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 BO07: MFE: Divertor and Scape-Off-LayerOn Demand
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Chair: Livia Casali, General Atomics Room: Rooms 315-316 |
Monday, November 8, 2021 9:30AM - 9:42AM |
BO07.00001: Benchmarking 2D Te and ne Measurements from Multi-Spectral Imaging of Helium in TCV's Divertor with 2D Thomson Scattering and Reciprocating Probe Measurements. Bryan Linehan, Artur Perek, Basil P Duval, Jorge M Munez-Burgos, Filippo Bagnato, Patrick Blancard, Claudia Colandrea, Hugo De Oliveira, Olivier Fevrier, Erik R Flom, Sophie Gorno, Earl S Marmar, Lorenzo Martinelli, Antoine Merle, Nicola Offeddu, Andreas Smolder, James L Terry, Christian Theiler, Cedric K Tsui, Oliver Schmitz, Benjamin Vincent, Mirko Wensing, Curdin Wüthrich This talk presents results of experiments that benchmark 2D poloidal measurements of Te and ne over the SOL using line ratios obtained from multi-spectral imaging of intrinsic He-I emission in TCV's divertor. Te and ne are obtained by matching multiple emissivity line ratios to a collisional radiative model. For non-recombining plasmas, the Te measurements showed very good agreement with those from Thomson scattering and reciprocating probes, while the agreement for the ne measurements was fair. Two multi-spectral cameras with fields of view tangent to the torus were used simultaneously. A camera in the lower divertor imaged He-I emission from 2 singlet transitions, 2 triplet transitions, and a He-II line. A second system imaged 2 singlet transitions, and a 3 transitions from a pupil 30 cm higher. Single-null L-mode plasmas were observed for both He and D majority plasmas, for both BT directions, and a range of core densities. The outer legs of the plasmas were nearly vertical allowing comparisons with 2D measurements from a vertical reciprocating probe and three Thomson scattering lasers. Following this benchmarking, the multi-spectral measurements were compared against SOLPS-ITER, and acquired for several TCV discharges with different divertor configurations. |
Monday, November 8, 2021 9:42AM - 9:54AM |
BO07.00002: Divertor detachment in negative-delta configurations in the TCV tokamak Olivier Fevrier, Cedric K Tsui, Stefano Coda, Basil P Duval, Sophie Gorno, Laurie Porte, Holger Reimerdes, Olivier Sauter, Christian Theiler Experimental observations on TCV and DIII-D have shown that negative triangularity (δ) discharges may have H-mode grade confinement in L-Mode operation, which opens the possibility for high confinement reactors side-stepping the challenges associated with H-mode such as ELMs, narrow scrape-off layer widths, and density control. To ensure safe power exhaust, partially or fully detached divertor operation will still be required. In this work, we investigate detachment on TCV in ohmic L-Mode δ<0 configurations and compare it to matched ohmic L-Mode δ>0 cases. Preliminary results indicate that detachment is generally harder to reach in negative δ, where sufficient cooling (< 5eV) of the outer target is achieved only with high levels of N2 seeding, and generally comes at the price of degraded core confinement. This behavior is currently ascribed to a connection length reduction at δ<0, resulting in a larger fraction of the exhaust power being diverted towards the outer target compared to δ>0. |
Monday, November 8, 2021 9:54AM - 10:06AM |
BO07.00003: Exploring the Edge/SOL Fluctuations in Negative Triangularity Plasmas on TCV Woonghee Han, Nicola Offeddu, Theodore Golfinopoulos, Christian Theiler, Cedric K Tsui, Jose A Boedo, James L Terry, Earl S Marmar, Randall A Pietersen, Rafael Villamor-Lora, Matthew J Beveridge, Iddo Drori Magnetically confined fusion plasmas with a negative triangularity (δ) core shape are known to feature enhanced confinement as compared to standard, D-shaped plasmas. Recently, based on Gas Puff Imaging (GPI) and probe measurements, sufficiently negative δ (δ≲-0.25) plasmas on TCV were also found to feature a complete suppression of first-wall interactions [1]. Reasons for this suppression, which could have important implications for the prospects of negative δ as a reactor solution, were explored, pointing towards the role of reduced connection length intrinsic to negative δ. This work is currently being extended in the upcoming experiment, from the previous range of Greenwald fraction of 0.13-0.42 towards the density limit and edge/SOL fluctuations, and their dependence on δ are explored with GPI. For the analysis of the GPI images, a new machine learning model is used for tracking the position and shape of filaments in GPI images, frame by frame, so that the filament statistics can be used to characterize the edge/SOL turbulence by the intermittency of filaments and other key parameters [2]. |
