Characteristics of grassy ELMs and their impact on the divertor heat flux width

Li, Nami; Xu, X. Q.; Wang, Yifeng; Niu, Y.; Zhang, Jiayuan; Qian, Jinping; Sun, Jizhong; Wang, Dezhen

doi:10.1088/1741-4326/ac83d9

Cited by 7 publications

(15 citation statements)

References 37 publications

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“…In this paper, two EAST discharges of small/grassy ELMs with different poloidal magnetic field are studied. These are shot #103745 with lower-single null divertor configuration (Li et al 2022a) and high poloidal magnetic field B p and #090949 with upper-single null divertor configuration (Li et al 2022b) and low B p . These two shots are simulated to investigate the impact of turbulence spreading on the ELM size and divertor heat flux width.…”

Section: Physics Models and Simulation Settingsmentioning

confidence: 99%

“…For shot #103745, the small ELM is triggered by the marginal intermediate-n peeling-ballooning instability (Li et al 2022a). For shot #090949, the small ELM is triggered by the low-n peeling mode (Li et al 2022b). The simulations are radially global with both edge and SOL regions, which range from normalized poloidal flux ψ n = 0.75 to ψ n = 1.05 for shot #103745 and from ψ n = 0.8 to ψ n = 1.1 for shot #090949.…”

Section: Physics Models and Simulation Settingsmentioning

confidence: 99%

“…The divertor heat flux width is ~2 times larger than that estimated by the ITPA multi-machines scaling law, due to strong turbulent transport (Li et al. 2022 b ). The pedestal density/collisionality and width scans for EAST indicate that small ELMs can be triggered by marginally unstable modes.…”

Section: Introductionmentioning

confidence: 96%

“…BOUT++ nonlinear simulations for EAST indicate that small ELM crashes are accompanied by the upward movement of the peeling boundary induced by an initial radially localized collapse in the pedestal, which stops the growth of instabilities (Xu et al 2019a). The linear and nonlinear simulations for a long-pulse, high-q 95 , high-B p grassy ELM shows that the ELM crash is triggered by low-n peeling modes, and fluctuations are generated at the peak pressure gradient position and then radially spread outward into the scrape-off-layer (SOL) (Li et al 2022b). The divertor heat flux width is ∼2 times larger than that estimated by the ITPA multi-machines scaling law, due to strong turbulent transport (Li et al 2022b).…”

Section: Introductionmentioning

confidence: 99%

“…The linear and nonlinear simulations for a long-pulse, high-q 95 , high-B p grassy ELM shows that the ELM crash is triggered by low-n peeling modes, and fluctuations are generated at the peak pressure gradient position and then radially spread outward into the scrape-off-layer (SOL) (Li et al 2022b). The divertor heat flux width is ∼2 times larger than that estimated by the ITPA multi-machines scaling law, due to strong turbulent transport (Li et al 2022b). The pedestal density/collisionality and width scans for EAST indicate that small ELMs can be triggered by marginally unstable modes.…”

Section: Introductionmentioning

confidence: 99%

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Turbulence spreading effects on the ELM size and SOL width

Li,

Xu,

Diamond

et al. 2024

J. Plasma Phys.

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BOUT++ turbulence simulations were performed to investigate the impact of turbulence spreading on the edge localized mode (ELM) size and divertor heat flux width $({\lambda _q})$ broadening in small ELM regimes. This study is motivated by EAST experiments. BOUT++ linear simulations of a pedestal radial electric field (Er) scan show that the dominant toroidal number mode (n) shifts from high-n to low-n, with a narrow mode spectrum, and the maximum linear growth rate increases as the pedestal Er well deepens. The nonlinear simulations show that as the net E × B pedestal flow increases, the pressure fluctuation level and its inward penetration beyond the top of the pedestal both increase. This leads to a transition from small ELMs to large ELMs. Both inward and outward turbulence spreading are sensitive to the scrape-off-layer (SOL) plasma profiles. The inward turbulence spreading increases for the steep SOL profiles, leading to increasing pedestal energy loss in the small ELM regime. The SOL width $({\lambda _q})$ is significantly broadened progressing from the ELM-free to small ELM regime, due to the onset of strong radial turbulent transport. The extent of the SOL width $({\lambda _q})$ broadening depends strongly on outward turbulence spreading. The fluctuation energy intensity flux ${\varGamma _\varepsilon }$ at the separatrix can be enhanced by increasing either pedestal Er flow shear or local SOL pressure gradient. The ${\lambda _q}$ is broadened as the fluctuation energy intensity flux ${\varGamma _\varepsilon }$ at the last close flux surface (LCFS) increases. Local SOL E × B flow shear will restrain outward turbulence spreading and the associated heat flux width broadening. Operating in H-mode with small ELMs has the potential to solve two critical problems: reducing the ELM size and broadening the SOL width.

