Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session UI01: MFE V: Pedestal and Edge-Localized ModesInvited Live
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Chair: Federico Halpern, General Atomics Room: Ballroom B |
Thursday, November 11, 2021 2:00PM - 2:30PM |
UI01.00001: Using the Super H-Mode as a Platform for Integrated Core-Edge Studies Invited Speaker: Theresa M Wilks DIII-D experiments assess compatibility of Super H-mode (SH) pedestals with up to 100% of injected power radiated dominantly in the divertor region using advanced feedback algorithms for density and radiated power control with impurity seeding. A current limited pedestal allows access to high density and low collisionality, providing high pedestal and separatrix density that produce a high pressure core plasma with a cold, dense divertor plasma. The SH regime maximizes physics parameters likely in future devices such as pedestal pressure, collisionality and separatrix density, providing a platform to study the impacts of a high-performance core on divertor conditions in present devices. Three optimization avenues for core edge integration have been developed: 1) Attached SH plasmas with high performance cores and pedestals (stored energy > 2MJ, pedestal normalized beta >1), but modestly cooled divertors (Te~15eV) represent a core-focused solution; 2) Nitrogen seeded partially detached plasmas with ~25% degradation to core performance represent a divertor-focused solution; 3) High recycling divertor at the onset of detachment with modest penalties (<5%) to core performance, 80% radiated power fraction, 30% reduced heat flux, and divertor temperatures ~5eV represent a path towards a balanced solution. At detachment onset, there is still plasma present at the strike point and in the scrape off layer, allowing most of the power to be radiated outside of the separatrix, maximizing confinement on closed flux surfaces and providing a target plasma for testing high heat flux scenarios on various divertor configurations. EPED predictions are consistent with experimental pedestal stability, and prospects for access to a stationary balanced core-edge solution are proposed for future experiments. |
Thursday, November 11, 2021 2:30PM - 3:00PM |
UI01.00002: Reduced predictive models for Micro-tearing modes in the pedestal Invited Speaker: Max Curie This talk will present a global reduced model for microtearing modes (MTM) in the H-mode pedestal, which reproduces distinctive features of experimentally observed magnetic fluctuations, such as chirping and discrete frequency bands with definite mode numbers. Our model, importantly, includes the global variation of the q profile and diamagnetic frequencies. In contrast, the local flux tube approach fails to reproduce these distinctive features. Our model includes a recently recognized feature for MTM stability; the MTM is enabled by the alignment of a rational surface with the peak in the profile of the diamagnetic frequency. Conversely, MTMs are strongly stabilized for toroidal mode numbers for which these quantities are misaligned. This feature explains the discrete fluctuation bands in several DIII-D and JET discharges. Gyrokinetic global simulations of those discharges using the GENE code have also demonstrated these concepts. In this talk, we present the application of the model to many representative experimental pedestal conditions from DIII-D, by comparing the experimental spectrogram with the global reduced model and GENE simulations. The experimentally observed frequency bands overlap with the predicted frequencies of the global reduced model. The growth rate calculated by such a reduced model has a qualitative agreement with GENE global linear simulations. We also characterize the parameter regimes and scenarios in which the MTM is expected to be active. Based on recent combined experimental, theoretical, and simulation studies, it is well established that MTMs are one of the prime candidates for electron heat transport in the pedestal. A fast yet accurate reduced model for MTMs enables rapid interpretation of magnetic fluctuation data from a wide range of experimental conditions to help assess the role of MTM in the pedestal. |
Thursday, November 11, 2021 3:00PM - 3:30PM |
UI01.00003: The pedestal density structure with reduced neutral fueling Invited Speaker: Saskia Mordijck We find that the H-mode pedestal density structure remains steep, mostly independent of the edge ionization source rate of the plasma edge in DIII-D and Alcator C-Mod high confinement plasmas, thus suggesting that a pinch is present to maintain the steep density profile. We were able to change the edge opaqueness from 1/10 to 1/2 the expected value on ITER. To calculate the magnitude of the pinch, we first confirm a reduction of neutral density by 50% in DIII-D at the separatrix and 20% in the pedestal structure using midplane filterscope measurements. The reduction of the neutral density in the pedestal with increasing opacity close to the separatrix is further confirmed by SOLPS-ITER modeling in both DIII-D and C-Mod experiments. The neutral density drops from ~1016 m-3 in DIII-D to ~1015 m-3 in C-Mod at the midplane just inside the separatrix, while the electron density rose from ~4x1019 m-3 in DIII-D to 4x1020 m-3 in C-Mod. Moreover, we find good agreement between SOLPS-ITER modeling and Lyman-alpha data of the neutral density profile in C-Mod L- and H-mode plasmas. As the steepness of the density gradient is unaffected by changes in neutral fueling, but the gradient is affected by changes in heating and plasma current, this suggests plasma transport and an inward pinch play a role. Using a perturbative Helium puff as well as a modulated Deuterium gas puff technique in DIII-D H-mode plasmas, we will investigate both ion as well as the electron averaged transport in the pedestal. Based on the normalized density gradient in the pedestal, without the presence of a source, the particle pinch should be an order of magnitude larger than the diffusion coefficient (D~0.01-0.1 m2/s), resulting in a v~-0.1-1 m/s. These results suggest that predictions of weak density gradients in future machines might not be realized. |
