S. Schweizer

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.

TECHNICAL FEATURES AND PROGRESS OF ASDEX UPGRADE

The objective of ASDEX Upgrade is to investigage open poloidal divertor configurations as produced by a reactor-compatible, poloidal field coil system. The accessible poloidal and toroidal fields and hence the plasma current provide a sufficiently large plasma parameter regime to ensure the functioning of the open divertor.

Actively cooled high-intensity heat shield (form-locked) design analysis

Abstract A feasibility study is performed with 2D-mathematical models for heat shield designs using carbon fibre composites and tungsten monoblocks, which are form-locked to the coolant tube instead of metallurgically bonded. The geometrical boundary conditions and the overall shape correspond to the design for the ITER divertor vertical target. Steady state thermo-mechanical analyses have been carried out for a limited number of cases to investigate the effects of the particular features of the proposed connection. The results demonstrate the principal feasibility of this approach and open prospects for further improvements.

High-intensity non-brazed heat shield for safe steady-state operation

Many plasma-facing components in todays experiments are inertially cooled. Graphite tiles are mounted on a water-cooled steel plate using a spring and bolt mechanism. When exposed to high heat flux, such a shield can take loads only for a few seconds without becoming overheated. To operate in high intensity long pulsed/steady-state conditions, new actively cooled heat shields of brazed and non-brazed types are under development. With clamped (form-locked) monoblock tiles a prototype of non-brazed heat shield structure underwent its first high heat flux tests for divertor applications in the Julich MARION ion source [1]. The test results are encouraging, and validated a finite element model as discussed in this paper.