摘要:

It has been known for some time that the range and phase normalized far field is proportional to the three‐dimensional spatial Fourier transform of the source distribution, taken on the surface of the Ewald sphere at a fixed frequency. Unfortunately, however, this very useful relationship is neither widely known, nor does there even exist for it a good, short, simple, and direct derivation in the open literature (thus making references to the relationship difficult). The purpose of this paper is to alleviate these difficulties by presenting a brief derivation of the relationship for both the scalar acoustic as well as the vector electromagnetic cases. A brief discussion of the implications of this relationship on the inverse source and inverse scattering problem is also presented.

ISSN:0021-8979    

DOI:10.1063/1.335709

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A general procedure of the deconvolution of noisy experimental data is proposed. The deconvoluted solution is represented in terms of a cubic spline functionf(x) with variable knots. The value of parameters and the location of knot points in the spline are determined by solving the nonlinear least‐squares problem of fittingg(x)=∫&kgr;(&tgr;)f(s(&tgr;,x))d&tgr; to the data, where experimental datag(x) represent the convolution off[s(&tgr;,x)] with an apparatus weighting functionk(&tgr;) ands(&tgr;,x) is a function related to the configuration of the experimental system. The profile of the fitted solution varies widely with different choices of the number and position of knots in the spline. To obtain a good solution, the optimum number of knots is determined based on the data by applying minimum Akaike’s Information Criterion procedure. Practical application of the variable‐knot spline is generally considered to be rather intricate. However, by taking advantage of an advanced technique of a quasi‐Newton method, the nonlinear least‐squares problem becomes tractable, and excellent smoothing in the deconvoluted solution can be achieved.

ISSN:0021-8979    

DOI:10.1063/1.335616

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

An improved understanding of the factors that control the axial focus of applied‐Bion diodes was obtained from time‐resolved diagnostics of ion‐beam trajectories. This resulted in a new selection of anode shape that produced a proton focus of 1.3‐mm diameter from a 4.5‐cm‐radius diode, which is a factor of 2 improvement over previous results. We have achieved a peak proton power density of 1.5±0.2 TW/cm2on the 1‐TW Proto I accelerator. The radial convergence of this proton beam, defined as the ratio of the anode diameter to focused beam FWHM, is 70. Time‐resolved information about virtual cathode evolution, the self‐ and applied‐magnetic‐field bending, and the horizontal focus of the beam was also obtained. In addition, the diffusion of the magnetic field into the anode plasma is estimated by measuring the horizontal focal position as a function of time. Finally, we discuss the effects of gas cell scattering on the beam focus.

ISSN:0021-8979    

DOI:10.1063/1.335697

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Microwave generation by a relativistic electron beam injected through a screen or foil into a cylindrical waveguide is investigated. The electron beam is generated by a field‐emission diode with an anode‐cathode gap of ∼1 cm. The electron density exceeds the space‐charge limit for the beam, which forms a virtual cathode inside the waveguide. The instability of the virtual cathode causes oscillations which couple to bunched reflexing electrons trapped in the potential well. Frequency locking between the oscillating virtual cathode and the reflexing electrons is found to generate a much purer, narrowband microwave output. Using diode mismatch to increase the beam impedance is shown numerically to be a useful technique to improve the microwave generation efficiency and spectral purity. The peak conversion efficiency is calculated to be 2.2% at 7 GHz. The numerical results are compared to experimental results, and the agreement is within 8% in frequency and 25% in peak power. Experimentally, we describe the qualitative effects of different cathode materials and dimensions. The anode materials used were screens and foils of various transparencies and thicknesses. Microwave generation efficiency dropped as the mean scattering angle increased with the thicker foils. Also, the increased transparency of fine screens caused an upward frequency shift due to the increased electron density.

ISSN:0021-8979    

DOI:10.1063/1.335673

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The shortest possible lasing wavelength and the lowest threshold current density of the GaInAsP visible injection laser diodes grown on (100) GaAs substrate by the liquid phase epitaxy using a two‐phase solution growth technique are discussed. The lasing wavelength and threshold current density at room temperature under pulsed conditions are 705 nm and 9.4 kA/cm2, respectively. Light output power of 5–6 mW from one facet and maximum differential quantum efficiency of 38% are realized under the condition of fundamental lateral and transverse mode operation.

