摘要:

The second harmonic generation technique has been used to evaluate combinations of third‐order elastic constants (TOEC) of NaCl between room temperature and liquid‐nitrogen temperature. TOEC data at room temperature are compared both with theory and other experimental results. The temperature variation ofC111is somewhat larger than predicted by theory. The temperature behavior suggests that a noncentral force and other exchange interactions predominate at 0 K.

ISSN:0021-8979    

DOI:10.1063/1.347005

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Irradiation of metals with laser pulses in the nanosecond to picosecond range may give rise to temperature fields which are significantly different from those predicted by the classical Fourier heat conduction model. In this work we examine the very short‐time temperature response of a semi‐infinite region to an axisymmetric laser surface source, which is either continuous or activated for a period &Dgr;tand has a spatial profile which is Gaussian, doughnut, or a combination of the Gaussian and doughnut modes. The material properties are assumed constant, and radiation and convection from the irradiated surface are neglected. The non‐Fourier temperature responses are compared with their diffusive counterparts and shown to give rise to steep local temperature concentrations.

ISSN:0021-8979    

DOI:10.1063/1.347006

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Three‐point bounds on the effective conductivity &sgr;eof isotropic two‐phase composites, that improve upon the well‐known two‐point Hashin–Shtrikman bounds [J. Appl. Phys.23, 779 (1962)], depend upon a key microstructural parameter &zgr;2. A highly accurate approximation for &sgr;edeveloped by Torquato [J. Appl. Phys.58, 3790 (1985)] also depends upon &zgr;2. This paper reports a new and accurate algorithm to compute the three‐point parameter &zgr;2for dispersions of hard spheres by Monte Carlo simulation. Data are reported up to values of the sphere volume fraction &fgr;2near random close‐packing and are used to assess the accuracy of previous analytical calculations of &zgr;2. A major finding is that the exact expansion of &zgr;2through second order in &fgr;2provides excellent agreement with the simulation data for the range 0≤&fgr;2≤0.5, i.e., this low‐volume‐fraction expansion is virtually exact, even in the high‐density region. For &fgr;2>0.5, this simple quadratic formula is still more accurate than other more sophisticated calculations of &zgr;2. The linear term of the quadratic formula is the dominant one. Using our simulation data for &zgr;2, we compute three‐point bounds on the conductivity &sgr;eand Torquato’s approximation for &sgr;e.

ISSN:0021-8979    

DOI:10.1063/1.347007

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Analytical theories and Monte Carlo calculations are used to treat the scattering and energy loss of electrons in the anode of a reflex triode. The solution of this scattering problem is combined with the equations for particle flow in vacuum to give a quantitative theory of triode operation. It is now possible to calculate several important properties of this device. These include the operating voltage in the constant voltage mode, the ratio of ion‐to‐electron current and the ion transit time.

ISSN:0021-8979    

DOI:10.1063/1.347008

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In this paper we study the creation process of a cylindrical pulsed argon plasma. A new numerical model to simulate the plasma evolution during the creation process has been developed. This model is based on the solution of the continuity equations for ions and electrons together with the Poisson’s equation. The good agreement obtained between numerical and experimental results of electron density as a function of the time confirms the validity of the hypothesis made in the model proposed.

ISSN:0021-8979    

DOI:10.1063/1.347009

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Extreme ultraviolet (XUV) spectral line emissivities, originating from resonant transitions with upper states excited mainly from ground or both ground and lowest metastable states by electron collisional excitations of highly ionized intrinsic impurities in tokamak plasmas, are linear functions of both electron density and temperature fluctuations,n˜eandT˜e, when the fluctuations are small and at high frequency. Correlations between measured intensities of spectral lines can thus provide localized measurements of the fluctuations. Newly developed XUV monochromators of high throughput enable accurate and fast diagnostics ofn˜eandT˜efor the study of the tokamak plasma microturbulence.

ISSN:0021-8979    

DOI:10.1063/1.347010

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In this work a filament‐assisted dc argon discharge is generated between parallel electrodes. A rf signal (<13.56 MHz) is applied to one electrode in order to modulate the plasma potential. The degree of modulation is monitored using an emissive Langmuir probe. The velocity distribution of ions arriving at the earthed electrode is observed to respond to the rf modulation of the local space potential, the effect diminishing as the rf frequency is increased above the ion plasma frequency. A model is developed to describe the behavior of the ions to the rf modulation of the space potential. Good agreement between the simulation and experimental results is observed. The modulation of ion velocities by nonsinusoidal potentials is briefly examined.

ISSN:0021-8979    

DOI:10.1063/1.347011

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Theoretical investigations have been performed to describe the discharges in rf‐excited CO2waveguide lasers. Especially, the formation of boundary layers has been considered as these regions are not yet understood completely. As characterization of the plasma, the spatial distributions of electron density and electric field have been calculated as these two quantities are suitable to describe the electrical properties of the discharge as well as the optical behavior. The electric‐field strength is split up into the rf field and a static field which forms as a consequence of ambipolar diffusion.

ISSN:0021-8979    

DOI:10.1063/1.347012

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Radio frequency glow discharges in SiH4(50%)/H2at 13.56 MHz and 100 kHz have been investigated by using time‐ and space‐resolved optical emission spectroscopy. The relative net‐excitation rate is obtained by deconvolving the spatiotemporal emission profile considering the influence of the radiative lifetime. From the results, we infer the behavior of electrons with energy greater than the threshold energy for the dissociative excitation and we discuss the discharge structure. Lines of SiI(4s 11P0→3p 21D), SiH(A2&Dgr;;v’=0→X2&Pgr;;v‘=0), H&agr;, and H2(d 3&Pgr;u→a 3&Sgr;+g) are selected for observation. It is concluded that there exists a considerable population of negative ions compared with positive ions, which has not been previously proposed in high‐frequency discharges in SiH4because of the small degree of electron attachment. At 13.56 MHz, the optical emission oscillates at 2&ohgr; in the bulk plasma region, in addition to the emissions at the plasma‐sheath boundaries. It implies that the field in the bulk is strengthened by the external sinusoidal field to make up for the loss of electrons by electron attachment. The bulk field is estimated at 69 sin(&ohgr;t)V cm−1fromV‐Icharacteristics. The existence of a double layer by the modulation of electrons is strongly suggested even in a high frequency at 13.56 MHz. At low frequency (100 kHz), a double layer is observed, which is formed by the large modulation of negative and positive ions. Consequently, excitation by electrons accelerated in the field at the double layer is observed in addition to excitation by secondary electrons from ion bombardment. Consideration of the role of negative ions in high‐frequency SiH4discharges at 13.56 MHz is necessary to understand the discharge structure of the silane plasma and the deposition of the hydrogenated amorphous silicon.

ISSN:0021-8979    

DOI:10.1063/1.347013

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A shielded plasma device has been used extensively in plasma synthesis and plasma processing. In order to optimize these processes, optical diagnostics were used to study the device operated at 545 kHz and pressures between 2.5 and 580 Torr. Plasma‐induced emission for Ar, N2, and N+2suggested that a local thermodynamic equilibrium point of view was inappropriate. Spatially resolved laser‐induced fluorescence and laser absorption measurements were performed on the metastable argon state3P2and the radiative state3P1. The metastable profiles were described in terms of a kinetic model dominated by electron‐atom kinetics. The model was used to determine the electron temperatureTeas a function of axial position and total gas pressure. These temperatures were found to lie in the range of 5000 K at 580 Torr to 11 000 K at 25 Torr.

ISSN:0021-8979    

DOI:10.1063/1.347014

出版商:AIP    

年代:1990

数据来源: AIP