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Guirlet R.,French Atomic Energy Commission | Guirlet R.,EURATOM | Sirinelli A.,French Atomic Energy Commission | Sirinelli A.,CNRS Laboratory of Electric Arc and Thermal Plasmas | And 11 more authors.
Nuclear Fusion | Year: 2010

Electron and impurity transport has been studied in sawtoothing plasmas in the Tore-Supra tokamak. High time and space resolution measurements of the electron density reveal the existence of a flat profile region encompassing the q = 1 surface, on which is superimposed a density peak building up between sawtooth relaxations. For the first time in this regime, we have determined the underlying transport of both nickel and electrons independently of the effect of sawteeth in the central part of the plasma. Electron transport is consistent with the neoclassical expectations only in the close vicinity of the magnetic axis. Further out, it exceeds the neoclassical values as calculated with the NCLASS code, although the turbulence level is very low in the whole central region. In contrast, nickel transport is in good agreement with the neoclassical calculations in the same region. The neoclassical effect on trapped particles of a persisting mode due to incomplete reconnection of the magnetic surfaces is consistent with these observations. © 2010 IAEA, Vienna Printed in the UK & the USA.

Molecular dynamics simulations are used to study the interaction between incident copper ions, SF6 molecules and a polyethylene surface. Average particle velocities from 15 to 21 km• s-1 are tested in steps of 2km• s-1. The damage to the polyethylene crystal is reviewed in terms of material heating, local molecular disorder and bond breaking accompanied by particle emissions. Two types of emitted particles are distinguished to explain the degradation of the polymer. The first corresponds to free radicals with molecular formula CnH2n+1 and the second takes into account all other possible molecules, especially hydrogen atoms and carbonaceous molecules. The formation of CuF molecules during the interaction is shown and this result is discussed on the basis of other theoretical results concerning the calculations of plasma composition. The self-diffusion coefficients of the emitted atoms are calculated by using two different methods: the alternative Green-Kubo expression, based on the integrated velocity autocorrelation function, and the Einstein expression, based on the mean-square displacement autocorrelation function. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Akbi M.,CNRS Laboratory of Electric Arc and Thermal Plasmas | Akbi M.,University of Boumerdes
Proceedings - European Conference on Heat Treatment and 21st IFHTSE Congress | Year: 2014

Contact materials used for electrical breakers are often made with silver alloys. Mechanical and thermodynamical properties as well as electron emission of such complicated alloys present a lack of reliable and accurate experimental data. At present, new types of contactors with longer duration are marketed, but manufacturers do not understand well why this improvement. This paper deals mainly with electron work function (EWF) measurements in UHV conditions. Also, the photoelectric emission from silver-metal (Ag-Me) electrical contacts (Ag-Ni (60/40), Ag-Ni (70/30) and Ag-W (50/50)) has been investigated in the spectral range of 196-256 nm. The effects of thermal surface treatment on EWF of the abovementioned contacts were studied at room temperature and residual gas pressure of 1.4 × 10-7 mbar. The EWF of the contact pastille made with silver alloys Ag-Ni and Ag-W varies with surface treatments: cleaning by vacuum outgassing and annealing (long heating time in UHV). By heating alloy contact in ultra high vacuum large variations of EWF have been observed, which result from material component vaporization by sheets. In addition, the influence of arcing on EWF of binary alloys has been investigated. Also, EWF varies with the number of applied arcs. Therefore, after 500 arcs in air, the observed EWF increasing is probably due to progressive inclusion of oxide on alloy surface. Microscopic examination is necessary to get better understandings on EWF of silver alloys, for both virgin and eroded electric contacts. So, surface analysis by SEM and EDS have been performed to produce proofs of these various phenomena.

Akbi M.,CNRS Laboratory of Electric Arc and Thermal Plasmas | Akbi M.,University of Boumerdes | Bouchou A.,University of Science and Technology Houari Boumediene | Ferhat-Taleb M.,University of Science and Technology Houari Boumediene
Vacuum | Year: 2014

Theoretical models of arc roots need a good knowledge of physical constants characterizing contact material. With pure metals, all the constants are well known, whereas for the new industrial materials made with silver alloys some of these parameters are still not known. The purpose of this paper is to get a better understanding of emission of electrons occurring in an alloyed cathode submitted to several vacuum outgassing cycles at room temperature and residual gas pressure of 1.4 × 10-7 mbar. The electron work function (EWF) of silver alloyed contacts, Ag-Ni (70/30), was measured photoelectrically, using Fowler's method of isothermal curves. Experimental results about silver-nickel alloys show a large dependence of obtained results with the preparation of contact surface. The EWF of the contact pastille made with silver alloys Ag-Ni varies with surface cleaning by vacuum outgassing cycles. For an unpolished contact, the EWF varies between 4.34 eV and 4.51 eV (the EWF of Nickel), after 7 cleaning cycles and cleaning time of 9 days. On the other hand, for a polished contact, the EWF varies between the EWF of the two components, namely from 4.26 eV for Ag to 4.51 eV for Ni, at room temperature, after 22 cleaning cycles and a cleaning time of 16 days. The error in determining EWF was ±0.03 eV. A multilayer model, taking into account the strong intergranular and volume segregation gives a good interpretation of the obtained results. In addition, a change of order of 0.1 eV was observed for silver alloys Ag-Ni (60/40) EWF after polishing. Afterwards, the microstructure of the contact surface was analyzed with scanning electron microscopy (SEM) and energy dispersive x-ray spectrometry (EDS). The analyzes of the cathode surfaces before and after polishing enabled us to have evidence about the decrease of the electron work function for polished samples. © 2013 Elsevier Ltd. All rights reserved.

Akbi M.,CNRS Laboratory of Electric Arc and Thermal Plasmas | Akbi M.,University of Boumerdes
IEEE Transactions on Components, Packaging and Manufacturing Technology | Year: 2014

The nature of the contact material plays a key role to determine the characteristics of electrical arcs and particularly those related to electronic emission. Mechanical and thermodynamic properties as well as electronic emission of such complicated alloys present a lack of reliable and accurate experimental data. The purpose of this paper is to present the development of a method for measuring photoelectric work functions of contact materials. Also reported in this paper are the results of experimental work whose purpose has been the buildup of a reliable photoelectric system and associated monochromatic ultraviolet radiations source, and the photoelectric measurement of the EWF of contact materials. As a first test of the experimental Ultra High Vacuum setup, the electron work functions (EWF) of silver contacts, namely pure polycrystalline metals that are actually used in relays, were measured photoelectrically, using both Fowler's method of isothermal curves and linearized Fowler plots. Ultrahigh vacuum techniques were employed to obtain residual gas pressure of about (5× 10-9) mbar that allows accurate and reliable photoelectric work function measurements. The EWF measured at room temperature of polycrystalline Ag contact (as commercially available) increased from 3.70 eV before heating, to 4.30 eV, and finally, stabilized at the vicinity of 4.26 eV after several vacuum heat treatments at 533 and 773 K. The EWF value obtained, i.e., 4.26 ± 0.03 eV, is in good agreement with the results found in the literature. This shows that the experimental method is valid and the experimental setup is usable. Furthermore, new photoelectric measurements versus temperature have shown a linear decrease of the EWF of silver contacts with increasing temperature, i.e., the temperature coefficient α =dφ/dt) is constant and negative α =-4.58 . 10-4) eV/K in the experiment temperature range 300-780 K. © 2014 IEEE.

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