Monday, November 8, 2021 10:06AM - 10:18AM |
BO07.00004: Interpretive modeling of SOL impurity transport using 13C deposition patterns on crown and midplane collector probes in DIII-D Jonah D Duran, Ezekial A Unterberg, Jake H Nichols, Shawn Zamperini, Dmitry L Rudakov, Jun Ren, David Donovan Recent experiments injected deuterated methane with isotopically enriched 13C into the outer divertor of upper single null discharges. The far scrape-off layer (SOL) impurity transport code, 3DLIM, was used to reproduce the resulting deposition patterns for multiple collector probes. This has allowed characterization of the near-SOL 13C poloidal distribution. The poloidal structure of SOL impurities is inferred using collector probes (CPs) installed on reciprocating drives at DIII-D. The deposition patterns on these CP sets along with the 3DLIM simulations suggest a near-SOL source of impurities that is poloidally bound between the outboard midplane and the outer strike point when operating in an unfavorable magnetic drift direction. This differs from previous observations of tungsten SOL transport, which suggested a near-SOL impurity source in the vicinity of the plasma crown for similar plasma conditions. Specifically, it is found that the deposition profile is influenced by the convective radial carbon velocity, and it is found that the connection length between each CP and surrounding wall structures strongly influences the deposited 13C magnitude. These results are providing needed validation studies for understanding and controlling SOL impurity transport. |
Monday, November 8, 2021 10:18AM - 10:30AM |
BO07.00005: Surface heat flux control by divertor impurity seeding on the DIII-D tokamak Himank Anand, David Eldon, David A Humphreys, Anthony W Leonard, Charles J Lasnier, Filippo Scotti, Jose A Boedo, Jayson L Barr, Brian Sammuli, Alan Hyatt Control of the intense heat flux to the divertor is one of the outstanding problems in tokamaks. One technique that has shown promise is impurity seeding, i.e., the injection of low-Z gaseous impurities to radiate and dissipate the power before it arrives to the divertor target plate via thermal conduction. As a first step in the development, the DIII-D team has implemented a feedback system to control the injection of seed gas based on real-time surface heat flux estimation, as injecting excess impurity gas is disadvantageous. A model-based approach, successfully benchmarked against off-line infrared camera measurements for H-mode plasma discharges, provides real-time estimation of the plasma heat flux on the divertor target. The real-time surface heat flux is estimated in the DIII-D plasma control system, which then uses a proportional–integral–derivative (PID) algorithm to control the injection of gas and mitigate divertor heat flux. This paper presents the design of the new feedback system, validation of the model-based estimation against IR camera measurements as well as the real-time regulation of the peak divertor heat flux using impurity seeding in H-mode plasma discharge. |
Monday, November 8, 2021 10:30AM - 10:42AM |
BO07.00006: Revealing the strong interplay between divertor geometry and E×B drifts on divertor dissipation in the DIII-D Small Angle Slot (SAS) divertor Xinxing Ma, Huiqian Wang, Houyang Y Guo, Dan M Thomas, Peter C Stangeby, Eric T Meier, Morgan W Shafer, Jonathan G Watkins, Jun Ren, Roberto Maurizio Experiments in DIII-D and SOLPS-ITER modeling with drifts find a strong interplay between drifts and divertor geometry in facilitating divertor dissipation. Supported by SOLPS-ITER modeling, experiments show that for ion B×▽B drift out of the divertor, ‘unfavorable-Bt’, cold plasma with Te≤10 eV across the entire SAS divertor target can be achieved at relatively low upstream