Scaling laws for the spatial distributions of the plasma parameters in the positive column of a dc oxygen discharge

Comprehensive self-consistent simulations of the positive column plasma of a dc oxygen discharge are performed with the help of commercial CFDRC software (), which enables one to carry out computations in an arbitrary 3D geometry using fluid equations for heavy components and a kinetic equation for electrons. The main scaling laws for the spatial distributions of charged particles are determined. These scaling laws are found to be quite different in the parameter ranges that are dominated by different physical processes. At low pressures, both the electrons and negative ions in the inner discharge region obey a Boltzmann distribution; as a result, a flat profile of the electron density and a parabolic profile of the ion density are established there. In the ion balance, transport processes prevail, so that ion heating in an electric field dramatically affects the spatial distribution of the charged particles. At elevated pressures, the volume processes prevail in the balance of negative ions and the profiles of the charged particle densities in the inner region turn out to be similar to each other.     

On wave radiation induced by a fast shock wave in an oblique magnetic field

The corrugation stability of the flat surface of a fast magnetohydrodynamic shock wave in a perfect monoatomic gas with a constant heat capacity is studied with numerical techniques. The magnetic field makes an arbitrary angle with the plane of discontinuity. It is shown that the shock wave remains stable only if it is strictly perpendicular to the magnetic field. At any other angle between the fast shock wave and magnetic field, the former may spontaneously radiate outwardly propagating magnetohydrodynamic waves under certain conditions. Incoming flow characteristics at which these waves are induced are determined.     

Charge manipulation and imaging of the Mn acceptor state in GaAs by cross-sectional scanning tunneling microscopy

An individual Mn acceptor in GaAs is mapped by cross-sectional scanning tunneling microscopy (X-STM) at room temperature and a strongly anisotropic shape of the acceptor state is observed. An acceptor state manifests itself as a cross-like feature which we attribute to a valence hole weakly bound to the Mn ion forming the (Mn²⁺3d⁵+hole) complex. We propose that the observed anisotropy of the Mn acceptor wavefunction is due to the d-wave present in the acceptor ground state.     

Two scenarios for phase-transformation in disordered media

The reasons for the existence of various scenarios for structural transformations in disordered condensed media, such as liquids and amorphous substances, where both smeared transformations and sharp first-order transitions may occur, were analyzed. The ratio between the spatial scale of structural correlations and the size of the smallest possible region occupied by a new phase in the matrix of initial modification is the key parameter determining the scenario for equilibrium phase transformations in liquids. In amorphous substances, the experimentally observed transformations occur far from equilibrium, and the possible size of the region occupied by the new phase corresponds to the minimal nucleus size. For some amorphous solids, quantitative analysis of the transformation width was carried out and the main classes of covalent substances, in which the smeared or sharp transitions occur, were revealed. Specific features of the interparticle interactions determining various transformation scenarios are discussed.     

Uniformity in radon exhalation from construction materials using can technique

The uniformity in radon exhalation rates for 46 tiles of granite, marble and ceramic used as construction materials were determined using “Can Technique” employing CR-39 nuclear track detectors (NTDs). On each tile, two sealed cans, each enclosing one NTD fixed at the center of the tile surface area covered by the can, were mounted at two different locations of each individual tiles. The track production rates on the NTDs representing radon exhalation rates were measured. The radon exhalation rates from the surface of individual tiles showed uniform exhalations within the calculated uncertainties of the measured values. This makes Can Technique an alternative simple method to measure radon exhalation rates. Calibration required to convert track production rates into radon exhalation rates for the used can and NTD was done using an active technique. The correlation between the measurements by the two techniques shows a good linear correlation coefficient (0.83).     

Possible lower bounds for the direct detection rate of SUSY dark matter

One can expect accessible lower bounds for the dark matter detection rate due to restrictions on masses of the SUSY partners. To explore this correlation, one needs a new-generation large-mass detector. The absolute lower bound for the detection rate can naturally be due to spin-dependent interaction. Aimed at detecting dark matter with sensitivity higher than 10^?5 event/kg/d, an experiment should have a nonzero-spin target. Perhaps, it is best to create a GENIUS-like detector with both ⁷³Ge (high spin) and ⁷⁶Ge nuclei.     

THE COUPLING OF NATURAL BOUNDARY ELEMENT AND FINITE ELEMENT METHOD FOR 2D HYPERBOLIC EQUATIONS

In this paper, we investigate the coupling of natural boundary element and finite element methods of exterior initial boundary value problems for hyperbolic equations. The governing equation is first discretized in time, leading to a time-step scheme, where an exterior elliptic problem has to be solved in each time step. Second, a circular artificial boundary FR consisting of a circle of radius R is introduced, the original problem in an unbounded domain is transformed into the nonlocal boundary value problem in abounded subdomain. And the natural integral equation and the Poisson integral formula are obtained in the infinite domainΩ2 outside circle of radius R. The coupled variational formulation is given. Only the function itself, not its normal derivative at artificial boundary ΓR, appears in the variational equation, so that the unknown numbers are reducedand the boundary element stiffness matrix has a few different elements. Such a coupled method is superior to the one based on direct boundary element method. This paper discusses finite element discretization for variational problem and its corresponding numerical technique, and the convergence for the numerical solutions. Finally, the numerical example is presented to illustrate feasibility and efficiency of this method.     

Experimental evidence for anisotropic response of a driven vortex lattice

We present kinetic inductance measurements in current driven Josephson junction arrays. Measurements performed with circular coils indicate that this technique is very sensitive to the vortex mobility. Results obtained with anisotropic detection coils provide experimental evidence for the anisotropic response of a driven vortex lattice. Anisotropic behavior is obtained when rising the temperature or the mean vortex velocity as indicated by the loss of the superconducting shielding capability, first in the direction of vortex motion and then in the perpendicular direction.     

Tunable narrow-band infrared emitters from hexagonal lattices

In this work, we present both the theoretical basis as well as supporting experimental measurements for development of a novel mid-infrared thermally stimulated narrow band emitter with a spectral bandwidth of less than 10%. To achieve this, we utilize a metallized-surface 2D photonic crystal of air voids in a silicon background with hexagonal structure symmetry. Our results are based on the generation of discrete surface plasmon (SP) modes in the thin metallized layer residing on the top surface. This yields a series of adequately spaced discrete peaks in the reflection spectrum, dominated by a single sharp feature corresponding to the lowest plasmon order, in an otherwise uniform highly reflective spectrum (>90%) over most of the IR spectrum. This, in turn, gives rise to a sharp absorption feature with a correspondingly narrow thermal emission peak in the emission spectrum. Transfer matrix calculations simulate well both the position and strengths of the absorption peaks. By altering the period of the surface photonic lattice, the SP peak and emissive band can be tuned to the desired wavelength. These devices promise a new class of tunable infrared emitters with high power in a narrow spectral bandwidth. Such narrow band sources are critical to achieving high efficiency gas sensors.