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Section: Physics Models and Simulation Settingsmentioning

confidence: 99%

Section: Physics Models and Simulation Settingsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Turbulence spreading effects on the ELM size and SOL width

Li,

Xu,

Diamond

et al. 2024

J. Plasma Phys.

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How turbulence spreading improves power handling in quiescent high confinement fusion plasmas

Li,

Chen,

Diamond

et al. 2024

Commun Phys

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Viable magnetic fusion devices necessitate combining good confinement with effective power flux handling. A major concern for ITER, and devices beyond, is the divertor heat load width, which sets peak boundary heat loads on the plasma-facing materials. Current estimates of the heat flux width are narrow for future reactors. Here, we demonstrate how pedestal turbulence can expand into, or entrain, the stable scrape-off-layer and so broaden the heat flux width beyond these neoclassical predictions. Employing combined theoretical, computational, and experimental approaches, we focus on quiescent high confinement discharges on the DIII-D tokamak, but the results are of broader significance. Our findings uncover common trends in the edge turbulence intensity flux, the pressure perturbation skewness, and the turbulence mixing length, which together determine the heat flux width. This research demonstrates the physics of scrape-off-layer broadening by turbulence and highlights the promise of a turbulent pedestal for successful core-edge integration in ITER and future fusion devices.

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Impact of pedestal density gradient and collisionality on ELM dynamics

Wang

et al. 2022

Physics of Plasmas

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BOUT++ turbulence simulations are conducted to capture the underlying physics of small ELM characteristics achieved by increasing separatrix density via controlling strike points from vertical to horizontal divertor plates for three EAST discharges. BOUT++ linear simulations show that the most unstable modes change from high-n ideal ballooning modes to intermediate-n peeling–ballooning modes and eventually to peeling–ballooning stable plasmas in the pedestal. Nonlinear simulations show that the fluctuation is saturated at a high level for the lowest separatrix density. The ELM size decreases with increasing separatrix density, until the fraction of this energy lost during the ELM crash becomes less than 1% of the pedestal stored energy, leading to small ELMs. Simulations indicate that small ELMs can be triggered either by the marginally peeling–ballooning instability near the peak pressure gradient position inside the pedestal or by a local instability in the pedestal foot with a larger separatrix density gradient. The pedestal collisionality scan for type-I ELMs with steep pedestal density gradient shows that both linear growth rate and ELM size decrease with increasing collisionality. On the contrary, the pedestal collisionality and pedestal density width scan with a weak pedestal density gradient indicate small ELMs can either be triggered by a high-n ballooning mode or by a low-n peeling mode in a low collisionality region 0.04–0.1. The simulations indicate the weaker the linear unstable modes near marginal stability with small linear growth rate, the lower nonlinearly saturated fluctuation intensity and the smaller turbulence spreading from the linear unstable zone to stable zone in the nonlinear saturation phase, leading to small ELMs.

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Characteristics of grassy ELMs and their impact on the divertor heat flux width

Cited by 7 publications

References 37 publications

Turbulence spreading effects on the ELM size and SOL width

Turbulence spreading effects on the ELM size and SOL width

How turbulence spreading improves power handling in quiescent high confinement fusion plasmas

Impact of pedestal density gradient and collisionality on ELM dynamics

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