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 PI01: MFE: H-mode, Pedestal, and FuelingInvited Session
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Chair: Ben Dudson, LLNL Room: Plaza F |
Wednesday, November 1, 2023 2:00PM - 2:30PM |
PI01.00001: The Role of Thermal Charge Exchange Neutrals in Fueling on DIII-D and Future Reactors Invited Speaker: Shaun R Haskey Spectrally resolved measurements of the Deuterium Balmer-$alpha$ (Ba-$alpha$) line have been used on DIII-D to uncover details of the neutral energy distribution, demonstrating the existence of neutrals with energies in excess of 1keV for typical DIII-D pedestals and showing good agreement with synthetic spectra from neutral transport modeling. Multistep charge exchange (CX) between ions and recycled neutrals transfers energy and momentum, creating thermal neutrals with energies approximating the ions at the pedestal top. This increases their mean free path by orders of magnitude, allowing fueling in what may otherwise be considered opaque scrape off layers. These atoms play a central role in core edge integration by affecting energy, momentum, impurity and particle transport as well as permitting increased spectral radiation from low-Z impurities and increasing the energy of neutrals impinging on the wall. The Doppler shifts of the Ba-$alpha$ emission from these neutrals allow them to be directly detected despite cold recycling neutral emission being orders of magnitude larger. Simulations constrained by these measurements demonstrate their importance for particle transport studies on DIII-D, accounting for 40% of the integrated ion source inside the pedestal top and being significantly larger than the source from typical NBI fueling. Simulation scans show that the particle source broadens with increased Ti, but narrows with increased ne, Zeff, and isotope mass. For ITER, main chamber thermal neutrals are attenuated more rapidly but will still play an important role. Additionally, the same CX process will apply to auxiliary fueling methods. Contrary to expectations, the benefits of smaller scales for fueling compact high field devices are largely offset by the higher densities assumed in modeling due to the larger nGW density limits. |
Wednesday, November 1, 2023 2:30PM - 3:00PM |
PI01.00002: The Role of Main Ion Isotope Mass on Neutral Fueling and the Density Pedestal Structure Invited Speaker: Ryan A Chaban Edge ionization source measurements and plasma profiles on the DIII-D tokamak for dimensionally matched hydrogen (H) and deuterium (D) H-mode pedestals show a clear isotope-mass effect and give experimental evidence for an influence on the density pedestal structure. For all electron pedestal densities studied, hydrogen penetrates 40% deeper than deuterium. Neutral penetration decreases with increasing electron density pedestal. For low density, we find that the hydrogen isotope-mass penetration increase widens the density pedestal in comparison to deuterium. As the electron pedestal density increases, the ratio of H/D penetration remains constant, but the overall magnitude of the isotope-mass penetration difference diminishes and projects to be negligible for a reactor density pedestal. Simultaneous High-Field Side (HFS) and Low-Field Side (LFS) ionization source measurements confirm an asymmetry in which the majority of the fueling occurs on the HFS, at the diagnostic location that demonstrates the isotope-mass fueling increase in H. However, on the LFS, differences in the neutral profiles suggest an additional isotope-mass driven transport effect in the pedestal is present. |
Wednesday, November 1, 2023 3:00PM - 3:30PM |
PI01.00003: Reconstruction and interpretation of edge line radiation asymmetry from first-principles kinetic simulation Invited Speaker: George J Wilkie With kinetic edge simulations, it is shown that the observed ionization asymmetry on DIII-D is due to reversal of ion flows which modifies the primary recycling location from either the outer or inner divertor plates. Here, we present the first successful reproduction of observed Lyman-alpha brightness on DIII-D directly from first-principles simulations. The XGC total-f particle-in-cell software suite is now fully coupled with the DEGAS2 neutral transport solver and was used to predict the plasma structure in the pedestal and SOL self-consistently with recycling neutrals. DEGAS2 and its associated synthetic diagnostics act as a validation mechanism for XGC and provides physical insight for the scrape-off layer and ionization fueling. |
Wednesday, November 1, 2023 3:30PM - 4:00PM |
