摘要:

The retarded Josephson equation, which is a second‐order differential‐integral equation, can be approximated by differential equations in different ways. There are the well‐known adiabatic approximations which are justifiable for small ac‐voltage amplitudes and in the slow‐motion regime. Another type of approximation makes use of the pendulum analogy, where the arguments of the functions entering in the adiabatic current phase relation are replaced by time averages. Several modifications of this average pendulum approximation will be discussed. In particular, the characteristics of these different approximations are compared with the exact result in a limit where the agreement should be rather good. In the same limit the solutions are also investigated by asymptotic expansions under very general conditions.

ISSN:0021-8979    

DOI:10.1063/1.325314

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The analysis of quasiacoustic‐wave excitation by the uniform interdigital transducer is approached from a field point of view. Fields within the transducer are expressed in terms of a set of normal‐mode functions which have been defined specifically for the interdigital array. Dispersion relations for these normal‐mode functions are found to display band‐gap regions with complex wave numbers. The complex wave number is responsible for the excitation process and permits a solution of the boundary conditions at both ends of the transducer structure. It is shown that the radiation fields excited by the transducer grow according to the dictates of a sine hyperbolic function from one end of the transducer to the other.

ISSN:0021-8979    

DOI:10.1063/1.325315

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Small‐area contrast fluctuations observed in cathodoluminescence‐mode SEM images of thin AlxGa1−xAs layers grown by liquid‐phase epitaxy on GaAs : Cr substrates are attributed to local variations in alloy composition. Quantitative estimates of the composition excursions are obtained from the variations in CL intensity by calibration against compositions known from electron‐probe microanalysis. In a typical sample, the CL variations are shown to correspond to peak‐to‐peak fluctuations of about 1 at.&percent; of Al and occur over irregular regions generally in the range 6–20 &mgr;m in diameter.

ISSN:0021-8979    

DOI:10.1063/1.325316

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A hot‐wire pyrometer for measuring gas temperatures with a high sensitivity has been developed. The method can easily be extended to measurements of surface temperature. The principal elements of a hot‐wire pyrometer are a fine wire inserted into the gas and a photomultiplier for measuring the radiation emitted from it. The output voltageVof the photomultiplier is shown both theoretically and experimentally to be the following function of wire temperatureT:V=&sgr;T7/4 exp(−&ggr;T−1/2), where &sgr; and &ggr; are the calibration constants unique to a particular system.

ISSN:0021-8979    

DOI:10.1063/1.325212

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A simple and efficient way to enhance the photoacoustic signal from solid or powder samples is described. The increase in signal is found to be related mainly to the diffusivity and the ratio of specific heats of the gas which is used to couple the heat variation at the sample surface to the microphone detector. The measured signal enhancement on various powder samples agrees fairly well with the one‐dimensional theory by Rosencwaig and Gersho. The effect of physical adsorption of gas by the sample is also examined.

ISSN:0021-8979    

DOI:10.1063/1.325317

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The time‐dependent response of photoacoustic cells to pulsed excitation is examined both theoretically and experimentally. The theoretical treatment includes both the response of conventional optical absorbers and absorbers containing metastable states. These results should be useful in dye laser applications of photoacoustic spectroscopy on both absorbing and luminescent materials. The experimental results were obtained using a newly developed device in which both electrical heating and optical heating have been shown to produce an equivalent acoustic response. This device should have applications in the design and calibration of cells for photoacoustic spectroscopy.

ISSN:0021-8979    

DOI:10.1063/1.325318

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

It has been asserted previously that isotope separation in mass‐spectrograph‐type separators will cease when the ion density becomes so large that the time for Coulomb collisions between the two isotopic species becomes comparable to the transit time through the machine. It is shown here that the first effect of such collisions will be to increase,notdestroy, the separation, but that the current densities required for the effect are so far from the state of the art as to be regarded as inaccessible.

ISSN:0021-8979    

DOI:10.1063/1.325319

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The parameters of a plasma produced by irradiation of alkali hydride targets with a ruby‐laser pulse are experimentally measured. Positive‐ and negative‐ion abundances are determined with a quadrupole mass analyzer. Electron temperature was measured with rapidly swept Langmuir probes. Negative (H−) ion temperature was obtained using the mass analyzer and focusing grids. The electron density near the target was determined by optical holography. The quantity of negative ions produced per laser pulse make the source attractive for the future development of high‐energy neutral‐beam systems.

ISSN:0021-8979    

DOI:10.1063/1.325320

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Atomic hydrogen particles from a Hall accelerator were utilized in low‐energy (<2 keV) negative‐hydrogen‐ion‐production experiments in cesium vapor. Cell thicknesses in the range 8×1011to 6×1015Cs atoms/cm2were studied for H−current densities on the order of 1 mA/cm2. Anomalously, large H−fractions of up to 30–60&percent; were observed at intermediate cell thicknesses when protons were not removed from the dominantly H0incident beam. These observations indicate either the presence of excitation of the H0beam or the existence of two space‐charge phenomena: a focusing effect inside of the cesium cell, due to a buildup of positive space charge in the beam line, and a divergence effect as the H−beam emerges from the cesium cell due to a lack of space‐charge neutralization.

ISSN:0021-8979    

DOI:10.1063/1.325294

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The effect of three‐dimensional high‐frequency turbulent electric fields on the &sgr;+, &sgr;−, and &pgr; components of the allowed 11S0‐31P1line and the forbidden 11S0‐31D2lines in the laser absorption spectra of a magnetoactive He I plasma is investigated by means of the coupled equations for the probability amplitudes of atomic states. The spectral shifts of &sgr;+, &sgr;−, and &pgr; components and the satellites about the forbidden lines are derived. The calculation reveals that the ratio of the intensity of the near satellite to that of the far satellite is different from the one obtained by second‐order perturbation theory or the three‐level rotating‐wave approximation. The observations of the line shifts permit a quantitative determination of the intensity of the stochastic electric field, the average electron density, and the relative direction of the local magnetic field with respect to the linearly polarized laser field.

ISSN:0021-8979    

DOI:10.1063/1.325295

出版商:AIP    

年代:1978

数据来源: AIP