摘要:

There has been a long history of the use of two electromagnetic techniques to measure surface‐breaking cracks in metals. Both the alternating current potential drop (ACPD) technique and the eddy current technique have given good agreement with experimental results, even though the theoretical models on which their interpretations are based use contrasting assumptions for the boundary condition on the metal surface. The model for the ACPD technique assumes that the magnetic scalar potential satisfies the 2D Laplace equation, while eddy current modeling assumes an approximation of Born type in which the surface field is unperturbed by the presence of the crack. This paper considers a general model matching the thin‐skin electromagnetic field around a surface‐breaking crack to that in the free space above and shows that the two contrasting boundary conditions are extremes of a more general one. The Laplace approximation is valid for high permeability materials such as mild steel, while the Born approximation is appropriate for materials of low permeability and high conductivity such as aluminum. Experimental investigations of the magnetic fields near semielliptical cracks in mild steel and aluminum show quantitative agreement with the theory.

ISSN:0021-8979    

DOI:10.1063/1.341384

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The scattering of electromagnetic waves by an obstacle is formulated as a boundary value problem analogous to the fluid dynamic problem of flows past the obstacle. The transformed Helmholtz equation in terms of Debye amplitude functions can be solved numerically in a manner similar to the solution of governing fluid flow equations. The far‐field radiation condition and media interface boundary condition can also be enforced in a manner similar to the enforcement of the free‐stream condition and flow‐tangency condition. Various numerical methods in computational fluid dynamics CFD can be carried over for the computation of scattering characteristics. An example problem of the scattering of plane electromagnetic waves by a perfectly conducting sphere is solved in the frequency domain by a finite difference method based on the concept of generalized scattering amplitude. Numerical results are presented forka=2.9, wherekis the wave number equaling 2&pgr; divided by the wavelength andais the radius of the sphere.

ISSN:0021-8979    

DOI:10.1063/1.342487

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Consistent preservation of ⟨01¯1¯⟩‐oriented grating corrugations with 2850‐A˚ period and 1500‐A˚ height on (100) InP substrates has been achieved during thermal cycling at 660 °C for 60 min in a H2ambient, using roughened GaAs cover slices. It is found that transport of Ga and As causes the formation of a graded 25‐A˚‐thick InGaAsP alloy on the surface of the gratings, as detected by Auger and x‐ray photoelectron spectroscopic analysis. This preservation technique eliminates the need for lower‐temperature crystal growth of distributed feedback laser wafers by liquid‐phase epitaxy (LPE). Successful growth of distributed feedback laser wafers using conventional high‐temperature LPE is demonstrated.

ISSN:0021-8979    

DOI:10.1063/1.341385

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A model is described for the heavy‐ion pumping of an XeF(B) laser by uranium fission fragments (FF).The model is a self‐consistent accounting of the generation and transport of the FF’s through the fission foils, slowing of the fragments in the gas, evolution of the secondary‐electron‐source function and distribution, and the XeF laser plasma kinetics. By simulating the same quantities for ane‐beam‐pumped plasma, direct comparisons can be made for laser performance. We found that the secondary‐electron source generated by thee‐beam is more energetic than that for direct ionization by FF’s due to a more favorable mass ratio for momentum transfer collisions with orbital electrons. This difference in the electron‐source functions significantly affectsWvalues and excitation fractions. The impact on laser performance, though, is not large due to the high efficiency of channeling deposited energy to the upper laser level in XeF lasers. For conditions typical of FF excitation (power deposition 1–3 kW cm−3, pulse length ≊200 &mgr;s),e‐beam excitation results in 10%–15% higher gain than heavy‐ion excitation.

ISSN:0021-8979    

DOI:10.1063/1.341386

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The electron density is determined in a laser tube that is formed by a hyperboloid of revolution and spherical‐end surfaces. The electron temperature variation is accounted for by allowing the ratio of the free‐electron production factor &agr; to the diffusion coefficientDato be a function of the local tube radius. Calculations are made assuming different symmetrical optical resonator configurations ranging from near planar to near confocal. The tube geometry is specified in terms of the optical resonator parameters: resonator lengthL, mirror radius of curvatureRm, and optical spot size at the mirror surfaced. The active medium is assumed to be low‐temperature, weakly ionized plasma. Schottky boundary conditions are assumed and the particle‐diffusion equation is solved using oblate spheroidal coordinates. The oblate spheroidal angular dependence (which in the cylindrical limit corresponds to the variation perpendicular to the axis of the cylinder) of the density is found to be that of a zero‐order Bessel function. The oblate spheroidal radial dependence (which in the cylindrical limit corresponds to the axial dependence) is obtained by means of numerical methods. Radial density profiles are calculated, assumingL=1.0 m, for variousRm/Lratios. The peak radial density is found to remain at the center of the tube (Z=0) for all values ofRm/L>0.5.

