Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Plasma Physics
Monday–Friday, October 30–November 3 2023; Denver, Colorado
Session GO09: MFE: Detachment, Power Handling, and Divertor Physics |
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Chair: Tess Bernard, General Atomics - San Diego Room: Governor's Square 16 |
Tuesday, October 31, 2023 9:30AM - 9:42AM |
GO09.00001: Overcoming challenges: leveraging machine learning for efficient modeling of divertor plasmas Ben Zhu, Menglong Zhao, Harsh Bhatia, Xueqiao Xu, David Eldon Modeling divertor plasma is challenging and time-consuming due to the inherent multi-scale, multi-physics nature of the problem. Consequently, application of high-fidelity divertor plasma model is often limited. The emerging machine learning technique offers an alternative solution to this challenge. A fast and fairly accurate data-driven surrogate model for complex physics is possible by leveraging the latent feature space concept as the intermediate step. This idea was first tested in the inertial fusion research [1], while the consequent study with simplified 1D flux-tube setup further demonstrated that complicated divertor plasma state has a low-dimensional representation in latent space [2]. Following the same methodology, application-specific surrogate models for divertor plasma (e.g., initial solution prediction for code acceleration, integrated tokamak divertor design, and divertor plasma detachment control) are constructed, trained, and tested based on more realistic 2D axisymmetric transport simulations. These models appear to be able to fulfill the designated tasks, indicating that machine learning could be a powerful tool for divertor plasma physics and fusion energy research. |
Tuesday, October 31, 2023 9:42AM - 9:54AM |
GO09.00002: Simulation of TCV-X21 and NSTX-U tokamak edge turbulence with Hermes-3 Benjamin Dudson, Mike Kryjak, Hasan Muhammed, Peter Hill, John Omotani, Vlad Soukhanovskii Anomalous transport in the edge and divertor of high-power tokamak plasmas remains a critical uncertainty in predictive simulations of divertor and wall heat and particle fluxes, but 3D fluid turbulence models have now matured to the point where detailed comparisons with experiment can be performed. Over the last 5 years we have developed and applied the Hermes-3 code[1,2], building on BOUT++ to model self-consistently both turbulence and transport processes in tokamak diverted X-point and other geometries. We will present Hermes-3 simulations of the TCV-X21 diverted L-mode reference case[3], in both forward and reversed field configurations, and comparison to this open experimental dataset. Simulations have been performed for models of varying complexity, including an isothermal model, hot ion model, and hot ion model with fluid neutral gas. This enables a quantitative assessment of the impact of model assumptions on experimental agreement. Results will also be presented of turbulence simulations in NSTX-U double-null configurations. These simulations aim to inform to edge transport models, and to make predictions that can be tested in future experiments. |
Tuesday, October 31, 2023 9:54AM - 10:06AM |
GO09.00003: Results From The TCV Plasma EXhaust (PEX) Upgrade Olivier Fevrier, Holger Reimerdes, Christian Theiler, Claudia Colandrea, Basil P Duval, Ambrogio Fasoli, Sophie Gorno, Benoit Labit, Lorenzo Martinelli, Artur Perek, Harshita Raj, Guangyu Sun, Cedric K Tsui To investigate the critical role of divertor neutral pressure in power exhaust, the TCV divertor was equipped with several sets of removable divertor gas baffles, altering neutral transport in and out of the divertor chamber. This upgrade was accompanied by diagnostic enhancements and plasma heating upgrades. |
Tuesday, October 31, 2023 10:06AM - 10:18AM |
GO09.00004: Wall conditioning of WEST after installation of the ITER grade, actively cooled lower divertor Alberto Gallo, David Douai, Nicolas Fedorczak, Thierry Alarcon, Kirill Afonin, Vivien Anzallo, Regis Bisson, Clarisse Bourdelle, Jerome Bucalossi, Eric Caprin, Yann Corre, Matthieu De Combarieu, Corinne Desgranges, Pascal Devynck, Annika Ekedahl, Jonathan Gaspar, Christophe Guillemaut, Remy Guirlet, James P Gunn, Julien Hillairet, Thierry Loarer, Patrick Maget, Pierre Manas, Jorge Morales, Philippe Moreau, Francis-Pierre Pellissier, Emmanuelle Tsitrone Future fusion reactors like ITER and DEMO will have all-tungsten (W) walls and perform long pulses. These features will make wall conditioning more challenging than in most of the existing devices. The W Environment in Steady-state Tokamak (WEST) is the only long pulse (~100 s) fusion device with W plasma-facing components in the EU. WEST is a unique test bed to study radiation losses mitigation and plasma density control via reactor relevant wall conditioning. The phase II of WEST operations has begun in 2022, after the installation of a new lower divertor, now entirely constituted by actively cooled, ITER grade, W monoblocks. After pump down, we baked WEST between 90 °C