Edge radiated temperature fluctuations across confinement regime transitions in favorable and unfavorable drift configurations at ASDEX Upgrade

Abstract

The physics behind the difference in heating power threshold for the low to high confinement (L–H) transition between favorable ion $B \times \nabla B$ drift (toward active X -point) and unfavorable ion $B \times \nabla B$ drift (away from active X -point) magnetic configurations is a longstanding open question. In this work we show differences in edge ( $\rho_\mathrm{pol} = 0.95$ –1.0) turbulence in matched heating power favorable and unfavorable magnetic configuration discharges using a correlation electron cyclotron emission diagnostic to measure radiated temperature ( $T_\mathrm{rad}$ ) fluctuations. During power matched L-mode phases, the edge $T_\mathrm{rad}$ fluctuations have higher amplitude in the unfavorable configuration plasma as compared to the favorable configuration plasma. The lower fluctuation amplitude in favorable configuration is observed concomitant with increased E × B shear, as compared to the unfavorable case. Although this turbulence is different in amplitude, it is similar in radial and frequency structure between favorable and unfavorable configurations and grows to the weakly coherent mode during the I-mode phase of the unfavorable configuration discharge. A broadband pedestal fluctuation is seen during an inter-ELM H-mode period of the favorable configuration discharge.

Keywords

• edge turbulence
• shear
• confinement
• electron cyclotron emission diagnostics