摘要:

According to the Biot theory and other models of propagation, two compressional waves can propagate in a porous medium saturated by air. At low frequencies each wave creates displacements of the same order of magnitude in the fluid and the frame. It is shown that for the case of thin materials bonded onto a rigid impervious backing, at low frequencies the acoustic field can nevertheless be described by only one wave which propagates in the air. This simple description is related to a mixing of both compressional waves which leaves the frame motionless. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362462

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Design of surface acoustic wave (SAW) devices requires accurate values of velocity shiftqand reflection coefficientrassociated with finger overlays placed on the substrate material. In previous treatments variational and perturbative approaches have been developed for calculating first‐order reflection coefficients and velocity shifts; however, the anisotropic substrate materials have until now been modeled by an effective isotropic substrate. We extend the variational and perturbative calculations of the first‐order quantities to fully anisotropic crystals for several standard SAW substrate and overlay materials. Moreover, we report results obtained with both electrically free and shorted trial functions, and suggest optimum parametrizations ofqandr, to first order inh/&lgr;, wherehis the finger thickness and &lgr; is the Rayleigh wavelength. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362463

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Experimental measurements of steady‐state discharge parameters are performed in a Cs–Ba tacitron equipped with Langmuir probes. Measured quantities include electron temperature, plasma density, and plasma potential, in both the emitter‐grid and grid‐collector regions. The effects of Cs pressure and electrode arrangements on the measured parameters are investigated. Measurements indicate that the electron energy distribution function in the emitter‐grid gap is Maxwellian, and that there is an electron beam in the grid‐collector gap, which relaxes to Maxwellian at high cesium pressure. Placing the grid closer to the collector not only increases the ion loss rate, but also makes ionization inefficient, lowering the plasma density in the grid‐collector gap. The low plasma density makes it easier to extinguish the discharge during current modulation. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362464

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An analytic model for a three‐component electronegative plasma has been developed within the framework of a diffusion–drift approximation. Anode fall voltage and anode sheath thickness are found for elevated‐pressure glow discharge. The predicted voltage–current dependencies are compared to available experimental and numerical results for dc glow discharges in air at pressures of 40–100 Torr. Electronegative ions increase the anode fall voltage at high current densities, including the values for anode spots, and reduce the anode fall voltage at low current densities, but pass through a minimum at a critical value of their concentration. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362465

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We measured the conversion efficiency of laser pulse energy into keV x rays from a variety of solid planar targets and a Xe gas puff target irradiated using a high average power Nd:glass slab laser capable of delivering 13 ns full width at half‐maximum pulses at up to 20 J at 1.053 &mgr;m and 12 J at 0.53 &mgr;m. Targets were chosen to optimize emission in the 10–15 A˚ wavelength band, includingL‐shell emission from materials with atomic numbers in the rangeZ=24–30 andM‐shell emission from Xe (Z=54). With 1.053 &mgr;m a maximum conversion of 11% into 2&pgr; sr was measured from solid Xe targets. At 0.527 &mgr;m efficiencies of 12%–18%/(2&pgr; sr) were measured for all of the solid targets in the same wavelength band. The x‐ray conversion efficiency from the Xe gas puff target was considerably lower, at about 3%/(2&pgr; sr) when irradiated with 1.053 &mgr;m. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362701

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The representation of plasma impedance of gas laser discharge and spark gap channels by an inductance–capacitance (Lp−Cp) tank circuit has been useful in describing the frequency response of a pulsed superradiant laser charging circuit. The impedance matching of these plasma channels can lead to resonant narrowing of the laser pulsewidth in superradiant nitrogen lasers. Using fluid equations to model the electron and ion plasmas, the impedance of plasma channels is formulated. This derived impedance supports the phenomenologicalLp−Cptank circuit configuration, and provides the means to calculate the inductance and capacitance in terms of the plasma density and discharge dimensions. Conjugating this model with the pulse width narrowing via plasma impedance matching, the electron density at the very moment of lasing in a superradiant gas laser channel can be estimated by using the measuredLpandCp. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362466

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Bipolar components that consist ofp+njunctions have been irradiated by MeV protons at fluences ranging from 1011to 1013particles cm−2. Capacitance‐voltage measurements have been used to investigate changes in the carrier concentration profiles. Shallow donors that can induce harmful effects in electronic devices have been studied as a function of fluence, flux, and annealing parameters. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362467

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The x‐ray rocking curve analysis was used to investigate damage accumulation with increasing dose in silicon implanted with 50 keV and 1 MeV11B+ions, and 50 keV, 180 keV, and 0.7 MeV28Si+ions. The damage buildup was studied by following the maximum value of lattice strain normal to the surface (&egr;⊥) and the depth integral of &egr;⊥. The &egr;⊥profiles were obtained with a dynamical diffraction formalism by using automated best fits to experimental x‐ray rocking curves. It is shown that, for doses below the amorphization limit, the damage increases sublinearly with dose and not very differently for B and Si ions. The sublinearity results from intercascade recombination of point defects produced under bombardment. The small differences, if real, in the sublinearities observed for the two ions could be explained by the different forms of aggregation to which the surviving defects evolve when the dose approaches the amorphization threshold. However, the study of damage growth must stop at the upper dose at which a continuous buried amorphous layer begins to form. In fact, starting from this dose, a simple semikinematical diffraction model shows that the determination of a unique peak value of &egr;⊥depth profile, and hence of its integral, is not possible. This is the consequence of the fact that the sample behaves as an x‐ray interferometer when an embedded amorphous layer is present. The analysis reported here is compared with similar studies recently published in the literature. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362482

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The Avrami–Johnson–Mehl [P. G. Shewmon,TransformationinMetals(McGraw Hill, New York (1969)] approach to phase transformations is extended to include finite area amorphous zone nucleation or generation by ion impact onto semiconductors together with bombardment induced zone expansion and thermal annealing zone contraction (recrystallization) in order to predict the variation of amorphized material fraction with ion fluence. The ion flux and substrate temperature are found to play decisive roles in determining this variation. The model predictions are shown to agree qualitatively with currently available experimental data with respect to the system variables of ion flux, fluence, and substrate temperature. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362468

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A persistent spectral hole‐burning technique has been employed to investigate the impact of uniaxial stress on the defects created by neutron irradiation in natural Ia‐type diamond. Shifts of the spectral holes in the 649, 681, and 774 nm zero‐phonon lines (ZPLs) have been measured. The values of the shifts, 2.2, 1.2, and 0.65 cm−1/kbar, obtained from these measurements have been compared with the stress shifts of the lines of defects with well‐known structure. It is suggested that vacancies are present in the defect responsible for the 681 nm ZPL. A combination of the results of polarized luminescence and uniaxial stress experiments leads to the conclusion of rhombic I symmetry of the defects that give rise to the 649 and 681 nm ZPLs. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362469

出版商:AIP    

年代:1996

数据来源: AIP