摘要:

In this paper, the backscattering of an electromagnetic (EM) missile the energy of which decays asr−&egr;by arbitrary three‐dimensional (3D) and two‐dimensional (2D) targets, such as perfect conductors, dielectrics, and a perfect conductor coated with dielectric, is analyzed by using geometrical optics. The analysis shows that the backscattered energy of the EM missile by arbitrary 3D and 2D targets is proportional tor−&egr;R1R2/[(R1+2r)(R2+2r)] andr−&egr;R/(R+2r), respectively, whereR1andR2are the two principal radii of curvature of the 3D surface at the specular point andRis the radius of curvature of the 2D surface at the specular point. This general result includes several special examples available, such as plates, perfectly conducting elliptical cylinders, and spheres. It is seen that the rate of energy decrease of the backscattered EM missile is slower than those for conventional 3D and 2D backscattered fields.  

ISSN:0021-8979    

DOI:10.1063/1.351688

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Permanent optical channel waveguides in single crystals of KNbO3are formed with low‐dose MeV He ion irradiation. A single energy implantation forms a confinement barrier at the projected range of the ions whereas multiple energy implantations combined with a positive shielding mask form the side walls. Guiding modes are produced and propagation losses as low as 1.0 dB/cm are measured without the need for any annealing. The irradiation induced refractive index changes that define the boundaries of the waveguiding channels are made visible with cross‐polarized light microscopy.

ISSN:0021-8979    

DOI:10.1063/1.351689

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We have carried out laser‐induced grating and nonlinear transmission measurements near the band gap of amorphous Si:H. The nonlinear four‐wave mixing response deviated substantially from traditional third‐order effects. An intensity dependence of less than cubic for the diffracted intensity was observed. The nonlinearity does not follow the absorption profile. An effective nonlinear susceptibility &khgr;(3)=5×10−9esu was found at 572 nm. The nonlinear transmission showed saturable absorption at low intensities, and induced absorption at high intensities. This behavior can be attributed to light‐induced defects.

ISSN:0021-8979    

DOI:10.1063/1.351690

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A novel analytical consideration is presented for dealing with phase conjugation by degenerate four‐TE wave mixing in a planar waveguide bounded by a nonlinear cladding, in which both phase‐conjugate power coupling and intensity‐dependent field distributions are discussed. New wave solutions for signal and phase‐conjugation reflections are obtained with anomalous power‐dependent field distributions, and are used to specific numerical example based on a nematic liquid crystal.

ISSN:0021-8979    

DOI:10.1063/1.351691

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Using the coupled mode theory, an analytical investigation of scattering of laser beam by an acoustohelicon wave and the consequent amplification of the scattered modes and the acoustohelicon wave is carried out in a longitudinally magnetized semiconductor of a noncentrosymmetric nature. We have assumed the origin of nonlinear interaction to be the second order optical susceptibility arising from the nonlinear induced current density. The numerical estimations are made forn‐InSb crystal at 77 K. The threshold pump electric fieldE0this found to decrease with increasing wave number and decreasing scattering angle. The nonlinear dispersion characteristics for a right hand circularly polarized mode become positive forE0<E0thand negative forE0≳E0th. The nature of a left hand circularly polarized mode is found to be just the reverse. The steady state gain constants for piezoelectric (gp), deformation (gd), and both cases (gb) are also obtained. The influence of the doping level on nonlinear dispersion characteristics and gain constants has also been considered.

ISSN:0021-8979    

DOI:10.1063/1.351692

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Hydrodynamic phenomena from KrF excimer laser ablation (10−3–20 J/cm2) of polyimide, polyethyleneterephthalate, and aluminum are diagnosed by schlieren photography, shadowgraphy, and dye laser resonance absorption photography (DLRAP). Experiments were performed both in vacuum and gaseous environments (10−5–760 Torr air, nitrogen, and argon). In vacuum, ablation plumes are observed to expand like a reflected rarefaction wave. As the background gas pressure is increased, shock waves and reduced‐density ablation plumes become visible. Below 10 Torr, the ablation plume follows closely behind the shock wave. Between 20 and 100 Torr, the plume recedes behind the shock wave. Below 10 Torr and above about 200 Torr, both the plume and the shock expand with the same temporal power law dependence. Agreement is found between these power law dependences and those predicted by ideal blast wave theory. The DLRAP diagnostic clearly shows that the ablated material (CN molecule from polyimide and ground state neutral aluminum atoms from laser‐ablated aluminum) resides in the ablation plume. CN molecules are detected in both argon and air environments proving that CN is generated as an ablation product and not by reaction with the background gas. As the background gas pressure and the time after ablation is increased, the film darkening due to the laser‐ablated material begins to fade leaving only the nonresonant shadowgraphy component of the plume. The plume dynamics observed by DLRAP are discussed in terms of gas dynamics, plume chemical kinetics, material diffusion in the plume, and cluster/particulate formation.

