ASDEX Upgrade Divertor Research Articles

A comparison of B2-Eirene code results and ASDEX Upgrade Divertor II

A comparison of B2-Eirene code results and ASDEX Upgrade Divertor II

First experimental determination of ion flow velocities and temperatures in the ASDEX Upgrade divertor

For the analysis of ion dynamics (i.e. ion temperature and ion flow velocity distributions) in the divertor I of ASDEX Upgrade an optical spectroscopic system with high spectral, temporal and spatial resolution was brought into operation. Integrated along up to 74 lines of sight with different orientations to the magnetic field lines, emission spectra of atoms and ions were recorded with high resolution by an Echelle spectrometer and a 2D CCD camera. With this setup, ion flow velocities and temperatures in a divertor plasma could be determined experimentally for the first time. In the attached high-recycling divertor regimes analysed, neutrals such as helium do not flow preferentially parallel to any of the installed lines of sight. In contrast, all the ions investigated always showed pronounced Doppler shifts. The corresponding ion flow is directed towards the target plates, in the outer as well as in the inner divertor. For typical L- and H-mode discharges (attached high-recycling divertor conditions) the ions flow at speeds of about in the L-mode and in the ELMy H-mode phases. The temperatures of the ions near the divertor plates range from about 5 eV (L-mode) to up to 20 eV (ELMy H-mode phases). The comparison of these results with computer code calculations showed good agreement.

Particle exhaust studies in ASDEX Upgrade

An experimental overview on pumping and particle exhaust studies for deuterium as well as for helium and neon in the ASDEX Upgrade divertor tokamak is presented. Strong turbomolecular pumps connected to the divertor region allow effective pumping of all these gases. Deuterium, however, is pumped effectively by the carbon walls in ASDEX Upgrade which dominate over the external pumping. Noble gases are hardly pumped by the walls, and their pumping efficiency and exhaust rate depends strongly on their respective density in the divertor chamber, i.e. on the effectivity of scrape-off layer transport and divertor retention. Compression of helium and neon in the divertor chamber increases with the divertor neutral gas density, and in high-power, high-density discharges in H- or CDH-mode pumping is very efficient. Helium exhaust in these scenarios is sufficiently fast to fulfill the requirements for ITER or a fusion reactor.

Hydrogen isotope inventory in the graphite divertor tiles of ASDEX Upgrade as measured by thermal desorption spectroscopy

The hydrogen and deuterium inventories of the ASDEX Upgrade divertor tiles were measured after the experimental period from December 1994 to July 1995 by thermal desorption spectroscopy (TDS) of samples cut out of the divertor tiles. The samples were heated by electron bombardment up to 2100 K; the released gases were measured by means of a calibrated quadrupole mass spectrometer. The measured hydrogen or deuterium inventories are of the order of 1023 m-2. They are larger for samples of the inner divertor than of the outer divertor by a factor of about 2. The largest inventory was found at the separatrix position of the inner divertor. Most of the released hydrogen (H) can be attributed to water adsorbed in the near surface region during the air exposure prior to the TDS measurements. The total inventories measured by TDS exceed the inventories in the near surface region (<25 mu m) measured by ion beam analysis methods by a factor of up to 10. Hence, the total hydrogen retention is governed by the diffusion out of the near surface region deep into the material. The hydrogen and deuterium inventories decrease with increasing surface temperature

Spectroscopic measurements of tungsten erosion in the ASDEX Upgrade divertor

Tungsten erosion in the ASDEX Upgrade divertor has been measured by observing the WI emission at 400.9 nm. The sputtering yield is in the region of atoms/ion for typical divertor plasma conditions. By comparison of the measured sputtering yield with modelled sputtering yields one can conclude that tungsten erosion is dominated by plasma impurities. Adding impurities without cooling the boundary plasma increases the sputtering yield due to the additional impact energy gained by the particle in the sheath potential caused by its higher net charge. Cooling the boundary plasma by adding more impurities or increasing the plasma density leads to a strong decrease in the tungsten erosion. By comparison of the tungsten influx with the measured net erosion the prompt redeposition was determined. The redeposited fraction of tungsten increases significantly with a higher divertor electron density as expected.

Spatial radiation profiles in the ASDEX Upgrade divertor for detached plasmas

Abstract In this paper we describe impurity line emission measurements in the divertor of ASDEX Upgrade during high power neutral beam heated discharges. We focus on detached conditions where the dominating part of the radiation comes from the X-point region. Spatially resolved line emission in the VUV and visible spectral region of the intrinsic carbon and additionally puffed impurities (neon and nitrogen) is presented. A simple interpretation of the line emission profiles is given and they are also compared to the results of bolometry.

Study of gross and net erosion in the ASDEX Upgrade divertor

Study of gross and net erosion in the ASDEX Upgrade divertor

Study of gross and net erosion in the ASDEX upgrade divertor

Erosion of carbon and tungsten in the divertor of ASDEX Upgrade was investigated by surface analysis of target tile probes. The probes were exposed in the divertor using a manipulator system. Erosion of carbon was measured by graphite probes covered with a 150 nm layer of 13C isotope to distinguish from the intrinsic carbon content. Tungsten erosion was measured by similar probes with 1–20 nm tungsten markers evaporated on the surface. From the measured erosion, sputtering yields were determined using plasma parameters at the target plates obtained from flush mounted Langmuir probes.