Thursday, November 11, 2021 3:30PM - 4:00PM |
UI01.00004: Simulation of pellet ELM triggering in low-collisionality, ITER-like discharges Invited Speaker: Andreas Wingen Unmitigated heat loads from edge localized modes (ELMs) are intolerable in ITER. Therefore, ITER operation relies on multiple approaches to control ELM heat fluxes. One is pellet ELM pacing. Predicting the performance of it is critical in ITER, which is expected to operate in a regime with low-collisionality, peeling-limited pedestals. The hypothesis for pellet ELM triggering is that the local pressure increase in the expanding pellet cloud pushes the equilibrium over the ballooning pedestal stability limit. This suggests that the distance of the equilibrium's operational point from the ballooning branch of the pedestal stability boundary could play a critical role. M3D-C1 simulations of DIII-D low-collisionality discharges are used to determine the pellet size threshold for ELM triggering using linear simulations. 3D nonlinear M3D-C1 simulations, which are more realistic but computationally expensive, confirm that a pellet larger than the threshold size successfully triggers an ELM. The pellet size threshold is determined for multiple times during a DIII-D discharge and compared against ELM triggering events in the experiment. In all cases the simulated threshold separates events of successfully triggered ELMs from failed ones. Further linear simulations confirm that growth rates are significantly larger when an equilibrium operates closer to the ballooning branch compared to the original equilibrium for the same pellet size. However, the same pellet fails to trigger an ELM for an equilibrium that operates further away from the ballooning branch. This result suggests that pellet ELM triggering in ITER could require large pellets, which makes ELM pacing mass flow rates challenging for divertor operation. |
Thursday, November 11, 2021 4:00PM - 4:30PM |
UI01.00005: Optimizing edge confinement and stability via adaptive ELM control using RMPs Invited Speaker: SangKyeun Kim We report an adaptive real-time approach to control edge-localized modes with resonant magnetic perturbations (RMP) in a way that optimizes both the pedestal stability and confinement by exploiting the hysteresis in RMP ELM suppression [1]. Such adaptive control is essential to maximize the plasma performance while also maintaining an ELM-stable edge. We recover up to 60% of the original confinement degradation and 45% of the fusion gain factor (G=H98βN /q952) by using our method. The adaptive ELM control uses a Dα-based ELM detector and adjusts the RMP accordingly. It iteratively increases and decreases the RMP amplitude during ELMy and ELM free phase, respectively, until it converges to a stable operating point that optimizes both ELM-free and confinement. Here, we find that RMP-induced ion-scale turbulence during ELM free phase widens the ion pedestal and improves pedestal stability [2], allowing a higher pedestal pressure and amplified field penetration. Nonlinear MHD simulations based on JOREK [3] and TM1 [4] show that collisional transport due to magnetic island is insufficient to explain the ion-pedestal behavior, supporting the idea of its broadening by edge turbulence. In general, the adaptive control is not trivial because the threshold characteristics of the bifurcation in and out of ELM suppression can lead to control system oscillations. Our approach has overcome these limitations by outcomes of turbulence, which lowers the RMP threshold for recovering ELM free operation and weakens the control oscillation. Such a favorable effect is difficult to be harnessed as turbulence quickly disappears with ELMs. However, the adaptive method fully exploits it by re-increasing RMP immediately after the reduction of turbulence and before the ELMs return, which is key to control convergence.[1]F. Laggner et al.,NF60,076004 [2]T. Osborne et al., NF55,063018 [3]G. Huysmans et al.,PPCF51,124012 [4]Q. Yu et al.,NF51,073030 |
Thursday, November 11, 2021 4:30PM - 5:00PM |
UI01.00006: New High-Speed Measurements of Pedestal Magnetic Field during the ELM cycle on DIII-D and Implications for Modeling Invited Speaker: Marcus G Burke New high speed pedestal localized measurements of the plasma magnetic field during the ELM cycle of a low input power DIII-D H-mode discharge imply a temporally and spatial complex redistribution of the edge current density profile (j[r]) and a magnitude that is inconsistent with theory. A novel spatial heterodyne spectroscopy technique extracts |B| (specifically the magnitude of the magnetic field perpendicular to the neutral beam velocity) from the Stark-split neutral beam radiation and enables these high time resolution (200 µs) measurements. Measurements across the pedestal, from normalized poloidal flux Ψ~0.9 to 1.0, have been made in repeated discharges. At Ψ=0.975±0.016, |B| appears saturated in the inter-ELM period and then rapidly decreases (in 200 µs) by ~1% at the crash. |B| then gradually recovers to the inter-ELM nominal value over ~10 ms. Inboard of the pedestal, |B| increases slightly after the ELM crash. This behavior is consistent with a rapid collapse of j[r] at the ELM crash and subsequent pedestal recovery. In some discharges, at Ψ>0.96, changes in |B| are observed approximately 5-10 ms before the ELM crash. Measurements of |B| during the H-mode transition show a large increase at Ψ =1 with little change at Ψ =0.9, consistent with the formation of the edge bootstrap current density peak. The measured change in |B| from Ψ =0.9 to 1 is ~2x larger than a predictive model of the edge current density based on a kinetic equilibrium reconstruction and Sauter bootstrap current model. While the increase is consistent, the discrepancy in the magnitude of the change across the pedestal region could have implications for pedestal stability, the ELM drive, and models of the edge current density. |
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