and 170 °C for about 2 weeks. After 82.5 h at 90 °C and 33 h at 170 °C, vacuum conditions were stable with a vessel pressure of 6x10-5 Pa and mass spectra dominated by H2 molecules. We then performed glow discharge cleaning (GDC) and boronization (GDB) at 170 °C, for the first time at such high temperature in WEST. Three sessions of D2 GDC, for a total of ~40 h, allowed for a further ~10 times reduction of the H2O signal in mass spectra. Then we carried out GDB for ~5 h using a 15%-85% B2D6-He mix, injecting a total boron mass of ~12 g. Once back to 70 °C, the vessel pressure was 5.5x10-6 Pa, lower than ever in WEST phase I. Plasma restart was seamless with ~30 s already cumulated over the very first 5 pulses, proving that the procedure described above was successful in conditioning a freshly installed ITER grade, actively cooled lower divertor. |
Tuesday, October 31, 2023 10:18AM - 10:30AM |
GO09.00005: Boron powder injection in WEST during long pulses with a fully actively cooled, ITER grade tungsten divertor Kirill Afonin, Alberto Gallo, Robert A Lunsford, Sayak Bose, Yannick Marandet, Philippe Moreau, Grant M Bodner, Hugo Bufferand, Guido Ciraolo, Corinne Desgranges, Pascal Devynck, Ahmed Diallo, Jonathan Gaspar, Christophe Guillemaut, Remy Guirlet, James Paul Gunn, Nicolas Fedorczak, Yann Corre, Federico Nespoli, Nicolas Rivals, Patrick Tamain, E.A. Unterberg The installation of a fully actively cooled ITER grade tungsten (W) divertor on WEST enables 1000 s pulses. Usage of W plasma facing components (PFCs) introduces high-Z impurities into the plasma, which can cause significant radiative losses. To prevent this, W PFCs are often coated with protective layers of low-Z material. Active wall conditioning using an impurity powder dropper (IPD) is being studied on WEST. Recently, boron (B) powder was dropped during a series of 20 s long pulses with mBtotal ~855 mg, more than double than previously achieved. The maximum drop rate achieved without disruption was 58 mg/s, far greater than in 2021. This discrepancy may be due to the new divertor: new actively cooled W monoblocks vs. used inertial W-coated CFC. Another important difference is the response of Prad to injected B, showing a rollover during B injection. Spectroscopy measurements suggest O gettering to be the cause. To interpret the experimental results, a modelling workflow with SOLEDGE-EIRENE fluid boundary code and Dust Injection Simulator code was employed to study the transport of B in the plasma and its effects on PFCs, e.g. reduced recycling of D and low-Z impurities, decreasing divertor plasma density and increasing confined energy WMHD. |
Tuesday, October 31, 2023 10:30AM - 10:42AM |
GO09.00006: Identification and radiated power control of the divertor plasma in the DIII-D tokamak Lennard Ceelen, Jesse T.W. Koenders, David Eldon, Himank Anand, Qiming Hu, Gijs L. Derks, Francesca Turco, Auna L Moser, Anthony W Leonard, Marco R. Baar, Matthijs van Berkel This is the first use of modern system identification for exhaust control on an American tokamak. With these experiments, we investigate the dynamics of, and successfully control, the divertor plasma exhaust in the DIII-D tokamak. |
Tuesday, October 31, 2023 10:42AM - 10:54AM |
GO09.00007: Observation of a double-peaked heat flux profile in the SAS-VW divertor Jun Ren, Dan M Thomas, Jonathan H Yu, Xinxing Ma, Roberto Maurizio, Huiqian Wang, Jon Watkins, Dinh Truong, Ryan T Hood, Anthony W Leonard, David C Donovan A series of experiments were carried out in the DIII-D SAS-VW divertor to investigate the interplay between divertor behavior, the impact of drifts, and divertor geometry. A double peak structure was observed in both q// derived from Langmuir probe (LP) measurements and q⊥ determined from surface eroding thermocouples for both BT directions with the outer strike point placed at the inner slant of the SAS-VW divertor. With ion directed out of the divertor, particles cross the vertex from the inner slant to the outer target region driven by the ion drift. This results in a considerable flux of particles being redirected from the separatrix to the outer target region in the SAS. The double-peak in the Jsat profile measured by LPs is interpreted as evidence of this drift effect. Alternatively, with ion directed into the divertor, the double peak in the heat flux profile is caused by the separation of electron temperature which is demonstrated by a double-peak in the Te profile from LP measurements and reproduced by SOLPS modeling. The double-peak in heat flux profile separates the deposited power into two radial bands, in which the second heat flux peak can be as high as ~60% of the first heat flux peak, consequently reducing the maximum heat flux at the strike point. |
Tuesday, October 31, 2023 10:54AM - 11:06AM |
