摘要:

Important effects of the finite size of the probe beam occur in optical beam deflection experiments when the probe beam radius is of the order of magnitude of the thermal diffusion length. The calculation of this effect is checked by experiments in air at frequencies around 10 kHz and in liquids around 100 Hz. In this last case, for certain values of the modulation frequency and of the probe beam‐sample surface distance, the amplitude of the deflection exhibits a sharp minimum and its phase varies drastically.

ISSN:0021-8979    

DOI:10.1063/1.341463

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A recent design concept for millimeter‐wave free‐electron lasers [J. Appl. Phys.60, 521 (1986)] would require the stable propagation of a sheet electron beam through a narrow waveguide channel. Experimental results reported in this article support the feasibility of such a configuration by demonstrating the stable propagation of relativistic sheet electron beams through a narrow waveguide gap (3.2 mm) using focusing by a short‐period electromagnet wiggler. 90% of the electron current in a 100‐keV sheet electron beam was transmitted through a 5‐cm‐long channel with peak wiggler fields of 800 G. Almost 80% of a 400‐keV beam was similarly confined with a 1600‐G wiggler field. The data were consistent with single electron trajectory models, indicating that space‐charge effects were minimal. No evidence of beam breakup or filamentation instabilities was observed.

ISSN:0021-8979    

DOI:10.1063/1.341221

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Reverse field injection was used to inject a 0.9‐MeV electron beam produced in a ring diode into a drift tube. The annular beam then was transported distances of up to 183 cm by gradientBdrift in an applied magnetic field. The electron beam struck a tantalum target at the end of the drift tube, producing bremsstrahlung x‐rays. An electron beam current of 1.1 MA was inferred from measurements of x‐ray production in the target. This implies an injection efficiency of 69%.

ISSN:0021-8979    

DOI:10.1063/1.341457

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We present a theoretical model for the effect of material pixellation on the operating characteristics of optical logic elements using nonlinear interference filters. From our calculations we predict pixel packing densities of 250×250 per cm2(an increase of three orders of magnitude over optical pixellation) and power levels of 100 &mgr;W. We define a figure of merit which describes the effects of pixellation of thermo‐optic logic elements. We also show numerically the effect on cooling rate.

ISSN:0021-8979    

DOI:10.1063/1.341467

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Signal‐beam amplification is achieved in degenerate two‐wave mixing experiments in liquid crystals at the wavelength &lgr;=10.6 &mgr;m of the cw CO2laser. The origin of the dynamic grating is the large thermal index change that arises when the cell operates near the phase transition temperature. Maxwell’s equations are analytically solved for the calculation of the amplitude variations of the incident waves (Ip@B:pump,Is@B:signal) and of the self‐diffracted wave (I−1). It is shown that energy transfer arises from the contribution of the &pgr;/2 component of the index modulation generated by the interference of (Ip,I−1). The resulting three‐wave model yields good agreement with the experimental results.

ISSN:0021-8979    

DOI:10.1063/1.341420

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

In this paper, a theoretical investigation of the locking of two weakly coupled semiconductor lasers is presented. The analysis begins with a set of rate equations for the electric fields and carrier densities inside each laser diode cavity. From these, the dynamic stability of the solutions as a function of the detuning between the lasers and the linewidth enhancement factor of each laser is determined. It is seen that when the linewidth enhancement factor is the same for both lasers, the entire locking bandwidth is dynamically stable. When the linewidth enhancement factor is different for the two lasers, asymmetric locking occurs, and only a portion of the locking bandwidth is dynamically stable.

ISSN:0021-8979    

DOI:10.1063/1.341431

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The tetragonal tungsten bronze ferroelectrics in the strontium barium niobate system have been extensively studied over many years. As for many of the bronzes, a crude interpretation of the experimental data has been attempted in the past using the simple Landau‐Ginsburg‐Devonshire expansion of the Gibbs free energy as a Taylor series in powers of the polarization, lumping all the temperature dependence into the lowest order term. In this paper new measurements are presented for the temperature dependence of dielectric polarization, permittivity, and theE‐field dependence of the permittivity. It is shown that for a realistic fitting of the data, the Taylor expansion must be taken to at least the eighth power term, and that the coefficients of terms up to the sixth power must be taken as functions of temperature. Since the phenomenology describes equilibrium behavior, it is the total static polarizability that is being explored in this treatment. The nature of this temperature dependence strongly suggests that the phase transition from a macropolar to a macrononpolar state is tetracritical.

ISSN:0021-8979    

DOI:10.1063/1.341438

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Fully developed, laminar liquid‐metal flows, currents, and power losses in a rectangular channel in a uniform,skewedhigh external magnetic field were studied for high Hartmann numbers, high interaction numbers, low magnetic Reynolds numbers, and different aspect ratios. The channel has insulating side walls that are skewed to the external magnetic field. Both the perfectly conductingmovingtopwallwith an external potential and the stationary perfectly conducting bottom wall at zero potential act as electrodes and are also skewed to the external magnetic field. A solution is obtained for high Hartmann numbers by dividing the flow into three core regions, connected by two free‐shear regions, and Hartmann layers along all the channel walls. Mathematical solutions are presented in each region in terms of singular perturbation expansions in negative powers of the Hartmann number. The free‐shear layers are treated rigorously and in detail with fundamental magnetohydrodynamic theory. Numerical calculations are presented for the total current carried by the core region between top and bottom electrodes, Joulean and viscous power losses, and channel resistance at different skewed external magnetic field angles. With the high external magnetic field, the current through the central core region between the electrodes must be parallel to the external magnetic field lines. The two side core regions carry no current to the zeroth order. The two free‐shear layers carry less current than the central core region. The theoretical magnetohydrodynamic model derived here was developed to provide data to help in the design of liquid‐metal current collectors.

ISSN:0021-8979    

DOI:10.1063/1.341219

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

This paper presents the results of an experiment dealing with the absorption of electromagnetic waves in a longitudinally magnetized plasma waveguide at electron cyclotron frequency. Our results demonstrate, under different operating conditions, the importance of the polarization of the incident waves as well as the linear behavior of the plasma density as a function of the absorbed power. The gas used in this study was argon.

ISSN:0021-8979    

DOI:10.1063/1.341220

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Propagation of a charge‐neutralized ion beam, in a transverse magnetic field (Bz<400 G) and in a magnetized plasma, has been studied. Measurements indicate that the beam propagation mechanism is due to theE×Bdrift in the region of high &bgr; (1<&bgr;<400), where &bgr; is the ratio of beam kinetic energy to transverse magnetic field energy. Diamagnetic measurements, both internal and external to the propagating beam, confirm the fast diffusion ofBzinto the beam on a time scale much shorter than the beam rise time of 10−7s. When the beam is injected into a magnetized plasma the electric field is shorted to a degree that increases with increasing background plasma density. When the plasma density reaches 1013/cm3(∼200×the beam density) complete shorting occurs and the beam is deflected by the transverse magnetic field.

ISSN:0021-8979    

DOI:10.1063/1.341222

出版商:AIP    

年代:1988

数据来源: AIP