Spatial radiation profiles in the ASDEX Upgrade divertor for detached plasmas

In this paper we describe impurity line emission measurements in the divertor of ASDEX Upgrade during high power neutral beam heated discharges. We focus on detached conditions where the dominating part of the radiation comes from the X-point region. Spatially resolved line emission in the VUV and visible spectral region of the intrinsic carbon and additionally puffed impurities (neon and nitrogen) is presented. A simple interpretation of the line emission profiles is given and they are also compared to the results of bolometry.

Manufacturing and high heat flux loading of tungsten coatings on fine grain graphite for the ASDEX-upgrade divertor

Fine grain graphite tiles coated with tungsten layers by plasma spray (PS, thickness 100–550 μm) and physical vapour deposition (PVD, 30–200 μm), respectively, were subjected to thermal loads up to 17 MW/m 2 and 2 s pulse duration. The damage limit was evaluated by increasing the heat flux and the pulse length stepwise. The results proved that PS coatings are capable of withstanding heat loads up to 15 MW/m 2 at 2 s pulse length without any structural changes, and cyclic loading with 1000 cycles at 10 MW/m 2. The highly dense PVD coatings suffered damage by crack formation at slightly lower heat loads, and thin PVD layers failed under cyclic loading with 1000 cycles at 10 MW/m 2 due to thermal fatigue and melting. The good performance of PS coatings is related to their porosity, which provides a crack arresting mechanism, and to their mechanical strength, depending on the density of the PS layer.

Spectroscopic measurement of target plate erosion in the ASDEX Upgrade divertor

The erosion of the graphite divertor plates in the ASDEX Upgrade tokamak is measured spectroscopically. Spatial profiles of the D0 and C+ influxes across the outer target plate are determined from measured absolute line intensities. Plasma parameters (ne, Te) at the target, which are required to determine the appropriate photon emission efficiencies for these lines, are obtained from an in-vessel reciprocating Langmuir probe above the target plate. Yields for the erosion of the graphite by the incident D+ flux are determined from the ratio of the measured C+ to D0 fluxes. Over a range of moderate densities the measured yields of ⩽4% are explicable in terms of physical sputtering alone. Chemical sputtering by low energy Franck-Condon neutrals probably contributes, however, to the total erosion. At higher densities detachment of the plasma from the targets occurs owing to formation of a MARFE near the X point. Under these conditions localized physical sputtering of the targets ceases. The impurity level (Zeff) is, however, maintained following detachment, indicating a corresponding maintenance of carbon influx, perhaps due to chemical erosion of the total graphite surface and/or an improvement in particle confinement in the detached state

Experimental investigations of high- Z materials in the ASDEX-Upgrade divertor

The erosion, plasma penetration and redeposition of divertor plate material such as C, Si, V, Mo and W have been studied in the ASDEX-Upgrade divertor. Long-term erosion and redeposition have been studied using evaporated markers, while time resolved measurements and plasma penetration were studied spectroscopically on W tiles. The expected reduction of erosion due to plasma ion sputtering could be confirmed. This reduced erosion is accompanied by largely enhanced deposition for high- Z ions, when the gyroradius exceeds the ionisation length.

Modelling of impurities in the ASDEX-Upgrade divertor with DIVIMP

The two-dimensional Monte Carlo code DIVIMP is used as an interpretative tool for the analysis of spectroscopic measurements in the edge and the divertpr plasma of ASDEX-Upgrade. For a series of standard Ohmic discharges, the spectral lines of C + and C 2+ ions, recorded by two spectrometers viewing perpendicular and tangential to the outer divertor plates, were modelled. The simulations show that carbon production by physical sputtering alone is not sufficient to explain the observed signals. Further sources, possibly caused by chemical erosion processes must be taken into account.

Energy transport to the divertor plates of ASDEX-Upgrade during ELMy H-mode phases

The energy flux to the ASDEX-Upgrade divertor plates is routinely measured by thermography and Langmuir probes. The thermographically observed power decay length at the target plate is about 1 cm near the inboard separatrix. During an edge localized mode (ELM) of type I the density profiles are significantly, changed; an additional contribution occurs characterized by a power decay length in the order of 10 cm outside the separatrix and additional power is deposited into the private flux region. It is supposed that this is due to the changing, contribution of energy conduction versus convection. Results of ELM-modelling using the coupled B2-EIRENE code reproduce the main features of the experimental observations. The sheath transmission factor is calculated by combining thermography and Langmuir probe data.

Time dependent neutral gas transport in tokamak edge plasmas

The effects of neutral particles on the edge plasma conditions play a key role in divertor and limiter physics. In computational models they are usually treated in steady state approximation (instantaneous relaxation). However, the characteristics transport time scale is comparable to the ion acustic time scale. Thus neutral atoms relax to their steady state distributions much slower than electron temperature profiles along the fieldlines are established. A computational assessment of divertor or limiter dynamics requires ultimately an extension to time dependent algorithms. The numerical procedure in the EIRENE Monte Carlo code is presented. A first numerical study of ELM's in the ASDEX-Upgrade divertor plasma has been carried out and the results are briefly discussed.

Erosion and redeposition in the ASDEX Upgrade divertor

The erosion and redeposition of silicon and tungsten evaporated onto the divertor plates of ASDEX Upgrade have been investigated. For comparison with the experimental results the computer program, ERO, has been used. The erosion of Si markers is smaller than expected from plasma ion sputtering, but still exceeds the W erosion by more than an order of magnitude. The observed erosion can only be explained by assuming a protective effect of carbon ions coming from the plasma which are deposited on the marker surfaces, thus reducing the marker erosion. A redeposition of the sputtered tungsten atoms has been found close to the original marker spots mainly because of prompt redeposition during the first gyration.