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

Journal Article · Mon Oct 21 00:00:00 EDT 2024 · Nuclear Fusion

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

Khabanov, F. O. ^[1]; Hong, R. ^[2]; Diamond, P. H. ^[3]; Tynan, G. R. ^[3]; Yan, Z. ^[1]; McKee, G. R. ^[1]; Chrystal, C. ^[4]; Scotti, F. ^[5]; Yu, G. ^[6]; Zamperini, S. A. ^[4]; Zhu, Y. ^[6]

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1. Univ. of Wisconsin, Madison, WI (United States)
2. Univ. of California, Los Angeles, CA (United States)
3. Univ. of California, San Diego, CA (United States)
4. General Atomics, San Diego, CA (United States)
5. Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
6. Univ. of California, Davis, CA (United States)

Long-wavelength density fluctuations ($$k{\rho _i}$$ <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 (ƒ ~ 20–120 kHz) have a non-Gaussian distribution. The skewness of $$\delta n/n$$ 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 $$\left\langle {{{\tilde v}_{\text{r}}}{{\tilde n}^2}} \right\rangle$$ is calculated to characterize turbulence spreading at the plasma edge. During ECH/NBI power ramps and at counter-I_p injected torque, $$\left\langle {{{\tilde v}_{\text{r}}}{{\tilde n}^2}} \right\rangle$$ 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 $$\left\langle {{{\tilde v}_{\text{r}}}{{\tilde n}^2}} \right\rangle$$ is observed with co-I_p torque. A correlation between co-I_p torque, turbulence intensity $$\delta n/n$$ at ρ = 0.97, and increased srape-off layer (SOL) heat flux decay length $${\lambda _q}$$ is found in the torque scan scenario, showing that edge turbulence plays a material role in determining the SOL conditions and heat flux width.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA); USDOE Office of Science (SC), Fusion Energy Sciences (FES); USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF)
• Grant/Contract Number: AC52-07NA27344; FC02-04ER54698; FG02-08ER54999; SC0021338; SC0019352; FG02-99ER54531; SC0023500; SC0022528; DESC0022528
• OSTI ID: 2472871
• Alternate ID(s): OSTI ID: 2452821; OSTI ID: 2511193
• Report Number(s): LLNL-JRNL-2001868
• Journal Information: Nuclear Fusion, Vol. 64, Issue 12; ISSN 0029-5515
• Publisher: IOP Science
• Country of Publication: United States
• Language: English

Related Subjects

plasma physics
tokamak
edge density fluctuations
turbulence spreading