Results 1 - 10 of 129
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[en] An interesting phenomenon observed in the near-anode region of a Hall thruster is that the anode fall changes from positive to negative upon removal of the dielectric coating, which is produced on the anode surface during the normal course of Hall thruster operation. The anode fall might affect the thruster lifetime and acceleration efficiency. The effect of the anode coating on the anode fall is studied experimentally using both biased and emissive probes. Measurements of discharge current oscillations indicate that thruster operation is more stable with the coated anode
[en] A quasi one-dimensional steady-state model of the Hall thruster is presented. For given mass flow rate, magnetic field profile, and discharge voltage the unique solution can be constructed, assuming that the thruster operates in one of the two regimes: with or without the anode sheath. It is shown that for a given temperature profile, the applied discharge voltage uniquely determines the operating regime; for discharge voltages greater than a certain value, the sheath disappears. That result is obtained over a wide range of incoming neutral velocities, channel lengths and widths, and cathode plane locations. A good correlation between the quasi one-dimensional model and experimental results can be achieved by selecting an appropriate temperature profile. We also show how the presented model can be used to obtain a two-dimensional potential distribution
[en] The process involving the entry of a vortex structure into a long Josephson junction whose parameters vary greatly but smoothly along the length of the structure is theoretically analyzed. A sine-Gordon-type equation that describes the rapid oscillations of the Josephson current along the junction is reduced with the aid of the asymptotic technique to an ''abridged'' equation that describes the slow variations of the parameters of such a vortex structure. The abridged equation admits of an analytic solution, and this allows us to find in its explicit form the critical junction current as a function of the magnetic field. The obtained results are in good agreement with the data obtained in recently performed experiments
[en] Theoretical, methodological and methodic aspects of the problem of radiation risk analysis are considered. It was shown, that the potential risk caused by technogenic exposure can't practically be selected for certain on the overall risk background relative to exposure of factors non-radiation nature. The structure both of the overall risk assessment and of its radiation component is given. The main factors limiting validity of the radiation component of overall risk finding are discussed. An importance of problems actual for an estimation of radiation safety of both individual and society as a whole is systematized. Some aspects of acceptable risk assessment are considered. Volume and influence of risk of these technogenic sources on health are compared. Some general theses characterized the modern State of the problem. (author)
[en] The beginning of XXI century is marked with increasing public concern over impact of man-made activity, including nuclear technologies, on the environment. Currently, the anthropocentric principle is applied in the course of the radioecological safety guaranteeing for the environment, which postulates that human protectability serves as guarantee of the environmental one. However, this principle correctness is called in question recently. The ecocentric principle is proposed as an alternative doctrine, defining balance between human importance and that of any other elements of biota. The system recommended isn't intended for the regulatory standards development yet, because of substantial gaps in scientific knowledge. Nevertheless, renunciation of the anthropocentric principle can result in unwarranted tightened regulatory basis, decreasing of nuclear industry evolution rates, and, consequently, breaching of societal and economical priorities. It is obvious that for the safety guaranteeing, nuclear industry shouldn't stand out against a background of other fields of human activity involved hazard factors. Therefore, new conceptions applying within the regulatory system is to be weighted and exclude formal using of discussion theses. More than semi-centennial experience of the anthropocentric approach applying serves as an evidence of safe protection of ecosystems against radiation exposure that ensures safe ecological development of nuclear power industry and other fields of nuclear technologies application. (author)
[en] We suggest a new approach to numerical modeling of electron distribution function in an electron cyclotron resonance (ECR) discharge. The method is based on a pointwise mapping of electron velocity over a single bounce oscillation. We limit our consideration to ECR heating, collisions, ionization, and ambipolar losses from the trap although other processes can be accounted for as well. The method gives a solution close to the brute-force particle-in-cell integration but is incomparably faster. Initial results of experiments are presented
[en] The nuclear technologies development is defined considerably by an adequate assessment presence of possible exposure negative consequences for the branch personnel health. Up today a number of analytical approaches had been worked out to estimate some radiation exposure effects, but model presentations tests are limited, as a whole, by the instantaneous exposure conditions. That's why the obtained dependencies expanding to the prolonged irradiation conditions, ignoring its exposure specifics, couldn't be voted as adequate enough. We propose a model option intended to assessment and prognosis of radiation induced leukemia morbidity dynamics. The model is based on the theses developed in the MVKconception for the description of the generation process of sequential stable mutations in target cells. The general form of the model equations describing the cell amount distribution, considering two or three mutagenesis stages preceding malignisation. a decision was obtained approximing the leukemia yield dynamics during both spontaneous mutagenesis and instantaneous and prolonged exposure. To model parameters assessment we used the information obtained for the LSS (DS86)-cohort and unique data from the register of IC Mayak (SUrlBPh) staff relating to the beginnings of nuclear branch in Russia. At that time, the staff was exposed by prolonged irradiation with doses exceeding considerably all of the modern limits. the model testing showed a satisfactory agreement between observed leukemia morbidity dynamics and calculated values. for the Mayak staff group irradiated with the total doses exceeded 3-4 Sv, the specifics of dynamical morbidity profile was determined related to the group of so-called early leukemia with the realization periods of 2-5 years from the exposure beginning. The obtained results confirm the correctness of the carcinogenesis process description by means of modeling of sequential stable mutation generations in the organism's critical system cells. (Author) 9 refs
[en] Hyperfine fields (hff) in Fe/V and Fe/Co superlattices have been investigated using angle dependent Moessbauer spectroscopy. Conversion electron Moessbauer (CEM) spectra were measured for different orientation of radiation beam relative to the crystallographic axes of the sample. Hff distributions were extracted from the texture-free spectra in which the ratio between different lines is independent on the hff directions. In Fe/V multilayers we found well-defined magnetic texture in the superlattice plane. To connect hff with magnetic moments we performed microscopic calculations of the electronic structure for Fe/V multilayers with alloyed interface. In Fe/Co multilayers the second and the fifth line of the spectra can be almost fully suppressed for appropriate beam direction in the plane of the sample. It means that all magnetic moments of Fe atoms are ordered in the same direction and, therefore, Fe/Co multilayers have to show strong magnetic anisotropy.
[en] Moessbauer spectroscopy with 57Fe (119Sn) probe layers is a useful method to study the local magnetic structures at buried interfaces. However interface alloying, which always exists in the real samples, have to be taken into account for accurate interpretation of experimental data. We developed an algorithm, which describes the interface intermixing in the multilayers. Substituting deposited atoms by atoms of substrate and floating of deposited atoms in the upper layers during epitaxial growth leads to the formation of asymmetric chemical and magnetic interfaces. This asymmetry in the M1/M2 superlattices can explain the difference between magnetic responses from M1 on M2 and M2 on M1 interfaces which were observed in experiments. Applying this intermixing model to the systems with probe layers located at different distances from the interfaces gives the natural explanation of hyperfine fields distributions on probe atoms and helps us clarify some discrepancies reported in the literature.