Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session UM10: Mini-Conference: The Integrated Tokamak Exhaust and Performance Gap IIILive Streamed
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Chair: David Hatch, UT-Austin Room: 206 CD |
Thursday, October 20, 2022 2:00PM - 2:05PM |
UM10.00001: Introductory Remarks
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Thursday, October 20, 2022 2:05PM - 2:25PM |
UM10.00002: Turbulence Broadening of Heat Flux Width in QH-Mode Plasmas and Future Relevance of Turbulence-Limited Pedestals Darin R Ernst, Alessandro Bortolon, Choongseok Chang, Xi Chen, Colin Chrystal, Shaun R Haskey, Seung Hoe Ku, Florian M. Laggner, Charles J Lasnier, Zeyu Li, Xinxing Ma, George R McKee, Terry L Rhodes, Filippo Scotti, Dinh D Truong, Huiqian Wang, Jonathan G Watkins, Zheng Yan Future devices including SPARC, ITER, and FPP will operate at low ρ* with reduced pedestal ExB shear suppression and high opacity, suggesting turbulence-limited pedestals like Wide Pedestal Quiescent H-Mode may naturally arise. Evidence for turbulence broadening of the divertor heat flux width in QH-Mode plasmas is presented. In the WPQH-Mode regime, measurements of the heat flux width λq =2.5-3.1 mm from divertor Langmuir probes are consistent with infra-red imaging and match the Eich scaling (2.7 mm). A torque increase forced a back-transition to Standard QH-Mode without coherent MHD, doubling turbulent density fluctuation levels in the pedestal and near SOL. Simultaneously, the measured heat flux width λq =5.2-5.3 mm from both diagnostics doubled relative to the unchanged Eich scaling value. Turbulence in the pedestal and near SOL was measured by Beam Emission Spectroscopy and Doppler Backscattering. Distinct ion- and electron-directed features are present in the BES spectrum. Their intensities are correlated with increased λq, while the stronger ion feature extends beyond the separatrix. Simulations using the XGC and BOUT++ codes are in progress. Finally, impurity injection reduced high WPQH-Mode SOL temperatures, reducing carbon sputtering to maintain high confinement with low carbon Zeff. |
Thursday, October 20, 2022 2:25PM - 2:45PM |
UM10.00003: Separatrix density scaling in JET with the ITER-like wall Bartosz Lomanowski, Giulio Rubino, Andrea Uccello, Mike Dunne, Nicola Vianello, Mathias Groth, Andrew G Meigs, James Simpson The electron separatrix density, ne,sep, in JET-ILW L-mode and H-mode unseeded plasmas exhibits a strong dependence on the averaged outer divertor target electron temperature, ne,sep ∝〈Te,ot〉-1/2, arising from parallel pressure balance, as well as on the ratio of the power and momentum volumetric loss factors, in line with expectation from the reversed two point model (rev-2PM). Rev-2PM ne,sep estimates are recovered to within ±25% of the measurements, with a scaling factor applied to account for use of 〈Te,ot〉, an averaged quantity, rather than flux tube resolved target values. Both the volumetric loss factor trends and recovery of ne,sep using the rev-2PM formatting are reproduced in EDGE2D-EIRENE L-mode-like and H-mode-like density scan simulations. Main chamber recycling likely plays a significant role in moderating the influence of divertor closure changes on neutral leakage from the divertor and the ne,sep trends. In the inner vertical, outer horizontal divertor configuration with significantly reduced clearance to the high-field side separatrix, the more pronounced high-field side high density region formation has no clear impact on the outer midplane ne,sep and ne,ped trends, and hence does not appear to play a strong role in fueling the outer midplane confined plasma. |
Thursday, October 20, 2022 2:45PM - 3:05PM |
UM10.00004: The case for toroidally symmetric limiters within DIII-D Shawn A Zamperini, Peter C Stangeby, E.A. Unterberg, Tyler Abrams, Jacob H Nichols, Andreas Wingen, Chris Crowe Toroidally symmetric limiters within DIII-D would symmetrize the SOL near the wall, enabling measurements taken at/near the wall to be representative of plasma conditions at every toroidal angle. This would create a significantly more diagnosable and interpretable far-SOL, analogous to how poloidal divertors create a toroidally symmetric near-SOL. This would enable DIII-D to make reliable measurements at/near the wall and to take a leading role in investigating the issue of unacceptably high levels of plasma-wall interactions in reactor-scale devices. The current DIII-D wall is protected by three discrete midplane "bumper limiters" at different toroidal angles. This creates a complicated 3D SOL wherein each measurement at the wall depends on which toroidal angle it was taken at. Field line tracing calculations show that the toroidal variation in the connection length can vary by a factor of ~3. Toroidal limiters are shown to eliminate this variation. Additionally, a 3D far-SOL does not allow 2D codes, such as SOLPS-ITER and OEDGE, to reliably simulate conditions near the wall. A 2D far-SOL would enable reliable studies of the filamentary nature of the far-SOL upon the main-wall, such as the relationship between density shoulder formation and connection length. |
