Impurity convection reversal caused by edge localized turbulence for generating a stationary edge radiative layer in the HL-2A tokamak

Abstract

The effect of impurity on edge turbulence was investigated on the HL-2A tokamak using neon supersonic molecular beam injection (SMBI). For the first time, a stationary radiative layer accompanied by edge localized turbulence (ELT) lasting more than 100 ms is observed at the edge region $\rho = 0.7\sim0.9$ . This duration is much longer than the SMBI pulse length (1.2 ms), indicating that the impurity diffusion is on the whole canceled out by the impurity convection without spreading of the localized turbulence during this period. This implies that the impurity convection is reversed from the positive to the negative impurity density gradient region. This reversal could be explained by the parallel impurity compression mechanism. The observed turbulence is a broadband electrostatic turbulence (25–70 kHz) propagating in the electron diamagnetic drift direction with a normalized poloidal wavenumber of $0.1 \unicode{x2A7D} {k_\theta }{\rho _{\text{s}}} \unicode{x2A7D} 0.4$ . It has been found that the excitation of the ELT strongly depends on the local collisionality ${\nu ^*}$ . This feature is qualitatively predicted by the spectral multiscale gyrokinetic turbulence simulation, which suggests that the turbulence could be a dissipative trapped electron mode. This experiment demonstrates that mutual interaction between externally injected impurity ions and edge turbulence can generate a stationary edge radiative layer and avoid impurity core accumulation. These results could provide a potential approach for the formation of a stable impurity radiative layer by impurity injection in tokamaks.

Keywords

• impurity injection
• edge turbulence
• impurity radiative layer