E. Wallura

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.

Studies on properties of low-Z ceramics as limiter materials —electron beam and textor limiter tests

Abstract Several types of low- Z ceramics are investigated with regard to their applicability as high heat-flux component materials for nuclear fusion reactors. Their thermomechanical behaviours under realistic plasma conditions are studied through both out-of-pile, laboratory test and in-pile, TEXTOR test. Thermal shock resistivities of several types of SiC. nitrides, graphite and coated systems are tested by bombardment with an electron beam at energy densities between 1 and 500 MJ/m 2 (out-of-pile test). The maximum tolerable pulse length at different power densities for total fractures of various qualities of SiC is determined. Candidate-material segments are “sandwiched” between Inconel 600 reference segments. Main damage observed on the test limiter surface after exposure to the TEXTOR plasma is due to unipolar arcing. Difference of both arcing behaviour and redeposited-metal behaviour on the test-limiter segments are discussed.

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.

Behaviour of boron-carbide materials in TEXTOR and under electron beam irradiation

Abstract Boron-carbon materials have been exposed to high heat fluxes in the TEXTOR tokamak and in the JUDITH electron beam facility. The materials were a B 4 C Coat Mix bulk material, a conversion-type B 4 C coating, and a plasma-sprayed B 4 C coating on graphite EK98. The boron-carbide Coat Mix limiter showed, due to the low thermal conductivity of 11 W/m K, surface melting already at a heat flux of about 1.5 kW/cm 2 after 0.4 s. Both in the tokamak experiments and the electron beam tests the surface temperature stayed close to the melting temperature under continued heating. The conversion coated limiter and specimens had a very irregular coating thickness and surface structure. At heat fluxes of 2.6 kW/cm 2 melting of the coating was reached after 0.5 s. The thermal behaviour and evaporation of impurities into the plasma were comparable to the Coat Mix limiter but shifted to higher heat fluxes. Both limiters fractured under the applied heat loads. The plasma sprayed coating showed early melting of the coating and agglomeration of the molten carbide to droplets on the graphite.

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.

Material damage in graphite by run-away electrons

Abstract Graphite limiters in the KFA tokamak TEXTOR have been hit by run-away electrons, which “burned” craters into the material surface. The craters have been formed by material erosion and/or sublimation and by chipping of thin laminates from the surface and additionally by redeposition of graphitic material. Moreover, thin metallic layers have been detected which may have been redeposited independently from the run-away electron event, and which could be found also outside the run-away electron crater. The redeposited carbon showed primarily two types of structure, namely oriented pyrocarbon and carbon-black-like material. The crater has been investigated by metallography, optical microscopy, secondary electron microscopy combined with energy-dispersive X-ray analysis, and automatic quantitative image analysis. The examined parameters are crater geometry, cracking behaviour and type and structure of redeposited material.

Surface damage in TiC coating layers on PDX wall armor tiles

Abstract Surface damage and material redepositions are investigated on 17 graphite tile surface of the PDX wall armor exposed to more than 6500 shots of neutral beam injection (NBI) shine-through and more than 700 shots of direct NBI. Titanium deposition layers, from 0.1 to 7 μm in thickness, redeposited metal droplets, or micro-projections with diameters of less than 2–3 μm and a number density at around 10 8 /cm 2 , are observed on the tile front faces. Aligned wedge-shaped pits are observed at all micro-projections, which are likely due to shadowed Ti-deposition at redeposited metal droplets by Ti-getter evaporation. Blistering observed in both the TiC coating and Ti-deposition layers is due to plasma shine-through D 0 beam irradiations to a fluence of more than 2 × 10 19 at./cm 2 .

Influence of thermal shocks on the strength of silicon carbide

Abstract Low-Z ceramics such as SiC have a potential as a material candidate for plasma-facing components in thermonuclear fusion reactors. To investigate the thermo-mechanical behaviour of SiC, high-heat flux experiments were performed on small rectangular test samples. To get additional information on the influence of thermal shocks on the mechanical properties of ceramics, a set of 4-point bending test specimens was exposed to heat loads just below the damaging threshold. Emphasis was laid on the detection of the point of crack initiation; a qualitative and quantitative evaluation was performed on the fracture surfaces. The strength of these pre-damaged materials was compared to the as-manufactured grades.