摘要:

The surface acoustic wave (SAW) propagation loss mechanism of piezoelectric ceramics is examined using lead titanate (PbTiO3) ceramics modified with additives of Nd2O3, MnO2, and In2O3, which have zero temperature coefficients of SAW delay time. The propagation lossLin the frequency range 30 to 300 MHz consists of two parts. One is the Rayleigh scattering lossLScaused by grain, and the other is the true dissipative lossLFcaused by internal friction. Thus, total loss results inL(dB/cm) =LS+LF= 3.2×103D3f4+6.8×10−4f2(whereDandfdenote average grain size in cm and frequency in MHz, respectively). It is also revealed experimentally that the internal friction causing the latter loss has a close correlation with the dielectric loss through piezoelectric combination. Therefore, materials with fine grain (<0.5 &mgr;m) and low dielectric loss (<10−3) must be developed to reduce SAW propagation losses which prevent the piezoelectric ceramics from being used in higher frequency (≳100 MHz) SAW applications. Based on these requirements, fine grained (∼0.5 &mgr;m) specimens are fabricated. The propagation losses obtained from these specimens, 9 dB/cm at 100 MHz, are the lowest values ever reported for piezoelectric ceramics. This result suggests that the present ceramics have high potential as materials for high‐frequency SAW devices.

ISSN:0021-8979    

DOI:10.1063/1.329374

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The surface acoustic wave (SAW) and piezoelectric properties of PbTi03ceramics modified by the partial substitution of rare earths for Pb, particularly (Pb, Ln) (Ti, Mn)03(Ln = La, Ce, Pr, Nd, Sm, or Gd) ceramics, are examined. The effects of the rare earths introduced on these properties are noted. Properties, such as velocities and electromechanical coupling factors, for bulk wave and SAW vary monotonically with the ionic sizes of the rare earths. On the other hand, the temperature coefficient of SAW delay time takes a minimum value when the Ln is Nd. It is shown that the variation of the temperature coefficient with rare earths corresponds well with those of the temperature coefficients of elastic constantsCE33andCE44.

ISSN:0021-8979    

DOI:10.1063/1.329375

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

As part of a program to determine the feasibility of inertial confinement fusion (ICF), the physics of implosion stability is being studied. Ablatively‐driven double‐shell cylinders with and without initial periodic perturbations on the outer edge of the pusher were imploded using a single electron beam. Four‐pulse holographic shadowgraphy yielded spatially and temporally resolved images of the implosions. The experiments are in a regime where fluidlike behavior is expected to dominate. A comparison of experimental data on the free‐surface motion with two‐dimensional, planar‐geometry numerical calculations which include materials effects indicates shock‐accelerated unstable growth of fabrication irregularities at the perturbed material interface. Peak pressures of 0.26 TPa (2.6 Mbars) are inferred in the high‐density pusher material. Both the experiment and the calculation show a decrease in the amplitude of the free‐surface perturbations at late time. In the experiment this decrease in amplitude begins earlier and the amount of the decrease is larger because of the enhanced interaction of adjacent perturbations due to convergence. The complex nonlinear development of both intentional and nonintentional target perturbations are demonstrated in the experiments.

ISSN:0021-8979    

DOI:10.1063/1.329376

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Cylindrically converging shock waves are produced by utilizing the detonation of cylindrical explosive shells. The production and the propagation of shock waves are observed by framing and streak camera photographs, and the trajectory of shock propagations is determined by using an electrical ionization probing system. The effect of the quantity of explosives on the stability, or the axial symmetry, of shock fronts and on the strength of shocks produced is investigated. It has been shown that, for practical purposes, the approximation of shock trajectories by Guderley’s formulas would be sufficiently acceptable in an unexpectedly wide region near the implosion center, and that the axial symmetry of the shock front is improved by increasing the quantity of explosives, and thus, strong shocks are produced by merely increasing the quantity of explosives. The reflected diverging shock seems to be very stable. Piezoelectric elements have also been used to detect reflected diverging waves.

ISSN:0021-8979    

DOI:10.1063/1.329377

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The effect of broadening the bandwidth with a dielectric load in a cylindrical gyrotron is investigated for a hollow electron beam. The linear dispersion relation for the azimuthally symmetric, transverse electric (TE) modes is obtained by the method of the wave impedance matching. It is found that the TE perturbations exhibit three unstable modes characterized by their phase velocitiesvph: one fast wave, the long wavelength mode (LWM,vph≳c), and two slow waves, the intermediate (IWM,c≳vph≳c&egr;−1/2) and the short (SWM,vph<c&egr;−1/2) wavelength modes. The optimum conditions for the wide band operation are obtained individually for each mode. Although the bandwidth in excess of 40% is possible for the slow waves (IWM, SWM) at a small axial momentum spread, it decreases rapidly as the spread increases. On the other hand, the LWM yields approximately 10% of the bandwidth insensitive to the spread. It is also shown that for a small spread (<5%), the slow wave (IWM) is preferable for the wide band operation, whereas for a large spread (≳5%) the fast wave (LWM) is desirable.

