4.6 MW) to show consistent edge fluctuation behavior. Density fluctuations associated with the Weakly Coherent Mode are observed to span the pedestal region, extending out to the separatrix, while the fluctuation associated with the Geodesic Acoustic Mode is observed on the profile reflectometer near the foot of the T_e pedestal. Trends in the edge $E_r$, ExB shear, and rotation in I-mode show little correlation with the behavior of the edge fluctuations, suggesting an alternate driver for destabilization of the WCM and GAM, as compared to the QH-modes analyzed for DIII-D with an EHO.
Country or International Organization United States of America EX/P6-19

### Primary author

Dr Theresa Wilks (UsMIT)

### Co-authors

Mr A. M. Garofalo (General Atomics) Dr Amanda Hubbard (Massachusetts Institute of Technology, Plasma Science and Fusion Center) Jerry Hughes (MIT PSFC) Dr Keith H. Burrell (General Atomics) Prof. Patrick Diamond (University of California San Diego) Dr Xi Chen (General Atomics) Dr ZHIBIN GUO (NUCLEAR FUSION RESEARCH INSTITUTE, DAEJEON, KOREA)

### Presentation Materials

 Wilks_IAEA_Proceedings.pdf Wilks_summary.pptx