Density fluctuation statistics and turbulence spreading at the edge of L–mode plasmas

Khabanov, F. O.; Hong, R.; Diamond, P. H.; Tynan, G. R.; Yan, Z.; McKee, G. R.; Chrystal, C.; Scotti, F.; Yu, G.; Zamperini, S. A.; Zhu, Y.

doi:10.1088/1741-4326/ad820d

Density fluctuation statistics and turbulence spreading at the edge of L–mode plasmas

Journal Article · Sun Oct 20 20:00:00 EDT 2024 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/ad820d· OSTI ID:2472871

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Abstract

Long-wavelength density fluctuations ( $k ρ_{i}$ <1) are studied using beam emission spectroscopy (BES) at the edge of DIII-D L-mode plasmas ( $ρ$ = 0.88–1.1) in scenarios with electron cyclotron heating (ECH) power ramp ( $P_{ECH}$ up to 1.5 MW), neutral beam injection (NBI) power ramp ( $P_{NBI}$ up to 2.5 MW), and injected torque scan (−1 < $T_{inj}$ <0.6 Nm). We find that broadband turbulent density fluctuations ( $f$ ∼ 20–120 kHz) have a non-Gaussian distribution. The skewness of $δ n / n$ changes sign from negative at $ρ$ <0.95–0.97 to positive at $ρ$ > 0.97, indicating the prevalence of density ‘voids’ at inner radii and density ‘blobs’ at outer radii and outside of the separatrix. The turbulence intensity flux $⟨ {\tilde{v}}_{r} {\tilde{n}}^{2} ⟩$ is calculated to characterize turbulence spreading at the plasma edge. During ECH/NBI power ramps and at counter- $I_{p}$ injected torque, $⟨ {\tilde{v}}_{r} {\tilde{n}}^{2} ⟩$ is directed inward inside the separatrix, which is evidence of inward spreading of turbulence intensity from the edge gradient region caused by the inner propagation of density ‘voids’. Significantly weaker $⟨ {\tilde{v}}_{r} {\tilde{n}}^{2} ⟩$ is observed with co- $I_{p}$ torque. A correlation between co- $I_{p}$ torque, turbulence intensity $δ n / n$ at $ρ$ = 0.97, and increased srape-off layer (SOL) heat flux decay length $λ_{q}$ is found in the torque scan scenario, showing that edge turbulence plays a material role in determining the SOL conditions and heat flux width.

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Sponsoring Organization:: USDOE

Grant/Contract Number:: AC52-07NA27344

OSTI ID:: 2472871

Alternate ID(s):: OSTI ID: 2452821
OSTI ID: 2511193
OSTI ID: 2570844

Journal Information:: Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 12 Vol. 64; ISSN 0029-5515

Publisher:: IOP PublishingCopyright Statement

Country of Publication:: IAEA

Language:: English

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