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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??s. 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.

Edge conditions have been varied on ASDEX by altering the divertor configuration and wall conditioning. Introduction of bypasses which allow neutral gas to enter the main chamber from the divertor by paths other than through the divertor neck greatly increased the power threshold for the H-mode and led to poorer H-mode quality. At the same time a regime of improved ohmic confinement (IOC) made its appearance. Closing the bypasses and boronization of the walls restored good H-mode behavior in a wide parameter range. The degree of improved ohmic confinement was found to be a function of recycling and plasma purity. The IOC effect with all its facets could be provoked in clean discharges by neon puffing. An explanation is provided in a simple theory which links impurity radiation and power flow Psol into the scrape-off layer (SOL) to the edge density. In like manner, observed isotopic effects of density at the separatrix can be related to an isotopic dependence of Psol. A subset of ASDEX density-limit discharges may also be understood in terms of edge parameters and Psol, thereby sheading light on the weak power scaling observed for the density limit with auxiliary heating.

In ohmic discharges in all tokamaks at low plasma densities the global energy confinement time, , increases almost linearly with the density (LOC, linear ohmic confinement). In tokamaks with sufficiently large dimensions, saturates at a critical density (ASDEX: ) and is nearly constant at higher densities (SOC, saturated ohmic confinement). In the same density region some experiments report a further confinement regime for deuterium discharges in which exceeds the saturated value and is further increased (IOC, improved ohmic confinement). There the global energy confinement time roughly behaves as in the LOC regime. For both the LOC and the SOC regimes an isotope effect, i.e. the dependence of on the ion mass, is reported as an additional aspect of the ohmic energy confinement. A statistical analysis is performed to identify the parameters which are responsible for the properties of the energy confinement in these discharges in ASDEX. In contrast to earlier reports on confinement time scalings in ASDEX OH, only discharges with a full experimental description of kinetic electron and ion parameters, i.e. profiles of densities, temperatures and , are used to evaluate the energy contents of both species. By means of statistics it is shown that the characteristics of are mainly caused by the behaviour of the electron energy flux and the ohmic input power. The ion energy flux, however, does not play a significant role. Furthermore, the IOC regime is explained as a continuation of the low-density LOC regime. Both the isotope effect and the density dependence of are caused by features of the electron energy transport.

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

Electrostatic fluctuations have been measured in the scrape-off layer of ASDEX and W7-AS by Langmuir probes and by observation of H? 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.

Electrostatic fluctuations have been measured in the scrape-off layer of ASDEX by Langmuir probes and by observation of H α light with high poloidal and temporal resolution. It was demonstrated that these fluctuations contribute a significant, if not dominant, fraction of the “anomalous” radial particle transport. 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.

A statistical test for extracting and identifying coherent structures in the scrape-off layer turbulence is used to analyze Langmuir probe data from the ADITYA and ASDEX tokamaks. This method, the biorthogonal decomposition, allows one to characterize large-scale coherent structures in plasma turbulence in an unambiguous manner. It is shown that such structures effectively contribute to radial transport and to intermittency. Results from numerical simulations of turbulence driven by the resistive interchange instability in the scrape-off layer are compared to the observed statistical properties.

The charge exchange (CX) neutral energy distributions from ASDEX measured with the conventional neutral particle analyzers (NPA) at energies >500 eV are combined with the low energy CX spectra from the low energy neutral analyzer (LENA). In the region of overlap their shapes fit each other very well. With the 3D EIRENE code the neutral gas was simulated and ion temperature ( T i ) profiles from the center to the edge are obtained. The T i values at the separatrix and the edge based on the LENA data are considerably lower than those suggested earlier from the NPA data. This is attributed to the different energy ranges—high energies for the NPA, low energies for LENA—that are used for the T i evaluation.

Sawtooth-free Ohmic discharges can serve as a model case of a quiescent tokamak plasma. The properties and global parameters of such discharges as observed in ASDEX are discussed as well as the mechanism apparently responsible for stabilization of the sawtooth instability. Stationary Ohmic discharges were used to investigate particle, impurity and energy transport in the absence of the sawtooth instability. Particular emphasis was placed on comparing these results with the predictions of neoclassical theories. The level of the ion energy transport is found to be that predicted by neoclassical theory. Particle transport and impurity transport are explained in terms of neoclassical inward drift velocities and diffusion coefficients with the same small anomalous contributions. In the central region of the plasma, where the power flux is low, the values of the electron heat conductivity were found to be very low.

