摘要:

The propagation of surface acoustic waves (SAW) in multilayered structures is studied by using Laguerre polynomials. Field distributions and velocities are obtained easily by this simple and noniterative method. Formulations are given for open‐ and short‐circuit boundary conditions. The SAW velocity of a ZnO film on a Si substrate is calculated and the result is compared against other calculations from the literature. Numerical results are also given for GaAs/AlGaAs multilayered structures.

ISSN:0021-8979    

DOI:10.1063/1.347086

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

An analysis of PZT‐7A plates driven into thickness‐extensional trapped energy vibrations by the application of a voltage to strip electrodes is performed. The plate waves in the unelectroded region that carry energy away from the trapped mode are included in the treatment. The solution consists of a sum of the driven thickness solution and the pertinent plate eigensolutions for shorted electrodes in the electroded region and for zero surface charge in the unelectroded region. Since the sum of the solutions in each region satisfies the differential equations and boundary conditions on the major surfaces exactly, the substitution into the appropriate variational principle of linear piezoelectricity yields integral conditions across the thickness of the plate at the edge of the electrodes. The resulting system of inhomogeneous linear algebraic equations yields the response from very low frequency to slightly above the first thickness‐extensional frequency of the unelectroded plate.

ISSN:0021-8979    

DOI:10.1063/1.347087

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

An analysis of PZT‐7A plates driven into thickness‐extensional trapped energy vibrations by the application of a voltage to strip electrodes and radiating into an adjacent fluid is performed. All previous treatments of thickness‐extensional trapped energy modes in piezoelectric plates have ignored the radiation into the surrounding medium and, hence, have been for resonators rather than transducers. The solution consists of a sum of a driven thickness solution and the pertinent plate eigensolutions for shorted electrodes in the electroded region and for zero surface charge in the unelectroded region, including in each instance the radiation into the adjacent fluid. Since the sum of the solutions in each region satisfies the differential equations and boundary conditions on the major surfaces including the fluid loading exactly, the substitution into the appropriate variational principle of linear piezoelectricity yields integral conditions across the thickness of the plate at the edge of the electrodes. The resulting system of inhomogeneous linear algebraic equations yields the response from very low frequency to slightly above the first thickness‐extensional frequency of the unelectroded plate.

ISSN:0021-8979    

DOI:10.1063/1.347088

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In this paper fully time‐dependent collisional‐radiative (CR) calculations are used to study the highly dynamic nonequilibrium anode plasma in a magnetically insulated‐diode experiment. The CR model for the C i–C ivatomic system is described in detail, including the radiative and collisional rates and the level structure of the system. The electron temperature is determined by comparing time‐dependent line intensities for C iiand C iiiions to calculations of level‐population ratios in which continuous particle injection from the anode surface into the plasma is taken into account. The electron temperature is approximately 7 eV. The time‐dependent injected fluxes and the fluxes accelerated away from the plasma for C i, C ii, and C iiiare determined by inverting the coupled CR rate equations. The calculated extracted flux near the end of the pulse is consistent with the measured carbon‐ion current drawn from the plasma. Injection of excited particles is also investigated and is found to be important during the rise of the current pulse (≤50 ns). The importance of including multiple branching for ionization into excited final states is also shown. The time‐dependent cooling rate due to inelastic electron‐ion collisions and radiative processes is derived for general level‐population distributions and is used to investigate the anode plasma. This work should also be relevant in the study of other pulsed‐power nonequilibrium plasmas, such as recombining plasmas that have applications for UV and x‐ray lasers.

ISSN:0021-8979    

DOI:10.1063/1.347089

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In this paper a method for treating electron collisional ionization in collisional‐radiative calculations is presented that includes the effects of direct ionization into multiple excited states. Ionization from a given state proceeds to terms of the next ionization state using appropriate cross sections and branching ratios. Level populations are calculated for the case of a low‐temperature (7 eV), moderate‐density (ne=5×1015cm−3) ionizing carbon plasma using a time‐dependent collisional‐radiative model. These results show appreciable differences when compared with calculations in which ionization is assumed to couple only into the ground state of the next ionization stage. Applications of these results to astrophysical and laboratory plasmas are discussed.

