The “Quiescent H-mode” (QH-mode) regime, originally discovered in the DIII-D tokamak, combines good stationary H-mode connement with the absence of Edge Localised Modes, which are replaced by a characteristic continuous MHD oscillation, the “Edge Harmonic Oscillation” (EHO). Quiescent H-mode has been studied in ASDEX Upgrade in three campaigns with reversed plasma current for neutral beam injection opposite to the plasma current (counter-injection). The regime has not been obtained so far with co-injection. QHmode is achieved with all neutral beam sources in ASDEX Upgrade, with injection angles ranging from near radial to tangential at mid-radius on the high field side. Strong gas fuelling results in a loss of QH-mode and re-appearance of ELMs. We have injected cryogenic pellets without losing QH-mode, however the density achieved so far was limit to about 40% of the Greenwald density. The pedestal pressure and stored energy in QH-mode are similar or higher than in ELMy H-modes at similar parameters. The radial electrical field in the H-mode barrier region, measured with a Doppler reflectometer, is about twice as large as in ELMy H-mode. The lowest effective charge with counter-injection so far was obtained after fresh boronisation, a value of Zeff= 2,5 in QH-mode, similar to that obtained in ELMy phases. The Edge Harmonic Oscillation (EHO) is studied in some detail. The EHO fundamental has an n=1 toroidal mode number and appears at frequencies of 7...12 kHz. The poloidal mode number adjusts itself according to a resonant surface in the H-mode edge barrier region. The characteristic harmonics as seen in spectograms of various types of measurements originate from a rigidly rotating anharmonic shape of the helical displacement. ECE measurements reveal the EHO does not have an island structure. All stationary ELM-free phases show EHO activity, and the disappearance of the EHO, for example after impurity injection by laser-blow-off, marks the onset of a density rise.
In addition to the EHO, a second characteristic type of magnetic activity is observed at much higher frequency, 350...500 kHz. This mode, termed the “High Frequency Oscillation” (HFO), occurs in bursts that have a fixed phase relationship with the EHO cycles. There is a strong correlation between EHO cycles, HFO bursts and oscillations of the Da light intensity in the outer divertor, suggesting that particle losses occur in a fixed phase relation with the EHO amplitude modulation. The ASCOT Monte-Carlo orbit following code code is used for simulations of the slowingdown process of injected neutral beam ions, taking into account the experimental plasma
configuration and experimental profiles of density, temperature and radial electrical field. Despite large orbit losses, the fast particle population in the H-mode barrier region is significantly larger with counter-injection than with co-injection. The radial electrical field in QH-mode is sufficiently large to reverse the direction of the precession drift and to create a resonance of the EHO with a fraction of the fast particle population. The significance of these two observations for the existence of the EHO is discussed. |
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