摘要:

The concepts of generalized polytropic processes and polytropic refrigeration cycles, which consist of two isothermal processes and two generalized polytropic processes of paramagnetic salt, are introduced. It is shown that such a class of general magnetic refrigeration cycles, which includes the Carnot, Stirling, and other useful magnetic refrigeration cycles, possesses the conditions of perfect regeneration and can have the same coefficient of performance as the Carnot cycle for the same temperature range. Thus, they have many applications in the research and manufacture of magnetic refrigerators.

ISSN:0021-8979    

DOI:10.1063/1.349472

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Raman spectroscopy, in which a laser beam serves the dual role of exciting the Raman spectrum and annealing the sample, shows promise as a means of investigating oxygen effects in the oxide superconductors. A technique is described, based on measurements of the ratios of the areas of corresponding peaks in the anti‐Stokes and Stokes spectra, whereby the temperature of the illuminated region of the sample can be determined as a function of the power in the incident laser beam. It is found that, for sintered samples of bismuth 2122, a small correction must be made for the departure from thermodynamic equilibrium induced by the pumping effect of the laser beam.

ISSN:0021-8979    

DOI:10.1063/1.349473

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Charged particle beams that are not perfectly matched or centered when injected into a focusing channel (or accelerator) have a higher total transverse energy than the equivalent ideal stationary beam. The excess energy &Dgr;Erepresents free energy that can cause emittance growth. A general relationship between the possible emittance growth and &Dgr;Eis derived, and analytical results are presented for nonuniform, mismatched, and off‐centered beams.

ISSN:0021-8979    

DOI:10.1063/1.349474

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The low‐frequency asymptotics of an electric field generated inside a conducting half‐space by an external time‐varying current source are derived. The conductivity of the half‐space may be either uniform or it may contain inhomogeneities (i.e., regions in which the conductivity varies). The dependence of the low‐frequency asymptotics on the dimensionality of the current source and the inhomogeneity are exhibited. We display those leading terms in the expansion that are given exactly by the quasistatic approximation. It is shown that in the case of variable conductivity, the asymptotics of the electric fields inside the half‐space are determined by a Fredholm integral equation of the second kind. For the general case, the solution of this integral equation is identical to solving a classical boundary‐value problem of electrostatics. We also show that there is an important class of problems for which the low‐frequency asymptotics are trivial and are given explicitly in terms of the external field via the Born approximation.

ISSN:0021-8979    

DOI:10.1063/1.349475

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The interaction of KrF excimer laser pulses with polyimide, poly(ethylene terephthalate), polystyrene, poly(methyl methacrylate), and polyethylene has been studied by time‐resolved reflectivity. The width, fluence, and peak intensity of a KrF (248 nm) laser pulse reflected from the polymer surface was determined over a large range of incident laser fluences, 1–3000 mJ/cm2. The reflected pulse was truncated once the incident fluence exceeded a critical value (threshold), characteristic for each polymer. Above the threshold the pulse decayed exponentially with time constants ranging from 2 to 6 ns. For polyethylene, such a critical value appears to lie above the highest fluence available in the present experiments. The threshold fluences for pulse truncation are usually somewhat lower than the ablation thresholds measured photoacoustically. The mechanism of the process is discussed in terms of time‐dependent absorption and scattering from particles emerging from the surface during ablation. Approximate calculations for ablation under our experimental conditions (ns wide excimer pulses) suggest that large particles are ejected initially which are continuously reduced in size during the laser pulse.

