Publications Scientifiques

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Author: search.filters.author.Akbi, MohamedEnd date: 1999

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    Work function measurements of contact materials for industrial use
    (IOP, 1998) Akbi, Mohamed; Lefort, André
    The nature of the contact material is important for the characteristics of electric arcs and particularly for the electronic emission. Work functions of new industrial materials made with silver alloys and silver oxide alloys are not known at present. An experimental set-up is described which allows work function measurements from room temperature up to 700 K. The Fowler method was used for the measurement of the work function by the photoelectric effect. As a first application of the experimental device, work functions of metals (Ag, Cu, Ni, Sn and Zn) were determined. Furthermore, the influence of industrial surface treatment such as the application of electric arcs and repeated mechanical shocks on the Ag contact work surface was studied by SEM to observe their effect on electron work function. Breaking arcs in air cause a remarkable increase in the work function of the silver contact material by progressive formation of silver oxides
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    Experimental determination of work function of silver alloys
    (1994) Lefort, Andre; Akbi, Mohamed; Parizet, Marie-Jose
    Models about arc roots need a good knowledge of physical constants characterizing contact material. With pure metal, all the constants are well known; but for alloys some are not known. In this paper we relate how we have measured work function of silver alloys. Measuring method used is the Fowler one where photoelectronic current produced by different ultraviolet radiations is measured in high vacuum. Experimental set-up have been tested for pure materials and then, have been applied to some silver alloys. Results about pure metals show an evolution of the work function with temperature; it decreases when temperature increases. Results about silver-nickel alloys show a large dependence of obtained results with the preparation of contact surface. For example, polishing and cleaning induce, because of atom migration, a large modification of the ratio of the two components on surface; this phenomenon induces a variation of work function comparatively with obtained values with prLtary product. By heating alloy contact in high vacuum we have observed large variations of work function, which result from material component vaporisation by sheets. Surface analyses by electron microscope produce proves of the phenomenon. An empirical relation is given to calculate work function of binary alloy when this constant is known for each alloy component
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    Deuterium trapping in divertor tiles of ASDEX-Upgrade
    (Elsevier, 1999) Hildebrandt, D.; Akbi, Mohamed; Juttner, B.; Schneider, W.
    The deuterium inventory of divertor target tiles used in ASDEX-Upgrade for up to 2000 discharges has been analyzed by thermodesorption spectrometry. In addition, surface analysis techniques as auger electron spectrometry, secondary ion mass spectrometry, nuclear reaction analysis, electron microscopy and optical surface pro®lometry have been applied for investigating erosion and deposition phenomena. The original plasma facing surfaces were graphite (EK98) and plasma sprayed tungsten, respectively. The total deuterium inventories have been found to vary between 4 ´ 1021 D-atoms/m2 and 3 ´ 1023 D-atoms/m2. The deuterium is contained in a deposit on the surface of the graphite and tungsten tiles consisting mainly of carbon, boron and the hydrogen isotopes. There is strong indication that morphological e ects in¯uence impurity deposition, deuterium trapping and re-erosion of the contamination
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    Influence of surface roughness on the deuterium inventoryof ASDEX-UPGRADE divertor tiles
    (1998) Hildebrandt, D.; Akbi, Mohamed; Jüttner, B.; Schneider, W.
    The gas inventory of divertor target tiles used in ASDEX-UPGRADE has been analyzed by thermodesorption spectrometry (TDS°. desorbed gaseous molecules have been measured by heating up complete and cut divertor tiles. the largest samplrs '80X80mm2)could be heated up to temperatures of 1500 K and smaller ones to even higher temperatures.in addition, surface analysis techniques as auger electron spectroscopy (AES),secondary ion mass spectrometry(SIMS), nuclear reaction analysis (NRA), electron microscopy and optical surface profiling have been applied for investigating erosion and deposition phenomena. the original plasma facing surface of the tiles were graphite (EK98) and plasma sprayed tungsten, respectively. the graphite tiles were used from 1991-1995 for about 1900 discharges and the tungsten tiles in 1996 for about 800 discharges...
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    Deuterium and impurity contamination of divertor tiles and collector probes of asdex-upgrade
    (Fusion and plasma physics, 1998) Hildebrandt, D.; Akbi, Mohamed; Jüttner, B.; Rohde, V.; Schneider, W.
    The uptake and release of hydrogen isotopes at the plasma-facing components in magnetic, confinement fusion devices affects the working gas recycling, the plasma behaviour and the tritium inventory [1]. This attracts considerable interest in the investigation of hydrogen trapping during plasma exposure. The most intensive plasma material interaction occurs on limiters and divertor plates. Post-mortem analysis of such components gives information on the total amount of hydrogen isotopes retained in the material after plasma exposure. Recent investigations of divertor tiles of ASDEX-UPGRADE have shown that the dominant trapped deuterium amount is contained in the deposited material at the surface [2]. This surface contamination consists mainly of carbon, boron and the hydrogenic isotopes. Movable collector probes have been applied to investigate the hydrogen trapping and impurity deposition under specific plasma conditions. In the present paper results on the impurity and deuterium contamination at the surface of collector samples are presented and compared with corresponding results from the divertor tiles. The collector samples were exposed to the scrape-off plasma of the main chamber (SOL-probe) and to the divertor plasma (DIV-probe)