ISSN:0021-8979    

DOI:10.1063/1.335688

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

We employ an optical technique requiring no electrodes to measure three parameters characterizing photorefractive traps in nominally undopedn‐type bismuth silicate at 515‐ and 488‐nm wavelengths. These parameters are (1) an effective density of active electron trap sites (∼1016cm−3), (2) the contribution &agr;pcm−1of the photorefractive excitations to the total absorption coefficient (∼3.8 cm−1or 54% at 488 nm and 1.4 cm−1or 86% at 515 nm), and (3) the average distance over which an optically excited electron moves in the absence of electric fields before being retrapped (∼4 &mgr;m). With these parameters (and the crystal properties) standard models give predictions for all photorefractive effects, with or without applied electric fields and crystal motion, provided that there is negligible change in the density of occupied traps during an electron recombination time (∼microseconds). Our technique employs measurement of the light‐induced exponential decays of spatial ‘‘gratings’’ of trapped charges as a function of the grating period. The expected three‐parameter function is observed from which we determine the above three parameters.

ISSN:0021-8979    

DOI:10.1063/1.335694

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Large values of the exponential gain coefficient &Ggr; are obtained (&Ggr;&bartil;8–12 cm−1) when recording with a moving grating in photorefractive BSO crystals (nearly degenerate two‐wave mixing; drift recording mode). The resolution of the Kukhtarev’s equations with a moving grating shows a resonance effect which at the optimum velocity makes the modulation of the photoinduced space charge fieldEschigher. An optimum of the grating spacing also exists: &Lgr;opt:2&pgr;(E0/NA)(&mgr;&egr;/e&ggr;R)1/2. In such conditions, the space charge field is phase shifted by &pgr;/2 with respect to the incident fringe pattern; this allows an efficient beam coupling between the two recording beams. The dependence of the gain &Ggr; versus the incident beam ratio &bgr; of the two interfering waves is interpreted by including the second‐order term in the Fourier development ofEsc. The conditions allowing one to obtain a reasonable agreement between the theory and experiments are presented and discussed, as well as the adopted values of the crystals’ parameters.

ISSN:0021-8979    

DOI:10.1063/1.335646

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

We present a self‐consistent theory of a low‐pressure positive column with two‐step ionization from a metastable state in which the coupling between the continuity equations for the electrons and the metastables is appropriately taken into account. The corresponding boundary value problem is solved numerically for both cylindrical and planar geometries and vanishing electron and metastable densities at the boundary, for a wide range of the two parameters on which the problem is shown to depend. Curves of the electron and metastable density profiles, of the eigenvalues, and of conveniently defined generalized diffusion lengths for both types of particles are presented. We also show that the eigenvalues together with Boltzmann calculations of the electron rate coefficients and transport parameters provide similarity laws for a steady‐state positive column with radiusR: ratio of the electric field to the gas density,E/NvsNRforI/R=const and ratio of the metastable density to the gas density,nM(0)/NvsI/RforNR=const, whereIis the discharge current. An application to argon is discussed.

ISSN:0021-8979    

DOI:10.1063/1.335668

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

We present two approximate analytical solutions of the boundary value problem that models the low‐pressure positive column with two‐step ionization from a metastable state in the case of planar geometry and compare them with the numerical results of Part I.

ISSN:0021-8979    

DOI:10.1063/1.335625

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Output and input grating couplers for step index planar dielectric waveguides are investigated taking into account the interaction between a guided mode with a finite number of radiative and an infinite number of nonradiative space harmonics. For transverse electric and transverse magnetic modes, and for shallow gratings of arbitrary profiles located at either the upper or the lower guide surface, the governing equation for the slowly varying amplitude of the guided wave is deduced, and therefrom the real and the imaginary parts of the correction to the wave number of unperturbed guide are evaluated, the power conservation relation is established, and the output and the input coupling efficiencies are defined. For the input coupler, the excitation by a two‐dimensional radiation beam is also included. The role of the space harmonics which, like the guided wave, carry power along the guide in modifying the real and the imaginary parts of the wave number of the guided wave is discussed. The effect of the choice of the profile and location of the grating on the distribution of the outgoing radiation between the cover and the substrate, and between harmonics of different orders are examined. In a multimode coupler, significant improvements in input coupling efficiency can be achieved by a suitable choice of the grating profile and the excitation region. The effect of tapering the relative depth of the grating on the amplitude profile of the radiated beam of the output coupler, and on the coupling efficiency of the input coupler are investigated.

ISSN:0021-8979    

DOI:10.1063/1.335626

出版商:AIP    

年代:1985

数据来源: AIP