ISSN:0021-8979    

DOI:10.1063/1.351693

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The time evolution of backscattered and sidescattered microwaves from a plane conductor covered by a pulsed‐plasma column has been studied; transmitted signals along the axial and transverse directions of the column have also been studied. The incident microwaves are at a cw power level varying from a few milliwatts to a few watts. The cw nature of the signal makes it possible to observe the fine structure in the time evolution of the interactions. The plasma is generated by an array of plasma guns embedded in the conducting plate. The plasma density has a gradient in the axial direction and contains regions of subcritical and supercritical plasma density. Reductions in the backscattered radiation down to the lowest measurable levels with available equipment are observed for durations that are an order of magnitude longer than the duration of the current pulse that generated the plasma. Variations caused by changes in the charging potential on the plasma gun capacitor bank, the pressure of the background gas, and the frequency and power level of the incident microwaves are reported. The attenuation in the backscattered radiation decreases with increasing pressure, especially at lower charging potentials. There is a close correlation between the timing and location of sidescattered signals and the transition of the critical density layer across the axial position of the detectors for the scattered signal. The amplitude of the scattered signal was also measured. Estimates have been made for the percentage of incident power accounted for by scattering. Models are discussed for absorption of microwaves in the plasma.

ISSN:0021-8979    

DOI:10.1063/1.351694

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A one‐dimensional kinetic code is used to study the effect of ion‐neutral (charge exchange and elastic scattering) and ion‐ion collisions on plasma flow in the downstream region of an electron‐cyclotron‐resonance plasma etching system. Ions are assumed to leave the source region at the Bohm velocity. Argon, nitrogen, and CF4plasmas are simulated, assuming that the dominant ion species are Ar+, N2+, and CF3+, respectively. Results show that charge exchange and elastic scattering collisions play a significant role in reducing the electrostatic potential variation in the downstream region. For neutral gas pressures above ∼1 mTorr, the potential drop in the downstream region is small, which means that most of the energy with which ions hit the substrate surface is gained while crossing the substrate sheath region. Although the effect of ion‐ion collisions on the plasma potential profile and on the ion distribution function is weak, ion‐ion collisions are responsible for transferring energy from the parallel to the perpendicular direction and hence increasing the perpendicular ion temperature.

ISSN:0021-8979    

DOI:10.1063/1.351695

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The mechanisms of high‐current transients in lightning stepped leaders and in long laboratory sparks at negative polarity are analyzed both from improved time resolution measurements and from systematic identifications of the various elements of the discharge during a pulse. A qualitative model was first constructed when the observations made were correlated with some previously known basic phenomena. Then the model was made quantitative by means of a computer simulation of spark formation, carried out from a modified program in electromagnetics. The relevance of the proposed model was checked for long sparks at negative polarity when computed and recorded current pulse wave forms were compared. Finally, the model of stepped‐leader development was applied to a full scale event: an in‐flight lightning strike on an instrumented aircraft. Slight discrepancies between computed and recorded current pulse wave forms may indicate possible underestimation of the electromagnetic high‐frequency threat to sensitive airborne equipment due to the frequency limitations of the transient recorders used for lightning characterization on aircraft.

ISSN:0021-8979    

DOI:10.1063/1.351696

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Ion transport through curved‐solenoid macroparticle filters is investigated using a parabolic potential model of the electric field. Consideration of the ion equations of motion shows that the filter efficiency will increase with the relative radius of curvature of the solenoid and decrease with the relative length of the input region. If the magnetic field is constant in the input region, transport efficiencies of up to 100% are predicted. We argue that the observed effect of axial magnetic field on the angular distribution of ions emitted from the cathode spot can be explained by electric fields changing ion trajectories after emission, rather than a change in spot behavior. Detailed calculations give the energy and angular distributions with which the ions exit the filter. Focusing effects are observed and seen to affect the ion transport−particularly when the ions are not emitted on the filter axis.

ISSN:0021-8979    

DOI:10.1063/1.351697

出版商:AIP    

年代:1992

数据来源: AIP