Repetitive hydrogen pellet injector utilizing a screw extruder

A new injector for the continuous formation of an unlimited supply of fuel pellets and their injection into the plasma of fusion reactors is described. A solid hydrogen rod of length in excess of 50 m has been extruded at an average rate of 20 mm/s, and more than a thousand pellets of diameter 2 mm have been formed from it in continuous operation at frequencies of 1 Hz and 2 Hz and have been accelerated to 0.6–0.8 km/s. Zh. Tekh. Fiz. 68, 117–120 (May 1998)     

Periodic character of the phase transformation in steel accompanying reflection of converging shock waves from the center of focusing

Special conditions in an experiment on the loading of low-carbon steel by converging shock waves made it possible to obtain residual microstructural changes of a periodic character, with a period of the order of 0.2 mm, which are associated with a phase transformation caused by waves reflected from the center of focusing. A model explaining the periodicity of the microstructure and making it possible to predict the behavior of materials under similar conditions is proposed. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 803–807 (25 May 1998)     

Giant magnetoresistance of vanadium films with a surface magnetic superlattice

Giant (up to 68%) room-temperature magnetoresistance is observed in (110) V films on mica with a periodic system of 5–25 nm wide, thin (⩽10 nm) Co strips, separated by 1–2 nm gaps, grown on the films. The effect is observed only for samples in which the magnetization tilts out of the plane of the film. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 5, 346–349 (10 September 1996)     

Hydrogen in amorphous and crystalline ytterbium films

Ytterbium vapor condensation on a liquid-helium cooled substrate in a hydrogen atmosphere is used to obtain Yb-H films containing up to 55 at.% hydrogen. Various thermodynamic and kinetic parameters of the transition of these films from the amorphous to the crystalline state (a→c transition) are investigated along with the electrical conductivity of these states. It is shown that the investigated properties of Yb-H films containing up to 40 at.% hydrogen are essentially indistinguishable from those of pure Yb films in the temperature interval 4.2–293 K. Increasing the hydrogen concentration to 55 at.% leads to an insignificant increase in the electrical resistivity, the kinetic temperature, and the activation energy of the a→c transition, and also to a decrease of the propagation speed of self-maintaining avalanche (explosive) crystallization. Reasons for the observed influence of hydrogen on the properties of Yb-H films are analyzed. The examined low-temperature Yb-H condensates can be characterized as a “frozen” solid solution of hydrogen in ytterbium in the temperature interval 4.2–293 K. Storing such films at room temperature leads to the formation of ionic ytterbium dihydride YbH₂. Fiz. Tverd. Tela (St. Petersburg) 41, 177–182 (February 1999)     

Amplification of the generation of the sum-frequency signal in multilayer periodic structures at the edges of the Bragg band gap

A new property of a one-dimensional periodic structure — amplification of the sum-frequency signal arising under the simultaneous action of two laser pulses on this structure with radiation frequencies corresponding to the edges of the fixed Bragg band gap — is experimentally observed and described. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 718–721 (10 December 1999)     

Suppression of the scattering of conduction electrons by lattice phonons in palladium in the presence of small hydrogen (deuterium) inclusions

An increase has been found in the electrical conductivity of electrochemically dehydrogenated palladium hydrides (deuterides) as compared with original samples of pure palladium in a wide temperature range (75–300 K). It is shown that this effect is due to the suppression of the scattering of conduction electrons by phonons in the palladium lattice for T⩾θ (θ is the Debye temperature) in the presence of clusters of quasimetallic hydrogen (deuterium). Fiz. Tverd. Tela (St. Petersburg) 39, 2113–2117 (December 1997)     

Production of fullerenes in an arc discharge in the presence of hydrogen and oxygen

The production of fullerenes in an arc with graphite electrodes in mixtures of helium with oxygen and hydrogen as well as in pure oxygen and hydrogen is investigated. The radiation spectra of the arc and the mass spectra of the soot obtained in the arc are recorded; the content of fullerenes in the soot is determined. It is shown that fullerenes are formed in appreciable quantities (∼1%) even in pure hydrogen. Zh. Tekh. Fiz. 69, 48–51 (December 1999)     

Limiting role of desorption in hydrogen transport across a deposited beryllium film

Hydrogen transport across a deposited beryllium layer has been investigated using the hydrogen permeability and concentration pulse methods. A layer of beryllium was deposited on a prepurified nickel membrane by cathode sputtering in a glow discharge plasma in “especially pure” grade hydrogen. An analysis of the experimental results showed that the main limiting process for hydrogen transport is desorption from the layer rather than diffusion in the bulk of the layer. A mathematical transport model is proposed and used to determine the rate constant of hydrogen desorption from beryllium. Zh. Tekh. Fiz. 68, 128–130 (January 1998)     

