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Studies of the ‘Quiescent H-mode’ regime in ASDEX Upgrade and JET

The stationary edge localized mode (ELM)-free ‘Quiescent H-mode’ (QH-mode) regime, obtained with counter neutral beam injection, is studied in ASDEX Upgrade and Joint European Torus. QH-mode plasmas have high pedestal and core ion temperatures together with good High-confinement mode (H-mode). ELMs are replaced by continuous MHD oscillations, the ‘edge harmonic oscillation’ (EHO) and the ‘high frequency oscillation’. Stationarity of particle and impurity densities is linked to the occurrence of these MHD modes. The EHO location in the steep-gradient region and its appearance with increasing edge pressure points towards the edge pressure or pressure gradient as possible drivers for the EHO. Injection of small cryogenic pellets can raise the plasma density to about 40% of the Greenwald density limit without triggering ELMs. Orbit following calculations of the slowing-down distribution show the presence of an enhanced fast particle density in the H-mode barrier region despite the large loss currents with counter-injection. The radial electrical field in the edge barrier region, about twice as large in QH-mode as in ELMy H-mode, is large enough to reverse the precession drift direction of injected beam ions, leading to a resonance of the EHO with the drift precession frequency.