Rotational shear effects on edge harmonic oscillations in DIII-D quiescent H-mode discharges

Abstract

In the quiescent H-mode (QH-mode) regime, edge harmonic oscillations (EHO) play an important role in avoiding transient edge localized mode (ELM) power fluxes by providing benign and continuous edge particle transport. A detailed theoretical, experimental and modeling comparison has been made of low-n (n≤5) EHO in DIII-D QH-mode plasmas. The calculated linear eigenmode structure from the extended MHD code M3D-C1 matches closely the coherent EHO properties from external magnetics data and internal measurements using the ECE, BES, ECE-Imaging and microwave imaging reflectometer (MIR) diagnostics, as well as the kink/peeling mode properties found by the ideal MHD code ELITE. Numerical investigations indicate that the low-n EHO-like solutions from M3D-C1 are destabilized by the rotation and/or rotational shear while high-n modes are stabilized. This effect is independent of the rotation direction, suggesting that EHO can be destabilized in principle with rotation in either direction. The modeling results are consistent with observations of the EHO, support the proposed theory of the EHO as a low-n kink/peeling mode destabilized by edge ExB rotational shear, and improve our understanding and confidence in creating and sustaining QHmode in present and future devices.

Analytic stability criteria for edge MHD oscillations in high performance ELM free tokamak regimes

“…Indeed intriguing similarities between such perturbations and EHOs are found: the structure of the radial eigenfunction (cf. (13)) resembles closely experimental measurements [5], the rotation frequency of the mode scales as nΩ * (with Ω * the value of toroidal rotation at the pedestal top) with the growth rate (and presumably the saturated non-linear state) being independent of the sign of Ω * . A further extension of this work will be to include X-point effects but this is beyond the scope of the present paper.…”

“…Since experimental evidence (see e.g. [1,5]) and nonlinear simulations [15] dictate that low-n (especially n = 1) modes are dominant, we concentrate on the linear stability properties, and marginal stability conditions, for low-n ideal modes. Critical to the work is the inclusion of low magnetic shear and steep pressure gradients in the edge and the extension of magnetic perturbations across a vacuum region to a conducting wall.…”

E × Bflow shear drive of the linear low-nmodes of EHO in the QH-mode regime

“…A linear eigenmode analysis of the EHO-like low-n modes using M3D-C1 indicated that these modes were destabilized by rotation and/or rotation shear, independently of the rotation direction [16]. This is consistent with the earlier observations of EHOs [8] and supports the view that the EHOs are saturated low-n kink/peeling modes destabilized by edge E × B flow shear [4,9].…”

“…Destabilization of the low-n modes was seen in recent MHD simulation using M3D-C1 [16] and BOUT++ [17] codes. A linear eigenmode analysis of the EHO-like low-n modes using M3D-C1 indicated that these modes were destabilized by rotation and/or rotation shear, independently of the rotation direction [16].…”

“…These in turn enhance the kink drive as the parallel wavenumber increases significantly away from the rational surface where the magnetic shear is also strong. The pedestal fluctuation measurements of electron density and temperature in DIII-D [16] indicate that the EHOs exhibit rather broad radial structure, covering the whole pedestal region or even extending inward beyond the pedestal. The predicted radial expansion of the mode structure is consistent with this observation.…”