摘要:

Modern techniques of functional minimization are applied to the one‐dimensional profile inversion problem. The procedure is applied to profiles that can be represented as a series of linear ramps. A band‐limited knowledge of the transmission function is used to determine the parameters that describe the profile. It is found that simple profiles can be readily determined. For more involved profiles, the method of simulated annealing proves to be helpful in obtaining the correct profile.

ISSN:0021-8979    

DOI:10.1063/1.349176

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A variety of diagnostics measure fluctuating quantities at a small number of locations. A common method to estimate the wave‐number spectrumS(k) at a given frequency is to make many measurements of the ‘‘local’’ wave numberkl, which is the phase difference between two locations divided by their separation. The random‐phase model allows the synthesis of a typical signal from a givenS(k). We apply local wave‐number analysis to such synthetic signals and show that, in the limit of small separation, the power‐weighted mean ofklaccurately reflects the mean ofS(k), and the width of the distribution is exactly &squflg; times smaller than the width ofS(k). We also show that no information on the shape ofS(k) exists in the measurements. Finally, we generalize these results to finite separations, finite measurement volumes, and any number of locations.

ISSN:0021-8979    

DOI:10.1063/1.349177

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The spatial and temporal variation of a fluctuating quantity can often be described with sufficient accuracy as a sum of independent modes, whose powers are given by a functionS(k) for each frequency. Many diagnostics are restricted to making measurements at two points, so that a determination ofS(k) by spatial Fourier transformation is not possible. Nevertheless, in the limit of point measurements with infinitesimal separation, the mean wave numberk¯ and the rms deviation &sgr; about the mean can be exactly determined by repeated measurements of the phase difference between the tips. If the separation is finite, the method can be extended in such a way that the correct results are obtained for any separation as long asS(k) is Gaussian. For other spectral shapes, the error in the measurement ofk¯ may be of the order of &sgr;, and the fractional error in the measurement of &sgr; may be substantial. A finite measurement volume reduces the sensitivity to short wavelength modes. Under certain assumptions, the measured values can be adjusted to yield better estimates ofk¯ and &sgr;.

ISSN:0021-8979    

DOI:10.1063/1.349178

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The effect of electron irradiation on ferroelectric poly(vinylidene‐trifluoroethylene) copolymers has been studied from both microscopic and macroscopic points of view. Microscopically, it is found by x‐ray photoemission spectroscopy analysis and ultraviolet absorption that the irradiation damaging is mainly through dehydrofluorination and leads to double‐bond production and crosslinking. On the other hand, dielectric permittivity, internal friction, x‐ray diffraction, and differential scanning calorimetry experiments show that both the ferroelectric transition temperatureTcand the melting temperatureTmare lowered under irradiation. These various experimental features are consistently analyzed taking into account three main mechanisms resulting from the electron irradiation: (i) reduction of the crystallite mean size, (ii) changes in the internal pressure, and (iii) reduction of the dipolar energy.

ISSN:0021-8979    

DOI:10.1063/1.349179

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The coupling of electromagnetic energy into a conical waveguide through an electrically small hole near the cone tip is considered. This problem is relevant to near‐field microscopy, where tapered metallic probes are used to either illuminate, or collect light from, the structure under investigation. The situation considered in detail here is simplified by assuming that the aperture is surrounded by a planar baffle, which approximates the flat ends of fabricated probes. An established formulation for small‐hole coupling into waveguides is used and the waveguide itself taken to be a truncated, perfectly conducting cone. As expected, the values obtained for the energy transmitted through an aperture with a diameter significantly less than that of the radiation being used are small. However, it is also found that the size of the waveguide at the hole position strongly affects the percentage of the incident energy transmitted.

ISSN:0021-8979    

DOI:10.1063/1.349180

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A dc high‐current and low‐energy electron beam gun has been developed. The maximum electron beam current is 63 A for an accelerating energy of 140 eV. The maximum ratio of the beam current to the discharge current is 78%. The ion current, which is extracted from the helium plasma produced by the electron beam, is 12.3 A. This value corresponds to the current density of about 7 A/cm2, which is the highest value among the ion sources in steady operation. The electron density of the plasma is estimated from the ion saturation current to be about 4.6×1013cm−3, which corresponds to the degree of ionization of 21%. This gun will open the way for many electron beam applications.

ISSN:0021-8979    

DOI:10.1063/1.349152

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

It has been shown recently that when an electromagnetic (EM) missile with an energy decreasing asr−&egr;is incident upon a perfectly conducting plate, the reflected energy will decrease asr−&egr;with the same exponent as the EM missile. The question of studying the backscattering properties of an EM missile by other objects is then left open to further investigation. In this paper we make a study of backscattering of an EM missile with an energy decreasing asr−&egr;by a perfectly conducting sphere, and the analysis indicates that the rate of energy decay for the backscattered field far from the sphere is proportional tor−(2+&egr;)(0<&egr;<2), which is faster thanr−&egr;for plate scattering but slower than the usual energy decayr−4for a three‐dimensional backscattered field.

ISSN:0021-8979    

DOI:10.1063/1.349153

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The laser‐induced damage thresholds for &lgr;=1.06 &mgr;m of commercially available (produced at Institute of Optics) dielectric optical coatings, both antireflective and high reflectance, have been determined. The dependence of the optical coatings stability on design and selection of materials has been investigated. An improvement of the coatings durability by using nonquarterwave layers in addition to the basic design of the mirrors has been obtained. The choice of the coating materials is also discussed.

ISSN:0021-8979    

DOI:10.1063/1.349154

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The effect of two‐wavelength oscillation for short‐optical‐pulse generation in laser diodes by sinusoidal current modulation has been investigated using a rate equation analysis. A high peak power and short pulse without a tail can be generated at the shorter wavelength &lgr;2with the help of carrier damping through carrier‐photon conversion at the longer wavelength &lgr;1. This can be obtained provided that the cw threshold current for &lgr;1is lower than that for &lgr;2and the gaing2larger thang1at the high carrier excitation levels that occur under the conditions of gain switching.

ISSN:0021-8979    

DOI:10.1063/1.349155

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

1,4‐dithioketo‐3,6‐diphenyl‐pyrrolo‐[3,4‐c]‐pyrrole, which is a good charge generation material for a near‐infrared electrophotographic photoreceptor, has three polymorphs called the I, II, and III forms. The I and II forms are transformed into the III form after having been kept in suitable organic solvents. On the other hand, the III form is changed to the I and/or II forms by heat treatment above 320 °C. Among the three polymorphs, only the highly ordered III form shows the necessary and characteristic absorption in the near‐infrared region.

ISSN:0021-8979    

DOI:10.1063/1.349156

出版商:AIP    

年代:1991

数据来源: AIP