The effect of plasma shape and neutral beam mix on the rotation threshold for RMP-ELM suppression

Abstract

Varying the neutral beam injection (NBI) mix reveals a clear pedestal-top rotation threshold for edge localized mode (ELM) suppression by resonant magnetic perturbations. Guided by expectations for the RMP penetration mechanism, the rotation threshold is found to correspond to a critical radius for the ExB rotation zero-crossing. No such critical radius is observed for the electron perpendicular rotation zero-crossing. Varying the amount and ratio of power in different NBI source geometries (termed the NBI mix) also reveals that the rotation threshold can be crossed at widely varying total injected NBI torques. Computing the local torque density at the edge, the rotation threshold is found to be crossed when the local edge NBI torque is negative in nearly all discharges. Reducing the upper triangularity from the ITERsimilar value of 0.3 to 0.1 significantly reduces the pedestal height and width. This in turn: 1) Increases the rotation threshold and yields a more outward critical ExB rotation zero-crossing location. 2) Decreases the density threshold, consistent with a comparable collisionality range at lower pedestal temperatures. 3) Increases the input torque requirement, due to observed lower confinement and smaller intrinsic torque. These findings represent an important step along the road to predicting ELM suppression access conditions in future tokamaks such as ITER, where the toroidal rotation is expected to be low and consequently the rotation zero-crossing far from the pedestal-top.

The density dependence of edge-localized-mode suppression and pump-out by resonant magnetic perturbations in the DIII-D tokamak

“…The threshold exceeds the available RMP amplitude for n e,ped >2.5×10 19 m -3 (black dashed line Fig. 5d), consistent with the ubiquitous density threshold for ELM suppression in DIII-D. 14,15 The three high density discharges with B r /B r,th < 1 are not ELM suppressed and have no observable field penetration event at the pedestal top. The increasing penetration threshold δB r,th /B T with increasing density is due to the increasing inertia of the plasma when the flow frequency is constant, requiring higher RMP levels that eventually exceed the experimentally available RMP amplitude for penetration.…”

“…One characteristic of ELM suppression in DIII-D low collisionality (ν * e <0.5) ITER-similar-shape (ISS) plasmas is the low pedestal density n e,ped ≈ 2-3×10 19 m -3 required for access to ELM suppression. 14,15 This requirement at low collisionality raises the question whether ELM suppression can be achieved for the much higher densities expected in ITER. A further characteristic of ISS plasmas is that RMPs can produce a strong density reduction (called pump-out) 6,16 that rapidly diminishes as the density increases.…”

Theory of edge localized mode suppression by static resonant magnetic perturbations in the DIII-D tokamak

“…It turns out that, for the case illustrated in Figure 3, −n ω E and the nonlinear natural phase velocity, 0 , take very similar values when −q 95 4.55. Hence, the observations reported in [17] are more consistent with the nonlinear mode penetration model presented in this paper than mode penetration models that employ linear layer physics (which, inevitably, conclude that mode penetration is correlated with ⊥ e passing through zero). Figure 4 illustrates what happens to the calculation shown in Figure 3 if we take the experimental resistivity profile and scale it up by a constant factor.…”

“…The application of a static RMP, resonant in the pedestal region, to an H-mode tokamak discharge is observed to give rise to two distinct phenomena [13,14,15,16,17]. The first of these is the so-called density pump-out, which is characterized by a reduction in the electron number density in the pedestal region that varies smoothly with the amplitude of the applied RMP, is accompanied by a similar, but significantly smaller, reduction in the electron and ion temperatures, but is not associated with ELM suppression.…”

Filamentary plasma eruptions and their control on the route to fusion energy

“…However, the production of such a well-localized island places stringent constraints on the flow profiles of the plasma that would produce very strict access conditions. Recent work has shown that such flow profiles are not a necessary constraint for ELM suppression 74,79 . These studies suggest that 3D plasma distortions may lead to turbulent transport effects playing a role.…”