摘要:

Methods of solution are given for the sonic resonances corresponding to the free flexural and torsional vibrations of an anisotropic bar of rectangular cross section. The resonance equations for the torsional vibrations of a symmetric, three‐piece, cylindrical compound oscillator are derived. The particular application of the methods and equations for the determination of some of the elastic constants of compression‐annealed pyrolytic graphite, a highly anisotropic material, is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1659429

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Compression‐annealed pyrolytic graphite and single‐crystal graphite specimens were irradiated with neutrons for total exposures of 3 to 71×1017neutrons/cm2(nvt) at 50°C, 3 to 8×1019nvt at 650°C and 1.2×1020nvt at 1000°C. The elastic constants of the materials were determined before and after irradiation by ultrasonic‐, sonic‐resonance‐, and static‐test methods. The only elastic constant that changed significantly with irradiation was the shear modulusc44although a small change was also seen inc33. The natural crystals after irradiation had shear moduli in the range of 0.16–0.46×1011dyn/cm2, the modulus after irradiation showing no correlation with irradiation temperature, exposure, or modulus before irradiation. The average values of the shear modulus of the pyrolytic graphite were 0.22−0.40×1011dyn/cm2after irradiation at 50°C, 0.030−0.033×1011dyn/cm2after irradiation at 650°C, and 0.042×1011dyn/cm2after irradiation at 1000°C; the shear modulusc44of graphite when all dislocations are pinned is thus equal to or greater than 0.40×1011dyn/cm2. The results can be correlated with electron‐microscope observations and interpreted in terms of nucleation theory of radiation damage in analogy to the interpretation of the electron‐microscope observations of W. N. Reynolds and P. A. Thrower [Phil. Mag. 12, 573 (1965)] or in terms of a difference in the availability of sinks for the displaced atoms.

ISSN:0021-8979    

DOI:10.1063/1.1659430

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The theory of an optical parametric oscillator located internal to the cavity of the pump laser has recently been presented by Oshman and Harris. An oscillator of this type has been constructed and the details of its operation are presented here. Several features predicted by the theory have been observed; in particular a form of relaxation oscillation peculiar to the internal oscillator has been observed for the first time. In addition to the experimental results the question of the stability of the various modes of operation and factors affecting the pulse repetition rate of the pulsing solution are considered.

ISSN:0021-8979    

DOI:10.1063/1.1659431

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The expression for transient photocurrent due to light pulse of arbitrary intensity has been derived in the presence of trapping and detrapping. The pulse duration is assumed to be short compared to the transit time and the analytic solutions are obtained for ``weak'' trapping case alone. Transcendental equations involving transit time and injected charge density have been solved numerically and the solutions are displayed graphically. In various limiting cases our expressions reduce to previously reported results. Analysis presented in this article may be useful in obtaining trapping parameters from experimental data.

ISSN:0021-8979    

DOI:10.1063/1.1659432

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The conventional technique for the measurement of mobility sandwiches the sample between two electrodes, at least one of which is transparent. The sample is illuminated through this surface and the resultant current is measured under constant voltage conditions. In the present work we describe a modified method for the determination of mobility where one surface of the sample is grounded and the other is left floating. The floating surface is charged to an initial voltage,V0and its subsequent decay upon exposure to intense light is observed experimentally. The relation between the instantaneous surface voltageV(t) and the exposure timetis referred to as the ``photoinduced discharge characteristic'' (PIDC) of the photoconductor plate. Knowing the PIDC and the theoretical expressions derived in this paper one can obtain an accurate value for the mobility of the charge carriers. As verification of our analysis we have measured the PIDC of amorphous Se films deposited onto Al substrates. Both the surface potential and its time rate of decay were measured simultaneously through a NESA surface potential probe. The agreement between theory and experiment is excellent and the value of the hole mobility obtained using our analysis is in good agreement with those already reported in the literature.

ISSN:0021-8979    

DOI:10.1063/1.1659433

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Previous studies of anodized InSb samples cooled in the dark to 77°K have shown that initiallyn‐type surfaces are changed toptype upon visible illumination. Forn‐type bulk material an infrared photovoltaic response can be obtained from the inverted surface after such illumination. A new response has been found inn‐type samples which was present initially but which then disappeared upon visible illumination. Measurements on both types of response have shown other distinguishing features, and several models are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1659434

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The effects of surface contamination of the surface of Langmuir probes have been studied experimentally. These experiments were carried out in three different argon glow discharge tubes. One tube was designed to simulate conditions found in large plasma facilities used in engineering studies, while the other two were designed and prepared in accordance with high‐purity laboratory standards. In addition, the effect of a small amount of air on an argon plasma was investigated. The data are presented in the form of original probe traces taken on a two‐axis recorder. These data were used to construct curves of contamination versus time and of the logarithm of electron current versus probe voltage. It was found that sufficient contamination is present, even in relatively well‐prepared systems, to cause gross errors in probe measurements. The errors are found to be especially insidious in that the self‐consistency checks commonly used in Langmuir probe applications do not reveal the presence of contamination. It is concluded that probe measurements will be compromised unless particular attention is given to the problem of maintaining a clean probe.

ISSN:0021-8979    

DOI:10.1063/1.1659435

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Electrostatic probe theories are traditionally aimed at predicting probe characteristic curves for specified sets of plasma density, temperature, and potential. The inverse process of determining the plasma properties from a measured current‐voltage curve is conventionally done in a fashion involving subjective judgement and resulting in one set of properties for one characteristic. A method is presented which eliminates subjective factors altogether and yields one set of properties for each point of the characteristic curve. By averaging, the accuracy and reliability of the data is considerably increased. The method is based on the replacement of conventional dimensionless variables by new, experimentally convenient ones and expressing the results of a given theory in terms of the new variables. The method is applied to a cylindrical probe in a collision‐free Maxwellian plasma at rest. It is demonstrated, by way of an example, that the results of the inverse solution are consistent with the direct problem. The principle of the inversion is applicable to a wide variety of probe theories.

ISSN:0021-8979    

DOI:10.1063/1.1659436

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Spectroscopic measurements in the extreme uv and visible have been carried out on a plasma produced by giant‐laser impact on small LiH pellets suspended in vacuum. The experimental results indicate a fast‐expanding periphery containing Li, H, and some oxygen impurities, and a plasma core with a smaller mean expansion velocity. The latter gives rise to a sequence of emission lines due to ions of successively decreasing charge. This plasma core is completely recombined after about 100 to 150 nsec.

ISSN:0021-8979    

DOI:10.1063/1.1659437

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Drift velocities of electrons in a gaseous neon plasma have been obtained by measuring the discharge current, radial density profile of electrons, and axial electric field in the positive column of a neon discharge forE/na(V cm2) values ranging between 2.8×10−17and 2.4×10−16at a temperature of 300°K. The data agree very well with that of the previous workers. The validity of the Langmuir probe measurement for the electron densities in this experiment is discussed in detail.

ISSN:0021-8979    

DOI:10.1063/1.1659438

出版商:AIP    

年代:1970

数据来源: AIP