Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session JM9: Mini-Conference on Plasma–Material Interactions in Fusion Devices: ITER and Beyond. II. Boundary Effects, Plasma Dynamics, and Alternative Divertor Solutions |
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Chair: Sergei Krasheninnikov, University of California, San Diego Room: OCC C123 |
Tuesday, November 6, 2018 2:00PM - 2:25PM |
JM9.00001: Long-legged divertors for addressing tokamak PMI challenges* M. V. Umansky, B. LaBombard, D. Brunner, T. Golfinopoulos, A. Q. Kuang, M. E. Rensink, T. D. Rognlien, J. L. Terry, M. Wigram, D. G. Whyte Recent modeling of a tightly-baffled long-legged divertor demonstrates a detached divertor regime where energy is dissipated on sidewalls of the divertor leg by radial transport and radiation [1]. This fully detached regime is found to be passively stable; it persist for a wide range of input power from the core, and as input power is varied, the location of the detachment front in the leg shifts closer to, or away from, the divertor target. For a sufficiently long divertor leg, the divertor remains detached, with benign power loads on the material surfaces, and the detachment front is located safe distance away from the target plate and the primary X-point. Calculations demonstrate that a long-legged divertor can accommodate up to an order of magnitude larger exhaust power than a standard divertor can, for otherwise similar parameters. This makes a long-legged divertor a potentially attractive option for a high-power tokamak, and it is currently considered for high-field designs (ADX, ARC). Physical mechanisms and sensitivity to model assumptions are examined for the long-legged detached regime, and its implications for fusion energy are discussed. [1] Umansky et al., Phys. Plasmas 25 (2017) 056112. |
Tuesday, November 6, 2018 2:25PM - 2:50PM |
JM9.00002: Considering a Novel Detached Divertor Solution Based on Thermionic Emission Michael D. Campanell Thermionic emission from tungsten has an important influence on the PMI in tokamaks. Emission is often viewed as unfavorable since it occurs when the surface is hot and is thought to weaken the sheath and further raise the heat flux. Here, we will show that if thermionic emission across the divertor plates exceeds a critical threshold, it will cause a dramatic cooling of the target plasma temperature, which can be used favorably to induce detachment. This idea stems from recent work showing that the classical sheath breaks down and transitions to an inverse sheath when the emission coefficient exceeds unity [1]. In the inverse regime, a high density of cold thermionic electrons (tenths of eV) dominates the quasineutral plasma near the surface. If induced in a divertor, it will ensure the formation of a cold target plasma with high recombination rates. This opens up the possibility of maintaining detachment without injecting impurities that contaminate the core. [1] M.D. Campanell and M.V. Umansky, PoP 24, 057101 (2017) and PRL 116, 085003 (2016). |
Tuesday, November 6, 2018 2:50PM - 3:15PM |
JM9.00003: Validated modeling of plasma boundary physics coupled to surface response for atmospheric arcs Igor Kaganovich, Alexander Khrabry, Andrei Khodak, Vladislav Vekselman, Shurik Yatom, Yevgeny Raitses, Brent Stratton Plasma boundary physics for tokamak divertors and atmospheric arcs have a lot of similarities: both require analysis of the heat and particle fluxes coupled to the surface response in the walls or electrodes mitigated by the space-charge sheaths modified by the electron and neutral emission from the surfaces. Resulting nonlinear equations for the plasma profiles are substantially complex and difficult to solve. We have developed several codes in multi-dimensions and verified each code against other to remove unavoidable bugs. Additional verification was performed against specially derived nonlinear analytical solution in the boundary layer where an approximate invariant of combination of density and temperature gradients was found [1]. In order to validate the predictions of the model, ablated species profiles and ablation rates of the wall material were compared with the measured experimental data in our Nanolab [2] and are in a good agreement, though some observed differences require further investigation. [1] A. Khrabry, et al., Phys. Plasmas 25, 013521 and 013522 (2018). [2] https://nano.pppl.gov/ V. Vekselman, et al., Plasma Sources Sci. T. 27, 025008 (2018). |
Tuesday, November 6, 2018 3:15PM - 3:40PM |
JM9.00004: Kinetic heat pulse propagation in the tokamak scrape-off layer Ilon Joseph, Mikhail Dorf, Milo Dorr In order to predict the heat and particle fluxes impinging on the target plate of a tokamak divertor, the transient behavior of a plasma heat pulse that travels along a magnetic flux tube is studied using the 4D long wavelength gyrokinetic COGENT code. Both kinetic electrons and kinetic ions are used along with gyrokinetic sheath boundary conditions [1]. Results are compared to heat pulse propagation test problems [2-3] that have been used to compare physics models and numerical algorithms. Simulations show that the particle distribution functions can become non-Maxwellian in the collisionless limit. Comparison to fluid models will be used to assess under which conditions kinetic effects become important. [1] E. L. Shi, G. W. Hammett, T. Stoltzfus-Dueck, and A. Hakim, J. Plasma Phys. 83, 905830304 (2017). [2] E. L. Shi, A. H. Hakim, and G. Hammett, Phys. Plasmas 22, 022504 (2015). [3] E. Havlickova, W. Fundamenski, D. Tskhakaya, et al., Plasma Phys. Control. Fusion 54, 045002 (2012). |
