7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

SOL Width Expansion driven by Fluctuation Energy Intensity FluxNami Li1, X. Q. Xu1, P. H. Diamond2, Y. F. Wang3, X. Liu3, T. Zhang3, N. Yan3 and G. S. Xu3
1

Lawrence Livermore National Laboratory, 2 University of California San Diego, 3 Institute ofPlasma Physics, Chinese Academy of Sciencese-mail (speaker): li55@llnl.gov

The pulsed heat load due to large ELMs is an existential
problem for future devices of ITER size and FPPs
because that load would produce unacceptable damage tothe divertor plates. Simultaneously controlling large
ELMs and divertor heat loads in H-mode plasma iscrucial for achieving steady-state operation of a tokamak
fusion reactor. Recently, both experiments andsimulations have shown that H-mode plasma regimes
with small/grassy ELMs can help to reduce the ELM sizeand broaden the SOL width 𝜆! , while maintaining highplasma confinement compared to type-I ELMs [1-3].
Investigating how turbulence spreading affects edge
profile evolution and the SOL width is essential.
Turbulence simulations using BOUT++ were conducted
to investigate the effects of turbulence spreading on
ELM size and the broadening of the divertor heat fluxwidth (𝜆! ) in small ELM regimes [4,5].

inward penetration depth. The results from BOUT++
nonlinear simulations indicate that turbulence energy
intensity flux 𝛤" is a crucial factor in the broadening ofthe SOL width 𝜆! [6]. As the fluctuation energy
intensity flux 𝛤" at the last close flux surface (LCFS)
increases, 𝜆! expands. The transition from ELM-free tosmall ELM regime leads to a significant broadening of𝜆! , due to the strong radial transport of turbulence energy,
which is consistent with EAST experimental results that𝜆! is broadened as the upstream density fluctuation
intensity increases.
The outward turbulence spreading from pedestal to theSOL is shown to be sensitive to pedestal parameters,such as pedestal pressure gradient ∇P# and pedestal∗
collisionality 𝜐$%&. The fluctuation energy density flux∗
𝛤" increases for increasing ∇P# and decreasing 𝜐$%&.
Peeling modes are seen to be especially effective for
spreading on account of their large radial extent, which
endows them with a large mixing length. The fluctuationenergy density flux 𝛤" induced by low-n peeling modes,
which dominate at low collisionality, is larger than for
high-n ballooning modes, which occur at highcollisionality, on account of the wider mode structure ofthe former.
Operating in H-mode with small ELMs has tremendous
potential to address two of the most critical problems for
Tokamak fusion reactors: significantly reducing the ELM
size and substantially broadening the SOL width.

Fig. 1. (a) 3D plot of heat flux width 𝜆! vs poloidal
magnetic field 𝐵$ and fluctuation energy intensity
flux 𝛤" ; 2D plot of heat flux width 𝜆! vs poloidal
magnetic field 𝐵$ (b1) and fluctuation energy
intensity flux 𝛤" (b2).

References

This study is motivated by EAST experiments, where thepedestal is near marginal stability for small ELMs andrelaxes into a linearly stable state after the initial ELM
crash. In small ELM regimes, the ELM size issignificantly smaller than that of the large ELMs, but it
can still increase with a corresponding increase of the[1] G.S. Xu, et al., Phys. Rev. Lett. 122 (2019) 255001[2] X.Q. Xu, H.Q. Wang, et al, IAEA FEC 2020
[3] N.M. Li, X.Q. Xu, et al., Nucl. Fusion 62 (2022)
096030[4] N.M. Li, X.Q. Xu, et al., Phys. of Plasmas 29 (2022)
122302[5] N.M. Li, X.Q. Xu, et al., submitted to Phys. ofPlasmas (2023)[6] N.M. Li, X.Q. Xu, et al., submitted to Phys. Rev. Lett.
(2023)

This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract
DE-AC52-07NA27344, LLNL-ABS-845665 and by UCSD under Contract No. DE-FG02-04ER54738.