SOL width broadening by spreading of pedestal turbulence

Chu, Xu; Diamond, P. H.; Guo, Zhifang

doi:10.1088/1741-4326/ac4f9f

“…Notably, the increase in inward spreading leads to a larger ELM size, while outward spreading can broaden the SOL width. These findings are consistent with the theory of turbulence spreading (Chu, Diamonds & Guo 2022). Figure 19 provides a comprehensive overview of the divertor heat flux width plotted against the fluctuation energy intensity flux for all the cases discussed in this paper.…”

Section: Impact Of Turbulence Spreading On the Divertor Heat Flux Widthsupporting

confidence: 92%

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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“…The radial turbulent transports in the HESEL simulations are observed to increase with the increase of n e,LCFS , while the parallel transports do not change much (see figure 11(b) in [22]). Since magnetic drifts set the lower limit of λ q , the obtained λ js scalings in equations ( 3) and ( 4) and the comparisons in figures 7 and 8 might imply the turbulence broadening of λ q /λ js (a hot topic in recent λ q study [17,21,24]) on EAST.…”

Section: Scaling Of the Sol Particle Flux Width For Deuterium Plasmasmentioning

confidence: 89%

Experimental scalings of scrape-off layer particle flux width by outboard divertor Langmuir probes for deuterium and helium plasmas on EAST

Liu,

Meng,

Xu

et al. 2023

Nucl. Fusion

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The scrape-off layer (SOL) power width (λq ) is an important parameter for predicting the heat load on divertor targets for future magnetically confined devices. Currently, the underlying physics for λq scaling is not fully understood. This paper extends the previous SOL particle flux width (λjs, λq ≈ λjs is assumed, js is the measured ion current density used as a proxy for particle flux) scaling measured by the inboard divertor Langmuir probes (Div-LPs) in deuterium plasmas (Liu et al 2019 Plasma Phys. Control. Fusion 61 045001) to the λjs scalings measured by the outboard Div-LPs in both deuterium and helium plasmas on EAST. A systematic method has been used to correct the upper-outer Div-LP measurements to reduce the measurement uncertainty of λjs . About 520 discharges are selected for the construction of six databases (H-mode, L-mode, and Ohmic for deuterium and helium plasmas) to scale λjs . Since the published λq scalings do not fit EAST λjs well in the databases, four separate λjs scalings are proposed using statistical method to minimize the scaling parameters. It is found that λjs has a robust scaling dependence on the plasma line-averaged density ( n ˉ e ) for both deuterium and helium plasmas and a strong positive scaling dependence on the divertor leg length (L div,leg). The scaling exponent of the stored energy agrees well with the previous λq /λjs scalings for deuterium plasmas. The near unity scaling exponents of edge safety factor and L div,leg reveal the importance of parallel connection length in determining λjs . The positive scaling dependence of λjs on n ˉ e and the 2 ∼ 4 times larger scaling amplitude compared with the published λq scalings probably indicate the turbulence broadening of λjs on EAST. The comparison of λjs for deuterium and helium plasmas using the constructed databases shows no significant difference. Statistical comparison of the normalized λjs with different ELM types suggests that the grassy-ELM λjs is slightly larger than the inter large-ELM (type-I ELM) λjs for both deuterium and helium plasmas.

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“…However, in the ELM regime, λ q is predominantly determined by the outward turbulence spreading at the separatrix. λ q is broadened for increasing edge fluctuation energy density flux Γ ε [36]. λ q ∼ B −1 p scaling persists for the broadened SOL.…”

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confidence: 83%

How fluctuation intensity flux drives SOL expansion

Li,

Xu,

Diamond

et al. 2023

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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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SOL width broadening by spreading of pedestal turbulence

Cited by 13 publications

References 31 publications

Turbulence spreading effects on the ELM size and SOL width

Turbulence spreading effects on the ELM size and SOL width

Experimental scalings of scrape-off layer particle flux width by outboard divertor Langmuir probes for deuterium and helium plasmas on EAST

How fluctuation intensity flux drives SOL expansion

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