In the edge plasma of the ASDEX tokamak, electrostatic fluctuations were observed with Langmuir probes and in Halpha light with high poloidal and temporal resolution. These fluctuations contribute a significant fraction to the 'anomalous' radial particle transport in the scrape-off layer (SOL). The basic properties and the dependence of the fluctuation parameters on the discharge conditions are documented. A model for an instability mechanism specific to the SOL is introduced and the experimentally observed fluctuation parameters are compared with the predictions of the linearized version of this model. For plasma temperatures above approximately 10 eV in the SOL the observed parameter dependences of the fluctuations are well reproduced by the model. By mixing length arguments the radial transport and the resulting density and pressure gradients in the SOL are estimated from the model. Their dependence on plasma temperature and density qualitatively agrees with the behaviour observed in ohmic discharges on ASDEX

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Edge-localized modes (ELMs) have been observed by diagnostic systems with high spatial and temporal resolution in the ASDEX and JET tokamaks. During the ELMs, substructures exist in the scrape-off layer (SOL) with sizes of 2-5 cm and lifetimes of the order of 20-50 µs. Their poloidal-temporal evolution is directly observed, and the radial transport due to turbulent E × B drift in these substructures is estimated. A comparison is made between 'normal' fluctuations between ELMs and these substructures in terms of poloidal size and velocity and related radial transport. The increased radial transport during ELMs is shown to be mainly due to these substructures. The observed fluctuation amplitudes and velocities of the substructures are found to be compatible with the model of radial transport by plasma 'blobs' and plasma potential gradients generated by the sheath boundary conditions. The transport due to turbulent radial E × B flows is compared during and between ELMs, and the observation of ELM substructures in the main plasma SOL is put into context with the observation of substructures on the target plates.

ASDEX is a large tokamak (R = 1.65 m, a = 0.4 m, I p < 500 kA) that started operation in 1980. Its distinctive features are a double-(or single-) null poloidal divertor, the capability for long-pulse operation (up to 10 s) and high.power neutral-beam (4.3 MW), ICRH (3 MW) and LH (2.4 MW) heating systems. Several highly significant experimental findings have been obtained, facilitated by the large flexibility of the machine. The high-recycling divertor regime is considered the most viable solution for handling the large power fluxes envisaged for the next generation of tokamak devices. Studies of impurity transport in the bulk plasma led to the postulation of an additional inward particle drift, while gas puffing experiments revealed the importance of thermoelectric forces along field lines for impurity retainment by the divertor. The highconfinement regime (H-mode) of neutral-beam-heated plasmas, discovered on ASDEX, is also intimately connected with the divertor configuration. The H-mode.confinement, in turn, enabled the investigation of /J-limits resulting in the experimental scaling / 3 ^^ 2.8 I p /(a Bj) [%; MA, m, T], in excellent agreement with theoretical predictions. The most important results from recent experiments with RF heating and current drive include the recharging of the OH transformer at constant plasma current, and the observed high heating efficiency obtained with combined second-harmonic ICRH and neutral-injection heating.

Electrostatic fluctuations have been measured in the scrape-off layer of ASDEX and W7-AS by Langmuir probes and by observation of Hα light and light emitted from a fast Li atom beam with high spatial and temporal resolution. It was demonstrated that these fluctuations contribute a significant, if not dominant, fraction of the anomalous radial particle transport. The basic properties of the fluctuations are the same in both experiments. A model for an instability mechanism specific to the SOL is presented including density, temperature and electric potential fluctuations. From this model mixing length estimates for the radial transport and resulting density and pressure gradients in the SOL are derived and compared to measurements in the mid-plane and in the divertor of ASDEX. In spite of several simplifications in the model a quantitative agreement up to factors of 1-3 and a qualitative agreement for variations of discharge parameters is achieved between the model predictions and the measurements. Data from poloidal multi-pin probe arrays are decomposed into a sum of spatial-temporal "events" by means of a fitting procedure. Centres of selected events serve as reference points for the analysis of the dynamics in their surroundings. Averaging shows that positive and negative potential events appear mostly in pairs with the E x B drift in between directed radially outward.