Puffing of impurities (neon, argon) and deuterium gas in the main chamber is used to feedback control the total radiated power fraction and the divertor neutral particle density simultaneously in the ASDEX Upgrade tokamak. The variation of Psep=Pheat-Prad(core) by impurity radiation during H mode shows a similar effect on the ELM behaviour as that obtained by a change of the heating power. For radiated power fractions above 90%, the ELM amplitude becomes very small and detachment from the divertor plates occurs, whilst no degradation of the global energy confinement is observed (completely detached high confinement mode). Additional deuterium gas puffing is found to increase the radiated power per impurity ion in the plasma core owing to the combined effect of a higher particle recycling rate and a lower core penetration probability. The outer divertor chamber, which is closed for deuterium neutrals, builds up a high neutral pressure, the magnitude of which is determined by the balance of particle sources and pumping. For this particular situation, the effective pumping time of neon and argon is considerably reduced, to less than 0.3 s, mainly owing to an improved divertor retention capability. The radiation characteristics of discharges with a neon driven radiative mantle are modelled using a 1-D radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The results of simulations are in good agreement with experiment
The H-mode obtained in ASDEX Upgrade by three heating methods (Ohmic, NBI, ICRF) is analysed. The power threshold is shown to be relatively low compared with other tokamaks and to be independent of the heating method The operational window for the different types of H-phases, (dithering, ELMing with type-LII and I E M S , ELM-free) is given and discussed. The features of the H-mode discharges, in particular quasi-stationary ELMing phases with type I ELMs, are described and discussed. Finally, it is shown that the power flux through the plasma edge is a key parameter for H-mode operation and conml.
In the ASDEX Upgrade, X-point marfe formation and its behaviour up to the density limit is investigated in gas-fuelled ohmically-heated single-null discharges over a wide range of pameters: Ip = 0.6-1.2 MA, Bw = I .35-2.4 T, a plasma elongation of 1.6 and 2 , ~ c 2. The standard-ion VB drifl is directed towards the X-point. At medium electron densities inevitably a marfe develops in the vicinity of the active X-point The marfe formation is consistent with a model of thermal instabilities in the radiating edge plasma. Moreover, stable steady-state operation is demonstrated with marfes which can extend significmtly into the bulk plasma. The density limit is always connected with quick m d e expansion and movement followed by mode-locking leading to a major disruption. The limit scales linwly with In md is in good agreement with the Greenwald density limit scaling. The resulting experimental Hugill limit is E,Rqv5 / B E = 2.8 x IO2" m-2 T I . Reversal of the ion VB drift direction away from the target has a detrimental effect an the density limit.
The results of divertor studies on ASDEX Upgrade, at currents of up to 1.2 MA and heating powers up to 10 M W are described, with emphasis on the ELMy H-mode. The spatial and temporal characteristics of their heat load, and the simulation of ELMs by a time-dependent scrape-off layer code are described. High gas puff rata were found to lead to a large increase in divertor neutral pressure, at modest changes in %, and to a strong reduction in timeaveraged power flow and complete detachment from both target plates in between ELMs. Using pre-programmed puffs of neon and argon, the radiative power losses could be raised to 75% of the heating power, in H-regime discharges, and the regime of enhanced divertor neutral pressure was found also to lead to an improved pumping of recycling impurities. 1.Introduction:ASDEX Upgrade is a mid-size tokamak w i t h non-circular cross-section (major radius R, , = 1.625m, horizontal minor radius a = 0.5 m, elongation b/a = 1.6), purpose-designed as a poloidal divertor device (Figure 1). Further distinguishing features of it are the poloidal field coils placed outside the toroidal ones, and the presence of a saddle coil ("PSL" .. pssive Stabilising loop) inside the vacuum vessel for stabilising the vertical displacement instability. Together, these two features provide a relatively large space between the vacuum vessel and the X-point of the poloidal field lines, although the present divertor configuration, selected to optimise the heat load distribution, places the target plates relatively close to the x-point.
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