densities. In contrast, for favorable-Bt Te remains >20 eV at the target until eventual onset of detachment at a comparatively high n/nGW. The coupling of divertor geometry and drift flows can strongly affect the path towards divertor detachment. With the strike point on the inner slanted surface and unfavorable-Bt, detachment bifurcations were observed with Te suddenly falling below 5eV. This differs from the open divertor where the Te cliff was only observed for favorable-Bt. SOLPS-ITER shows that, at a higher density, both the radial and poloidal E×B flows reverse direction causing a rapid density accumulation right near the separatrix which eventually results in low Te across entire divertor target plate. These results indicate that the interplay between geometry and drifts needs to be fully considered in future fusion reactor divertor designs. |
Monday, November 8, 2021 10:42AM - 10:54AM Not Participating |
BO07.00007: Increased heat exhaust by a sas-like divertor in a large-scale tokamak Ookjoo Ra, Kyu Been Kwon, Min Sup Hur Development of a reactor-compatible divertor is a challenging and important task for the realization of nuclear fusion, as the divertor is a part that directly interacts with hot plasma particles and neutrals. It is believed that the heat flux imposed on the divertor region in DEMO-class reactors will be more than 20 which is a technological limit of PFC for maintaining steady state. One promising solution to the reduction of heat flux on the divertor target is using and controlling the detachment phenomenon. From lots of previous studies, it has been confirmed that the change of the divertor structure (especially the shape) directly affects the detachment. In this study, we investigate the effect of SAS-like divertor shape on detachment in a large-scale Tokamak. We carried out comparison studies of original and SAS-like shapes on the ITER environment using the SOLPS-ITER package. From our simulations, we observed that the heat flux decreases by less than 1/3 in the SAS-like cases for similar upstream densities. Interestingly, although the SAS-like shape was employed only in the outer target, a considerable decrease of the heat was observed in the inner target also. We discuss the origins that leads to those results by analysis of neutral trajectories in the different shapes of divertors. |
Monday, November 8, 2021 10:54AM - 11:06AM |
BO07.00008: SOLPS-ITER studies of Neon seeding in EAST Dieter Boeyaert, Stefano Carli, Kristel Ghoos, Wouter Dekeyser, Sven Wiesen, Liang Wang, Fang Ding, Kedong Li, Yunfeng Liang, Martine Baelmans In order to decrease power and particle loads towardsthe divertor targets in future fusion devices, active extrinsic impurity seeding is required [1]. A dedicated experimental program on neon seeding in H-mode plasmas was performed at EAST [2]. Experimental observations show a factor of three reduction of the power flux towards the targets, but this is not sufficient to determine whether detachment is reached. Therefore, the SOLPS-ITER code package [3] is employed for assessing the plasma edge transport in the EAST discharges of [2]. Drift terms are successfully included for the first time in SOLPS-ITER simulations of EAST, resulting in upstream agreement within the experimental error bars and downstream agreement within a factor three or better between simulations and experiments, and showing that Ne seeding induces detachment. Sensitivity studies towards the main unknown input parameters for the code are executed. In order to quantify the precision of the performed simulations, the numerical errors affecting the simulation results are examined. Similar to previous ITER simulations with B2-EIRENE [4], the main error is the discretization error due to the finite plasma grid. By making an appropriate choice of the remaining numerical input parameters, the error contributions induced by the Monte Carlo noise of the EIRENE code are kept sufficiently small. [1] M. Wischmeier et al., J. Nucl. Mater. 2015 [2] D. Boeyaert et al., Nucl. Mater. Energy 2021 [3] S. Wiesen et al., J. Nucl. Mater. 2015 [4] K. Ghoos et al., Nucl. Fusion 2018 |
Monday, November 8, 2021 11:06AM - 11:18AM |