The authors present a brief review of collisional (classical and neoclassical) and anomalous transport. Particular emphasis is devoted to the question of charge independence of the anomalous transport coefficients and the combined action of anomalous and collisional transport. In the light of these results the experimental facts are analysed and interpreted. It is found that impurity accumulation-characterized by peaked zeff-profiles-is caused by the combined effects of improved confinement (i.e. reduction of anomalous transport) and peaking of the electron density profile. For the cases of pellet refuelled plasmas and counter neutral injection heating quantitative comparisons are performed which show good agreement between the experimental measurements and simulations based upon neoclassical theory.

The behaviour of the (2, 1) tearing mode in lower hybrid current drive (LHCD) discharges in ASDEX is examined. Due to the change in the current profile j(r), this mode can be destabilized during LHCD. This often leads to mode locking and a disruption, thereby limiting the amount of LH power that can be transferred to the plasma during current drive. A statistical analysis for two different current drive spectra shows that this power limit is dependent on both plasma current Ip and line-averaged electron density nc. The authors discuss the possibilities of disruption control and show how tailoring of the LHCD spectra might avoid the occurrence of the (2, 1) mode.

A database of high density (0.3 < e[1020 m−3] < 0.8), low qa(1.9 < qa < 3.4), Ohmic discharges from the ASDEX experiment is analysed statistically. Bulk parameter scalings and parameterized temperature and density profile shapes are presented. The total plasma kinetic energy, assuming Ti = Te, scales as e0.54±0.01 IpO.90±0.04 and is almost independent of the toroidal magnetic field. The electron temperature profile peaking factor scales as T03/2/(T3/2) = 0.94(± 0.04)qa1.07±0.04, in close agreement with the assumption of classical resistive equilibrium. In the inner half of the plasma, the inverse fall-off length for both temperature and density has a strong dependence on qa, with the temperature dependence being more pronounced. Outside the half-radius, the qa dependence disappears, but the density profile broadens near the edge with increasing plasma current. A second database of moderate density, moderate qa discharges (0.2 < e/[1020 m−3] < 0.4, 2.4 < qa < 4.2), is presented for comparison.

Angular momentum transport studies were conducted for nearly stationary situations of about 50 neutral-beam-heated ASDEX discharges under a variety of experimental conditions. Comparison of the confinement times of angular momentum and energy reveals close similarities between thermal and perpendicular momentum transport. Scaling laws are derived for the dependence of the central rotation speed, the momentum confinement time and the radially-averaged momentum diffusivity from the main experimental parameters of L-mode discharges. A well-developed isotope effect, significant power degradation and a favourable current scaling are found to be characteristic features of momentum confinement. The L-mode results are compared with the momentum transport behaviour during the improved confinement phase of H-mode, pellet-fuelled and counter-NI discharges. For the H-mode, the confinement improvement is comparable for momentum and energy. The peaked density profile scenarios exhibit a considerably more pronounced increase in momentum confinement than in energy confinement.

The scaling of the energy confinement time with plasma density and current has been investigated for ohmically heated tokamak discharges in ASDEX. The linear dependence τE e is maintained in the high density improved Ohmic confinement (IOC) regime with peaked density profiles. The peaking of the radial density profile can be brought about by reducing the net power flow through the plasma surface, thereby leading to a reduction of the edge density. Tailoring of the radiation profile with the addition of low-Z impurities, for example neon, gives access to the IOC regime under conditions where otherwise the degraded saturated Ohmic confinement (SOC) behaviour prevails. The energy confinement time τE increases with current and decreases with heating power also in Ohmic discharges, as is shown by a statistical analysis. However, with the intrinsic coupling between power and current, the two relationships cancel and τE becomes independent of POH and Ip. The two most prominent features of Ohmic confinement can therefore be explained on the basis of simple physical models.

