A. Geier

Key ITER plasma edge and plasma–material interaction issues

Abstract Some of the remaining crucial plasma edge physics and plasma–material interaction issues of the ITER tokamak are discussed in this paper, using either modelling or projections of experimental results from existing tokamak operation or relevant laboratory simulations. The paper covers the following subject areas at issue in the design of the ITER device: (1) plasma thermal loads during Type I ELMs and disruptions, ensuing erosion effects and prospects for mitigating measures, (2) control of co-deposited tritium inventory when carbon is used even on small areas in the divertor near the strike points, (3) efficiency of edge and core fuelling for expected pedestal densities in ITER, and (4) erosion and impurity transport with a full tungsten divertor. Directions and priorities of future research are proposed to narrow remaining uncertainties in the above areas.

Impurity behaviour in the ASDEX Upgrade divertor tokamak with large area tungsten walls

At the central column of ASDEX Upgrade, an area of 5.5 m2 of graphite tiles was replaced by tungsten-coated tiles representing about two-thirds of the total area of the central column. No negative influence on the plasma performance was found, except for internal transport barrier limiter discharges. The tungsten influx ΓW stayed below the detection limit only during direct plasma wall contact or for reduced clearance in divertor discharges spectroscopic evidence for ΓW could be found. From these observations a penetration factor of the order of 1% and effective sputtering yields of about 10-3 could be derived, pointing to a strong contribution by light intrinsic impurities to the total \mbox{W-sputtering}. The tungsten concentrations ranged from below 10-6 up to a few times 10-5. Generally, in discharges with increased density peaking, a tendency for increased central tungsten concentrations or even accumulation was observed. Central heating (mostly) by ECRH led to a strong reduction of the central impurity content, accompanied by a very benign reduction of the energy confinement. The observations suggest that the W-source strength plays only an inferior role for the central W-content compared to the transport, since in the discharges with increased W-concentration neither an increase in the W-influx nor a change in the edge parameters was observed. In contrast, there is strong experimental evidence, that the central impurity concentration can be controlled externally by central heating.

New results from the tungsten programme at ASDEX Upgrade

Abstract Since 1998 an increasing area of tungsten coated tiles has been installed at the central column of ASDEX Upgrade, reaching 7.1 m 2 in 2001/2002. The tiles were coated commercially by plasma arc deposition to a thickness of 1 μm. Post mortem analysis of the W-coating showed a strong erosion, which is attributed to ion sputtering, due to its two-dimensional variation. During the campaigns with W-coated central column, almost no negative influence on the plasma performance was found. Only during direct plasma wall contact or for reduced clearance in divertor discharges spectroscopic evidence for tungsten influx could be found. Effective sputtering yields of about 10 −3 were derived, pointing to a strong contribution by light intrinsic impurities to the total W-sputtering. The increased W-content during plasma current ramp up decreases after X-point formation and the flux consumption during current ramp is only marginally enhanced compared to the graphite wall case. The W concentrations ranged from below 10 −6 up to a few times 10 −5 . In discharges with increased density peaking, a tendency for impurity accumulation was observed, which affected only a closely localized central region. Central heating led to a strong reduction of the central impurity content, consistent with neoclassical impurity transport.

Tungsten as plasma-facing material in ASDEX Upgrade

Since 1993, a tungsten programme is run at ASDEX Upgrade, leading to the installation of a complete W-divertor in 1995 followed by 1 year of operation. From 1998 onwards, an increasing area of tungsten-coated tiles has been installed at the central column of ASDEX Upgrade, reaching an area of 7.1 m2 in 2001/2002 representing about 85% of the total area of the central column. The programme comprises not only the operation with large area W-coatings but also the qualification and testing of the coated tiles, W-erosion and -migration investigations, development of spectroscopic diagnostics and interpretation of the observed W-behaviour by impurity transport calculations. During the campaigns with the W-divertor as well as with W-coated central column, almost no negative influence on the plasma performance was found. The W-erosion is dominated by light intrinsic impurities and typical tungsten concentrations in the main plasma range from below 10−6 up to a few times 10−5, which would be sufficiently low for a burning plasma.

Determination of the tungsten divertor retention at ASDEX Upgrade using a sublimation probe

For the measurement of the divertor retention at ASDEX Upgrade, a sublimation probe has been built with which it is possible to puff metallic impurities into the divertor plasma. This probe offers the opportunity to cross calibrate the influx of the metal with the influx of carbon or oxygen. For the measurement of the divertor retention R, a special deposition probe was used in the main chamber. The value found for R is in good agreement with earlier estimations from long-term deposition probes and computer modelling. The different measurements suggest a propagation of tungsten within the scrape-off layer from the divertor into the main chamber and from there across the separatrix into the confined plasma.

