摘要:

A small CO2‐N2‐He laser excited by a pulsed microwave source at 2.45 GHz has been operated at low powers. Microwave power flux density was increased by using tapered sections to modify the aspect ratio. Pulsed microwave excitation of the gases resulted in laser output pulses characterized by time delays to threshold and long pulse lengths. The efficiency of conversion of microwave energy to laser energy was over 4&percent;.

ISSN:0021-8979    

DOI:10.1063/1.325429

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The transfer function of the title is derived first for the case of a stationary source‐receiver and a stationary target, with each in the far field of the other. Then, a quasi‐transfer‐function is constructed for the case in which the two bodies, separated as before, are moving at speeds which are small relative to the speed of sound, the flow around them being assumed to be derivable from velocity potentials. In both cases, the problem is formulated in terms of Helmholtz‐Kirchhoff integrals. Conditions related to the distance to boundaries are deduced through the method of images. Theorems due to Blokhintsev and Oestreicher are used to treat the radiation, scattering, and reception of sound by a moving body.

ISSN:0021-8979    

DOI:10.1063/1.325430

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A simple circuit model for the multistrip coupler is presented. The transmission matrix of a single strip relating the output‐ and input‐wave amplitudes in different tracks is obtained from circuit theory. The overall transmission matrix of the coupler is obtained by cascading the transmission matrices of successive strips. The model is shown to yield the correct results for common coupler configurations. This model because of its strip‐by‐strip approach makes it straightforward to analyze nonperiodic couplers which are difficult to analyze by usual techniques. Numerical solutions for a strip‐coupled UDT are presented in agreement with previous experimental results.

ISSN:0021-8979    

DOI:10.1063/1.325431

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Simple rigid crack‐front propagation and the mechanism of double‐kink nucleation and spreading are analyzed in a periodic crystallographic potential‐energy field. For the analysis the classical Griffith model for crack propagation is superposed on the periodic energy field of the crystal lattice. The energy‐condition theory of the time‐dependent crack propagation is developed in terms of the absolute rate theory. A complete kinetics analysis is carried out for the description of the rigid and kinked crack‐front propagation rate over the consecutive energy‐barrier system of the crystal lattice. The rate is expressed as a function of the applied stress, temperature, the free energy of fracture, and the lattice parameters.

ISSN:0021-8979    

DOI:10.1063/1.325432

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The fracture of CaF2, SrF2, and BaF2crystals by {111} cleavage requires critical fracture energies (&ggr;IC) which increase with decrease in cation size, as would be predicted. The &ggr;ICof single crystals, however, can be affected by the crystallographic direction of crack propagation and by annealing treatment. The introduction of grain boundaries can also result in some increase in &ggr;IC, particularly for finer grain sizes. Slow crack growth, which is sensitive to the environment, is observed but differs from that noted in other brittle materials in that it is often accompanied by crack arrest. This observation, as well as the stair‐step crack motion during slow crack growth in concentrated HF, indicates that dislocation motion at the crack tip is associated with the slow‐crack‐growth phenomenon. Because of the extremely strong influence of the stress intensity on crack growth velocity, the effects of delayed failure should be minimal in CaF2, SrF2, and BaF2.

ISSN:0021-8979    

DOI:10.1063/1.325433

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

This paper describes the experimental procedure we have developed for producing spherical deuterated polyethylene targets with a continuum of densities in the range 20–60&percent; that of solid (&rgr;solid?1.0 g/cm3). These spheres (termed ’’microsponges’’) are typically 80–300 &mgr;m in diameter and are highly porous networks with average pore sizes of a few microns or smaller. The technique consists of (i) forming solid (i.e., normal density) microspheres by pulverizing and spheroidizing the (CD2)nstarting material, (ii) cross linking the polyethylene chains in a &ggr;‐ray flux, (iii) swelling the cross‐linked network in an appropriate hot solvent such asp‐xylene, and (iv) rapidly extracting the solvent to ’’freeze‐in’’ the expanded structure in the final target. A quantitative model based upon the theory of swollen networks has been employed to correlate the measured densities with the experimental parameters, such as radiation dosage, solvent type, and swelling temperature.

ISSN:0021-8979    

DOI:10.1063/1.325327

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The nonlinear development of the negative‐mass instability is investigated for a thinElayer (&Dgr;≪R0) located inside a cylindrical waveguide. A quasilinear analysis shows that the instability saturates by the development of an energy spread and a corresponding increase in thickness &Dgr; of theElayer. The efficiency of microwave generation by this instability is also calculated.

ISSN:0021-8979    

DOI:10.1063/1.325434

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The electron density is derived from the half‐width of an optically thin Stark‐broadened spectral line. A small nonperturbing amount of K is introduced, and the emission profile of the K 6s‐4p(6939 A˚) is measured. The side‐on intensity distribution is recorded with a scanning Fabry‐Pe´rot interferometer. Fourier transformation and Abel inversion yields the local profiles. Thus, the electron densityneis found as a function of the radial coordinate. The plasma temperature is calculated fromneand the sodium pressurepNa. Axis values arene= (1.0±0.2) ×1022m−3andT=4100±120 K forpNa=100 Torr;ne= (2.1±0.4) ×1022m−3andT=4350±120 K forpNa=200 Torr.

ISSN:0021-8979    

DOI:10.1063/1.325435

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The plasma etching of silicon and silicon dioxide in CF4‐O2mixtures has been studied as a function of feed‐gas composition in a 13.56‐MHz plasma generated in a radial‐flow reactor at 200 W and 0.35 Torr. Conversion of CF4to stable products (CO, CO2, COF2, and SiF4) and the concentration of free F atoms ([F]) in the plasma were measured using a number of different diagnostics. The rate of etching, the concentration of F atoms, and the intensity of emission from electronically excited F atoms (3s2P–3p2P° transition at 703.7 nm) each exhibit a maximum value as a function of feed‐gas composition ([O2]); these respective maxima occur at distinct oxygen concentrations. For SiO2, the variation in etching rate with [O2] is accounted for by a proportional variation in [F], the active etchant. The etching of silicon also occurs by a reaction with F atoms, but oxygen competes with F for active surface sites. A quantitative model which takes oxygen adsorption into account is used to relate the etch rate to [F]. The initial increase of [F] with [O2] is accounted for by a sequence of reactions initiating with the production of CF3radicals by electron impact and followed by a reaction of CF3with oxygen. When [O2] exceeds ∼23&percent; (under the present discharge conditions), [F] decreases due, probably, to a decrease in electron energy with an increase of oxygen in the feed gas.

ISSN:0021-8979    

DOI:10.1063/1.325382

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The leader stage of the discharge in a long rod‐plane air gap was investigated. Based on the assumption that optimum conditions for the advancement of the leader are obtained when the field prevailing around the leader tip maintains a constant optimum value, mathematical expressions are developed relating the gradient along the leader with quantities directly measurable during breakdown tests.

ISSN:0021-8979    

DOI:10.1063/1.325383

出版商:AIP    

年代:1978

数据来源: AIP