BO07.00009: SOLPS-ITER simulations of large power handling in the divertor for CFETR with full drifts Hang Si One of the major challenges for the 1GW class fusion reactor Chinese Fusion Engineering Testing Reactor (CFETR) is to handle huge power exhaust (200MW) efficiently. In the present work, the effects of seeding radiation impurities, such as argon (Ar) and neon (Ne) and increasing divertor leg length in CFETR are systemically investigated to evaluate the efficient reduction of the maximum steady-state power load at the divertor target to an engineering design level (less than 10 MWm-2) by the SOLPS-ITER code package with full drifts. The modeling results show clearly that increasing radiation impurities Ar and Ne seeding rate with the fixed D2 fueling gas injection rate can significantly reduce the target electron temperature and heat flux density for the reference divertor geometry, which can be both reduced further with higher D2 injection rate. Moreover, the radiation efficiency for Ar is better than that for Ne. However, higher impurity seeding rate will cause higher impurity concentration in the core region, which can be controlled well by selecting the proper D2 fueling gas injection rate and impurity seeding rate. Based on the reference geometry in CFETR, increasing the divertor leg length from 1.7m at present to 2.4m can also benefit reducing electron temperature and heat load at the target further. These results show that the divertor design with a longer leg and higher Z seeding such as Ar is appropriate to obtain the divertor scenario for CFETR divertor design. |
Monday, November 8, 2021 11:18AM - 11:30AM |
BO07.00010: New HL2M tokamak operation foreseeing to support the next step fusion energy research Lei Xue, Xuru Duan, Min Xu, Wulyu ZHONG, Jeronimo Garcia Olaya, Tuong Hoang, Guoyao Zheng, Jean-Francois Artaud, Jiaxian Li, Xiao Song, Shuo Wang, Hailong Du, Miao Xue New tokamak HL-2M (Ip=2.5~3MA, B=2.2T, R=1.78m, a=0.65m) is dedicated to address both the plasma physics and technology issues in ITER and future fusion reactors. First plasma has generated in 2020. A combination of the flexible magnet coil system and the heating power of 27 MW (15MW NBI, 8MW ECRF, 4MW LHCD) allows HL-2M to be an excellent platform for tests and qualification of various advanced divertor concepts (such as snowflakes, tripod) and high heat flux plasma facing components. Heat flux on the divertor target could reach 10~20MW/m2. Scenarios, such as inductive, hybrid and full non-inductive regimes, achieved via various auxiliary heating combinations. Present paper reports the key performance foreseeing during HL-2M operation, including divertor heat and particle flux, as well as various operation scenarios. In the inductive regime with high plasma current (2.5MA/2.2T), the central plasma temperature can reach up to ~10 keV at density of . Hybrid scenarios, with bootstrap current fBS=30%~50%, can reach in plasma current Ip =1.0~1.4MA, Greenwald fraction fG ~0.5 by combining NBCD with ECCD or ECCD+LHCD. Full non-inductive regimes, such as the hybrid steady state regime and the regime with a reversed magnetic shear, can reach in Ip=1MA with fBS>60%, H98(y,2) ~1.3, βN >3. Therefore, HL-2M can provide scientific and technical solutions of the divertor and the operation issues for ITER, especially in its pre-fusion power operation phase. |
Monday, November 8, 2021 11:30AM - 11:42AM |
BO07.00011: Simulations of molecular and impurity transport effects on divertor detachment Yulin Zhou, Benjamin Dudson, Fulvio Militello Divertor detachment is an important process in high power fusion reactors by which the power loads onto material surfaces may be reduced. Achieving this state typically requires a combination of power and momentum loss processes, from a complex combination of impurity species radiation (energy loss), and hydrogenic reactions (energy and momentum loss). In this work the SD1D and Hermes models built on BOUT++ have been extended to simulate multiple ion species, and applied to study both the role of molecular species in detachment, and the transport of impurities. We find that molecular processes contribute significantly to a strong rise of photon emission intensity (e.g. H-alpha) around detachment, in qualitative agreement with experimental results on TCV [Verhaegh et al 2020]. The different molecular reaction channels are decomposed, finding that for MAST-U like conditions the negative ion contribution only becomes significant in deep detachment. Multiple charge states of injected neon impurity are then studied, and used to test the impact of the boundary condition [Tskhakaya & Kuhn 2005] and parallel transport processes on the divertor solution. By evolving a separate ion temperature for each charge state, these simulations enable the validity of the common ion temperature approximation to be investigated. The aim of this work is to motivate future efforts to validate impurity transport models and atomic data against experiment. |
