The role of shear flow collapse and enhanced turbulence spreading in edge cooling approaching the density limit

• Published in: Nuclear fusion 2024-04, Vol.64 (6)
• Main Authors: Long, Ting, Diamond, P. H., Ke, Rui, Chen, Zhipeng, Xu, Xin, Tian, Wenjing, Hong, Rongjie, Cao, Mingyun, Liu, Yanmin, Xu, Min, Wang, Lu, Yang, Zhoujun, Yuan, Jinbang, Zhou, Yongkang, Yan, Qinghao, Yang, Qinghu, Shen, Chengshuo, Nie, Lin, Wang, Zhanhui, Hao, Guangzhou, Wang, Nengchao, Chen, Zhongyong, Li, Jiquan, Chen, Wei, Zhong, Wulyu
• Format: Article
• Language: English
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; density limit; edge cooling; shear flow; tokamak; turbulence spreading
• ISSN: 0029-5515

Experimental studies of the dynamics of shear flow and turbulence spreading at the edge of tokamak plasmas are reported. Scans of line-averaged density and plasma current are carried out while approaching the Greenwald density limit on the J-TEXT tokamak. In all scans, when the Greenwald fraction $f_G = \bar {n}/n_G = \bar {n}/ (I_{\text{p}}/\pi a^2)$ increases, a common feature of enhanced turbulence spreading and edge cooling is found. The result suggests that turbulence spreading is a good indicator of edge cooling, indeed better than turbulent particle transport is. The normalized turbulence spreading power increases significantly when the normalized $E \times B$ shearing rate decreases. This indicates that turbulence spreading becomes prominent when the shearing rate is weaker than the turbulence scattering rate. The asymmetry between positive/negative (blobs/holes) spreading events, turbulence spreading power and shear flow are discussed. These results elucidate the important effects of interaction between shear flow and turbulence spreading on plasma edge cooling.