ISSN:0021-8979    

DOI:10.1063/1.347196

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

This experimental study evaluates the potential of a cesium plasma as an emitter for a thermophotovoltaic (TPV) energy conversion system. A cesium plasma, as a result of the ground‐state transitions of its single outer‐shell electron, produces large amounts of radiation in the 850–890‐nm wavelength region. This would provide excellent coupling to silicon, gallium arsenide, and indium phosphide photovoltaic cells. Measurements of the radiative efficiency, the sum of the power at the 852 and 894 nm wavelengths relative to the total emitted power were made and correlated to the plasma operating variables. It was determined that for atomic density in the range (3–6)×1021/m3and electron temperature in the range 2000–3000 K, radiative efficiencies in excess of 70% are attainable. This would indicate that a cesium plasma with its selective emission characteristics and low electron operating temperatures of 2000–3000 K would be an excellent candidate as an emitter in a TPV system.

ISSN:0021-8979    

DOI:10.1063/1.347090

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We report on the importance of various sources of power imbalance of the 24 beam OMEGA laser system in contributing to laser irradiation nonuniformity. Long spatial wavelength irradiation nonuniformities on target are investigated numerically by adapting existing uniformity codes to incorporate time‐dependent density and temperature profiles. The analysis shows that power imbalance due to slightly detuned (≤100 &mgr;rad) frequency conversion crystals and small differences in the ultraviolet (UV)‐transport losses (&Dgr;E/E≤5%) makes only a modest contribution to the overall nonuniformity. However, power imbalance from pulse shape distortion was found to create substantial irradiation nonuniformity on the target early in the implosion. The predictions of the present analysis were verified by an experiment in which a well controlled power imbalance was applied. Good agreement was found between the experimentally observed core positions and those predicted by the simulation.

ISSN:0021-8979    

DOI:10.1063/1.347091

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The effect of partial screening due to electric microfields within the shielding cloud is examined and used as the basis for determination of electrical conductivity and certain thermodynamic functions of weakly nonideal plasma. A modified screening radius (non‐Debye radius) that is greater than the Debye radius is used to take into account short‐range Debye–Huckel‐type interactions. The evaluation of nonideal plasma electrical conductivity includes corrections to the Spitzer model and accounts for localization of low‐energy electrons in the continuum due to incomplete screening. Excellent argeement with experimental results is obtained in the weakly nonideal plasma regime. Corrections to certain thermodynamic functions, such as, chemical potential and internal energy, are evaluated using numerical methods. The effect of charge state on transport coefficients and thermodynamic functions is also analyzed.

ISSN:0021-8979    

DOI:10.1063/1.347092

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A method for analyzing the dynamic behavior of a low‐pressure mercury rare‐gas positive column is presented. An eight‐state scheme analysis for mercury and argon mixture under a non‐Maxwell distribution of electron energy is established. The computing time is shortened by using the two‐temperature model for electron energy distribution which I have developed to give an analytical expression for the non‐Maxwellian distribution. Two‐temperature parametersTtandTbfor the tail and bulk temperature, respectively, are determined from a two‐term Legendre polynomial expansion of the Boltzmann equation. Then a useful empirical result is that the ratiorT=Tt/Tbis a function of a single variable of electron density times electric field strength. By storingrTcalculated as a function of this variable, the electron energy distribution, which should be calculated from the Boltzmann equation at each iteration step of solving the rate equations, is found simply by a table‐looking‐up procedure, whereby the computing time is shortened. The examination of the eight‐state scheme is done by comparing calculated and experimental results, discussing physical processes occurring among the eight states.

ISSN:0021-8979    

DOI:10.1063/1.347067

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

This article reports the observation of intense x‐ray emission by a nonequilibrium energetic electron flow that can be produced when an explosive formed‐ferrite filament is used as a plasma initiator in a high vacuum of 10−5–10−3Torr. Exposure dose measurements and pinhole photographs for the x‐ray emission have shown that x rays originate linearly along a 14.5‐cm filament with a high exposure dose level of 50 mR(mili‐roentgen)/pulse at 6.5 keV, the predominant x‐ray energy. The x‐ray emission characteristics are described in terms of the time‐resolved and ‐integrated observations in order to infer the x‐ray emission mechanism in which an interaction between energetic electron flow and a cloud of ions or neutral atoms thermally produced from the filament has to be taken into account in the close vicinity of the filament.

ISSN:0021-8979    

DOI:10.1063/1.347068

出版商:AIP    

年代:1990

数据来源: AIP