Monday, November 8, 2021 11:42AM - 11:54AM |
BO07.00012: SOLT3D modeling of plasma instabilities and turbulence in tokamak scrape-off layer and divertor Maxim V Umansky, Bruce I Cohen, James R Myra, Andris M Dimits, Ilon Joseph SOLT3D is a plasma model for tokamak scrape-off layer and divertor, implemented in the BOUT++ framework [1]. The model supports basic fluid instabilities relevant to scrape-off layer (SOL) and divertor plasmas: the drift-resistive-ballooning mode instability driven by the magnetic curvature and the radial gradient of plasma pressure, and the conducting-wall mode instability driven by the end-plate sheath boundary conditions and the radial gradient of plasma temperature. SOLT3D has been benchmarked against analytic linear dispersion relations and against some known nonlinear results, demonstrating excellent agreement. The model is used to develop some new insights on linear plasma instabilities in the tokamak edge, in particular in the low-fields side (LFS) and high-field side (HFS). For nonlinear regimes, SOLT3D is used to assess the role of different drive mechanisms for fluid plasma turbulence for parameters relevant to tokamak SOL and divertor. [1] B. D. Dudson et al. Comput. Phys. Commun. 180, 1467–1480 (2009). |
Monday, November 8, 2021 11:54AM - 12:06PM Not Participating |
BO07.00013: A simulation-data derived reduced model for near-wall molecule-plasma interactions George J Wilkie, Robert Hager, Choongseok Chang Molecular hydrogen plays an important role in divertor detachment by providing additional momentum and energy sink mechanisms. For plasma temperatures greater than the Franck-Condon energy, molecules exist in a narrow region adjacent to the wall. The limited spatial extent of this "molecular presheath" permits the use of simplified models to characterize its effects on the plasma. Here, a reduced model for the momentum and energy sink of the plasma due to molecular interactions, as well as an effective atomic production energy, is informed by fitting to thousands of kinetic-neutral DEGAS2 simulations. Free parameters include: the plasma density and temperature, the angle the magnetic field makes with the wall, the wall composition, and its recycling coefficient. This model is implemented in XGC to provide a simple but accurate estimate of the effects of desorbed molecular hydrogen. Implications for plasma boundary conditions are also discussed. |
Monday, November 8, 2021 12:06PM - 12:18PM |
BO07.00014: Equilibrium radial ExB effects on ion temperature gradient instability in the scrape-off layer of a field-reversed configuration Wenhao Wang, Xishuo Wei, Zhihong Lin Linear and nonlinear effects of the equilibrium radial electric field on the ITG instability in the SOL of a FRC have been studied using gyrokinetic particle simulations for a single toroidal mode. Linear simulations with adiabatic electrons find that the E×B flow shear reduces the growth rate and causes a radial tilting of the mode structure on the toroidal plane. Nonlinear simulations find that the E×B flow shear significantly decreases ITG saturation amplitude and ion heat transport in the SOL by reducing both turbulence intensity and eddy size. The turbulence intensity is determined by fluid eddy rotation, which dominates the saturation mechanism for the SOL ITG. On the other hand, parallel wave-particle decorrelation determines the SOL ITG turbulent transport. A random walk model using the guiding center radial excursion as the characteristic length scale and the eddy turnover time as the characteristic time scale fits well to the scaling of ion heat conductivity with the E×B flow shear. |
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