A high-purity graphite probe for collecting impurities in ASDEX has been used for the first time. The probe head consists of a rotatable graphite cylinder housed in a fixed molybdenum shield containing an exposure slit. Measurements were made on the scrape-off layer plasma streaming to the upper outer divertor plate and, after extraction of the probe, collected impurities were identified and quantified using surface physics techniques. Single null ohmic discharges produce a threefold increase in erosion of the Cu plate of the active upper divertor compared to similiar double null discharges. The radial distributions of scrape-off layer impurities change in both shape and intensity with vertical plasma position. Boronization of the main vessel reduces the amount of Fe collected by more than a factor of two but the oxygen deposit increases. Radial distributions of Cu and Fe deposits as a function of time for a neutral beam heated discharge are also discussed.

The closed divertor configuration of ASDEX isolates the zone of dominant plasma-wall interaction in a rather well defined manner from the plasma in the main chamber. Studies have shown that for recycling gases the effective conductance between divertor and plasma chamber is closely represented by the values for molecular flow also during a discharge. Hence, from partial pressure measurements of contaminant gases in the divertor impurity recycling fluxes can be obtained. In deuterium discharges the problem of mass peak interferences, especially for methane and watervapour, has to be resolved. Data are shown for various ASDEX scenarios: stainless steel walls, carbon wall elements, Ti-gettering and boronization. The results expose the production of CO as main culprit, as long as no gettering or boronization is employed. Then, however, with carbon still present in the machine, the hydrocarbons limit the attainment of optimum performance parameters. What are the conclusions?

Large toroidal and poloidal asymmetries with characteristics which are sensitively dependent on qa, the vertical position of the plasma, and the type of additional heating are observed in the energy flow to the ASDEX divertor target plates. The largest asymmetries and total energy depositions are observed during lower hybrid wave injection experiments with approximately 50% of the input energy going to the combined divertor targets and shields. A maximum localized energy density loading of 10 MJ/m2 is typical under these conditions. Measurements of the asymmetries are consistent with a model in which magnetic islands and ergodicity due to intrinsic magnetic perturbations dominate the energy transport across the primary magnetic separatrix. These results emphasize the essential role of resonant magnetic perturbations in determining the performance of tokamaks and demonstrate that non-axisymmetric effects caused by small perturbations become increasingly important in determining the transport properties as the injected power is increased.

The use of deuterium instead of hydrogen as the fuel gas in tokamak discharges improves the confinement of particles as well as of energy. This isotope effect is also observed in the edge plasma: The separatrix density is higher in hydrogen discharges compared to deuterium discharges and the separatrix temperature is slightly lower. Moreover, the total particle flux across the scrape-off layer surface is smaller in deuterium compared to hydrogen. All isotopic scalings are reduced, however, when going from carbonized to boronized wall conditions; additional heating yields further assimilation. Edge modelling has been successfully performed to explain this behaviour. The separatrix scaling derived from this model agrees reasonably well with a statistical analysis of a large database.

Impurity transport coefficients were determined by means of sinusoidally modulated gas puffing. The principles of the method are outlined, and results for impurity diffusion coefficients, evaluated with respect to a simplified transport model, are presented for measurements on ASDEX during stationary phases. The dependence of the diffusion coefficient on various plasma parameters is found to be in striking contrast to neoclassical theory.

In magnetically confined plasmas density fluctuations of apparently turbulent nature with broad spectra in wave number and frequency space are observed which are thought to be the cause for anomalous energy and particle transport across the confining magnetic field. Collective laser light scattering has been used to study the nature of these fluctuations. Specific problems of scattering from fusion plasmas are addressed and illustrated with experimental results from the 119 μm far infrared laser scattering experiment operated on the ASDEX tokamak. Using the system in the heterodyne mode the direction of propagation with respect to the laboratory frame can be determined. Spatial resolution has bean improved by making use of the change in pitch of the total magnetic field across the minor plasma radius. Special emphasis is placed on the ohmic phase where a number of parameter variations including electron density, electron temperature, toroidal magnetic field, and filling gas were performed.

