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

Turbulence spreading and flow shearing dynamics in high density operationTing Long1, Patrick H. Diamond2, Rui Ke1, Zhipeng Chen3, Xin Xu3, Wenjing Tian1,4, Yanmin Liu1,Qinghu Yang3, Jinbang Yuan1, Min Xu1, Jiquan Li1, Wei Chen1, Wulyu Zhong1
12Southwestern Institute of Physics, Chengdu, China
CASS and Department of Physics, University of California, San Diego, CA, USA3
Huazhong University of Science and Technology, Wuhan, China
4Tsinghua University, Beijing, Chinae-mail (speaker): longt@swip.ac.cn

Turbulence spreading generally refers to the spatial
propagation of turbulence energy due to nonlinear
interactions[1]. It plays a significant role in thecomprehensive performance of magnetic fusion devices.
It is related to the breaking of gyro-Bohm scaling[2,3]. It
is also important in the core-boundary coupling to affectthe pedestal structure as well as the divertor heat load
width[4-6]. Recent studies show that, the density limit canbe regarded as a state linked to the breakdown of
turbulence self-regulation[7-9]. In this report, we present
the recent experimental study about the dynamics ofturbulence spreading and shear flow in the high density
operation on the J-TEXT tokamak.
It is found that as collision rate increases, turbulence
spreading flux 〈𝑣̃𝑟 𝑛̃2 〉/2 increases. Turbulence
spreadingpower
𝒫𝑆 = −𝜕𝑟 〈𝑣̃𝑟 𝑛̃2 〉⁄2 increases
significantly in the region just inside the last close flux
surface (LCFS). The normalized turbulence spreading
power 𝒫𝑆 /𝒫𝐼 = (−𝜕𝑟 〈𝑣̃𝑟 𝑛̃2 〉⁄2)/(−〈𝑣̃𝑟 𝑛̃〉𝜕𝑟 〈𝑛〉) , a
measure of turbulence internal energy increments due tothe turbulence spreading relative to the local turbulence
production, increases with the normalized collisionality𝜈 /𝜀𝜈𝑖∗ ≡ 1/2 𝑖𝑖, as shown in Figure 1. The enhanced
𝜀𝑣𝑡ℎ,𝑖 /(𝑞𝑅)

2021, 63(8): 085017.
[4] Hahm T. et al. Phys Plasmas, 2005, 12(9): 090903.
[5] Singh R. et al. Phys Plasmas, 2020, 27(4): 042308.
[6] Chu X. et al. Nuclear Fusion, 2022, 62(6): 066021.
[7] Long T. et al. Nuclear Fusion, 2021, 61(12): 126066.
[8] Ke R. et al. Nuclear Fusion, 2022, 62(7): 076014.
[9] Diamond P. H. et al. Philosophical Transactions ofthe Royal Society A: Mathematical, Physical andEngineering Sciences, 2023, 381(2242): 20210227.

Figure 1. The normalized edge turbulence spreading
power VS the collisionality.

edge turbulence spreading plays an important role in theedge cooling approaching the density limit. This isstrongly associated with the interaction between the𝐸 × 𝐵 shear flow and the turbulence random scattering.
For high Greenwald fraction 𝑛̅/𝑛𝐺 , a region of ∆𝑟~𝑙𝑐𝑟
is observed at the edge, where the turbulence intensity
flux 〈𝑣̃𝑟 𝑛̃2 〉/2 is large but the turbulence intensity
gradient 𝜕𝑟 〈𝑛̃2 〉 is near zero. This implies thebreakdown of Fick’s law of local flux-gradient
formulation. Besides, the “mean jet velocity” ofturbulence spreading 𝑉𝐼 = 〈𝑣̃𝑟 𝑛̃2 〉/〈𝑛̃2 〉 shows the linear
positive correlation with the skewness of density
fluctuations and presents an increasing trend with theauto-correlation time of density fluctuations, as shown in
Figure 2(a-b). It is also found that the increasing
symmetry breaking between positive and negative
density fluctuations (especially blobs and holes)
coincides with the enhanced turbulence spreading.
References[1] Gürcan Ö. et al. Physical review letters, 2006, 97(2):024502[2] Lin Z. et al. Phys Plasmas, 2004, 11(3): 1099-108.
[3] Yan Q. et al. Plasma Physics and Controlled Fusion,

Figure 2. The “mean jet velocity” of turbulence
spreading VS (a) the skewness of density fluctuationsand (b) the auto-correlation time of density fluctuations.