摘要:

An analytical treatment of the electron’s motion in a magnetic monopole field results in useful expressions for both the lens action and the mirror action of the field. Using an appropriate definition of the magnetic moment of the electron, it is shown that there is an exact conservation of this parameter in the monopole field, implying that the motion is perfectly adiabatic. This property is important when the field is used for directing Auger electrons from a target to a detector; that is, when it is used as a parallelizer in a through‐the‐lens detection scheme. Regarding the monopole field as an electron lens, the image position and magnification are derived for an arbitrary object position. Expressions for both the axial aberrations (chromatic and spherical) and the image aberrations (coma, field curvature, astigmatism, distortion, and transverse chromatic) are derived for an arbitrary number of intermediate images between object and final image. The chromatic aberration turns out to be independent of the number of intermediate images and the spherical aberration decreases slightly with this number. This property is important when an electron beam must be focused to a small probe in a strong magnetic field. It is shown that if a certain combination of deflectors is used in conjunction with the monopole field, an ideal swinging objective lens is obtained: All image aberrations except field curvature disappear. Designs are presented in which the monopole field is used in the objective lenses of a transmission electron microscope and a scanning electron microscope.  

ISSN:0021-8979    

DOI:10.1063/1.352103

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A three‐dimensional calculation of Cherenkov‐type frequency doubling in an optical channel waveguide is implemented by slicing the lateral profile of the interacting fields into a number of small segments. In each segment, lateral field variations of the harmonic signal are neglected. This method is more simple and requires less computational effort in comparison with a direct three‐dimensional beam simulation. With the unidirectional‐radiation model one can predict an optimum configuration and the conversion efficiency of miniaturized frequency doublers that utilize guided–to–radiation‐mode interactions in nonlinear channel waveguides. Typical results are shown for blue‐light generation through frequency doubling in a proton‐exchanged MgO‐doped lithium niobate channel waveguide. Analyses of green‐light generation in organic channel waveguides are reported as well.

ISSN:0021-8979    

DOI:10.1063/1.352104

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A nonlinear optical susceptibility (&khgr;(2)) inversion structure in the thickness direction (NIST) is proposed for an optical waveguiding layer to achieve efficient second‐harmonic generation (SHG) in the form of Cerenkov radiation. Using a coupled‐mode analysis, it is found that the SHG efficiency of the Langmuir–Blodgett (LB) film waveguide incorporating NIST is several tens of times larger than that of the conventional LB film waveguide. 2‐docosylamino‐5‐nitropyridine (DCANP) was used as the LB film material. High precision is not so strongly required for the inversion position in the waveguiding layer thickness as for the total thickness of the waveguiding layer, if the refractive indices are almost uniform in the layer. A guiding principle is shown for selecting suitable materials for the substrate and the clad to achieve the highest efficiency with Cerenkov angle approaching zero. A waveguide structure was designed using a kind of perfluoropolymer for the clad. Several fabrication methods are proposed for the NIST waveguide. An enhancement of 20–30 times in the efficiency of Cerenkov radiation SHG due to the NIST is also experimentally demonstrated using DCANP‐LB film as the waveguiding layer.

ISSN:0021-8979    

DOI:10.1063/1.352105

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Nearly diffraction‐free beams have been generated by a CO2laser with a new type of cavity composed of a concave mirror and an output mirror with high reflectivity (99.4%) in the marginal part and low reflectivity (94.5%) in the central part. The beam propagation behavior is reproduced fairly well by the Fresnel–Kirchhoff integral of the Bessel–Gauss beam.

ISSN:0021-8979    

DOI:10.1063/1.352106

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Simple analytical representations of the ionization source terms in argon, helium, nitrogen, and silane dc glow discharges for steady‐state and quasisteady‐state conditions are presented. These analytical forms express well the highly nonequilibrium nature of the ionization in the cathode fall and negative glow regions which cannot be described by a Townsend ionization coefficient depending on the local value of the reduced electric field. These source terms can be easily incorporated into fluid models of gas discharges.

ISSN:0021-8979    

DOI:10.1063/1.352107

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A model has been developed to examine the effects of a 314.5 nm wavelength laser pulse directed vertically through the atmosphere on the local ion and electron concentrations in the beam path. The 314.5 nm wavelength was selected to exploit a three‐photon excitation resonance with the 3p54s 1P10excited state in argon. Absorption of a fourth photon of the same wavelength will ionize the excited atom. Using a rate equation formalism and a detailed collection of atmospheric chemistry reactions the model provides the concentrations of electrons and ions as functions of altitude, time, laser energy, relative humidity, focal characteristics of the pulse, and ambient atmospheric electric‐field conditions. Both linear and nonlinear effects on the propagation of the laser pulse have been taken into account. The calculated charged particle concentrations are used to estimate the conductivity of the ionized column. Results presented indicate peak electron densities up to 108cm−3can be created using laser intensities on the order of 108W/cm2. Electron lifetimes in the column are typically 50–200 ns, however, the lifetime of the conductive channel is long in comparison (100 &mgr;s). Long‐lived ions created by charge transfer and attachment reactions between laser‐produced argon ions and electrons and other atmospheric species provide a persistent enhanced conductivity in the beam path. Model results demonstrate that laser pulses having peak powers attainable with current technology can perturb the conductivity of an ionized column by as much as seven orders of magnitude in comparison to ambient conditions. A distinct tradeoff between column length and the extent of ionization is evident in the results presented. The tradeoff is a result of the competing mechanisms of multiphoton ionization and stimulated Raman scattering and is discussed.