GO09.00008: Effect of D2 mass injection rates on intra-ELM and inter-ELM W erosion and ELM peak heat flux in SAS-VW divertor in DIII-D Alec Cacheris, Tyler W Abrams, Daisuke Shiraki, Robert Wilcox, Jeffrey L Herfindal, Jun Ren, Seth H Messer, Jeremy D Mateja, David C Donovan The effect of D2 injection via gas puffing and pellets at the midplane on intra- and inter-edge localized mode (ELM) tungsten (W) erosion and peak heat flux in the Small Angle Slot (SAS) divertor has been studied in the DIII-D tokamak. The experiment distinguished the effect that D2 injection has on divertor W erosion and ELM peak heat flux reduction. While a greater D2 injection rate decreased the ELM peak heat flux, the impact on W erosion at the progressive angle outer strike point (OSP) position was not monotonic. The average intra-ELM W erosion per ELM was about 30% lower for D2 injection shots compared to the no D2 injection case. However, a larger D2 injection rate led to more frequent, reduced amplitude ELMs and increased both the total intra- and inter-ELM W erosion rate. Dα (656.19 nm) and WI filterscopes (400.9 nm) detected ELM start times and inferred photon emission from eroded W atoms, respectively. Langmuir probes measured divertor ne and Te to convert the photon emission rate to W atoms/s using the S/XB method, surface eroding thermocouples (SETCs) measured ELM peak heat flux, and impurity collector probes (CPs) assessed the upstream W content. W erosion at the SAS vertex and collector probe W content is currently under investigation and will also be discussed. |
Tuesday, October 31, 2023 11:06AM - 11:18AM |
GO09.00009: Effects of the small magnetic field incidence angle on the sheath drop in Wendelstein 7-X (W7-X) Arun Pandey, Michael Endler, Yu Gao, Felix Reimold Under typically used magnetic field configurations in W7-X, the flux tubes carrying the highest particle and heat flux in the scrape-off layer (SOL) make grazing contact with the grounded divertor surface (incidence angle ≤ 3o). The shallow incidence significantly affects the sheath drop (δVs) and floating potential (Vf) on the divertor surface. In W7-X, it has been observed that at ground potential (Vg), the divertor Langmuir probes (LP) collect significant current (Ig). Ig can have contributions from non-ambipolar drifts, thermoelectric currents and potential difference between non-floating wall (or LP) and plasma. For addressing the third contribution, it is necessary to calculate (δVs) for shallow incidence because Ig depends on the difference between plasma potential (= Vf + δVs) and Vg. The conventional methods fail to calculate δVs for shallow incidence. They neither explain the larger than expected ion saturation current on the LPs nor do they appropriately account for a sheath reversal at near parallel incidence. We propose a simple model based on the idea that for ions, below a certain incidence angle, exiting the quasineutral region is faster along the gyration path than along the parallel to field direction. The calculated Ig based on δVs from the model, shows a very good qualitative and quantitative agreement with experiments. Same calculation with the conventional sheathdrop give results which are far from qualitative agreement. This shows that our model is successful in capturing the modified ion and electron fluxes at the grazing field incidences. This also implies that the major part of the parallel SOL current in W7-X depends on δVs on the divertor in attached plasmas. The model is also extended to calculate the current and heat flux received by the divertor and shows good quantitative agreement with the observed heat flux. |
Tuesday, October 31, 2023 11:18AM - 11:30AM |
GO09.00010: Spatial structure of the magnetic island plasma in the island divertor scrape-off layer in Wendelstein 7-X Carsten Killer, Dario Cipciar, Olaf Grulke The W7-X stellarator utilizes an island divertor, where the scrape-off layer (SOL) is formed by a chain of intrinsic magnetic islands that are intersected by (modular) divertor targets. Compared to a tokamak SOL, the island divertor SOL features a much more complex magnetic geometry and a fine-structured distribution of connection lengths that can reach up to 1km due to the small field line pitch angle. As a consequence, we observe a 3D distribution of plasma parameters (e.g. density, temperature, potential, electric fields) interplaying with 3D transport processes: |
Tuesday, October 31, 2023 11:30AM - 11:42AM |
GO09.00011: Investigation of radiation distribution in W7-X with an infrared video bolometer camera Gabriele Partesotti, Felix Reimold, Glen A Wurden, Kiyofumi Mukai, Byron J Peterson, Victoria R Winters, Daihong Zhang, Aysia Demby Radiative cooling is one of the main energy loss channels in magnetically confined fusion plasmas. Hence, the radiation distribution and its emissivity are crucial aspects of heat transport, heat load mitigation, and plasma performance. An InfraRed Video Bolometer (IRVB) camera has been recently installed in W7-X to measure the radiated power, similarly to LHD [1]. |
Tuesday, October 31, 2023 11:42AM - 11:54AM |
