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≈0.87k⊥) and high₋kθ (k⊥=8,20 cm₋1, and k⊥ρs≤0.73, 1.86, respectively; kθ≈k⊥) turbulence behavior during an 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. In addition, 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 flat-top phase (Ip= 0.4 MA): when the ne shows continuous increase/decrease, the high-kr/kθ turbulence power increases/decreases correspondingly and the τΕ 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. Furthermore, 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.