Investigations on tungsten heavy alloys for use as plasma facing material
Introduction
Since 2014 ASDEX Upgrade (AUG) is using bulk tungsten (W) tiles at the outer divertor strike-point. In two experimental campaigns more than 2000 plasma discharges with up to 10 s duration and 100 MJ plasma heating were successfully conducted, without impairment by the W tiles. However, inspections after the campaigns revealed that a large number of tiles suffered from deep cracking attributed to brittle fracture [1]. A possible option – at least for non-nuclear fusion devices – could be the use of more ductile W heavy alloys as they are produced commercially by several companies. It consists of up to 97% (weight) W and Ni/Fe (or Ni/Cu) admixtures. According to [2], [3], the endurance limits of the W heavy alloys should be considerably larger than that of W (see for example [4]) which should help to avoid cracks under the cyclic load. Their major drawbacks, in view of the application in fusion experiments, are the rather low melting temperature (≈1500 °C or <1100 °C, respectively) and the higher vapour pressure of the binder phase and their magnetic properties (in the case of Ni/Fe admixture).
In order to explore their feasibility, W heavy alloys from two suppliers (Plansee Composite Materials GmbH and HC Starck Hermsdorf GmbH) were investigated concerning their thermal and magnetic properties and subjected to screening tests and cyclic loading in the high heat flux test facility GLADIS. In order to gain first experience under cyclic plasma load and to compare its behaviour to the divertor tiles made of bulk tungsten, one tungsten heavy alloy tile was exposed with the divertor manipulator in high power AUG discharges.
In Section 2 the measured properties (magnetic, thermal and surface composition) of the investigated materials will be presented. Section 3 presents the results of high heat flux screening and cycling loading including metallographic investigations, whereas Section 4 reports on the behaviour observed in divertor of AUG. Section 5 concludes the paper and provides the outlook on further activities.
Section snippets
Properties of tungsten heavy alloys
Tungsten heavy alloys are compounds with a large fraction of tungsten and admixtures of either Ni/Fe or Ni/Cu produced by pressing and sintering of powders or liquid phase sintering. Several commercial suppliers offer various grades of these W heavy alloys and they are widely used as shielding material and balancing weight because of the high atomic number (shielding) and high density (shielding, balancing). Table 1 summarizes the names, suppliers, composition and density of the tungsten heavy
High heat flux tests in GLADIS
In order to characterise the behaviour of tungsten heavy alloys under high heat fluxes (HHF), tiles with identical geometry as the AUG divertor components were exposed in the high heat flux test facility GLADIS [8]. During these adiabatical loading tests the tiles are exposed to a hydrogen beam with a gaussian shape of 75 mm FWHM and a maximum central power density of 20 MW m−2. Since the tiles have the dimensions of 227 mm × 77 mm × 15 mm, two thermally independent test series per tile could be
Exposure in the ASDEX Upgrade divertor
After the encouraging results of the HHF test in GLADIS one D185 tile was mounted in AUG using the divertor manipulator (DIM II) [9]. This unique device allows the exposure of two complete tiles (which could be elaborately instrumented or even actively heated/cooled) for single experimental days or for long experimental periods. Before mounting, the maximum forces originating from the magnetisation of the tile were calculated. Since saturation is reached already at very small magnetic field the
Conclusions and outlook
Tungsten heavy alloys are compounds with a large fraction of tungsten and admixtures of either Ni/Fe or Ni/Cu. In respect to bulk W, these materials are considerably cheaper due to the facilitated sintering process and they show improved machinability and ductility at room temperature. The latter could be beneficial when being used in PFCs because of the thermal cycles due to the pulsed operation and the cyclic loading through ELMs. The low melting point of the alloying elements Fe, Ni and Cu
Acknowledgement
The authors want to thank M. Li for his help concerning the interpretation of the mechanical fatigue data and B. Böswirth for assisting the GLADIS tests.
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