ISSN:0021-8979    

DOI:10.1063/1.349476

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Characteristics of waveguides with periodically domain‐inverted regions in LiNbO3and LiTaO3for quasi‐phase‐matched (QPM) second harmonic generation are investigated. The domain‐inverted regions in LiNbO3are formed by heat treatment using selective SiO2mask and those in LiTaO3are formed by heat treatment using selective proton exchange. The domain‐inverted region in LiTaO3is fabricated up to 2.7 &mgr;m depth, which is twice as deep as that in LiNbO3. The channel waveguides incorporating the regions are fabricated using proton exchange by pyrophosphoric acid. The propagation loss (0.9 dB/cm) of LiTaO3waveguide is much lower than that of LiNbO3waveguide (2.9 dB/cm). The second harmonic power generated in LiTaO3waveguide by third‐order QPM consequently has three time higher conversion efficiency, compared to that in LiNbO3waveguide. In domain‐inverted LiNbO3waveguide, we have observed optical damage. On the other hand, in domain‐inverted LiTaO3waveguide, stable blue light (421 nm) has been obtained without optical damage.

ISSN:0021-8979    

DOI:10.1063/1.349477

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A twist reflector is idealized by a surface which supports electric current in one direction only. When a transverse electric mode or a transverse magnetic mode is incident on a twist reflector, both types of modes are reflected. This property of the twist reflector to generate the cross‐polarized mode is used in the development of a device that can rotate the plane of polarization of the incident electromagnetic wave by the rotation of the twist reflector about an axis normal to its surface. A ray‐tracing technique is used to deduce the angle of rotation of the plane of polarization as a function of the orientation angle of the twist reflector. Numerical results are provided to illustrate the characteristics of the polarizer.

ISSN:0021-8979    

DOI:10.1063/1.349478

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The aluminum‐magnesium photoresonant x‐ray laser scheme is investigated using detailed atomic models for both elements. The aluminum pump source is assumed to be aZpinch driven by a current of several MA. It is found that monochromatic pump powers of 100 GW/eV are obtainable within the 48‐A˚ aluminum pump line over a broad range (125–400 eV) of pump plasma temperatures. The gain in Mgixis calculated assuming separations of 1 or 2 cm from the pump plasma. The highest gain appears in the 2s3d1D2‐2s4f1F3line at 228 A˚. However, photoionization as well as resonant photoexcitation appears to be crucial to the success of this scheme. If photoionization by lines other than the pump line is neglected, Mgixdoes not exist below∼60 eV. In the 60–100‐eV Mg plasma temperature range, collisional excitation from low‐lying states to the lower laser level severely degrades the gain. However, when photoionization of Mg by all of the strong Alxilines is taken into account, Mgixis found to exist at much lower temperatures, leading to respectable gain of ∼3 cm−1at 1‐cm Al‐Mg separation.

ISSN:0021-8979    

DOI:10.1063/1.349479

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Using a comprehensive theoretical CO2laser model, we calculated the amount of the catalytic conversion from CO to CO2, which is required for the stable and long‐life operation of the closed‐cycle, transversely excited atmospheric CO2laser. The amount of the catalytic conversion was evaluated by the fractional conversion (&eegr;) defined as &eegr;=([CO]in−[CO]out)/[CO]in, where [CO]inand [CO]outare the CO molecules number density at the entrance and exit of the CO2regenerator in the laser chamber, respectively. For the laser gas mixture of CO2/N2/He=15/15/70(%) and the output coupler reflectivity of 80%, an &eegr; of the CO2regenerator greater than 0.22 is theoretically required to maintain the 95% level of the initial laser output energy at an input energy density and a laser gas clearing ratio CR of 176.1 J/land 2.0, respectively.

ISSN:0021-8979    

DOI:10.1063/1.349480

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The paper presents both the theoretical investigation of refractive index profile generation of stripe waveguides by field‐assisted ion exchange in glass and a comparison with experimental results obtained by Ag+‐Na+exchange in a special glass type. The internal electric field of a periodical structure of mask openings is calculated and applied to ion‐exchange processes through a single mask opening as well as to processes using so‐called guard bands. These guard bands are especially suitable for profile matching to step index fibers. Matching to graded index fibers requires burying steps, which are modeled with plausible physical assumptions.

ISSN:0021-8979    

DOI:10.1063/1.349481

出版商:AIP    

年代:1991

数据来源: AIP