In the regime of Improved Ohmic Confinement (IOC) in ASDEX the energy confinement time tau E increases linearly with increasing line-averaged density ne up to the density limit. The establishment of the IOC is accompanied by a substantial reduction of the external gas feed, concomitant with large decreases of all plasma edge fluxes. However, the data do not supply conclusive evidence that the IOC is primarily connected with the recycling conditions. More recent observations with very clean machine conditions seem to indicate that the impurity radiation plays a significant role.

An experimental investigation of metallic impurities has been made in the ASDEX divertor in order to better understand their likely sources and production mechanisms. The impurities were collected on rotatable carbon collector strips which were exposed to the scrape-off layer plasma near the upper outer divertor plate under various discharge conditions. Quantitative ion beam analysis of exposed samples was subsequently carried out by Rutherford backscattering spectroscopy, proton induced X-ray emission and nuclear reaction spectroscopy. Time resolved measurements and spatial profiles of collected impurity fluxes reveal an increase in collected metallic impurities during auxiliary neutral beam heating of plasma discharges causing strong erosion of the copper divertor plate. To a rough approximation, 0.2% of this auxiliary heating power is invested in the production of stainless steel impurities. Measurements made for single-null divertor discharges show a copper flux of 6 × 1017 atomss−1 in the scrape-off layer which is not directly sputtered from the target plates. Collected impurity fluxes in the divertor chamber are found to increase with both the main plasma current and the density. Measurements made after boronization reveal an initial reduction in collected stainless steel impurities although boronization does not have any long term effect on the suppression of such impurities. Some limitations of the collector probe technique are also pointed out, and planned improvements of the existing setup are outlined.

The analysis of the heating properties with ICRH in ASDEX is complemented by the study of edge physics, aimed at understanding and avoiding some of the deleterious effects which presently limit in some cases the usefulness of this heating method. We review the advances that have been made on ASDEX, thereby concentrating on second harmonic heating. Experimental observations and several hardware modifications, made over the years in the machine are in agreement with the assumption that in the divertor tokamak ASDEX both additional impurity generation and a change in the transport of impurities in the scrape-off layer have to considered. The increased impurity generation is localized on the multipole protective cover near the entrance to the divertor. The need to properly design the structure close to the scrape-off layer, for an ICRF heated divertor tokamak, is emphasized.

To compare different Ohmic confinement regimes in ASDEX, the edge conditions are analyzed in detail. The results show that the improved Ohmic confinement coincides with a drop of the separatrix density. This drop allows the density profile to peak and seems to be the trigger of a change in the transport. A universal scaling between the electron temperature and the electron density at the separatrix prevails for all Ohmic scenarios. In addition, the total particle flux across the separatrix is evaluated and found to be strongly correlated to the separatrix density. Thus, the convective energy loss contributes less to the total energy losses when the confinement is improved. Since the correlations between the edge parameters do not change in different Ohmic confinement regimes of ASDEX, the edge physics appears to remain the same. Improved Ohmic confinement is characterized by an optimum separatrix density which provides a sufficiently high edge temperature together with low particle fluxes. These optimum conditions yield the maximum particle confinement.

The properties of ICRF heating applied to a boronized or carbonized machine are compared. Results from radiation, impurity production, density changes and heating are presented and discussed. The experimental observation that more ICRH power could be applied to an ohmic plasma with boronization is related to edge and divertor observation.

Impurity production and transport in ASDEX have been studied using a rotatable graphite collector probe which was exposed to the scrape-off layer plasma in the upper divertor chamber. Metallic deposits on the probe were later analysed using Rutherford backscattering spectroscopy and nuclear reaction analysis. Time resolved information (during tokamak discharges) and spatial profiles of collected impurity fluxes indicate strong erosion of the copper divertor plate during neutral beam heating of the plasma. Both stainless steel and copper deposits increase with neutral beam power. It is shown that, to a rough approximation, 0.2% of this auxiliary heating contributes to the increase in collected stainless steel impurities. Measurements made for single-null divertor discharges show that a copper flux of 6 × 1017 atoms s−1 is present in the scape-off layer which is not directly sputtered from the target plates. Measurements made subsequent to boronization reveal an initial reduction in collected heavy impurities although boronization does not appear to have any significant long-term effect on the suppression of such impurities.

