Effect of island overlap on edge localized mode suppression by resonant magnetic perturbations in DIII-D

Abstract

Recent DIII-D [J. L. Luxon et al., Nucl. Fusion 43, 1813 (2003)] experiments show a correlation between the extent of overlap of magnetic islands induced in the edge plasma by perturbation coils and complete suppression of Type-I edge localized modes (ELMs) in plasmas with ITER-like electron pedestal collisionality νe*∼0.1, flux surface shape and low edge safety factor (q95≈3.6). With fixed amplitude n=3 resonant magnetic perturbation (RMP), ELM suppression is obtained only in a finite window in the edge safety factor (q95) consistent with maximizing the resonant component of the applied helical field. ELM suppression is obtained over an increasing range of q95 by either increasing the n=3 RMP strength, or by adding n=1 perturbations to “fill in” gaps between islands across the edge plasma. The suppression of Type-I ELMs correlates with a minimum width of the edge region having magnetic islands with Chirikov parameter >1.0, based on vacuum calculations of RMP mode components excluding the plasma response or rotational shielding. The fraction of vacuum magnetic field lines that are lost from the plasma, with connection length to the divertor targets comparable to an electron-ion collisional mean free path, increases throughout the island overlap region in the ELM suppressed case compared with the ELMing case.

Plasma transport in stochastic magnetic field caused by vacuum resonant magnetic perturbations at diverted tokamak edge

“…Experiments also observe that a significantly enhanced particle transport deep in the plasma, either by unscreened magnetic field stochasticity or by RMP-induced turbulenct transport, is a necessary condition for a successful widening of the steep edge ∇p region [9]. Figure 1 displays the plasma profiles for DIII-D discharge 126006 versus the poloidal magnetic flux ψ N , a radial coordinate normalized to be 0 at the magnetic axis and 1 at the magnetic separatrix.…”

ELM control strategies and tools: status and potential for ITER

“…An empirical criterion was proposed for the suppression of type-I ELMs at low collisonalities at high triangularities based on the DIII-D experiments and vacuum modelling (i.e. without plasma response) which correlated the disappearance of the type-I ELMs with the width of the edge ergodised region, as characterised by the Chirikov parameter being larger than one in the region of normalised flux  N ≥ 0.835 [63]. Sound demonstration of type-I ELM suppression in a plasma with ITER-like plasma shape for the Q=10 baseline scenario was demonstrated in this way, as shown in figure 4.…”

“…The maximum current capability of the ITER ELM control coil system is 90 kAt and is determined on the basis of the criteria for minimum island overlap that is associated with ELM suppression in DIII-D applied to the 15 MA Q DT = 10 scenario in ITER [63] with an additional 20% margin in the maximum coil current to account for uncertainties. The values of the magnitude of the perturbation field, the magnitude of the various harmonics obtained in the plasma for a current distribution in the coils with n = 4 symmetry and the resulting Chirikov parameter across the plasma cross section are shown in Figure 7 [10].…”

Resonant Pedestal Pressure Reduction Induced by a Thermal Transport Enhancement due to Stochastic Magnetic Boundary Layers in High Temperature Plasmas

“…It is shown that the alignment of the internal field line pitch angle with the external field dominates for considerably high perturbation amplitudes the plasma response in ISS plasmas at high triangularity in DIII-D. The first hint for a pitch resonance is the robust observation that for a given plasma scenario and for fixed RMP spectral properties, ELM suppression at DIII-D depends strongly on the value of the edge safety factor q 95 [6,16]. As the typical RMP spectrum used at DIII-D has dominant poloidal (m) and toroidal (n) mode numbers of 8 < m < 14 and n ¼ 3, ELM suppression is observed for ISS plasmas at low à e in a narrow resonant window with a width of Áq 95 ¼ 0:1-0:48 around q 95 ¼ 3:55 [17].…”

“…This new finding was confirmed for comparable discharges at fixed q 95 ¼ 3:5, i.e., the most reliable ELM suppression working point. As the actual values of T e , n e , and p e depend strongly on wall condition [19], collisionality [16], and plasma shape [7], the results are taken from discharge FIG. 1 (color).…”