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 krho_s = 2, 3.6, 5.2, respectively; kr≈ 0.87k_theta) and high-k_theta (k= 8, 20 cm−1, and krho_s ≈ 0.73, 1.86, respectively;) 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-k_theta turbulence from densityfluctuation at r/a ~ 0.7–0.97 were measured by tangential and poloidal CO2 laser collective scattering diagnostics, respectively. Note that kr, k_theta, k_perp and rho_s are radial wavenumber, poloidal wavenumber, perpendicular wavenumber and ion gyroradius at electron temperature, respectively. Both high-kr/k_theta turbulence power and energy confinement time tau_E 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/k_theta turbulence power increases/decreases correspondingly and the tau_E shows corresponding decrease/increase;the stable ne is related to stable both tau_E and high-kr/k_theta turbulence power. Statistical results of high-kr/k_theta turbulence power versus tau_E further imply that high-kr/k_theta turbulence shows a strong correlation with plasma energy confinement degradation in high line-averaged ne condition, but high-kr and high-k_theta turbulence have relatively weak and no dependence on the transition between linear range tau_E 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-k_theta density fluctuation measurement region also have been given to qualitatively explain the enhancement of turbulence power with the increase of line-averaged ne.