GO09.00012: Mitigating severe damages to divertor and nearby components during plasma transients in ITER-like reactors Ahmed Hassanein, Valeryi Sizyuk Successful operation of thermonuclear reactors such as ITER, DEMO, and future commercial plants is critically determined by the choice of materials for various components. We comprehensively simulate the entire tokamak with exact 3D geometry to predict various materials with innovative designs to predict future ITER-like and DEMO performances during plasma instabilities. We used our HEIGHTS simulation package to predict ITER-like components response during transient events. HEIGHTS integrate models starting from the lost hot core plasma particles through SOL, deposition on the divertor surface, and the resulting evolution and generation of a secondary plasma of divertor materials. The simulation showed significant reduction in the heat loading and damage to all the divertor nearby and internal components in the case when lithium is used on the divertor plates compared to the case when either tungsten or carbon are used on the divertor plate. Significant damage can occur on the reflector, Dome structure, stainless steel tubes, and even parts of the first walls due to the high radiation power of the secondary plasma generated from high-Z divertor materials. Photon radiation deposition into the divertor and nearby components was decreased by two order of magnitude. This analysis showed that using liquid lithium on top or small C strip embedded in W can lead to substantial mitigation of transient events and significant enhancement in components lifetime. Comments of neutral gas injection pros and cons to mitigate transient events will also be addressed |
Tuesday, October 31, 2023 11:54AM - 12:06PM |
GO09.00013: Plasma characterization of tin-enriched clouds generated during the exposure of a liquid tin Capillary Porous System target at the OLMAT High Heat Flux facility Alfonso de Castro, Eider Oyarzabal, David Tafalla, Daniel Alegre, Kieran McCarthy, Matteo Iafrati, Igor Voldiner, Francisco Tabarés In future magnetic fusion devices, where extended pulse operation will be required, Liquid Metal (LM) Plasma Facing Components (PFCs), in particular tin, have been proposed as alternative to tungsten (W) ones. This is done in an attempt to overcome the limitations of W in terms of PFC lifetime and resilience to transient events. An advantage of LMs rely on the possibility of harnessing vapor shielding effects to dissipate part of the power exhaust while also offering natural protection to the underlying armor against disruptions [1]. The OLMAT (Optimization of Liquid Metal Advanced Targets) High Heat Flux (HHF) facility [2] is used to pursue the experimental development of LM PFCs by enabling the exposition to heat fluxes up to 58 MW/m2 in pulsed operation (30-150 ms duration, frequency up to 2 pulses/min). The results obtained with a W felt target filled with liquid tin and embedded single Langmuir Probe (LP) are presented. This allowed diagnosing the plasma cloud (Te and ne) created in front of the target during the exposure at temporal scales where tin vaporization contributed to induce shielding effects on the incoming heat fluxes. Challenges regarding the LP interpretation due to possible thermionic electron emission and by the presence of tin content in the plasma are discussed. The results are complemented with line spectroscopy, fast-frame/infrared camera and pyrometry measures attempting the pioneering, in-situ diagnosis of tin-enriched plasmas of primary interest for alternative LM divertor scenarios |
Tuesday, October 31, 2023 12:06PM - 12:18PM |
GO09.00014: Lithium Vapor Shielding: Experiment and Modeling Daniel Andruczyk, Rabel Rizkallah, Davide Curreli, Andrew J Shone, Rajesh Maingi, Fabio Romano, Thomas W Morgan Li has been shown to help mitigate the high incident plasma fluxes by a behavior known as Li vapor shielding (LVS) successfully protecting W targets for heat fluxes up to 29 MW/m2. The exact mechanics and limitations with LVS are still not well understood. Experiments have been conducted on Magnum-PSI to help understand the LVS phenomenon. Another experimental campaign on HIDRA was undertaken with the specific aim of measuring Li redeposition and recreating vapor shielding regimes in a toroidal environment. The measured redeposition rates lower than the expected > 0.999 from literature. A plasma chemistry model was developed to compute the power loss per Li particle due to electron impact excitation, ionization and recombination. A global plasma reaction solver named CRANE was used to solve for the steady-state content of a 0-D vapor cloud. Calculations also show that redeposition rates below 0.99 are needed to justify LVS from a plasma chemistry description and is in agreement with experimentally measured rates on HIDRA. CRANE was then coupled to Zapdos, a plasma transport solver to expand to a 1-D model. The Zapdos-CRANE implementation was tuned to reproduce similar Li clouds to the ones observed during the Magnum experiments and successfully demonstrated the ability of the vapor cloud to radiate enough power away. |
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