Toroidal modelling of RMP response in ASDEX Upgrade: coil phase scan, q₉₅ dependence, and toroidal torques

Abstract

. A systematic investigation is carried out, considering various plasma and coil configurations as in the ELM control experiments. The low q plasmas, with q 95 ∼ 3.8 ( q 95 is the safety factor q value at 95% of the equilibrium poloidal flux), responding to low n (n is the toroidal mode number) field perturbations from each single row of the ELM coils, generates a core kink amplification effect. Combining two rows, with different toroidal phasing, thus leads to either cancellation or reinforcement of the core kink response, which in turn determines the poloidal location of the peak plasma surface displacement. The core kink response is typically weak for the n = 4 coil configuration at low q, or for the n = 2 configuration but at high q (q 95 ∼ 5.5). A phase shift of around 60 degrees for low q plasmas, and around 90 degrees for high q plasmas, is found in the coil phasing, between the plasma response field and the vacuum RMP field, that maximizes the edge resonant field component. This leads to an optimal coil phasing of about 100 (-100) degrees for low (high) q plasmas, that maximizes both the edge resonant field component and the plasma surface displacement near the X-point of the separatrix. A strong parallel sound wave damping moderately reduces the core kink response but has minor effect on the edge peeling response. For low q plasmas, modelling shows that both the resonant electromagnetic torque and the neoclassical toroidal viscous (NTV) torque (due to the presence of 3D magnetic field perturbations) contribute to the toroidal flow damping, in particular near the plasma edge region. For high q plasmas, however, significant amount of torque is also produced in the bulk plasma region, and the contributions from the electromagnetic, the NTV, and the torque associated with the Reynolds stress, all play significant roles.

ELM control with RMP: plasma response models and the role of edge peeling response

“…[8] is a clear correlation between the fluid model predicted edge peeling response on one hand, and the experimentally observed large density pump out in both L-and H-mode plasmas in MAST and the ELM mitigation in H-mode plasmas. Since then, more modelling work has been performed on various devices [9,10,11,12,13,14,15], further confirming the basic physics that has been found in Ref. [8].…”

Three dimensional boundary displacement due to stable ideal kink modes excited by external n = 2 magnetic perturbations

“…Note, the LFS and the HFS responses of this plasma configuration have a very similar dependence on ∆ϕ UL , which is indicated in figure 7(c) and the following sections. This is a feature of the investigated plasma configuration and does not hold generally as suggested by calculations based on other ASDEX Upgrade configurations [50] and other machines [6]. One should also keep in mind that the displacement in the static experiments are expected to be roughly two times larger than the one in the rigid rotation experiments.…”

“…This change is marginal suggesting a marginal impact on the plasma response [49]. The comparison between MARS-F calculations [50] and the axisymmetric plasma response (e.g. n e , ELM frequency) is shown in figure 7 in Ref.…”

Plasma response measurements of external magnetic perturbations using electron cyclotron emission and comparisons to 3D ideal MHD equilibrium

“…11 in Ref. [20]). It is clearly seen in Figure 3 that core densities and temperatures do not change significantly during the rigid rotation (less than 6%).…”