Antipolarization effect in quantum wells in a strong external alternating field

It is shown that when a strong ac electric field acts on an electron in a double quantum well, the dipole moment is an almost periodic function of the dc voltage applied to the structure. An antipolarization effect — the structure is polarized in a direction opposite to the external field — appears during one half of the period. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 6, 386–391 (25 September 1999)     

Characterizing the Evolution of Porosity during Controlled Drug Release

Nuclear magnetic resonance (NMR) techniques have been successfully used to characterize the evolving pore structure of partially soluble pharmaceutical pellets as they absorb water and release soluble components. The restricted diffusivity of water trapped within pellets, which have been immersed in water for differing times, has been measured by pulsed field gradient NMR. These measurements have been used to calculate the surface-to-volume ratio and tortuosity of the pore structure. A one-shot Carr–Purcell–Meiboom–Gill sequence has been used to measure the spin–spin (T ₂) relaxation time of water trapped within the pellets. These data have been regularized and then analyzed by the Brownstein–Tarr model to provide a pore size distribution for the pellets as a function of increasing immersion time. It has been found that pore structure changes significantly as water enters the pellet matrix. Two pellet formulations (herein referred to as placebo and drug-loaded) were studied and showed the same trends of a decreasing surface-to-volume ratio and tortuosity with increasing immersion time. At an immersion time of 10 min, both of these parameters decreased to approximately 70% of their values compared to an immersion time of 2 min. The placebo material tested consistently had both a higher tortuosity and surface-to-volume ratio than the drug-loaded material. At an immersion time of 2 min, the tortuosity for the placebo and drug-loaded materials were about 18 and about 10, respectively, and surface-to-volume ratios of about 6 μm⁻¹ and about 5 μm⁻¹, respectively. The materials tested also show changes in their pore size distribution with immersion time. In both formulations the mean and modal pore sizes increase with immersion time. The placebo material maintains an approximately similar mean and modal pore size, about 2 μm over the timescales studied, suggesting a more symmetric pore size distribution. In the drug-loaded pellets the mean pore size is much higher than the modal pore size, their values being 6.5 and 2.1 μm after 10 min immersion time, respectively. Authors' address: Michael D. Mantle, Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK     

High-order perturbation theory for the hydrogen atom in a magnetic field

The states of a hydrogen atom with principal quantum numbers n⩽3 in a constant uniform magnetic field ℋ are studied. Coefficients in the expansion of the energy of these states in powers of ℋ² up to the 75th order are obtained. Series for the energies of the states and the wave functions are summed to values of ℋ on the order of the atomic magnetic field. A generalization of the moment method upon which these calculations are based can be used in other cases in which a hydrogen atom is perturbed by a potential with a polynomial dependence on the coordinates. Zh. éksp. Teor. Fiz. 113, 550–562 (February 1998)     

Stationary soliton on a charged surface of liquid helium and hydrogen films

A change in the shape of a charged surface of liquid hydrogen and helium — the formation of a solitary wave (a positively charged hump for hydrogen and a negatively charged dimple for helium)-is observed in an electric field exceeding a critical value under conditions of total compensation of the applied field by the surface charge. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 7, 547–552 (10 April 1997)     

Existence of an incommensurate phase in a Li2B4O7 crystal

The x-ray diffraction spectra of Li₂B₄O₇ single crystals are investigated in the temperature range 80–300 K, and the lattice parameter c is determined in the same temperature range in the presence of a periodically varying temperature field. An incommensurate phase is not observed anywhere in the temperature range investigated, regardless of whether the crystals are subjected to a periodic temperature field. Fiz. Tverd. Tela (St. Petersburg) 39, 1461–1463 (August 1997)     

Characteristic features of the formation of the vibrational distribution function of H2 molecules in a hydrogen stream

A theoretical analysis is made of the flow of vibrationally excited hydrogen in a channel. It is shown that coverage of the channel walls with adsorbed hydrogen atoms can substantially increase the concentration of vibrationally excited molecules in the stream. The possibility of applying these results to bulk sources of negative H⁻ hydrogen ions is discussed. It is shown that the rate of H⁻ ion generation in the source may be enhanced appreciably under conditions where this generation is achieved by dissociative attachment of thermal electrons to H₂ molecules injected into the discharge chamber, whose vibrational distribution function has been pre-enriched in excited molecules by suitably organizing the hydrogen flow in the channel. Zh. Tekh. Fiz. 69, 15–21 (June 1999)     

On hydrogen localization in the β′ phase of VO0.2

The results of an investigation of the dynamics of hydrogen atoms in the body-centered tetragonal lattice of the interstitial β′ phase of vanadium-oxygen by the method of inelastic scattering of slow neutrons are reported. It is concluded on the basis of an analysis of the structure of the alloy investigated that hydrogen in the lattice is localized in tetrahedral interstices with the point symmetry of an almost ideal tetrahedron. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 326–328 (10 March 1998)     

