R. Pugno

Tungsten: an option for divertor and main chamber plasma facing components in future fusion devices

The tungsten programme in ASDEX Upgrade is pursued towards a full high-Z device. The spectroscopic diagnostic of W has been extended and refined and the cooling factor of W has been re-evaluated. The W coated surfaces now represent a fraction of 65% of all plasma facing components (24.8 m(2)). The only two major components that are not yet coated are the strikepoint region of the lower divertor as well as the limiters at the low field side. While extending the W surfaces, the W concentration and the discharge behaviour have changed gradually pointing to critical issues when operating with a W wall: anomalous transport in the plasma centre should not be too low, otherwise neoclassical accumulation can occur. One very successful remedy is the addition of central RF heating at the 20-30% level. Regimes with low ELM activity show increased impurity concentration over the whole plasma radius. These discharges can be cured by increasing the ELM frequency through pellet ELM pacemaking or by higher heating power. Moderate gas puffing also mitigates the impurity influx and penetration, however, at the expense of lower confinement. The erosion yield at the low field side guard limiter can be as high as 10(-3) and fast particle losses from NBI were identified to contribute a significant part to the W sputtering. Discharges run in the upper W coated divertor do not show higher W concentrations than comparable discharges in the lower C based divertor. According to impurity transport calculations no strong high-Z accumulation is expected for the ITER standard scenario as long as the anomalous transport is at least as high as the neoclassical one.

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.

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.

Spectroscopic investigation on the impurity influxes of carbon and silicon in the ASDEX upgrade experiment

Abstract Emission profiles of carbon and silicon along the heat shield (HS) were measured before and after siliconization of the vessel. Siliconization does not affect the divertor and therefore large changes in impurity concentrations and influxes cannot be attributed to it. A strong transient increase of the silicon concentration was observed immediately after the siliconization lasting for a few discharges. However, no large change was observed in the silicon influx from the HS, excluding it as possible source of the concentration increase. The fast decrease in the silicon concentration can be attributed to strong erosion at the outer limiter. The impurity influxes for C2+ and Si2+ from the HS are estimated and compared with the corresponding core concentrations. The relative contribution to the plasma impurity content from divertor, HS and outer protection limiter is discussed. Zeeman splitting analysis permits one to identify the emitting region of carbon radiation. Anomalously high silicon sputtering yields are measured, which do not however cause high core silicon content.

An overview of JET edge modelling activities

A number of codes are in use at JET to model the edge plasma. The range of edge codes is described as is the range of physics issues being explored by these codes. The balance between focussed modelling (that looking at particular physics effects) and integrated modelling (attempting to combine codes or encapsulate the physics from some codes into other codes) is examined.

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.

Extinction of CD band emission in the divertor of ASDEX Upgrade

We found a complete extinction of the CD molecule emission in the inboard divertor of ASDEX Upgrade at high density with the transition to a recombining divertor plasma. The electron temperature in the recombining divertor plasma ranges from 1.2 down to 0.3 eV. Two effects may explain the extinction at such low temperatures, (1) a steep decay of the yield for chemical sputtering as theoretically predicted and (2) a suppression of the optical emission while the methane molecules dissociate unchanged via charge exchange.

Heat flux decay length in the midplane of ASDEX Upgrade

Power exhaust is a crucial issue for next step fusion devices since the heat flux to first wall elements has to be kept below a technically feasible limit. A crucial parameter in this context is the perpendicular heat flux decay length in the scrape-off layer (SOL). In this paper, a direct measurement of the midplane decay length in the poloidal divertor tokamak ASDEX Upgrade by continuously varying the magnetic configuration from a lower single null (SNL) through a double null configuration to upper single null (SNU), i.e., switching of the particle and heat flux continuously from the lower to the upper divertor, while monitoring the divertor particle and heat flux in the lower divertor is presented. The decrease of the signal measured at the outer lower divertor target plotted versus the midplane distance of the two separatrices results in e-folding lengths for the midplane heat flux. The decay lengths found are comparable to decay lengths derived from heat flux profiles measured at the divertor plates and mapped to the midplane.

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.