Session BI2: Invited MF: Resonant Magnetic Perturbation Effects

Optimization of ELM control coil configuration for various ITER scenarios

Large bursts of edge localized modes (ELMs) can severely damage plasma facing components in ITER. Three-dimensional resonant magnetic perturbation (RMP) has been proposed as a promising technique to mitigate or suppress Type-I ELMs in ITER. Plasma response to the applied vacuum RMP field has been shown to play a significant role in understanding and predicting the ELM control behavior. In this work, plasma response is computed for five plasma scenarios, covering the pre-nuclear and nuclear stages of ITER exploration. The plasma response, computed by MARS-F based on a resistive full magneto-hydrodynamic model in toroidal geometry, is used to construct both linear and quasi-linear figures of merit, such as the plasma displacement near the X-point of the plasma separatrix and the toroidal torque distributions along the plasma minor radius. These figures of merit are then utilized to optimize the RMP coil configuration, such as the coil phasing, for the purpose of (i) achieving the best ELM control in various ITER scenarios at the given coil current level, and (ii) avoiding plasma core flow damping by RMP whilst allowing sufficient field penetration through the plasma pedestal region. The optimal coil configurations between the two high-Q deuterium-tritium (DT) scenarios (at the same plasma current of 15 MA and magnetic field of 5.3 T but different fusion gains, Q$=$5 and 10) are predicted to be similar. Such optima also hold for other ITER scenarios with similar edge safety factor q95\textasciitilde 3, but are substantially different for the half-current full-field (7.5 MA/5.3 T) scenario. Toroidal torque density optimization, including the Maxwell and Reynolds stresses as well as the neoclassical toroidal viscosity, reveals a generally strong coupling of torque contributions between the plasma core and edge. The best decoupling is achieved by emphasing the role of the two off-middle rows of ELM control coils and de-emphasing the role of the middle row coils in ITER. Initial value MARS-Q quasi-linear simulations are also performed to investigate the effect of RMP fields on the plasma momentum and particle transport.   

First-time analysis of detached divertor conditions in RMP ELM suppressed H-mode plasmas in ITER

The ITER divertor has been designed for axisymmetric configurations, yet symmetry breaking resonant magnetic perturbations (RMPs) will be applied for control of edge localized modes (ELMs). Recently, the numerical capability to investigate the predicted detached divertor scenario at ITER with such 3-D deformations has been made available after stabilization of the iterative framework [H. Frerichs et al., NME 18 (2019) 62] of the 3-D edge plasma and neutral gas code EMC3-EIRENE. We have applied an n=3 RMP field to the baseline discharge for the pre-fusion power operation phase and included the plasma response from calculations with the single fluid resistive MHD code MARS-F. The divertor state is found to be sensitive to the toroidal flow impact on the plasma response. Even though screening of most of the resonances leads to a narrower region of broken flux surfaces, radial extension of the divertor footprint occurs due to field amplification near the separatrix. Detachment transition with RMP occurs at a lower gas puff rate and lower peak particle flux at the original strike zone, consistent with a lower upstream heat flux that it connects to along the 3-D scrape-off layer. However, a secondary non-axisymmetric strike location exists radially further outward, which remains attached because of a magnetic connection to higher upstream temperatures further inside the bulk plasma, carrying significant heat fluxes to this previously low flux domain in the divertor. This strongly reduces the potential for complete power dissipation, and it is a new feature in the ITER divertor revealing a challenge for the integration of RMP ELM control with a feasible divertor operation regime.   

Effect of resonant magnetic perturbations on turbulence-flow dynamics at the L-H transition on DIII-D

Applied n=3 resonant magnetic perturbations (RMPs) reduce the Reynolds stress drive for mean shear flow in the edge of L-mode plasmas, likely contributing to the RMP-induced increase in L-H power threshold. This L-H power threshold increase is a concern for ITER since RMPs must be applied before the L-H transition to ensure ELM suppression. To understand the cause of this effect, density and turbulence velocity fluctuations spanning $\rho$=0.85--1.05 are measured by the beam emission spectroscopy (BES) and Doppler backscattering (DBS) diagnostics in ITER-shaped, low-rotation, lower single null (favorable $\nabla B$ direction) DIII-D plasmas. Density fluctuation amplitudes measured by BES in the L-mode edge region, $\rho$=0.93--1.00, are amplified by 20\% by RMPs. In contrast, non-resonant magnetic perturbations, which raise the L-H power threshold only slightly, reduce density fluctuation amplitudes by 10\%. RMPs reduce the L-mode $E_r$ well depth, correspondingly reducing velocity shear, and raise the turbulence decorrelation rate. Thus, suppression of turbulence by shear flow, parametrized as $\omega_{Shear}/\omega_{Decorr}$, is significantly diminished by RMPs. In addition, radial and poloidal velocity fluctuation amplitudes, estimated by velocimetry analysis of 2D BES data, are reduced by RMPs, leading to a reduction of inferred Reynolds stress $\left\langle \widetilde{v}_r \widetilde{v}_\theta \right\rangle$. A simple poloidal flow calculation including terms for Reynolds stress drive and neoclassical damping shows that the resulting reduction of Reynolds stress drive for mean flow, $-\partial_r \left\langle \widetilde{v}_r \widetilde{v}_\theta \right\rangle$, accounts for a significant fraction of the L-mode $E_r$ well depth reduction with RMPs.   

