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 PI01: Magnetic Confinement Fusion VLive Streamed
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Chair: Kathreen Thome, GA Room: Ballroom 100 A |
Wednesday, October 19, 2022 2:00PM - 2:30PM |
PI01.00001: Pedestal transport validation in NSTX and development of a general ETG pedestal transport model Invited Speaker: Walter Guttenfelder Experimental validation of nonlinear gyrokinetic simulations finds that transport from electron temperature gradient (ETG) and microtearing mode (MTM) turbulence, in addition to neoclassical transport, can account for total power flow in NSTX wide-pedestal H-modes. Gyrokinetic analysis (CGYRO) predicts that a variety of NSTX H-modes, from narrow ELMy to wide ELM-free cases, are within 10% of kinetic ballooning mode (KBM) stability thresholds across the entire pedestal. This indicates KBM remains a viable candidate for constraining the maximum pressure gradient at low aspect ratio. However, the experimentally inferred ratio of electron particle to heat diffusivity (from SOLPS) is smaller than predicted by KBM. Both MTM and ETG, expected to transport primarily electron heat flux, are also predicted unstable. Nonlinear electron-scale ETG simulations predict a range of electron heat flux (Qe,ETG≈0-2 MW) depending on local parameters. Combined with neoclassical (NC) ion thermal transport (Qi,NC≈1 MW, from NEO), transport from ETG + NC accounts for 50-75% of the power flow for the wide pedestal and progressively less for the narrow pedestal. Nonlinear ion-scale MTM simulations predict electron heat flux comparable to that from ETG. Together, ETG + MTM + NC accounts for the total power flow in the wide-pedestal discharge in which the largest deviations from KBM transport ratios are observed. To develop a predictive capability, we use additional simulations to develop a reduced ETG pedestal transport model that reproduces many of the dependencies with driving gradients and equilibrium parameters. The resulting model unifies the NSTX results with those from recently published DIII-D analysis. |
Wednesday, October 19, 2022 2:30PM - 3:00PM |
PI01.00002: Ideal MHD limited electron temperature in spherical tokamaks Invited Speaker: Stephen C Jardin It is well documented that the central electron temperature in the National Spherical Torus Experiment (NSTX) remains largely unchanged as the external heating power, and hence the normalized volume averaged plasma pressure, $\beta$, increases.[D. Stutman, Phys. Rev. Lett. {\bf 102},115002 (2009)]. Here we present a hypothesis that low-n, pressure driven ideal magnetohydrodynamic (MHD) instabilities that are non-disruptive, can break magnetic surfaces in the central region and thereby flatten the electron temperature profiles. We demonstrate this mechanism in a 3D resistive MHD simulation of a NSTX discharge. By varying the toroidal magnetic field strength, and/othe heating power, we show that there is a critical value of $\beta$, above which the central temperature profile no longer peaks on axis. |
Wednesday, October 19, 2022 3:00PM - 3:30PM |
PI01.00003: A higher fidelity model for ELM onset in spherical tokamaks Invited Speaker: Andreas Kleiner We present developments towards a higher fidelity model to consistently predict edge-localized mode (ELM) onset in spherical torus (ST) configurations, such as NSTX/-U and MAST/-U. ELMs are typically associated with macroscopic peeling-ballooning (PB) modes in the edge pedestal, which arise due to strong pressure and current density gradients. In large aspect ratio devices these modes have ideal character and are well understood. However, a long-standing problem has been the reliable modeling of such stability boundaries in some ST scenarios, where ideal-MHD models often predict stability for ELMing discharges. We investigate current- and pressure-driven stability limits in ELMing discharges in NSTX and MAST, as well as ELM-free wide-pedestal H-mode and enhanced pedestal H-mode scenarios in NSTX. In simulations with the state of the art extended-MHD code M3D-C1, it is found that plasma resistivity can significantly alter macroscopic edge-stability in ELMing H-mode discharges in NSTX. These discharges are limited by resistive kink-peeling modes, while both ELM-free scenarios appear limited by ideal ballooning modes. While MAST discharges are often seen to be unstable to ballooning modes, recent results in MAST-U indicate the presence of kink-peeling modes. We investigate whether the impact of resistivity on PB stability is a result of aspect ratio or profile alterations due to Li coating in NSTX. We find that the simulation predictions are consistent with experimental observations in the considered discharges. The model thus enables higher fidelity predictions for ELM thresholds and presents a valuable basis in the quest for a predictive model for ELMs in low-aspect ratio tokamaks. This is an important step towards a compact fusion power plant. |
Wednesday, October 19, 2022 3:30PM - 4:00PM |
PI01.00004: An overview of the first physics results from MAST Upgrade Invited Speaker: Jack Lovell MAST Upgrade is a major enhancement of the MAST spherical tokamak, featuring 19 new poloidal field coils for improved plasma shaping, a new central solenoid with increased inductive flux, tightly baffled upper and lower divertor chambers and an improved diagnostic suite. First plasma was achieved in October 2020. By the end of the first physics campaign 12 months later real time feedback control of the plasma current, vertical position, outer radius and divertor strike points were all achieved and operating routinely. |
Wednesday, October 19, 2022 4:00PM - 4:30PM |
PI01.00005: The physics of plasma detachment in the novel MAST-Upgrade Super-X divertor Invited Speaker: Kevin Verhaegh MAST-Upgrade is a new spherical tokamak with a tightly baffled, double-null Super-X divertor. Its first results show a ~ x2 reduction in the detachment threshold (line-averaged density) compared to a conventional divertor. In this presentation we show the first MAST-U Super-X detachment analysis obtained utilising visible spectroscopy to explain the macroscopic divertor plasma behaviour from its microscopic origin: plasma-atom/molecule interactions. Our measurements show that the tightly baffled divertor elevates the molecular density, resulting in an unprecedented impact of plasma-molecular interactions on plasma detachment. This work has implications for plasma-edge modelling and data analysis of tightly baffled divertor designs. |
Wednesday, October 19, 2022 4:30PM - 5:00PM |
PI01.00006: RF Sheath Mitigation with Insulating Antenna Enclosures on the LAPD Invited Speaker: Gurleen Bal A single strap, high-power (~150kW), RF (2.4MHz) antenna was used to study RF sheaths in a magnetized helium plasma with plasma parameters ne ~ 1018 – 1019 m-3, Te ~ 1 – 10 eV and B0 ~ 0.1 T. Three experiments were carried out on the Large Plasma Device (LAPD) using different plasma-facing materials for ICRF antenna enclosure. These experiments demonstrated that electrically isolating the antenna walls from the bulk plasma can significantly reduce near-field rectification. The three different enclosure materials included copper, MACOR (electrically isolating), and MACOR over copper (MACOR-copper). In the case of the MACOR-copper side walls, the non-conductive MACOR material was exposed to the bulk plasma but a layer of copper was added below to allow for image currents. In the case of the copper enclosure, RF rectified potentials and associated convective cells were observed and reported [1]. In the experiments with MACOR and MACOR-copper enclosures, RF rectification was significantly reduced. Additionally, these experiments showed no evidence of convective cell formation. |
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