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 UI01: Magnetic Confinement Fusion VILive Streamed
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Chair: Sebastian De Pascuale, Oak Ridge National Lab Room: Ballroom 100 A |
Thursday, October 20, 2022 2:00PM - 2:30PM |
UI01.00001: Small ELM dynamics and its impact on the SOL width scaling Invited Speaker: Nami Li Simultaneous control of large ELMs and divertor heat loads in H-mode plasma is crucial for steady-state operation of a tokamak fusion reactor. BOUT++ turbulence simulations are performed to capture the physics of the small ELM characteristics with different pedestal density profiles achieved via controlling strike points from vertical to horizontal divertor plates in EAST discharges. When the strike point shifts from the vertical to horizontal divertor plates, the separatrix density increases, and the pedestal pressure gradient and bootstrap current decrease. Linear simulations show that the most unstable modes change from high-n ideal ballooning modes to the intermediate-n peeling-ballooning modes and eventually to peeling-ballooning stable plasmas in the pedestal as separartrix density increases. Nonlinear simulations show that both the fluctuation level and the elm size decrease as the separatrix density increases, leading to small ELMs. To project from the current tokamak to ITER relevant parameters, pedestal collisionality scans are performed with a fixed pedestal pressure. The ELM size increases as the collisionality decreases for type-I ELMs, which shows a good agreement with an experimental scaling. However, starting from a stable pedestal plasma at high collisionality, small ELMs can be easily triggered either by high-n ballooning modes with very weak pedestal density gradient or by low-n peeling modes with relatively large pedestal density gradient by decreasing the normalized collisionality to <0.1. The SOL width is calculated as a function of separatrix density fluctuation intensity flux from the pedestal to the SOL, showing positive correlations between them which is consistent with the theory of turbulence spreading. Operating in H-mode with small ELMs will solve two critical problems: reducing the ELM size and broadening the SOL width. |
Thursday, October 20, 2022 2:30PM - 3:00PM |
UI01.00002: On the formation and control of an X-point radiator in ASDEX Upgrade: SOLPS-ITER simulations and experiments Invited Speaker: Ou Pan The X-point radiator (XPR) is an attractive scenario to solve the power exhaust problem in future fusion devices. In the ASDEX Upgrade tokamak (AUG), experiments with an XPR showed a dissipated power fraction larger than 90 %, fully detached divertor targets and ELM suppression with a moderate confinement degradation [Bernert, NF, 2021]. Recently, a reduced model [Stroth, NF, 2022] was derived to explain the physical mechanisms for initiating a stable XPR. However, 2D numerical simulations are required to interpret the features not caught by the reduced model, including the spatial distribution of particle and power sources, cross-field transport and drifts in an XPR. In this work, the transport code SOLPS-ITER [Wiesen, JNM, 2015] was applied to reproduce the experimentally measured plasma conditions in AUG and to study the parameters relevant for the formation and control of an XPR. |
Thursday, October 20, 2022 3:00PM - 3:30PM |
UI01.00003: Low recycling, low collisionality and high performance with lithium conditioning in LTX-β Invited Speaker: Anurag Maan
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Thursday, October 20, 2022 3:30PM - 4:00PM |
UI01.00004: Impact of closure on the deeply detached divertor in DIII-D Invited Speaker: John Canik Experiments on DIII-D have shown that divertor closure has only a weak impact on the deeply detached state where the divertor ion flux is reduced to a small level, including the associated core performance. While closure has been shown to impact detachment onset conditions, it is unclear that it will have a strong effect under strong detachment as required for wall armor integrity in magnetic fusion devices beyond ITER. This has been tested in DIII-D, where intrinsic carbon is the primary radiator, through fueling scans in a) an open divertor geometry, b) a moderately closed geometry with nearby baffling, and c) a tightly baffled geometry. As in previous studies, closure is found to impact the onset of detachment, with changes of ~25-35% in the line-averaged density at which roll-over of the divertor ion saturation current is observed. However, when the divertor is pushed into deep detachment at high density, all configurations behave similarly, with strong radiation localized near the X-point, highly reduced divertor ion flux profiles, high divertor neutral pressure, and degraded global confinement. The more closed divertors show an increased ratio of divertor to midplane neutral pressure, indicating that closure can aid pumping even in highly detached conditions. In all cases high divertor neutral compression was maintained with deep detachment, with a transition out of H-mode providing the practical limit to detachment. The pedestal pressure is degraded as the density increases towards deep detachment regardless of closure, consistent with a ballooning-limited pedestal as indicated by stability calculations. All configurations show a reduction in core confinement (~20%) when deeply detached, although confinement is higher prior to detachment with more closed divertors. |
Thursday, October 20, 2022 4:00PM - 4:30PM |
UI01.00005: Magnetic fluctuation dominated electron heat transport in DIII-D ELMy H-mode pedestal Invited Speaker: Jie Chen Experimental evidence from DIII-D shows that micro-tearing modes (MTMs) can dominate electron heat transport in ELMy H-mode pedestal. Pedestal electron temperature gradient is observed to decrease as pedestal-top electron collisionality υe* is increased from ~0.4 (low) to ~1 (high), and is clamped during the entire inter-ELM period at high collisionality. Power balance analysis shows pedestal electron heat diffusivity increases 76% from 0.41 (low υe*) to 0.72 m2/s (high υe*). Experimentally-measured, internal, line-averaged magnetic fluctuations originating from MTMs increase from 3.1±0.4 to 4.1±0.5 Gauss. Estimated stochastic electron heat diffusivity using the measured magnetic fluctuation amplitudes and stochastic-field theory increases 43%, comparable to the increase of experimental electron heat diffusivity. Measured pedestal ion heat diffusivity and electron density profile evolution show little change, consistent with transport predicted by theory of micro-tearing modes. Local, nonlinear gyrokinetic simulations find MTMs are dominant in the pedestal steep gradient region at both low and high collisionalities. Simulated MTM magnetic fluctuation amplitudes and electron heat flux increase at high collisionality, qualitatively agreeing with experimental observations. This work supports gyrokinetic simulations that claim MTMs can generate significant electron heat transport in the pedestal. Furthermore, this work provides new insight into long standing issues at high collisionality, such as, H-mode pedestal degradation with high gas puffing and high density. |
Thursday, October 20, 2022 4:30PM - 5:00PM |
UI01.00006: Separating Convective and Diffusive Main Ion Particle Transport in the Tokamak Pedestal with Experimental Ionization Source Measurements Invited Speaker: Aaron M Rosenthal Quantitative edge ionization source measurements and plasma profiles during dynamic events, such as the edge localized mode (ELM) cycle and edge gas puff modulation, allows disentanglement of the relative contributions of diffusion and convection to forming the edge density profile. H-mode density pedestal rebuild after ELMs on the DIII-D tokamak show quantitative evidence of a 5 m/s inward particle pinch and 0.2 m2/s diffusion near the top of the density pedestal. Throughout the density pedestal rebuild and during steady state, inboard and outboard ionization source measurements show a significant asymmetry confirming strong poloidal variation of the ionization source. |
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