Understanding the RMP-Driven Transport in Tokamak Edge Plasma from Gyrokinetic-MHD Coupled Simulation in Realistic Divertor Geometry

ITER plans to rely on RMP coils as the primary means for ELM control. However, puzzling observations on present-day experiments complicate the physics understanding: plasma density is pumped out, which can lower the fusion efficiency in ITER, while the electron heat transport is still low in the pedestal. Kinetic level understanding including most of the important physics is needed but has not been available. Gyrokinetic total-f simulation of the plasma transport driven by n$=$3 resonant magnetic perturbations (RMPs) in a DIII-D H-mode plasma is performed using the gyrokinetic code XGC. The RMP field is calculated in M3D-C1 and coupled into XGC in realistic divertor geometry with neutral particle recycling. The RMP field is stochastic around the pedestal foot but exhibits good KAM surfaces at pedestal top and steep-slope. The simulation qualitatively reproduces the experimental phenomena: plasma density is pumped-out due to enhanced electrostatic turbulence while electron heat transport is low. Different from earlier gyrokinetic studies, the present simulation consistently combines neoclassical and turbulent transport, a fully nonlinear Fokker-Planck collision, neutral particle recycling, and the full 3-D electric field. Density pump-out is not seen without turbulence effects. Around the pedestal foot the heat transport rate is found to be much lower than the Rechester-Rosenbluth rate. Reduction of the ExB shearing rate is identified to be responsible, mostly, for the enhanced edge turbulence, which is found to be from trapped electron modes. Comparison with experimental results will also be presented.   

Demonstration and analysis of helium exhaust enhancement during resonant magnetic perturbation field application at DIII-D

It is shown for the first time in a diverted tokamak that key metrics for continuous fusion, effective helium particle confinement time $\tau _{\mathrm{p}}$*$_{\mathrm{,He}}$, and its ratio with energy confinement, $\tau_{\mathrm{p}}$*$_{\mathrm{,He}}$/$\tau_{\mathrm{E}}$, were reduced by more than a factor of two during suppression of edge localized modes (ELMs) by resonant magnetic field perturbations (RMPs) over the unperturbed ELMy case. Reduced $\tau_{\mathrm{p}}$*$_{\mathrm{,He}}$ during RMPs was observed in the core, edge, and pump plenum, where higher helium pressure and concentration were also found. Elevated ionized helium line emission during ELM suppression, with a decay time matching $\tau _{\mathrm{p}}$*$_{\mathrm{,He}}$, was measured at the inner strike point, suggesting more helium in the unconfined region. These observations suggest that processes during application of RMPs better retain helium at the edge, in excess of that expected from deuterium `pumpout,' where it is pumped more quickly. A multi-reservoir model, derived from a finite volume approximation of the continuity equation with diffusive and convective transport, fits the helium time evolution at each of the above-mentioned measurements well. Transport parameters obtained from these fits indicate the data are consistent with strongly increased transport of helium in the separatrix region during RMP application, although higher core transport is also found. EMC3-EIRENE modeling suggests magnetic field stochastization near the separatrix reduces the upstream thermal force relative to the friction force, enhancing outward transport in the edge. These findings, which were obtained in ITER-shaped plasmas, provide evidence that application of RMP ELM suppression in future devices such as ITER may be capable of matching or exceeding the helium impurity exhaust produced by the ELM events themselves, thereby helping to avoid helium ash buildup in a burning regime, and maintain high fusion gain.   

Gradually Increased Turbulent Fluctuations and Rapidly Changing $E \times B$ Flow at the Onset of RMP-Driven ELM-Crash Suppression

Dynamic influence of static resonant magnetic perturbation (RMP) on turbulent fluctuations and $E \times B$ flow ($v_{E \times B}$) has been extensively studied using the electron cyclotron emission imaging (ECEI) system in the KSTAR. In this study, we have directly measured the turbulent fluctuations and $v_{E \times B}$ near the pedestal at the transition of the RMP-driven ELM-crash suppression. Detailed analysis is in remarkable agreement with the previous studies that the RMP gradually enhanced small-scale turbulent fluctuations near the pedestal, while preventing a large collapse of pedestal due to a nonlinear interaction with coexisting ELM filaments [1]. Recently, direct evidence of rapidly changing $v_{E \times B}$ bifurcation has been measured at the onset of RMP-driven ELM-crash suppression by tracking the high-speed motion of turbulent eddies [2]. Specifically, we have identified a rapid change of perpendicular electron flow, attributable to $v_{E \times B}$ close to zero, nearly synchronous to the sudden disappearance of ELM bursts. Since the local poloidal shear flow was also reduced, this might have increased turbulent fluctuations. Interestingly, the $v_{E \times B}$ remains the smallest near edge pedestal at $\psi_N \sim 0.95$ during the ELM-crash suppression, possibly suggesting a strong plasma response associated with fully penetrated resonant field necessary for ELM-crash suppression. It is worthwhile to note that the RMP field strengths at the entrance and exit of the ELM-crash suppression are vastly different, showing a hysteresis of RMP influence on ELM-crash-suppression. Such turbulent fluctuations and $v_{E \times B}$ changes are expected to help us clarify the underlying physics mechanisms of RMP-driven ELM-crash suppression and elaborate the onset conditions of critical transition, in particular, in an ITER-like low torque plasma. [1] J. Lee et al, Phys. Rev. Lett. 117, 075001 (2016). [2] J. Lee et al, Nucl. Fusion 59, 066033 (2019).