Impact of pedestal density gradient and collisionality on ELM dynamics

Li, N M; Xu, X. Q.; Wang, Y F; Lin, Xin; Niu, Y.; Xu, G. S.

doi:10.1063/5.0111669

Cited by 2 publications

(20 citation statements)

References 23 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%

“…These two shots are simulated to investigate the impact of turbulence spreading on the ELM size and divertor heat flux width. 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).…”

Section: Physics Models and Simulation Settingsmentioning

confidence: 99%

“…EAST observed a grassy ELM regime in a metal wall at high density/collisionality ν * (Xu et al 2019a). A wide pedestal with a low pedestal density gradient is an essential condition for access to the grassy ELM regime, in addition to high q 95 or high poloidal magnetic field B p (Li et al 2022a;Xu 2022). In AUG, the type-II ELM regime, where high confinement and small ELMs occur simultaneously, was achieved with high SOL density (Stober et al 2001;Wolfrum et al 2011).…”

Section: Introductionmentioning

confidence: 99%

“…The pedestal density/collisionality and width scans for EAST indicate that small ELMs can be triggered by marginally unstable modes. These modes include low-n peeling modes, high-n ballooning modes, intermediate-n peeling-ballooning modes or a local ballooning instability in the pedestal foot, particularly in the presence of a larger separatrix density gradient (Li et al 2022a). The size of the ELM, whether small or large, strongly depends on the inward avalanching and turbulence spreading from the linearly unstable zone to the stable zone during the nonlinear saturation phase (Li et al 2022a).…”

Section: Introductionmentioning

confidence: 99%

“…These modes include low-n peeling modes, high-n ballooning modes, intermediate-n peeling-ballooning modes or a local ballooning instability in the pedestal foot, particularly in the presence of a larger separatrix density gradient (Li et al 2022a). The size of the ELM, whether small or large, strongly depends on the inward avalanching and turbulence spreading from the linearly unstable zone to the stable zone during the nonlinear saturation phase (Li et al 2022a). In the small/grassy ELM regime, the SOL turbulent thermal diffusivity increases due to larger turbulent fluxes being ejected from the pedestal into the SOL, resulting in a broadening of the SOL width (Li et al 2019;Xu et al 2019b;Li et al 2020).…”

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: 99%

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 fluctuation intensity flux drives SOL expansion

Li,

Xu,

Diamond

et al. 2023

Nucl. Fusion

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Predictions of heat load widths λ_q based on particle orbits alone are very pessimistic. This paper shows that pedestal peeling-ballooning (P-B) MHD turbulence broadens the stable SOL by the transport, or spreading, of fluctuation energy from the pedestal. λ_q is seen to increase with Γ_ε, the fluctuation energy density flux. We elucidate the fundamental physics of the spreading process. Γ_ε increases with pressure fluctuation correlation length. P-B turbulence is seen to be especially effective at spreading, on account of its large effective mixing length. Spreading is shown to be a multiscale process, which is enhanced by the synergy of large and small-scale modes. Pressure fluctuation skewness correlates well with the spreading flux – with the zero crossing of skewness and Γ_ε spatially coincident – suggesting the role of coherent fluctuation structures and the presence of intermittency in λ_q broadening. λ_q~B_p^(-1) scaling persists for the broadened SOL. We show that the spreading flux increases for increasing pedestal pressure gradient ∇P_0 and for decreasing pedestal collisionality υ_ped^*. This trend is due to the dominance of peeling modes for large ∇P_0 and low υ_ped^*. Ultimately, we see that a state of weak MHD turbulence, as for small ELMs, is very attractive for heat load management. Our findings have transformative implications for future fusion reactor designs and call for experimental investigations to validate the observed trends.

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Impact of pedestal density gradient and collisionality on ELM dynamics

Cited by 2 publications

References 23 publications

Turbulence spreading effects on the ELM size and SOL width

Turbulence spreading effects on the ELM size and SOL width

How fluctuation intensity flux drives SOL expansion

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