Tuesday, November 6, 2018 3:40PM - 4:00PM |
JM9.00005: SOL ballooning instabilities due to sheath boundary conditions and X-point geometry. Andrei I Smolyakov, Sergei Krasheninnikov The overview of the ballooning instabilities in the SOL driven by the sheath boundary conditions and temperature effects is presented. Particular emphasis is placed on the effect of the X-point geometry which adds additional dissipation thus modifying the nature of the modes. |
Tuesday, November 6, 2018 4:00PM - 4:20PM |
JM9.00006: Simulations of divertor heat flux widths using BOUT++ transport code with drifts N.M. Li, X.Q. Xu, Dan Brunner, J. W. Hughes, B. LaBombard, J. L. Terry, J.Z. Sun, D.Z. Wang The BOUT++ fluid transport code has been developed with all drifts and the sheath potential in the SOL. The calculated steady state radial electric field (Er) has been compared with experimental measurements from a C-Mod discharge using charge exchange recombination spectroscopy. The simulated Er profile is similar to the main ion diamagnetic term inferred from Thomson scattering profiles of electron temperature and density. In order to understand the impacts of drifts vs. turbulent transport on the divertor heat flux width, a set of four C-Mod EDA H-mode discharges with lower single null divertor configuration are simulated. BOUT++ transport simulations with all drifts included yield similar divertor heat flux width to experimental measurements varying within a factor of 2 and show a similar trend to the Goldston’s HD model. Magnetic drift has a significant influence on the divertor heat-flux widths, while the ExB drift further decreases the heat flux width by 10%~25%. In simulations of C-Mod discharges, the drifts dominate the cross-field transport and the heat flux width is sensitive to the separatrix electron temperature. |
Tuesday, November 6, 2018 4:20PM - 4:40PM |
JM9.00007: Prediction of Divertor Heat Flux width on ITER and CFETR Using BOUT++ Zeyu Li, Xueqiao Xu, Nami Li, Vincent Chan Investigation on the turbulent transport dynamics in Scrape-off-layer (SOL) and divertor heat flux width prediction is performed for ITER and China Fusion Engineering Test Reactor (CFETR). Both BOUT++ transport and turbulence codes are applied to capture the physics on different spatial-temporal scale. Simulations start with ITER 15MA baseline scenario and CFETR R7.2 Hybrid scenario (R=7.2m, BT =6.5T) respectively. In BOUT++ transport code, parametric scan for the SOL anomalous thermal diffusivity is performed which shows that when diffusivity is smaller than a critical value, heat flux width is almost unchanged, which is roughly consistent with Goldston’s HD model. Otherwise it would increase following the square-root of diffusivity scaling, indicating a transition from drift dominant regime to turbulence dominant regime. Using BOUT++ 6-field turbulence code, pedestal structure is found to be important in determining the effective SOL thermal diffusivity and in setting divertor heat flux width. Radial transport by electro-magnetic turbulence spreading is found to be the main contributor of the transport across separatrix in these turbulence simulations. |
Tuesday, November 6, 2018 4:40PM - 5:00PM |
JM9.00008: Investigations of Novel Liquid Lithium TEMHD Flow Geometries Matthew Szott, Steven Stemmley, David N Ruzic The use of flowing liquid lithium in plasma facing components (PFCs) has been shown to reduce erosion and thermal stress damage, prolong device lifetime, decrease edge recycling, reduce impurities, and increase plasma performance, all while providing a clean and self-healing surface. The Liquid Metal Infused Trench (LiMIT) system has proven the concept of controlled thermoelectric magnetohydrodynamic (TEMHD)-driven lithium flow for use in fusion relevant conditions, through tests at UIUC, HT-7, and Magnum PSI. One difficulty that arises is the phenomenon of lithium dryout, which exposes solid trench material due to strong local TEMHD acceleration in the areas with the highest heat flux. Novel geometries are being developed that maintain propensity for TEMHD flow while eliminating the risk of dryout. These include tailoring trench shaping to account for expected dryout regions, as well as large pore metallic foams, which aim to couple the surface stability of capillary porous systems with TEMHD flow. Maintaining a steady flowing liquid surface in the face of extreme heat fluxes is imperative for continued application of flowing liquid lithium PFCs. Designs, models, and experimental progress will be discussed. |
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