ISSN:0021-8979    

DOI:10.1063/1.341387

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The analytical solution of temperature profiles in complex laser‐heated structures is only possible for a few simple cases. This is partly due to the nature of heat‐source terms that preclude the use of conventional series‐type solutions. In this study we utilize a Laplace‐transform method in conjunction with Fourier integrals to obtain solutions for the temperature profiles in a laser‐irradiated rare‐earth–transition‐metal slab. The results provide a convenient qualitative tool to study the thermal response of laser‐irradiated solids.

ISSN:0021-8979    

DOI:10.1063/1.341388

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Transient, combined conduction and radiation is solved in an absorbing, emitting, and isotropically scattering solid cylinder initially at uniform temperature and for timest>0 subjected to a constant temperature at the black cylindrical surface. The collocation method is used to solve the radiation part of the problem and the implicit finite difference scheme is used to solve the conduction part. The effects of the conduction to radiation parameter, the single scattering albedo, the optical thickness of the medium on the temperature distribution, and the heat flux are examined.

ISSN:0021-8979    

DOI:10.1063/1.341389

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Using an arbitrary initial unipolar space‐charge distribution consisting of two species of charge carriers of different but constant mobilities in a medium, relations for the electric fields and charge‐carrier densities are derived as functions of positions and time. The highly nonlinear, one‐dimensional equations, which are derived for swarms of charge carriers between parallel plane electrodes with a fixed potential difference, include the effects of the space‐charge fields. A general method is outlined which, in principle, can be used to generate a second order differential equation whose solution predicts the time‐dependent current caused by the drifting space‐charge swarm. The general equations are applied to the special case where the initial space‐charge distributions are uniform in a solid or fluid medium. Although the resulting differential equation is complicated, the equation is in a form such that its solutions could be computer generated.

ISSN:0021-8979    

DOI:10.1063/1.341390

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Controlled, ultrahigh axial magnetic fields have been produced and measured in a gas‐puffZpinch. A 0.5‐MA, 2‐cm‐radius annular gas‐puffZpinch with a 3‐min repetition rate was imploded radially onto an axial seed field, causing the field to compress. Axial magnetic field compressions up to 180 and peak magnetic fields up to 1.6 MG were measured. Faraday rotation of an Argon laser (5154 A˚) in a quartz fiber on‐axis was the principal magnetic field diagnostic. Other diagnostics included a nitrogen laser interferometer, x‐ray diodes, and magnetic field probes. The magnetic field compression results are consistent with simple snowplow and self‐similar analytic models, which are presented here. Even small axial fields help stabilize the pinches, some of which exhibit several stable radial bounces during a current pulse. The method of compressing axial fields in a gas‐puffZpinch is extrapolable to the order of 100 MG. Scaling laws are presented. Potential applications of ultrahigh axial fields inZpinches are discussed for x‐ray lasers, inertial confinement fusion, gamma‐ray generators, and atomic physics studies.

ISSN:0021-8979    

DOI:10.1063/1.341391

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

This paper describes the application of phase‐amplitude imaging to the measurement of self‐generated magnetic fields in laser‐produced plasmas. The magnitude of the magnetic field is determined by polarimetry and the plasma density profile by interferometry. Both pieces of information are recorded on a single interferogram in a form that can be analyzed automatically using computer‐based image processing. We describe a method for successfully determining the magnetic field profile in plasmas generated by 70‐ps Nd laser pulses at high intensity on planar targets when plasma motion or turbulence introduces an additional difficulty by reducing the visibility of the fringes.

ISSN:0021-8979    

DOI:10.1063/1.341364

出版商:AIP    

年代:1988

数据来源: AIP