Recent experimental results of zonal flows in edge tokamak plasmas

1. Recent experimental results of zonal flows in edge tokamak plasmas Liu A Di MCF group of USTC EAST team and HL-2A team

2. Outline • Experiments on EAST • The arrangement of probe arrays • Change of floating potential during H-L transition • Summary • Experiments on HL-2A • The arrangement of probe arrays • The role of GAMs on deducing the radial particle transport • Summary

3. Experimental arrangement • Two groups of fast reciprocating probe system (FRPS), radial range for measurement is ρ=0.9~0.95; • Long distance separation (3m, toroidal angle between port A and E is 900); • Six probes with adjacent radial separation of 6mm, all used for measuring floating potential; • Inserted from the equatorial plane; Probe structure R=1.7m a=0.4m

4. raw probe data • The H mode was achieved through LHCD, and the two probe arrays simultaneously arrive at same radial position during the H-L transition; • no remarkable difference between fluctuation levels of H-mode and L-mode; relative fluctuation amplitude 8.4% (ELMy-free) 7.8% (L-mode) local profile unchanged?

5. Cross power and coherent coefficient spectra • How the spectra of cross power and coherence between two s on the same flux surface change during the transition; • The low frequency components (<10kHz) with large amplitudes are strongly correlated during ELMy-free H-mode; • Low frequency, large scale coherent structure;

6. The toroidal mode number • The phase delays between the two below 10kHz are <0.1π , the mode numbers are |n|<0.2; • The low frequency coherent structure is toroidally symmetric, consistent with the theoretical prediction for the low frequency zonal flow;

7. The percentage of Zonal flows power • During H-L transition, the percentage of Zonal flows decreases from ~0.9 to ~0.2; when ELMs appear/disappear, the percentage decreases/increases, ELM bursts break the symmetric structure; • In ELMy-free stage, the zonal flows dominate the spectra; S(f) is the power spectra; is the coherence spectra; is the frequency resolution;

8. Compared with ZFs on LMP • In linear magnetic plasma (LMP), how the zonal flows power change with the axial magnetic field were investigated in detail; • According to the predator-prey model: • The conditions in Tokamak edge are more complex (especially considering about GAMs); roughly say, L-mode refer to the small situation, H-mode refer to the big situation in LMP;

9. Summary I • Two floating potential fluctuations on the same flux surface was measured during H-L transition; • Large scale coherent structure with f<10kHz was observed, which occupied nearly 90% of the power spectrum in ELMy-free H-mode; • ELMs would break the toroidal symmetry and suppress zonal flows; • Suppression of the turbulence is not manifested on the relative fluctuation levels;

10. Experimental arrangement • HL-2A tokamak: • Major radius: R = 1.6m; • Minor radius: r = 0.4m; • Probe position: 0.95 • Safety factor: q = 3.2; • Ohmic discharging; • Limiter configuration. • Principles of four-tip probe: θ

11. Spectra characteristic • The spectra of cross power, coherence and phase delay between A1 and A3 floating potential were shown; • GAM dominates the low frequency components under such plasma discharge parameters, the coherence is close to 1 at the central frequency;

12. Intermittency characteristic of GAMs time • The amplitude of GAMs changes drastically with time, when the plasma parameters change very little; and it is nonperiodic; • The intermittency characteristic of GAM has also been observed on other devices; • Then we could separate the signal into two parts: turbulence with strong GAM and turbulence with weak GAM; and find the difference of the respective radial particle fluxes;

13. Suppression of turbulence by GAMs • the radial particle flux under strong GAMs has been suppressed by 13% compared with that under weak GAMs; • Four terms could influence the radial flux, which one is the main contributor to the particle transport suppression?

14. Role of GAM on deducing the turbulence • The power of density fluctuation and coherence of density and potential fluctuations contribute the main part of the deducing, while the cross phase of density and potential fluctuations and power of potential fluctuations contribute few to it.

15. Summary II • Using the intermittency characteristic of GAMs, a statistical analysis of the radial particle flux under strong and weak GAMs was estimated; • The experimental results reveal that the radial particle flux has been suppressed by 13% under strong GAMs than that under weak GAMs; • The power of density fluctuation and coherence of density and potential fluctuations contribute the main part to the radial particle flux suppression;

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