In W 7-AS the H mode has been observed for the first time in a currentless stellarator plasma. H modes are achieved with 0.4 MW electron cyclotron resonance heating at 140 GHz at high density. The H phases display all characteristics known from tokamak H modes including edge localized modes (ELMs). The achievement of the H mode in a shear-free stellarator without toroidal current has consequences on //-mode transition and ELM theories.

Counter injection into ASDEX leads to good particle, momentum, and also energy confinement with XE -80 ms at 1 MW (43 ms for co-injection). The improved confinement develops gradually during the heating phase and correlates with a simultaneous peaking of the density profile. The ion heat transport has to be reduced for a consistent transport analysis, in agreement with theoretical expectations. The sawtooth instability flattens the density profile and transiently reduces the energy content.PACS numbers: 52.55.Fa, 52.50.Gj An important goal of tokamak research is the development of plasma regimes with good confinement under auxiliary heating conditions to provide well established scenarios for the upcoming deuterium-tritium experiments. An important and challenging task is the understanding of the energy and particle transport in a tokamak plasma. In particular, it is the electron transport which is much higher than neoclassical theory predicts and which shows an unexplained variation with plasma parameters. But there is also accumulating experimental evidence that the ion heat transport is enhanced as well above the neoclassical expectation. 1 The effort to increase the plasma temperatures by auxiliary heating generally causes the electron transport to increase even further, resulting in the degraded confinement properties of the L mode. An alternative to the L-mode confinement is the H mode which shows good confinement properties even at high heating power 2 (L denotes low and H high confinement properties).In this paper we report on another operational regime characterized by good confinement properties. It develops when neutral injection as an auxiliary heating method is applied in the counter direction (against the direction of the plasma current). The injection of energetic neutral atoms into the plasma is presently the most successful and reliable auxiliary heating method. The orbits of the energetic ions after ionization depend strongly on the injection geometry. In case of counter injection (ctr-NI) the power and particle deposition profiles are broader and the fraction of ions which is lost during the slowing down phase is somewhat larger. There have been many attempts in the past to study ctr-NI heated plasmas and to compare their characteristics with those under co-injection (co-NI). 3,4 The main results were that particle confinement improves and remarkable differences in impurity transport have been noted. While co-NI reduces the impurity concentration, ctr-NI causes the impurities to accumulate in the plasma center. The different transport behavior was explained on the basis of neoclassical impurity transport in a situa-tion with momentum input and plasma rotation. 5 The energy confinement time T£, however, was found to degrade like that of L-mode plasmas heated with co-NI.On ASDEX we observe improved particle confinement with ctr-NI both affecting the bulk plasma (leading to a density increase without any external gas puffing only fueled by the beam) and giving rise to low-Z and high-Z impurity acc...

We give an overview of the different confinement regimes observed on ASDEX and compare the changes during the transition phases with qualitative tendencies suggested by theoretical models. The transitions discussed are those between purely Ohmic heating and additional heating in the L-regime, between the Land the H-regime, and between discharges with flat and peaked electron density profiles.

Strongly peaked electron density profiles have been obtained in ASDEX by different refuelling methods: pellet fuelling (ohmic and co-injection heating), NBI counter-injection and recently by reduced gas puff fuelling scenarios. These discharges show in common increased density limits, a canonical electron temperature profile independent of the density profile and an improvement of the particle and energy confinement. Whereas the changes in particle transport are not fully understood, transport analyses point out that the improved energy transport can be explained by reduced ion conduction losses coming close to the neoclassical ones. The different results for the ion transport with flat and peaked density profiles are quantitatively consistent with that expected from qi-driven modes. The analyses cannot yet explain the anomalous electron energy transport, apart from identified continuous trends such as inverse scaling with the isotope mass and enhancement with heating power.

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The Divertor Tokamak ASDEX, its neutral injection system and its ICRH system have been modified to pennit additional heating with a power of 6 MW for pulse lengths up to 10 s. The paper summarizes the arguments for long-pulse heating, describes the technical modifications of the divertor performed, their effect on the operational behaviour of the tokamak and presents a few typical results of recent experiments exploiting the long-pulse heating facilities.