J. Linke

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

Abstract 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/m2 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/m2 at 2 s pulse length without any structural changes, and cyclic loading with 1000 cycles at 10 MW/m2. 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/m2 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.

Performance of boron/carbon first wall materials under fusion relevant conditions

Abstract The conditioning of the plasma facing wall in thermonuclear confinement experiments has been performed very successfully by the application of amorphous boron containing hydrogenated carbon films. Boronization leads to tokamak discharges with significantly reduced oxygen and carbon contaminations. For high heat flux components (especially in future quasi-stationary confinement experiments) new boron/carbon materials have to be developed: monolithic tiles of boronated graphites which can be brazed to watercooled substrates or thick B 4 C-coatings on graphite or high-Z coolant tubes. A variety of bulk materials (boronated graphites with boron contents in the range from 3 to 30%, so-called coat mix material on the basis of B 4 C) and coatings (amorphous B/C films, thick B 4 C layers applied by LPPS or CVD methods) were characterized systematically. In addition the behaviour of these materials was investigated under thermal loads; erosion and disruption simulation experiments were performed in electron and ion beam high heat flux test facilities. Physical and chemical sputtering of the coat-mix-material was studied in the PISCES-B facility in dependence on the hydrogen ions fluence.

Simulation of disruptions on coatings and bulk materials

An important issue for the next step thermonuclear fusion devices is the development of a reliable engineering solution for the first wall and in particular for the divertor. Besides severe mechanical loads, these plasma facing components (PFC) will be subjected to energetic pulses during plasma disruptions. To evaluate the performance of plasma facing materials and thus to predict the lifetime of the PFCs, simulation experiments have been performed in the JAERI electron beam irradiation stand (JEBIS). Here different candidate plasma facing materials were evaluated in short beam pulses (1.2–10 ms) with an energy deposition of 2–9 MJ m −2 . The response of the individual materials to single and multiple shot beam pulses was investigated, and in these analyses special attention was given to the material erosion (melting, sublimation, particle emission). The quantification of these effects was done by weight loss measurements and by optical profilometry on the ablation craters. In addition, microstructural and morphological changes in the loaded surface were investigated.

Non-metallic materials for plasma-facing first wall components: Status and further development

During the last few years, systematic investigations of the response of the plasma-facing material to the plasma-wall interaction have been carried out. The goal of this work is to find materials which, on the one hand, withstand the plasma attacks, and, on the other hand, do not contaminate the plasma. The available results allow first systematic conclusions on the status of materials development and on the way that has still to be covered to reach acceptable solutions for this problem area. In this paper, the available test facilities are presented, together with the target values on which the material tests are aiming. The first systematic test results are discussed for non-metallic materials and the conclusions that can be drawn on the status of the already reached position. This is done on the basis of the materials work performed at the Institute for Reactor Materials in cooperation with the Institute for Plasma Physics of the Nuclear Research Center, Julich.

High heat flux experiment on B4C-overlaid C/C composites for plasma facing materials of JT-60U

High heat flux experiments (5–40 MW/m 2 , 5 s and 550 MW/m 2 , 5–10 ms) in the JAERI electron beam irradiation stand (JEBIS) have been carried out on three kinds (conversion, CVD and LLPS) of B 4 C-overlaid C/C composites, on which B 4 C is overlaid with a thickness of 100–250 μm. Measurements were made with respect to the weight loss, changes of the surface morphology and of the surface atomic composition, and the surface temperature. As a result of these experiments, it is found that B 4 C layers of all samples have no damages except small weight losses up to 12 MW/m 2 heat loads, which are estimated at the divertor tiles of JT-60U in normal plasma operation, and that the conversion method is the best of the three methods applied in the present tests, since no exfoliation has occurred even under the disruption conditions.

DISRUPTION SIMULATION EXPERIMENTS IN ELECTRON AND LASER BEAM FACILITIES

To design next step thermonuclear fusion devices a profound data base is needed for materials which are considered as prime candidates for the plasma facing side of the torus. Here especially reliable data on the performance of these materials under short heat pulses with an energy deposition of several MJ/m2 are required. Up to now apart from theoretical data only very limited exerimental results are available. Both laser and electron beam facilities have recently been used to perform disruption simulation tests on graphites, pyrographite, carbon carbon composites, and boron doped carbon materials; the deposited energies were 6 and 9 MJ/m2, resp.. Emphasis was laid on the quantification of the erosion depth during a single disruption event. To get additional information on the damaging mechanism in dependence on the specific material parameters a careful characterization of the damaged surfaces was performed using different analytical methods.

High heat flux experiments on ceramics: material selection for neutron irradiation tests

In the next generation of fusion devices an additional process has to be taken into account besides the well known material damage due to plasma wall interaction namely the irradiation with energetic neutrons. In NET, e.g. an integrated neutron fluence of approx. 1026 m−2 corresponding to 10 dpa is expected in graphite or low-Z ceramics. Significant changes in the physical properties in the first wall structural materials will occur at these dose limits. In order to identify the most promising material candidates for the preparation of neutron irradiation experiments high heat flux screening tests were performed on a variety of bulk ceramics in an electron beam device and in so-called sandwich limiter tests in TEXTOR. Selected results and specific problems with screening tests on ceramic materials are discussed in this paper.

Energy deposition during disruption simulation experiments in a plasma accelerator

Plasma accelerators are often proposed as test beds for disruption simulation experiments on plasma facing materials for fusion applications. The incident energy fluxes and the discharge duration are of similar order to those expected during disruptions in ITER. In this work the VIKA facility of the D.V. Efremov Institute was used for calorimetric measurements and for material tests. The calorimetric measurements showed that below the threshold for the ablation of material only a small fraction of the total power of the plasma is deposited in the target. Carbon materials which were exposed to energy depositions above the ablation threshold exhibited a nearly proportional increase of the erosion with increasing total power. From these experiments it is concluded that the shielding effect of the material surface from the incident power is mainly due to the formation of a dense cloud of recycled gas in front of the target and to a smaller extent due to the ablation of target material.

OPERATION EXPERIENCES WITH JT-60U PLASMA FACING COMPONENTS AND EVALUATION TESTS OF B4C-OVERLAID CFC/GRAPHITES

Erosion of carbon fiber composite divertor tiles of JT-60U has been reduced significantly by the precise alignment and insitu taper-shaping of tile edges. The divertor tiles are coated with redeposited carbon films. None of graphite first wall tiles has been broken. Evaluation tests of B4C-converted and -coated CFC/graphite have been performed from viewpoints of high heat fluxdurability, thermal shock, deuterium retention, and erosion yields. JT-60U in-pile test has also been carried out. The results exhibit satisfactory performance for the divertor plate and first wall of JT-60U.

Behaviour and structural changes of redeposited material due to plasma-wall interaction

During plasma-wall interaction in tokamaks erosion and redeposition processes substantially influence plasma parameters as well as the properties of the first wall. During disruptions or run-away-electron accidents hot spots will form on limiters or other protective installations of the first wall. In the case of metallic components beside erosion and evaporation melting processes are becoming essential: liquid metal droplets with diameters in the range of some 10μm up to some 100 μm are deposited onto the vacuum vessel and the limiters. Another important redeposition process is the formation of thin films by atomic condensation during normal operation. Influenced by high heat fluxes during successive shots these films are forced to undergo structural changes which can result in the formation of metallic agglomerates with mean diameters in the range of some microns.