The impact of ELM activity and density limit disruptions on the plasma density as well as the use of reflectometry for plasma position and shape control are discussed here. Fast density measurements allow the definition of three characteristic phases of the ELM: precursor, collapse and recovery. At the precursor phase low frequency oscillations starting approximately 3 ms prior to the ELM collapse are observed in the density profiles. Fluctuation measurements show a continuous rise of the broadband fluctuations prior to the ELM collapse. In the collapse phase the crash of the density profile is observed. In the recovery phase, the density profile starts to recover its pre-ELM shape. A comparison between HFS and LFS density behaviour shows a delay on the density collapse, comparable to the ion parallel transport time indicating a ballooning character of the ELM. For a set of ASDEX Upgrade ELMy H-modes, where plasma parameters such as plasma current, plasma density, triangularity and input power were varied, the radial velocity, ELM particle losses and affected depth, density pedestal width and edge density gradient are determined and its dependence with the plasma parameters determined. A comparison between intrinsic and pellet triggered ELMs shows no relevant differences suggesting that the triggering mechanism for both ELM type cannot be distinguished by the density profile dynamics or the density fluctuations. The changes observed on the density profiles during density limit disruptions support the observation that the erosion of the temperature is due to convection and not by stochastization. The information about the density profiles also provides a powerful tool for plasma position and shape control, as proposed for ITER to complement the magnetic measurements. Provided that the density is constant within the magnetic flux surfaces, a scaling factor between the line average density and the density at the separatrix can be used to estimate the density at the separatrix. The separatrix position can then be found from the measured density profiles. Software tools developed to quantify the errors involved in reflectometry gap evolution were used in ASDEX Upgrade ELMy H-mode discharges. The estimated position of the separatrix is found within 1 cm of accuracy when compared tosimilar magnetic data.

Tokamak major density limit disruptions are usually preceded by a m/n=2/1 tearing mode, that may or may not lock to the vessel wall. The complete destruction of energy confinement in these major disruptions is known to have a characteristic m/n=1/1 form. Initially only a small degradation of the confinement occurs, as is observed by the slow increase of the electron temperature at the plasma edge. But at some stage of the evolution of the 2/1 mode the degradation of the energy confinement shows an abrupt increase that quickly leads to a cold plasma. It was observed in circular plasmas that this abrupt increase of the degradation of the energy confinement starts at the low field side, in the neighborhood of the core facing side of the O point of the 2/1 mode. At this position a region with electron temperature similar to the temperature in the 2/1, O point, is observed to expand quickly towards the plasma core. Thomson scattering showed that at the time the electron temperature profile is flat over the plasma radius, the electron density is ~ 50% higher inside the 2/1 mode, compared at the onset of the 2/1, O point T_e erosion. In this paper we report on the 2/1, O point T_e erosion that we also observed in ASDEX Upgrade major density limit disruptions. The evolution of the temperature profile is measured with a heterodyne radiometer with 60 channels and 32 \mu s time resolution. We plan to investigate also the evolution of the density profile both from the high field side and from the low field side, measured with broad band multi channel reflectometer with 35 \mu s time resolution. The observation of the density increase inside the 2/1 mode associated with the already observed 2/1, O point T_e erosion will be an important indication that convection is involved in the abrupt destruction of the energy confinement, characteristic of major density limit disruptions.

ASDEX was in operation from February 6, 1980, till August 10, 1990. This very long period was recurrently interrupted in the course of the years when major components were newly developed, installed and commissioned, thus enhancing the experimental possibilities of the project. The many improvements and some, more or less, troublesome system faults requiring repair work limited the experimentation time of ASDEX. The reliability and availability of ASDEX, however, was very good over the years and increased with operational experience. This paper presents an overview of major modifications and operational experience. It also reports the most relevant events which hampered the experiment as well as ways of repairing and/or improving the systems concerned.

Divertor impurity retention and exhaust capability were studied at ASDEX for two different divertor geometries. In one of these, bypass openings allow neutral gas exchange between divertor and main chamber (open divertor), in the other the bypasses are closed (closed divertor). Experiments were made with a closed upper divertor and an open lower one, as well as with both divertor closed, and discharges with double-null or single-null (up or down) magnetic configuration where measured. From the analysis of the observed up-down asymmetries for the different configurations, information is deduced on the performance of different divertor geometries. Results show that a closed divertor has a much better retention and exhaust capability and the plasma impurity content is drastically reduced.