Plasma operation with tungsten tiles at the central column of ASDEX Upgrade

Abstract Two rows of the graphite tiles (≈1.2 m2) at the lower end of the central column of ASDEX Upgrade were coated with tungsten. Since no high heat fluxes are present in this region, the tiles were coated by a physical vapour deposition technique to a thickness of only 500 nm. The experimental campaign was started without wall conditioning by coating of the vacuum vessel. This allowed to measure the W-erosion and the W-concentration with an almost pure tungsten surface exposed to the plasma. The influx as well as the concentration were monitored by spectroscopic methods. The penetration probability for tungsten from the central column is deduced from direct laser ablation of the coating. The measurements were complimented by deposition probe measurements in the midplane and in the divertor. In all important discharge scenarios, which were performed already during the phase without wall conditioning, no concentrations above the detection limit of about 5×10−6 were found. This result is supported by the deposition probe measurements, which showed no or very small amounts of W deposited in single discharges. The concentrations are at least a factor of 10 below the acceptable concentration in ASDEX Upgrade. Consequently, no influence on the overall plasma behaviour was found and W seems also to be suited for use on larger surfaces in ASDEX Upgrade. Extrapolation to ITER conditions yields concentrations, which will not prohibit successful operation.

A SUBLIMATION PROBE FOR THE INJECTION OF HIGH-Z IMPURITIES INTO FUSION DEVICES

For the investigation of high-Z impurities in the ASDEX Upgrade tokamak a sublimation probe was developed and tested. With this probe it is possible to inject materials, that sublimate at temperatures from about 50 to 150 °C into the plasma through a controllable valve. For the investigation of the tungsten transport in ASDEX Upgrade the probe was operated with tungsten carbonyl. The flux of tungsten, which is difficult to determine directly because of the uncertain atomic data, can be determined using the fluxes of oxygen and carbon, the atomic data of which are better known. In this article the setup of the probe and first experiments are described. Here the layer of deposited tungsten was investigated and the number of emitted photons per ionization (the S/XB ratio) for the 400.8 nm line of W I was estimated.

Erosion and migration of tungsten employed at the main chamber first wall of ASDEX Upgrade

In ASDEX Upgrade, tungsten is employed as plasma facing material at the central column of the plasma main chamber. The campaign averaged tungsten erosion flux was determined by measuring the difference of the W-layer thickness before and after experimental campaigns using ion beam analysis methods. The lateral variations of the observed tungsten erosion flux reflects the pattern of magnetic field lines intersecting the respective plasma facing surfaces. Long term migration and redeposition of eroded tungsten were investigated by ion beam analysis of deposited tungsten on wall samples from main chamber and divertor. The obtained results, as well as the spectroscopically observed low tungsten plasma penetration probability, indicate that a major fraction of the eroded tungsten migrates predominantly through direct transport channels in the outer plasma scrape-off layer without entering the confined plasma.

Modeling of tungsten transport in the SOL for sources at the central column of ASDEX Upgrade using DIVIMP

Tungsten erosion at the central column and tungsten transport in the scrape-off layer of ASDEX Upgrade have been modeled with the 2D Monte-Carlo impurity transport code DIVIMP. The main objectives were the reproduction of the erosion and migration patterns that are observed experimentally. This was accomplished by implementing a new erosion routine which allows to calculate the tungsten source term at the central column and subsequently the poloidal distribution of the W density. First results on W transport in the boundary plasma as well as erosion and migration patterns of W are discussed in relation to different models of the plasma in front of the central column. A comparison between measured and modeled deposition of W in the divertor already shows satisfactory qualitative agreement.

Modelling of tungsten migration during limiter ramp-down in the ASDEX Upgrade divertor tokamak

The suitability of tungsten as a plasma facing material in magnetic fusion devices is currently under investigation at the ASDEX Upgrade divertor tokamak. Recently, the complete central column, which is also used as a ramp-up and ramp-down limiter was covered with W coated carbon tiles. Experimentally, the transient limiter phases were identified to dominate the gross influx of eroded tungsten and thus they could have an effect on the post-campaign surface analysis measurements. Therefore, the migration of W during limiter contact was assessed using the DIVIMP impurity transport code. The results indicate that most of the tungsten eroded during the limiter phases is redeposited locally on the central column and that only an average of less than 10% of the eroded W is deposited somewhere else on the wall. Accordingly, campaign integrated surface analysis measurements of tungsten deposition in the divertor are not influenced by tungsten deposited during ramp-up or ramp-down. Moreover, the comparison of tungsten migration during the limiter phases and the much longer flat-top divertor phases is not contradictory to the experimental result that only about 10% of the tungsten, eroded at the central column, is found in the divertor by post-campaign tile analysis.