The improved energy confinement mode (I-mode) is a potential candidate for future fusion power plants, as it combines ELM-free operation with good confinement. The unusual edge transport and turbulence in this regime is still not fully understood. This study analyzes the turbulent structure of the weakly coherent mode (WCM) in ASDEX Upgrade. Measurements from Doppler back-scattering and a thermal helium beam diagnostic are used to determine velocities of the background plasma and the WCM over multiple discharges. A phase velocity of the WCM of the order of 2–5 km s−1 in the electron diamagnetic drift direction is found, quantitatively close to a drift wave assuming negligible temperature fluctuations. A good agreement with a previously proposed mechanism behind the I-mode regime is observed. This marks the first experimental verification of a specific understanding of the WCM and the I-mode regime.
This study analyzes the turbulent structure of the weakly coherent mode (WCM) in ASDEX Upgrade, a fusion device. Measurements show the WCM has a phase velocity similar to drift waves, providing experimental evidence that the WCM is a drift wave. This helps understand the unusual edge transport and turbulence in the I-mode regime, a promising fusion confinement mode.