Overview of I-mode studies on ASDEX Upgrade: access, confinement properties and turbulence characterization

The I-mode is an improved confinement regime of tokamak plasmas where an edge transport barrier is observed in the energy channel but not in the particle transport [1]. This is in contrast to H-mode confinement, which is characterized by transport barriers in both energy and particles. Consequently, in I-mode the impurity confinement time is low, which results in low values of the effective charge Zeff. The I-mode can only be obtained if the power threshold for H-mode access is kept high, e.g. by using unfavorable magnetic configurations. After substantial I-mode research by the fusion community in the last years, the mechanism which selectively reduces only one of the transport channels is still not understood. Another characteristic feature is an instability called the weakly coherent mode (WCM), which resides in the plasma edge.

An overview of I-mode studies performed on the ASDEX Upgrade tokamak is given, including power threshold, pedestal development and confinement properties. Detailed investigations of turbulence properties in the I-mode edge have been carried out. It is shown that the WCM is coupled to the geodesic acoustic mode (GAM), which explains the width of the WCM. Stability analysis shows that the I-mode edge is peeling-ballooning stable and thus does not exhibit type-I ELMs. The confinement improvement in I-mode is accompanied by a deepening of the edge radial electric field well, suggesting a reduction of turbulence amplitudes due to E×B shear flow. Moreover, while low-amplitude density fluctuations are reduced in I-mode, strongly intermittent high-amplitude density bursts are observed by Doppler reflectometry at various turbulence wavenumbers in the plasma edge inside the separatrix. They have a solitary structure and are connected to the WCM. After their generation, they are expelled from the plasma and cause energy input into the divertor.