On the turbulence characterization of the quasicoherent mode in EDA high confinement discharges in ASDEX Upgrade

Intrinsic ELM-free regimes that combine a high-performance fusion core with a compatible power exhaustion solution are desirable in a fusion reactor. The enhanced D-Alpha (EDA) H-mode is an ELM-free regime that fulfills several requirements for ITER and a DEMO [1,2]. The principal signature of the EDA H-mode is an edge oscillation called quasicoherent mode (QCM) that is believed to prevent the pressure gradient from overcoming the peeling-ballooning boundary. However, the driven instability responsible for the appearance of the QCM is not well understood.

In this contribution, we present the latest experimental results on the turbulence characterization of the QCM in ASDEX Upgrade, mainly focusing on the data from a scanning probe and the thermal helium beam diagnostic in the outer midplane in EDA H-mode discharges. The QCM is localized radially in the pedestal foot, extending to the open field line region [3,4]. The mode is observed with frequency in the range 13-70 kHz and normalized poloidal wavenumber k_θρ_s = 0.018-0.075 [3,4], suggesting ion scale driven instability. The QCM induces radially outward transport across the separatrix, with an anti-correlated cross-phase between density and potential fluctuations in the confined region (a fingerprint of electromagnetic instabilities) and more correlated further out, consistent with a drift-wave [4]. Finally, the experimental observations are compared with initial results from turbulence simulations with the gyrokinetic code GENE.