Boronization is the plasmachemical deposition of thin boron/carbon or pure boron films onto the inner wall of a tokamak device. Since mid-April 1989 the ASDEX vessel has been boronized seven times in glow discharges, first with a gas mixture of 10% B 2 H 6 + 10% CH 4 +80% He (A), once with 10% B 2 +90% H 2 (B) and once with 10% B 2 H 6 +90% He (C). All boronization runs were done at room temperature vessel walls. Local surface analysis of the deposited layers yielded thicknesses of up to 2000A. Boronization provided considerably improved plasma conditions. In ohmic discharges, reductions of CO by a factor of more than five in the divertor chamber and of two for H 2 O were observed, in relation to a reduction of oxygen by a factor of more than five as measured by spectroscopy, but the reduction of hydrocarbons was only marginal. Iron disappeared from the spectra and even copper, the target plate material, was significantly reduced. As a consequence, Z eff is e > 3 × 10 19 m −3 . The density limit was improved by 25% (boronization A) and more (boronization B and C). The oxygen-diminishing effect of boron/carbon and pure boron layers was found to be longer lasting than the suppression of metal impurities. Deteriorated wall conditions occurred only after hundreds of discharges, including numerous additional heating shots with ICRH, NI, LH and combined scenarios, but pre-boronization conditions were never reached. The main emphasis of this paper is to describe the impact of boronization on aspects of plasma operation

Density measurements in the peripheral region of a NI heated discharge are presented. Most of the results are obtained via a new technique utilizing a 60 keV neutral lithium beam which yields profiles — on a continuous basis — that can be related to the density profile. The initial consequence of NI is a deterioration of the particle confinement time (L-phase) which manifests itself as an ∼ 40% increase in the density e-folding length λn outside the separatrix; possibly, there is a slight decrease in density inside the separatrix. Attainment of the H-phase with its better confinement properties is accompanied by an abrupt decrease in λn to a value typical of the Ohmic phase; there are indications that the density gradient near the separatrix is even steeper. The subsequent increase in edge density is then intermittently interrupted by bursts which, at the conclusion of each burst, lead to a reduction in ne of the scrape-off layer and at least the region immediately inside the separatrix.

The response of the electron temperature profiles to changes in heating profile is investigated analytically and by transport code simulations. A heat conduction model with local empirical scalings of the electron thermal diffusivity χe is used. It is found that the Te-profile responds rather insensitively, owing to double radial integration of the heating power density. By scanning the target density and injection energy of neutral beam heated ASDEX plasmas it is shown that only small profile changes in the conductive electron heat flux are achieved. In density scans these changes are further diminished by the convective power loss. The theoretically predicted variation in Te(r)/Te(0) is correspondingly weak and does not exceed the measured profile change. The observed Te profile invariance is thus produced by local transport models with a diffusive electron heat flux proportional to dTe/dr. Consequently, it is not necessary to introduce a χe(r) which adjusts to the profile of the conductive electron heat flux or which non-linearly depends on the electron temperature gradient.

Hydrogen and deuterium fluxes parallel to the toroidal magnetic field were measured in the plasma boundary of ASDEX using graphite collector probes. Time resolution of the order of 100 ms was obtained by rotating the cylindrical probes behind an aperture during the discharge. The trapped amount of hydrogen was determined by subsequent thermal desorption, in the analysis of deuterium the D( 3 He, p) 4 He nuclear reaction was used. Both methods yield quantitative results. Measurements were done for limiter and divertor discharges in the range of 4 to 20 cm outside the limiter or separatrix. The time distributions show a maximum flux at the beginning and at the end of the discharge. The relatively lower flux during the plateau phase of the discharge is in the range of 10 15 to 2 × 10 17 cm −2 /s , depending on the radial probe position, the maximum values are higher by a factor of 5 to 50. During neutral hydrogen injection an additional maximum can be observed. The radial 1/e- decay length is about 0.9 cm in front and 0.4 cm behind the fixed limiter. The results are compared with independent measurements in ASDEX and other plasma machines.