Magnetic structure and frequency scaling of limit-cycle oscillations close to L- to H-mode transitions

Abstract

Limit-cycle oscillations (LCOs) close to the power threshold of L-to H-mode transitions are investigated in plasmas of ASDEX Upgrade. During this phase, referred to as I-phase, a strong magnetic activity in the poloidal magnetic fieldḂ θ with an up-down asymmetry is found. In some cases, the regular LCOs during I-phase transition smoothly into a phase with intermittent bursts which have similar properties to type-III edge localised modes (ELMs). Indications of precursors during the intermittent phase as well as in the regular LCO phase point to a common nature of the I-phase and type-III ELMs. The LCO frequency measured in a set of discharges with different plasma currents and magnetic fields scales as f ∼ (B 1/2 t I 3/2 p )/(nT ).

The role of turbulence–flow interactions in L- to H-mode transition dynamics: recent progress

“…It has been pointed out in several experiments that LCO close to the H-mode transition can resemble type III ELMs, with a characteristic magnetic precursor (n = 0, m = 1) [48,49]. In AUG, precursor oscillations and an m = 1 magnetic perturbation propagating in the ion diamagnetic direction, corresponding to or very similar to a type III ELM signature, has also been recently detected during extended LCO periods [44].…”

“…The radial inward propagation changes the phase shift between density-fluctuation level and E × B drift, and is consistent with a reversal of the limit-cycle direction near or inside the bottom of the E r well. Outward propagation in the scrape-off layer (SOL) was recently also revealed in AUG [44]. It is not known at this time whether this radial dependence is universal in tokamaks.…”

Poloidal asymmetric flow and current relaxation of ballooned transport during I-phase in ASDEX Upgrade

“…Magnetic fluctuations measured by pick up coils sensitive to the poloidal magnetic componentḂ θ appear during Iphase in ASDEX Upgrade [20,22]. These fluctuations are strongly correlated with the flow represented by the Doppler shift from Doppler reflectometry [20,22]. To obtain a global picture of the poloidal mode structure the cross-correlation between different poloidally displaced Mirnov coils on a poloidal cross-section has been calculated.…”

“…They show up as a very robust feature and occur during low [1] as well as medium densities [20,22]. In order to interpret the experiment, the evolution equations for these magnetic fluctuations are derived in the present contribution.…”

“…Whereas the basic mechanism of turbulence suppression by zonal flows [13][14][15][16], the existence of zonal flows during the L-H transition [12] and the predatorprey dynamics in general [17] have been experimentally demonstrated, turbulence suppression by flow generation appears strong enough to trigger a transition into the Hmode [1,2,4,5,9,18] but some experiments show only weak or insufficient zonal flow activity at the L-H transition [6,7,19,20]. Recently also magnetic fluctuations are observed during these limit cycle oscillations [7,8,[20][21][22] even though the predator-prey model [10,11] is electrostatic in nature.…”