ISSN:0021-8979    

DOI:10.1063/1.329378

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The dynamics of a rotating plasma in a transverse electrode geometry appropriate to high power CO2laser discharge pumping is examined. The magneto‐gas‐dynamic fluid model used reveals that the spatially nonuniform electric and magnetic fields within the discharge cause the plasma and neutral gas rotational velocities to be strongly sheared within the cathode fall and the positive column regions. This sheared rotation serves to both stabilize the discharge against glow to arc transitions and to drive a centrifugal neutral gas flow within the volume. The concomitant bulk gas cooling effect so produced is particularly beneficial in a high powered CO2electrical discharge laser as it inhibits thermal bottle necking. An order of magnitude increase in discharge power loading has been achieved utilizing a simple ’’magnetic electrode’’ discharge configuration.

ISSN:0021-8979    

DOI:10.1063/1.329379

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The presence of high‐energy ions (with potentials greater than the overall arc voltage) in the flux from the cathode region of vacuum arcs has been well established. While much excellent theoretical work has been done on the cathode region of a vacuum arc, only a modest amount of such work has predicted the energies of the ions leaving the cathode region. Two vacuum arc cathode theories which do predict the ion energies are the potential hump theory—where a positive space charge at the cathode spot accelerates the ions electrostatically, and the gas dynamic theory—where the ions are accelerated by a flow process. Few theorists have taken advantage of the available mass spectrometric data concerning individual ion energies. In this paper I compare such data with theoretical predictions. The best confirmed ion energy data is that for a copper cathode. Additional data are presented concerning the ion flux from a vacuum arc with a ten percent Bi/Cu cathode. Comparison of the copper and bismuth/copper data with the theoretical predictions shows that the potential hump and gas dynamic theories are in rough agreement with experiment, but are definitely not correct. Modifying the potential hump and gas dynamic theories by considering time‐dependent phenomena (i.e., incorporating nonequilibrium states into the theory) is shown to offer the promise of bringing either theory into better agreement with experiment. I conclude that the available data on the energies of the ions leaving the cathode region of a vacuum arc can serve as strong constraints on theoretical analyses.

ISSN:0021-8979    

DOI:10.1063/1.329380

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

In this paper we present a comprehensive account of our results on streamer propagation in dielectric fluids in point‐plane geometries. Propagation velocities for both positive and negative streamers have been determined as a function of the following parameters: temperature, pressure, density, viscosity, composition, and conductivity. Effects of voltage and interelectrode spacing were examined. Current and light emission during streamer growth were measured. The relation between shock wave and streamer velocities was investigated. Small concentrations of low‐ionization potential additives markedly accelerated the positive streamers, while electron scavengers accelerated the negative streamers. Mechanisms to account for these observations are discussed.

ISSN:0021-8979    

DOI:10.1063/1.329327

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Surface‐potential decay characteristics of corona‐charged insulators are analyzed theoretically using a theory developed before by Rudenko. It was assumed that the charges are deposited in the surface of the insulator by a short corona pulse and that some amount of charge is instantaneously injected in the bulk of material. An analytical treatment of the standard transport equations in the presence of traps is presented. The model assumed for the traps is one in which they can be characterized by a single trapping time; liberation from traps was neglected, and also it was assumed that the traps are far from fully occupied. Using the theory it was possible to explain the fast decay of surface potential after a negative corona discharge in naphthalene single crystals. The value found for the trap‐modulated mobility at room temperature was 10−8cm2V−1s−1and 110 s for the trapping time used as a parameter. With measurements of thermally stimulated currents, it was possible to find two sets of traps with activation energies of 0.6 and 1.2 eV.

ISSN:0021-8979    

DOI:10.1063/1.329328

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The work presented here deals with the application of the theoretical model of the nonequilibrium discharge developed by one of us to a free‐burning discharge in argon subjected to a current impulse. The paper includes a description of the system of transport equations and a discussion of the boundary conditions and numerical solutions. The response to a 0.5‐&mgr;s current ramp from 0.4 to 1.2 A is studied in detail. The variation of the basic parameters, such as the electron density, temperature, heat flux and diffusion velocity, the heavy‐particle temperature, and heat flux and the average mass velocity is shown as a function of time and space. Radial profiles of the initial and final states are shown to agree well with the results of the earlier steady‐state model.

ISSN:0021-8979    

DOI:10.1063/1.329329

出版商:AIP    

年代:1981

数据来源: AIP