Sherwood 2015

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Edge harmonic oscillation studies with the NIMROD code

Author: Scott E Kruger
Requested Type: Poster Only
Submitted: 2015-01-19 16:45:57

Co-authors: J.R.King, A.M.Garofalo, P.B.Snyder

Contact Info:
Tech-X Corporation
5621 Arapahoe Avenue
Boulder, CO   80303
United States

Abstract Text:
It is desirable to have an ITER H-mode regime that is quiescent to edge-localized modes (ELMs). ELMs deposit large, localized, impulsive, surface heat loads that can damage the divertor. One such quiescent regime with edge harmonic oscillations (EHO) is observed on DIII-D, JET, JT-60U, and ASDEX-U [1]. These ELM-free discharges have the edge-plasma confinement necessary for burning-plasma operation on ITER. The EHO is characterized by small toroidal-mode numbers (n≈1-5); measurements from beam-emission spectroscopy, electron-cyclotron emission, and magnetic probe diagnostics show highly coherent density, temperature and magnetic oscillations. These measurements show that the EHO is a saturated macroscopic mode with perturbations peaking at the magnetic separatrix. The particle transport is enhanced compared to discharges without EHO, leading to essentially steady-state profiles in the pedestal region. Finally, the operation regime of the QH-mode is dependent on the rotation profile, and QH-mode discharges are produced with an applied torque through either co- or counter-neutral-beam injection and/or neoclassical toroidal viscosity from plasma interaction with non-resonant magnetic fields [1]. We investigate a reconstruction from DIII-D shot 14098 during a period of broadband-wavenumber EHO and compare our results to the known phenomenology.

Our preliminary EHO results are tantalizing. The linear growth rates as computed with a resistive-MHD model have a ballooning-like toroidal-mode spectrum that does not explain the low wavenumber phenomena of EHO. Given the importance of the flow profile to the experimental observations, we add the reconstructed flows to our computations. With flows, there is full stabilization at intermediate-n and destabilization at high-n. Independent of the flow effects, when drift effects are included through two-fluid, two-temperature modeling with ion gyroviscosity there is a a stabilizing effect on the high-n modes consistent with analytic theory [2]. Current modeling is focuses on computations with both flow and drift effects where a low-n mode may be most unstable.

This work is currently supported by the US DOE Office of Science and the SciDAC Center for Extended MHD Modeling.

[1] Burrell et al., PoP (2012); Garofalo et al, NF (2011) and refs. within.
[2] Hastie et al., PoP (2003).


March 16-18, 2015
The Courant Institute, New York University