Formation of amorphous carbon on melting of microcrystalline graphite by picosecond laser pulses

Observations of microcrystalline graphite subjected to picosecond laser pulses reveal the formation of a liquid phase with a subsequent transition to a uniform amorphous state of a surface layer upon solidification. This phenomenon is observed on a definite type of graphite and with the radiation incident on a plane parallel to the sixfold symmetry axis, and only for certain parameters of the laser pulse. A structural analysis of the amorphous phase is performed by electron microscopy and Raman scattering spectroscopy. A periodic structure with a period of the order of the wavelength of the heating pulse is formed in the heating region. The “rulings” of this periodic structure are oriented in the direction of polarization of the heating pulse. A study of the reflection kinetics of the probe laser pulse showed that the characteristic existence time of the liquid phase and of the solidification process is ∼10⁻¹⁰ s. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 10, 661–665 (25 November 1997)     

Effect of the loading rate on the mechanical properties of polycrystalline samples of a ZrO2-3 mol. % Y2O3 solid solution

The dependence of the bending strength and fracture toughness of polycrystalline samples of a solid solution of ZrO₂-3 mol. % Y₂O₃ on the traverse rate of a test machine has been studied in the velocity interval of 0.005–50 mm/min. It is shown that the strength and the fracture toughness are virtually constant in the interval of loading rates of 0.05–5 mm/min. An increase of the loading rate to 50 mm/min is accompanied by a significant decrease (by a factor of 2 or 3) in both the strength and the fracture toughness. When the loading rate is decreased to 0.005 mm/min, a slight increase (10–15%) in the fracture toughness and a comparable decrease in the strength is observed. An experiment has also been done to see how the mechanical properties measured at a loading rate of 0.5 mm/min are affected by holding polycrystalline samples of a solid solution of ZrO₂-3 mol. % Y₂O₃ under load for several hours. Holding the samples under a load causes a significant increase (by 30–50%) in both the strength and the fracture toughness. The observed phenomena are explained in terms of the variation of the amount of Y₂O₃, which stabilizes the metastable tetragonal phase of a solid solution of ZrO₂. Fiz. Tverd. Tela (St. Petersburg) 39, 880–884 (May 1997)     

Nonlinear laser-induced regime of surface-electromagnetic-wave generation and submicron periodic relief during liquid-phase photochemical etching of n-III–V semiconductors

This paper is devoted to an experimental study of the physical processes underlying the phenomenon of laser-induced generation of periodic relief on the surface of n-III–V semiconductors during liquid-phase photochemical or photoelectrochemical etching accompanying the resonance interaction of surface electromagnetic waves (SEWs). The increments of the exponentially increasing amplitudes of the dominant Fourier harmonics of the relief have been measured at the initial (linear) stage of the time evolution of the surface profile. It is proven by comparing the theoretical and experimental results that the mechanism for forming periodic structures that we have proposed is adequate. Ways of monochromatizing the generated relief and controlling the line shape of the surface grating are studied. It is experimentally detected for the first time that the nonlinear stage of the time evolution of the relief is characterized, in accordance with the predictions of the theory developed by the authors, by amplitude and phase oscillations of the first and second Fourier harmonics of the surface profile. It is shown to be possible to generate relief that suppresses specular reflection from the surface. A new nonmasked laser method is developed for forming high-quality submicron relief diffraction gratings, combining a holographic method and a method involving laser-induced relief generation during resonance excitation of SEWs. Diffraction gratings with a period of d=0.24–0.54 μm and a depth of h=0.1–0.2 μm over an area of 0.5×31 cm have been created on an n-InP surface. Zh. éksp. Teor. Fiz. 111, 174–198 (January 1997)     

Pellet-injection-based technologies for fusion reactors

An analysis is made of results obtained in the development of various technologies based on injecting pellets into tokamak and stellarator plasmas. Pellet injection is now used for fuel injection, plasma diagnostics, burn control, mode switching, and conditioning of the first wall. Physical and engineering problems encountered in the development of these technologies and results of recent applications are discussed. Zh. Tekh. Fiz. 69, 63–67 (September 1999)     

Dissociation energy and ionization potentials of molecules and molecular ions in a collisional hydrogen plasma

It is shown that the dissociation energies of a hydrogen molecule and a hydrogen molecular ion are virtually independent of the screening length of the Coulomb potential in plasma even at densities for which the decrease in the ionization potential of the molecule reaches 30%. For this reason, ionization of hydrogen in high-temperature shock-wave experiments should occur in a molecular phase with the formation of the molecular ion. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 397–401 (10 March 1997)