摘要:

Calculations of the x‐ray optics of four types of doubly curved diffractors are described and the results are compared with similar results for singly curved diffractors (Johann and Johansson geometry). In these calculations the locus of points on a curved diffractor’s surface is determined for a given deviation from the Bragg angle when the diffractor is used at Bragg angles ranging from 15 to 75 degrees. The six cases discussed include cylindrical, spherical, and toroidal geometries. The calculations are used as a basis for comparing the collection efficiency and the spectral resolution when these diffractors are used in scanning monochromators. It is shown that doubly curved diffractors are superior to singly curved ones and that a diffractor with spherical planes and toroidal surface provides the best performance of all of the diffractors considered if a wide range of Bragg angles must be covered.

ISSN:0021-8979    

DOI:10.1063/1.345629

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Previous measurements of the stopping power of Kapton and Havar for 1.1–4.4‐MeV protons have been analyzed with modified Bethe–Bloch theory in order to extract some of the parameters required in the formalism. Results suggest the possibility of systematic errors, in that the Havar data appear to have been slightly high and the Kapton data quite low when compared with other reported measurements. Currently recommended values of the mean excitation energy and Barkas‐effect parameter are, respectively, 79 eV and 1.34 (Kapton) and 296 eV and 1.36 (Havar). The need for further accurate measurements for Kapton in particular is emphasized.

ISSN:0021-8979    

DOI:10.1063/1.345630

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The three‐dimensional boundary‐element method (3D BEM) has been discussed in some aspects involving the subdivision of boundary elements, the quadrature of surface charge density integrals, and their singularities. The 3D BEM has been applied to the analysis of nonrotationally symmetric perturbations in electrostatic lenses. The perturbations due to machining defects (including elliptical distortions, inclined end faces, misalignments, and their combinations) have been calculated and also the asymmetric aberrations have been computed. A set of computer programs has been developed. The computed perturbation potential distributions have been compared with the theoretical ones.

ISSN:0021-8979    

DOI:10.1063/1.345631

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In experiments in which an intense relativistic electron beam is injected into an evacuated drift tube with a localized gas cloud located near the anode, ions with energies several times the electron beam energy have been observed. These experiments have been simulated using a particle‐in‐cell code which realistically models ionization of the gas. It was found that when the injected electron beam current exceeds the space‐charge limiting current, ions are accelerated to energies several times the electron beam energy by coherent motion of the ions and the intense virtual cathode electric fields. The dependence of the peak ion energy on the system parameters as observed in the simulations is also discussed. For the parameter regimes investigated with beam energies up to 3 MV, beam currents up to 35 kA, gas pressures up to 600 mTorr, and gas cloud widths up to 6 cm, peak ion energies of 5–6 times the electron beam energy have been observed.

ISSN:0021-8979    

DOI:10.1063/1.345632

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In the present work, the three‐dimensional thermal analysis of the process of the catastrophic mirror damage in double‐heterostructure stripe‐geometry GaAs/(AlGa)As diode lasers has been carried out for the case of their high‐power operation. In the analysis, the temperature dependencies of both the thermal conductivity and the thermal diffusivity have been taken into account. The analytical solution has been obtained with the aid of the Green’s function method. The catastrophic mirror damage has been explained from the thermal point of view as a result of a local overheating of the active area close to or at the mirror surface. It has been proved that a determination of an exact value of the critical mirror temperature from the catastrophic degradation point of view is of minor importance because the considered temperature increase is of an avalanche type. The analytical solution obtained in the work has enabled us to show a dependence of a catastrophic‐degradation time, i.e., a permissible length of current pulses from a point of view of the catastrophic mirror damage, on the amplitude of the pulses for the standard stripe diode lasers. Experimentally observed catastrophic mirror damage in diode lasers caused by an increase in their facet reflectivity has been also investigated. It has been confirmed that an increase in the reflectivity of an unprotected mirror may be accompanied by a very rapid facet failure. It has been also shown in which way one can avoid the above phenomenon.

ISSN:0021-8979    

DOI:10.1063/1.345633

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The cavity transfer matrix for the round trip of a light pulse in the optical resonator of the National Institute of Standards and Technology/Naval Research Laboratory free‐electron laser oscillator is derived. The cavity eigenmodes and the corresponding eigenvalues are obtained, using an expansion in Gaussian–Laguerre vacuum modes, by numerical diagonalization. The fractional power loss per eigenmode, caused by the finite sizes of the cavity mirrors and apertures, is determined. It is found that the losses are very small over the entire wavelength regime of operation.

ISSN:0021-8979    

DOI:10.1063/1.345634

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Ten linear combinations of the fundamental material constants of electroelasticity and electrostriction of quartz have been determined. They were obtained by the least‐squares fit to 134 observations of the change in the resonance frequency of the thickness modes of quartz plates caused by a dc field. With one exception, there is very good agreement between our results and their counterparts obtained independently by the transit‐time method (pulse propagation in bulk quartz). The differences between comparable constants are no longer out of proportion with their standard errors. This work supersedes earlier results [J. Appl. Phys.65, 715 (1989)].

ISSN:0021-8979    

DOI:10.1063/1.345635

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

An acoustic microscope has been developed which makes use of pressurized superfluid helium as an acoustic coupling medium. Acoustic micrographs were taken using pure superfluid4He under saturated vapor pressure at 0.1 K and under 25 atm at 0.1 and 1.9 K, for the frequency of 380 MHz. By pressurizing liquid4He, the acoustic attenuation decreases, the efficiency of the antireflection coating increases, and the power level of saturation caused by nonlinearity of the superfluid4He is raised. A good signal‐to‐noise ratio of imaging is thus obtained. The entire imaging instrument set is within a closed cell. The focusing mechanism is composed of two voice coils of superconducting wire. Using a servocontrol mechanism with a sensor and drive coil,Z‐direction stability was better than 0.05 &mgr;m. The manipulation sensitivity is 0.5 &mgr;m/mV, and the whole stroke is about 2 mm.

ISSN:0021-8979    

DOI:10.1063/1.345636

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The effective conductivity &sgr;eof a dispersion of long, parallel cylinders (or two‐dimensional systems), distributed in a second continuous matrix phase, is determined as a function of its microscopic structure. We obtained an expression for the effective conductivity of two‐dimensional systems (based on an approximate scheme) that includes the one‐body and two‐body terms in the cluster expansion for &sgr;e. The effective conductivities of two‐dimensional hard disks and square‐well particles are computed. It is found that this expression yields excellent estimates for the effective conductivity over a wide range of cylinder volume fractions, and for cases in which the cylinder to matrix phase conductivity ratios are high.

ISSN:0021-8979    

DOI:10.1063/1.345637

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

It is shown that, over a narrow range of amplitudes, applied voltage pulses can deposit wall charges inside a discharge tube without causing breakdown of the interelectrode gas: these charges enhance the breakdown voltage of the interelectrode gas for subsequent pulses, and the enhancement decays exponentially with the pulse interval. The enhancement decays more rapidly when the temperature of the discharge tube is increased so as to reduce its surface resistance, which furnishes the main leakage path of the wall charges. It is suggested that breakdown voltages of closely packed, repetitively pulsed discharge tubes are frequently enhanced by wall charges.

ISSN:0021-8979    

DOI:10.1063/1.345638

出版商:AIP    

年代:1990

数据来源: AIP