An experimental investigation of the effect of oxygen on the erosion of a multichannel tungsten cathode

The problem of the preservation of electrodes and the reduction of their depletion to a minimum has decisive significance among the practical questions of maintaining the efficiency of steady sources and plasma accelerators during an extended period of operation. For this purpose electrodes are made, as a rule, out of the refractory metals in familiar designs. In particular, tungsten, which has the highest phase-transition; temperatures among all the metals, thus permitting an appreciable reduction in the evaporation rate of the material under the thermally stressed operating conditions of these devices, is attractive. However, there exist other factors besides evaporation which affect the integrity of hot tungsten electrodes and their ablation (chemical processes, erosion in microarcs, ion bombardment, and so on). One of these factors requiring special investigation is the chemical interaction of tungsten with the oxygen contained in the working medium as an industrial impurity. Such an impurity can, for example, be present in industrially pure alkali and alkali-earth metals used as the working media. In addition, these metals can be contaminated by the oxygen of the air in the process of different industrial operations, in particular, in connection with the servicing of the supply system. The entrance into the working material of oxygen liberated from the structural elements of the supply route and the operating cavities of the plasma source is not excluded. In practice it is impossible to eliminate oxygen, even when using contemporary methods of deep cleaning of lithium, due to its great affinity for oxygen. The actual role of the oxygen impurity and the necessity for taking it into account in connection with the creation and operation of plasma devices can only be revealed as a result of direct experimental investigation on samples having an electrode. geometry similar to the class of sources or accelerators in question. Results are presented in this article of an experimental investigation of the erosion of a multichannel tungsten electrode [1–4] and a coaxial source of lithium plasma [5]. The use of lithium as the main plasma-forming material ensured the maintainance of a discharge distributed over the electrodes and having a small value of the potential drop next to the cathode, which lies below the threshold of cathode erosion upon bombardment of the surface of polycrystalline tungsten by lithium ions [6]. Thus, the phenomena of cathode destruction associated with microarcs and ion bombardment could be exluded from consideration in practice. Thus evaporation and chemical ablation due to interaction with the oxygen of the working medium remain the principal, competing processes contributing to the erosion of the cathode mass. Therefore, in order to obtain reliable quantitative relationships which characterize differential erosion due to oxidation and evaporation, the oxygen content in the working medium was varied in the experiments described within limits extending beyond the framework of the usual industrial impurity, which does not exceed tenths of a percent. One should note that in the process of investigating the role of an industrial oxygen impurity in causing the erosion of a tungsten cathode in a lithium plasma its direct effect was revealed on the anomalous current of a hollow cathode, which is characterized by the recorded average densities of the discharge current exceeding significantly the values of the emission current given at the same surface temperature by the Richardson-Schottky equation [7–9].Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 67–73, May–June, 1979.The authors express their gratitude to S. S. Kellin, N. P. Mezhevov, and V. N. Belinskii for their participation in the preparation and performance of the experiment.     

Distribution of Zinc Oxide Nanocrystals in a Polymer Film

The structure of polymer films modified with zinc oxide nanocrystals was studied by the optical absorption and small-angle X-ray scattering methods. Small-angle X-ray scattering allows determination of the nanoparticle size distribution function, which is the decisive factor in predicting the optical properties of heterophase materials, including filled polymer films. The optical absorption spectrum of an acrylate polymer film doped with 1.6 wt % zinc oxide nanocrystals approaches the ideal absorber spectrum, which allows this material to be recommended for use as a protective color filter.     

Drag associated with transitional flow regime of a fiber suspension in a tube

A semlempirical model is constructed of the flow of a fiber suspension of low and medium concentration in regimes that are usually called mixed and undeveloped turbulent regimes [1–4]. It is shown that although the flow of fiber suspensions in these regimes has features similar to those of the turbulent flow of a Newtonian fluid, for example, a logarithmic velocity profile, the characteristic features of the flow in both regimes can be better explained, not by turbulence of the flow, but by orientation of the fibers in it and by plastic flow of the fiber continuum. For this reason, to distinguish the mixed and undeveloped turbulent regimes from a truly turbulent regime it is proposed here to describe them by a general name — transitional flow. The obtained expressions agree qualitatively and quantitatively with the experimental results of Lee and Duffy [2], Sanders and Meyer [3], and Mih and Parker [4].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 40–45, May–June, 1984.I thank V. N. Nikolaevskii and A. N. Golubyatnikov for interest in the work and helpful comments.     

Transformation of aromatic compounds under the action of the basidiomycetesPhanerochaete sanguinea andCoriolus villosus

The pathways of the transformation of some aromatic compounds by rhe basidiomycetesPhanerochaete sanguinea andCoriolus villosus have been studied. It has been shown that the degradation of these compounds has an oxidative nature and depends on the type of substituents in the benzene ring and the propane chain. A difference has been found in the mechanisms of the reactions of the two fungi that is a consequence of the different compositions of their enzyme complexes.Wood Chemistry Division, Institute of Organic Chemistry, Siberian Division of the Russian Academy of Sciences, Irkutsk. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 808–819, November–December, 1994.     

Possibility of using recombination reactions and alkali metals to obtain inversion on atomic iodine

The possibility of producing a continuous-flow laser operating on an atomic iodine transition is considered. Population inversion is the result of depletion of the lower laser level on account of predominant recombination into iodine molecules in the ground state. Alkali-metal atoms are used to obtain atomic iodine and free radicals without consumption of electric energy. The use of an electric discharge is proposed for the excitation of atomic iodine. The kinetics of the basic chemical reactions is considered and the possibility is demonstrated of obtaining inversion at reasonable working-mixture parameters. The schematic diagram of the laser setup is discussed.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 125, pp. 202–209, 1980.