PI01.00004: Kinetic ballooning mode constraints for tokamak pedestals with varying aspect ratio and plasma shaping Invited Speaker: Jason F Parisi We find the pedestal width-height scaling [0] for multiple tokamaks using a new kinetic ballooning mode (KBM) gyrokinetic threshold model. At tight aspect ratio, we reproduce NSTX’s experimental linear pedestal width-height scaling for ELMy H-modes [1], overcoming previous issues with tight aspect ratio pedestal prediction [2]. We reproduce the square root pedestal width-height scaling [0] at regular aspect ratio for previously published DIII-D discharges [3]. Our model uses EFIT-AI [4] to calculate global equilibria with self-consistent bootstrap current and can be applied to any H-mode equilibria. For ELMy NSTX discharges, KBM physics is needed to match the experimental data: we find that infinite-n MHD stability overpredicts pedestal pressure and underpredicts pedestal width. In addition to device-specific results, we report the effect of aspect ratio and plasma shaping on width-height scalings, showing the dependence on various shaping parameters. Combined with peeling ballooning mode (PBM) stability [5,6], our model will calculate a maximum inter-ELM pedestal width and height based on KBM and non-ideal PBM stability. This work is an important step towards a unified predictive capability of pedestal stability and transport for tokamak equilibria across a range of operating space. |
Wednesday, November 1, 2023 4:00PM - 4:30PM |
PI01.00005: Advances in RMP ELM Suppression through Establishment of Record Pressure and Temperature Pedestals Invited Speaker: Matthias Knolker .Recent experiments dedicated to the control of edge localized modes (ELMs) using resonant magnetic perturbations (RMPs) on DIII-D have achieved significant advances towards ITER and future fusion power plants, achieving record pedestal pressures. For the first time, ELMs were suppressed at maximum toroidal field BT=2.17 T and plasma current IP=2.0 MA in the ITER similar shape (ISS). Moreover, the experiment followed a successful Predict First Approach: Pedestal pressure and behavior was predicted using the EPED model, ELM suppression windows in q95 were targeted based on previous GPEC simulations. Discharges started from a peeling-limited Super H-mode high performance phase, and then transitioned into ELM suppression. Typically, RMP experiments start with lower density/performance by early RMP coil activation. Combining high field and current allows exploration of lower collisionality pedestals at high density (here νe,ped~0.2), the expected environment for future fusion reactors. The suppression lasted for multiple energy confinement times. ELM suppression in the ISS plasma was achieved for both q95 = 3.3 and q95 = 3.6, the former achieving record stationary pedestal pressures of pped=14 kPa (pe,ped=6.5 kPa, pav=52 kPa), exceeding previous RMP suppression records by 30 %. These experimental results and the EPED codes are used to extrapolate to ITER, resulting in a pedestal height of 60 kPa with RMPs and show a peeling limited pedestal. Using TGLF and TGYRO to predict the core plasma, this results in 300 MW of fusion power, and Q=6 for ITER’s active phase. These results are based on a viable core-edge-solution and more realistic than the 500 MW/Q=10, since those assumes fully stationary type I ELM pedestals. Our work raises confidence in combining high pedestals and ELM control for developing the updated ITER research plan. |
Wednesday, November 1, 2023 4:30PM - 5:00PM |
PI01.00006: Understanding the L-H transition isotope effect in DIII-D Invited Speaker: Kyle Callahan Recent database and gyro-kinetic analysis of DIII-D plasmas in low-confinement mode (L-mode) just before transition to high-confinement mode (H-mode) have identified decreased carbon impurity content as the dominant isotope effect responsible for increasing the L-H power threshold (PLH) in hydrogen at ITER-relevant low collisionality. Increased Zeff in deuterium, due to enhanced (mass dependent) physical and chemical sputtering of graphite (carbon) from diverter and main chamber tiles, is found to increase the ITG critical gradient, stabilizing ITG modes [1]. Gyro-fluid and gyro-kinetic simulations capture the observed impurity isotope effect and its influence on electron and ion heat transport and power loss, via the TGLF and CGYRO codes, respectively. Edge simulations identify subdominant, intrinsic main ion mass effects due to electron non-adiabaticity [2] and differences in normalized E⨉B shear, that also contribute significantly to the isotope scaling of PLH. At high collisionality, no isotope effect is observed, as previously documented [3]. Measurements of density and electron temperature fluctuations using the Beam Emission Spectroscopy (BES) and Correlation Electron Cyclotron Emission (CECE) diagnostics, respectively, were found to agree well with turbulence predictions from flux matched CGYRO simulations at ρ = 0.7, 0.9, & 0.95. This comparison was made possible via advanced synthetic diagnostics. The observed PLH reduction with low-Z impurity dilution opens the important prospect of improving H-mode access in ITER hydrogen plasmas via Ne or N seeding. |
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