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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.

The effect of a radial electric field on nonthermal ripple-trapped ions is investigated using toroidal Monte Carlo simulations for edge tokamak plasmas. The increase in the neutral particle flux from the ions trapped in local magnetic wells observed by the charge exchange (CX) detector at a low confinement to high confinement transition at ASDEX (Axially Symmetric Divertor Experiment). Upgrade tokamak [Proceedings of the 20th European Conference on Controlled Fusion and Plasma Physics, Lisbon (European Physical Society, Petit-Lancy, Switzerland, 1993), Vol. 17C, Part I, p. 267] is reproduced in the simulations by turning on a radial electric field near the plasma periphery. The poloidal and toroidal angles at which the CX detector signal is most sensitive to the radial electric field are determined. A fast response time of the signal in the range of 50–100 μs to the appearance of the electric field can be found in the simulations with a relatively large half-width of the negative electric field region.

Feedback-controlled puffing of neon and deuterium has been applied to control the edge-localizedmode behavior and the target plate power deposition during high-power H-mode discharges in ASDEX Upgrade.A regime has been found in which more than 90% of the heating power is lost through radiation and divertor detachment occurs, without deterioration of the energy confinement. The plasma remains in the 0 mode, exhibiting small-amplitude, high-frequency ELM's, which do not penetrate to the target plates in the strike zone region. PACS numbers: 52.55.Fa Reduction of the energy Aux density to the target plates to below the values attainable by purely geometric spreading of the divertor fan is one of the most critical requirements for a fusion reactor [1]. Impurity radiation losses from the outer regions of the main plasma and scrape-off layer are, at present, considered the most viable option for attaining such a mode of operation [2]. The resulting state corresponds to a low-power Bow to the target plates (~0.1 of the total heating power), and low plasma densities and pressures in front of them, and has been termed detachment [3]. Experimentally, this aim has been pursued by raising the edge plasma density by strong gas puffing, and by the controlled introduction of light impurities. The most successful previous experiments, with strong additional heating, had involved feedback control of either the impurity puff (in TEXTOR [4]) or of the deuterium puff rate (in JET [5]). While continuous detachmentcould thereby be achieved in limiter and L-mode divertor discharges, experiments in H-mode resulted in either a relapse into the L regime or a reduction only of the time-averaged power Row, with heat pulses associated with edge localized modes (ELM's) still penetrating to the target plates [6,7].The experiments on the ASDEX Upgrade reported here, were employed for the first time in a divertor tokamak feedback control of the radiated power losses through impurity (neon) addition. Simultaneously, we applied deuterium gas puffing, feedback controlling, and also the divertor neutral density. In the most successful operating mode, we attained divertor detachment both in between and during ELM s, while maintaining standard H-regime energy confinement.Device, operating range, and diagnostics description -ASDEX Up.grade is a midsize tokamak (Ro = 1.65 m, a = 0.5 m, and plasma elongation b/a = 1.6) with a single null divertor (Fig. 1). All plasma-facing components are graphite-covered, the vessel is routinely boronized, and turbomolecular pumps allow control of the hydrogen and noble gas particle content of the vessel. The experiments described here were carried out in deuterium, with n,~1.2 X 10 m