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

Observation of diffusion and convection balanced impurity distribution at thetokamak edge plasma caused by localized turbulence
G.Q. Xue1,2, W.L. Zhong2, Z.X. Wang1, X.L. Zou3, J.M. Gao2, R. Ke2, G.L. Xiao2, Z.C. Yang2, L.
Liu2, X.X. He2, M.K. Han2, S.Q. Wang2, C.Y. Chen2, D.M. Fan2, W.P. Guo2, M. Jiang2, Y.G. Li2, A.S.
Liang2, T.B. Wang2,4, J. Wen2, J. Yin2, X. Yu2, Y.R. Zhu2, D.L. Yu2, L. Nie2, Z.B. Shi2, M. Xu2 andHL-2A team2
1
School of Physics, Dalian University of Technology, China
2Southwestern Institute of Physics, China
3
CEA,IRFM, France
4
ITER Organization, France
e-mail: xueguanqun@swip.ac.cn
Impurity injection in the toroidal fusion devices wasshown to be an effective method for controlling the edge
localized mode [1], improving the plasma energy
confinement [2], and reducing the heat flux on the divertor[3]. It has been observed that plasma instabilities can beexcited or changed by the impurity ions in tokamaks.
Understanding the mechanisms of the reciprocalinteraction among impurities and plasma instabilities is ofgreat interest to these topics.
On HL-2A, specific experiments have been
performed to investigate the effect of Neon (Ne) impurity
injection on the edge plasma instabilities. For the first time,strong interactions between the localized turbulence anddiffusion/convection balanced impurity distribution areobserved in the tokamak. As shown in Figure 1(a), theplasma radiation power increases significantly after theNe impurity injection by SMBI. It is interesting to notethat the increase in the radiation power is mainly localized
at = 0.7~0.9 and persists for hundreds of milliseconds，much longer than SMBI pulse length (1.2ms). This
indicates that the Ne impurity is concentrated in a
localized region, with little or no radial transport (impurityflux equals zero) under free source of impurity. Thisshows that the diffusion and convection terms of theimpurity ions are compensated. A broad-band spectrumof turbulence at frequencies ranging from 25 to 70 kHz isexcited after Ne impurity injection, lasting the same timewith the impurities as shown in Figure 1(b). Furthermore,this broad-band turbulence is localized at about =0.7~0.9, where the Ne impurity is concentrated, as shownin Figure 2. This indicates that the turbulence is excited by
the Ne impurity injection. It is inferred that the turbulence
could play a role in maintaining edge peaked impurity
ions. These results provide reliable support for thedevelopment of divertor steady-state and localizedradiation layers control by impurity injection in tokamaks.

Figure 1. (a) Monitor of the SMBI pulse; and spatiotemporal
evolution of the increment of plasma radiation power density
after Ne impurity injection, (b) amplitude spectrogram of density
fluctuation at ρ = 0.85, and (c) integrated turbulence intensity inthe frequency range of 25-70kHz.

Figure 2. Profiles of the relative intensity of theturbulence (25-70kHz) measured by BES before andafter SMBI.

References[1] W.L. Zhong et al, Nucl. Fusion 59 (2019) 076033[2] G.Q. Xue et al, Nucl. Fusion 61 (2021) 116048[3] J.M. Gao et al, Nucl. Fusion 63(2023) 036006