Density fluctuation statistics and turbulence spreading at the edge of L–mode plasmas

Abstract

Long-wavelength density fluctuations ( $k{\rho _i}$ 0.97, indicating the prevalence of density ‘voids’ at inner radii and density ‘blobs’ at outer radii and outside of the separatrix. The turbulence intensity flux $\left\langle {{{\tilde v}_{\text{r}}}{{\tilde n}^2}} \right\rangle $ is calculated to characterize turbulence spreading at the plasma edge. During ECH/NBI power ramps and at counter- ${I_{\text{p}}}$ injected torque, $\left\langle {{{\tilde v}_{\text{r}}}{{\tilde n}^2}} \right\rangle $ is directed inward inside the separatrix, which is evidence of inward spreading of turbulence intensity from the edge gradient region caused by the inner propagation of density ‘voids’. Significantly weaker $\left\langle {{{\tilde v}_{\text{r}}}{{\tilde n}^2}} \right\rangle $ is observed with co- ${I_{\text{p}}}$ torque. A correlation between co- ${I_{\text{p}}}$ torque, turbulence intensity $\delta n/n$ at $\rho $ = 0.97, and increased srape-off layer (SOL) heat flux decay length ${\lambda _q}$ is found in the torque scan scenario, showing that edge turbulence plays a material role in determining the SOL conditions and heat flux width.

Keywords

• tokamak
• edge density fluctuations
• turbulence spreading