The results of runaway electron confinement experiments from ASDEX are analysed to elucidate the structure of electromagnetic turbulence which may cause anomalous electron heat transport in the L-mode confinement regime. From a simple model, the radial correlation length (W) of the magnetic turbulence is determined to be about 1 mm. Using this value and that of the experimentally deduced electron thermal diffusivity, the authors determine the radial magnetic fluctuation level at the plasma edge in the L-mode to be (B r /B 0 ) ~ 2 X 10~4. Scalings of W and B r /B 0 are deduced from parameter scans. From a comparison of these results with the predictions of various theoretical models, it is concluded that skin depth turbulence, electromagnetic drift wave turbulence, rippling modes, and microtearing modes are inferior candidates and that resistive ballooning modes offer the best possibility for a consistent interpretation of the data.

The radiation and transport characteristics of ASDEX Upgrade discharges with a neon-driven radiative mantle are modelled using a one-dimensional radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The code is well suited to describing the measured impurity line radiation, total, soft x-ray and bremsstrahlung radiation in regions of the plasma that are not dominated by two-dimensional effects. The recycling and pumping behaviour of neon as well as the bulk transport of neon for radiative boundary scenarios are discussed.

Divertor plasmas with strong external gas puffing in ASDEX Upgrade have shown very efficient impurity retention, increasing with the divertor neutral gas density. The experiments presented here use feedback-controlled gas puffs in discharges with different pumping speed to keep the divertor neutral gas flux density the same. This allows for the first time a decoupling of the divertor neutral gas flux density and the external gas flow. The resulting plasmas are almost identical and show identical impurity retention, clearly demonstrating the importance of the divertor neutral gas density over the externally induced flow. PACS numbers: 52.55.Fa, 52.40.Hf Strong external gas puffing into the edge of a fusion plasma strongly affects its properties. Generally, in Ohmically heated and L-mode plasmas, the line averaged density is increased, eventually leading to a density limit disruption. The energy and particle confinement times decrease with external gas puffing. More directly, however, gas puffing modifies the scrape-off layer and divertor plasma in a divertor tokamak, and increasing the neutral gas density in the divertor has been proposed for a long time as a means of increasing power losses via radiation or collisions with the neutral gas [1,2]. Experiments with strong puffing of D 2 in the DIII-D tokamak [3,4] and in JET [5] resulted in a power load reduction at the divertor target plates and in a detached plasma at the outer target plate [5]. In all these cases, however, the energy confinement time was degraded by the gas puffing.Strong gas puffing has also often been proposed as a means of inducing particle flows in the scrape-off layer. Such flows should entrain impurities by friction, thereby flushing them into the divertor [6]. Recently the flow of impurities in the scrape-off layer of DIII-D was investigated using the "puff and pump" technique, where Ar was puffed either at the top of the vacuum vessel or in the private flux region [7]. Additional puffing of deuterium reduced the Ar content in the plasma strongly, and this effect was attributed to the streaming of Ar with the D 2 into the divertor chamber. Similar experiments on ASDEX Upgrade have also shown a strong increase of the exhaust rate for Ne and Ar [8] and for He [9] with external D 2 gas puffing.These ASDEX Upgrade results were interpreted, however, rather as an effect of the associated increase of the neutral gas and plasma densities in the divertor, which would tend to shift the ionization zone for the recycling noble gas atoms closer to the target plates. In this region, the strong background plasma flow towards the target plates would increasingly dominate over the thermal force trying to push impurity ions out of the divertor chamber, back towards the bulk plasma. This explanation of the observed variation of divertor retention with neutral density is supported by earlier, 1D scrape-off layer modeling calculations for ASDEX [10], but requires obvious confirmation by the 2D modeling effort currently under way.Experiments carried out at f...

The H-mode operating boundaries and confinement regimes in ASDEX Upgrade are described. The L to H mode threshold power is shown to linearly increase with the product of line-averaged electron density times magnetic field. The analyses of the boundaries between different ELM types show that mainly the steady-state ELMing H-mode discharges with type-I ELMs are achieved above the power threshold. The global confinement time of such discharges is analysed. Discharges of the same type with high edge radiation power (80% of the input power radiated) obtained by neon puffing remain in the H-mode and have a good confinement. The importance of strong deuterium puffing in addition to neon is demonstrated. These promising results for power load reduction of the divertor in ITER are discussed.

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