Experimental study of high-k turbulence during an energy confinement degradation phase in EAST ohmic plasmas

In this paper, we present experimental studies of both high-kr (k⊥ = 10,18,26 cm-1, and k⊥ρs = 2,3.6,5.2, respectively; kr ≈ 0.87 k⊥) and high-kθ (k⊥ = 8,20 cm-1, and k⊥ρs ≤ 0.73,1.86, respectively; kθ ≈ k⊥) turbulence behavior during a ohmic energy confinement degradation phase in Experimental Advanced Superconducting Tokamak (EAST). High-kr turbulence from density fluctuation at r/a 0.5 and high- kθ turbulence from density fluctuation at r/a 0.7-0.97 were measured by tangential and poloidal CO2 laser collective scattering diagnostics, respectively. Note that kr, kθ, k⊥ and ρs are radial wavenumber, poloidal wavenumber, perpendicular wavenumber and ion gyroradius at electron temperature, respectively. Both high-kr/kθ turbulence power and energy confinement time τE are found to be temporally correlated to line-averaged electron density ne in the plasma current flattop phase (Ip =0.4 MA): when the ne shows continuous increase/decrease, the high-kr/kθ turbulence power increases/decreases correspondingly and the τE shows corresponding decrease/increase; the stable ne is related to stable both τE and high-kr/kθ turbulence power. Statistical results of high-kr/kθ turbulence power versus τE further imply that high-kr/kθ turbulence shows a strong correlation with plasma energy confinement degradation in high line-averaged ne condition, but high-kr and high-kθ turbulence have relatively weak and no dependence on the transition between linear range τE in low-ne condition and energy confinement degradation in the high-ne condition, respectively. Moreover, profiles of electron temperature Te and ne as well as their normalized gradients in a part of high-kθ density fluctuation measurement region also have been given to qualitatively explain the enhancement of turbulence power with the increase of line-averaged ne.