ISSN:0021-8979    

DOI:10.1063/1.352108

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The origins of electrons and ions in uranium vapor generated by electron‐beam evaporation have been determined. Measurements were made for the electron emission current due to high‐energy electron‐beam irradiation on a uranium surface (backscattered electrons, etc.), thermionic emission current from the melt surface, and electron current due to vapor ionization. Comparison of these currents confirmed that vapor ionization was the main electron generation process at evaporation surface temperatures above 2200 K. The ionized vapor formed a weakly ionized plasma of very low electron temperature: The degree of ionization ≤1%, electron temperature ≤0.3 eV. The electron‐impact ionization process contributed mainly to plasma formation. Beam electrons, their backscattered electrons, and secondary electrons from the beam‐irradiated uranium surface were the source electrons for this process. Thermal ionization was the next major process. In addition to the plasma formation model, plasma behavior in vapor was described by a one‐dimensional symmetric expansion model. The calculated degree of ionization was in good agreement with the measured value over a wide range of evaporation temperatures and electron‐beam currents.

ISSN:0021-8979    

DOI:10.1063/1.352334

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The operation characteristics of the Cs‐Ba tacitron as a switch are investigated experimentally in three modes: (a) breakdown mode; (b)I‐Vmode; and (c) current modulation mode. The switching frequency, grid potentials for ignition and extinguishing of discharge, and the Cs pressure and emission conditions (Ba pressure and emitter temperature) for stable current modulation are determined. The experimental data is also used to determine the off time required for successful ignition, and the effects of the aforementioned operation parameters on the ignition duty cycle threshold for stable modulation. Operation parameters measured include switching frequency up to 8 kHz, hold‐off voltage up to 180 V, current densities in excess of 15 A/cm2, switch power density of 1 kW/cm2, and a switching efficiency in excess of 90% at collector voltages greater than 30 V. The voltage drop strongly depends on the Cs pressure and to a lesser extent on the emission conditions. Increasing the Cs pressure and/or the emission current lowers the voltage drop, however, for the same initial Cs pressure and emission conditions, the voltage drop in theI‐Vmode is usually lower than that during current modulation. As long as the discharge current is kept lower than the emission current, the voltage drop during stable current modulation could be as low as 3 V.

ISSN:0021-8979    

DOI:10.1063/1.352109

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Kinetic‐energy distributions have been measured for different mass‐selected ions sampled from 13.56 MHz rf glow discharges in argon inside a ‘‘GEC rf reference cell.’’ The electrode geometry of this cell produces an asymmetric discharge and the cell is operated in a pressure regime where ion‐molecule collisions in the sheath region of the discharge are significant. Ions are sampled from the side of the plasma perpendicular to the interelectrode axis using an electrostatic energy analyzer coupled to a quadrupole mass spectrometer. Kinetic‐energy distributions for Ar+, Ar2+, Ar++, and ArH+are presented as functions of applied rf voltage, gas pressure, and distance of the mass spectrometer entrance aperture from the edge of the electrodes. The distributions obtained for the sampling orifice placed close enough to the electrodes to allow formation of a sheath in front of the orifice exhibit features similar to those observed previously when sampling ions through the grounded electrode of a parallel‐plate reactor. The Ar+and Ar++distributions exhibit secondary maxima predicted to result from the formation of low‐energy (thermal) ions in the sheath region, such as by charge‐exchange and high‐energy electron collisions. Kinetic‐energy distributions for Ar2+and ArH+exhibit no secondary maxima and are peaked at high energies indicative of the sheath potential, and consistent with a formation mechanism involving relatively low‐energy collisions in the bulk plasma (glow region).

ISSN:0021-8979    

DOI:10.1063/1.352110

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The effect of switching a plasma medium of finite width (slab) on a time‐harmonic plane electromagnetic wave propagating in free space is considered. The transient solution is obtained through the use of Laplace transforms. The solution is broken into two components:Acomponent which in steady state has the same frequency as the incident wave frequency (&ohgr;0) and aBcomponent which has an upshifted frequency &ohgr;1=&sqrt;&ohgr;20+&ohgr;2p. Here &ohgr;pis the plasma frequency of the switched plasma slab. Numerical results are presented to show the effect of the slab width on the reflected and transmitted waves. A proper choice of &ohgr;0, &ohgr;pand the slab width will yield a transmitted electromagnetic pulse of significant strength at the upshifted frequency.

ISSN:0021-8979    

DOI:10.1063/1.352111

出版商:AIP    

年代:1992

数据来源: AIP