Thursday, October 20, 2022 3:05PM - 3:25PM |
UM10.00005: Broadening the SOL width with small ELMs Xueqiao Xu The pulsed heat load due to large ELMs is an existential problem for future devices of ITER size and compact FPPs because that load would produce unacceptable damage to the divertor plates. On the other hand, the divertor heat flux width for inter-ELMs is too narrow as predicted by the heuristic drift-based (HD) model and an experimental (Eich) scaling due to the suppressed turbulence transport in typical H-mode plasmas. The research for small ELMs becomes important because of following desired features: (1) Good energy confinement; (2) Comparable with inter-ELM level of divertor heat flux; (3) Broadening divertor heat flux due to larger turbulent fluxes ejected from the pedestal into the SOL; (4) Compatible with divertor plasma detachment operation; and (5) Quasi-continuous particle and power exhaust. In this presentation, we will briefly overview recent small ELM experimental results from multiple ITPA tokamaks and theory and simulations on current physics understanding of small ELMs. In order to identify the key drivers of small ELM instabilities, such as ideal, or resistive, or drift, or kinetic peeling-ballooning modes, we will compare simulations results from BOUT++ two-fluid and gyro-Landau-fluid (GLF) models for ITER 15 MA baseline scenario and 10 MA steady-state operation scenario. From nonlinear simulations, we will also dicuss the connection between small ELMs and marginal peeling-ballooning instabilities. |
Thursday, October 20, 2022 3:25PM - 3:45PM |
UM10.00006: What can an edge gyrokinetic code do about the integrated tokamak exhaust and pedestal performance gap? Choongseok Chang, Seung-Hoe Ku, Robert Hager, Michael Churchill, Julien Dominski, George J Wilkie Edge pedestal an important object in determining the core plasma performance that is experimentally observed to be coupled to the power exhaust issue. Multiple methods can predict the performance of the core plasma from the pedestal performance. The unknown gap is in the integrated relationship between the tokamak exhaust and the pedestal performance. To date, the edge gyrokinetic code XGC reproduces [1] the experimental heat-load width observed in various tokamaks (Eich/Goldston) and predicts that ITER’s heat-load width, during fusion operation phase (FPO), will be much wider than the one predicted by extrapolation of experimental data. This is due to the enhanced kinetic turbulent transport occurring at ITER’s extremely large value of a/ρi [1]. This enhanced near-SOL turbulence across the magnetic separatrix could yield more intimate relationship between power-exhaust and pedestal performance than what have been observed in the present tokamaks. Moreover, a gyrokinetic evidence says that the parallel force-balance relationship between upstream and downstream plasmas may not be well-described by simplified fluid equations [2]. The relationship between the pedestal performance - including the RMPs and the onset windows of ELM and density-limit electromagnetic instabilities - and the power exhaust in detached/semi-detached divertor condition in FPO ITER may be best predicted by gyrokinetic simulations that include neutral particles, heavy and light impurity particles with power radiation, and atomic physics. Recent XGC discovery of the micro-turbulent homoclinic tangle offers further evidence for closer connection between pedestal and divertor plasma in FPO ITER [3]. |
Thursday, October 20, 2022 3:45PM - 4:05PM |
UM10.00007: Data-driven model for divertor plasma detachment prediction Ben Zhu, Menglong Zhao, Harsh Bhatia, Xueqiao Xu, Peer-Timo Bremer, William H Meyer, Nami Li, Thomas D Rognlien So far the most successful method of reducing divertor heat load in tokamaks is achieved by detachment and yet a fast and accurate detachment prediction model is not available. We present a data-driven surrogate model for divertor plasma detachment prediction leveraging the latent space concept in machine learning research [1]. Our approach involves constructing and training two neural networks - an autoencoder that finds a proper latent space representation (LSR) of plasma state by compressing the multiple diagnostic measurements, and a forward model using multi-layer perception (MLP) that projects a set of plasma control parameters to its corresponding LSR. By combining the forward model and the decoder network from autoencoder, this new data-driven surrogate model predicts a consistent set of diagnostic measurements based on a few plasma control parameters. Benchmark between the data-driven surrogate model and 1D UEDGE simulations shows that our surrogate model is capable to provide accurate detachment prediction (usually in a few percent relative error margin) but with at least 10,000 times speed-up, indicating that performance-wise, it is adequate for integrated tokamak design and plasma control. Data-driven surrogate model using 2D UEDGE simulations will also be reported. |
Thursday, October 20, 2022 4:05PM - 5:00PM |
UM10.00008: Panel Disucssions
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