Turbulence characteristics in the edge region of ECH L-mode plasmas with different densities*

Characteristics of low-k (k$_{\mathrm{\theta }}$r$_{\mathrm{i}}$ \textless 0.5) electron temperature fluctuations has been studied in the edge region of ECH L-mode plasmas with different line averaged densities (n$_{\mathrm{e}})$. The spectral analysis of electron temperature fluctuations revealed the existence of dual modes having opposite phase velocities in the lab frame (v$_{\mathrm{L}})$. Modes with v$_{\mathrm{L}}$ \textless 0 (in the ion diamagnetic direction) are observed in the broad radial region (r/a $=$ 0.6---0.9), while modes with v$_{\mathrm{L}}$ \textgreater 0 (in the electron diamagnetic direction) are observed locally in the edge region (r/a \textasciitilde 0.9). The amplitude of the ion modes depends strongly on n$_{\mathrm{e}}$, i.e. it decreases as n$_{\mathrm{e}}$ increases with the additional gas puffing and becomes undetectably small when n$_{\mathrm{e}}$ is high (3.0e19 m$^{\mathrm{-3}})$, while the amplitude of the electron modes is affected little by n$_{\mathrm{e}}$. Therefore, in the edge region the ion modes are dominant when n$_{\mathrm{e}}$ is low (1.5e19 m$^{\mathrm{-3}})$ and vice versa. The co-existence of dual modes is observed in the L-mode edge with the intermediate n$_{\mathrm{e}}$ (1.5 \textless n$_{\mathrm{e}}$ \textless 2.0e19 m$^{\mathrm{-3}})$. This observation shows that the turbulence distribution in the ECH L-mode edge can change with n$_{\mathrm{e}}$ (or T$_{\mathrm{e}}$ since the ECH power was kept constant during the gas puffing).