摘要:

The thermoreflectance technique is applied for imaging electric current distributions and thermal transfer in a temperature reference resistor heated by an alternating current. High‐frequency scans (30 kHz) allow imaging of the current density distribution in conducting strips of the resistor while scans of amplitude and phase of the surface temperature variation at lower frequencies reveal plane, cylindrical, and spherical thermal waves. We investigate wave dimensionality as a function of heating geometry and thermal length, and present a simple method allowing a quantitative thermal analysis by exploiting the phase profile of cylindrical thermal waves. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363094

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A theoretical consideration of two‐dimensional ac temperature waves in a two‐layered platelike sample for application to thermal diffusivity measurement parallel to a sample surface using an ac calorimetric method is presented. The analysis is based on the Fourier‐transformed Green’s function technique. It is concluded that this thermal system deviates from an apparent one‐layered system of which the thermal diffusivity parallel to a sample surface is calculated based on steady state heat flow under the following conditions: (a) the frequency of the supplied ac thermal energy is high, (b) the difference between the thermal diffusivities of the layers is large, and (c) the layer with higher thermal diffusivity is thin. This deviation is due to the inhomogeneous ac temperature distribution normal to the sample surface. If experimental conditions are the reverse of conditions (a)–(c), an ac calorimetric method can be used to measure the thermal diffusivity of thin samples on substrates. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363095

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Based on new experimental results of Chpoun, Passerel, Li, and Ben‐Dor [J. Fluid Mech.301, 19 (1995)] and numerical results of Vuillon, Zeitoun, and Ben‐Dor [J. Fluid Mech.301, 37 (1995)] regarding the shock wave reflection transitions from regular to Mach reflection and from Mach to regular reflection in steady flows of perfect gases which contradicted the present understanding regarding these processes [Hornung and Robinson, J. Fluid Mech.123, 155 (1982)], we present a new approach for better understanding the supersonic flow deflection over straight wedges and the transition of steady shock wave reflections over straight wedges. The new approach is based on the principle of minimum entropy production which enables one to choose the physical (stable) solutions out of a variety of possible mathematical ones. To the best of our knowledge the new transition criteria which are proposed in this study agree with all the available experimental results. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363096

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The stability of both regular reflection and Mach reflection wave configurations in pseudosteady flows has been investigated by applying the principle of minimum entropy production, in a way similar to that done by us in Part I of this study [Li and Ben‐Dor, J. Appl. Phys.80, 2027 (1996)] where the stability of steady flow reflection configurations was considered. As a result, it was found that Mach reflection wave configurations are stable inside the dual solution domain, although, based on the state of the art when this study started, they were considered not to be stable. Consequently, a new general transition criterion from regular to Mach reflection was proposed. With the aid of this criterion the domains of regular, direct‐Mach and stationary‐Mach reflections, in the domain of the incident shock‐wave Mach number and the reflecting wedge angle, were reestablished. All the analytical findings were supported by experimental results from different sources. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363097

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Very small bubbles are generated in chemically different insulating liquids in a repeatable and controlled way. A constant dc voltage is applied between a sharp point and a plane electrode. Above a threshold voltage, the current is ‘‘impulsional.’’ The origin of each impulse is an electron avalanche while the consequence is the formation of a small bubble at the point electrode. The relationship between the injected electrical energy and the maximum bubble size is examined. Depending on the liquid used, and therefore the level of energy injected, different behaviors were observed. In particular, results for very low energy injections were explained by mechanical rather than thermal processes. The nonequilibrium plasma formed following an electron avalanche, brings about the emission of a pressure transient, behind which there is an ‘‘afterflow’’ of matter. The liquid travelling outwards from this point source causes the liquid to break down mechanically, thus forming an almost empty cavity, which disappears on collapse. In the case of high energy injection, energy is lost to local heating of the liquid. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363098

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We discuss the variant of a method for the determination of absolute hydrogen atom densities based on the absorption of the hydrogen Lyman‐alpha line. Previously, we demonstrated that by using a simple vacuum ultraviolet spectrometer with low resolution an accuracy of better than 50% for the determined hydrogen atom densities can be obtained for transmissions ranging from 10% to 90%. For transmissions outside of this range excessive errors occur, thus, limiting the usefulness of the method to a certain range of hydrogen atom densities, depending on absorber length and temperature. This range of atomic hydrogen densities accessible to the measurement can be extended by the new method that consists of using well‐defined mixtures of hydrogen and deuterium in the absorber. Using a source of either hydrogen or deuterium Lyman‐alpha radiation, only one sort of atoms contributes to the absorption. Thus, by selecting an appropriate mixture of hydrogen and deuterium, the range of atom densities accessible to the measurement can be extended to higher densities. Using well‐defined mixtures ranging from 1% hydrogen in deuterium to 2% deuterium in hydrogen as absorbers, we were able to determine hydrogen atom densities up to a factor 100 higher than those measured previously in pure hydrogen. Using mixtures down to the natural abundance of deuterium in hydrogen, the measurement of even higher atom densities seems possible. The effect of the difference of the energies of dissociation of hydrogen and deuterium has been investigated using a computer model of relevant chemical processes. In most situations, this effect is smaller than the experimental error. The main limitation of the method is its sensitivity to absorption by impurities, as many molecular gases absorb Lyman‐alpha radiation. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363099

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

To determine self‐consistently the time evolution of particle size and their number densityinsitumulti‐angle polarization‐sensitive laser light scattering was used. Cross‐polarization intensities (incident and scattered light intensities with opposite polarization) measured at 135° andexsitutransmission electronic microscopy analysis demonstrate the existence of nonspherical agglomerates during the early phase of agglomeration. Later in the particle time development both techniques reveal spherical particles again. The presence of strong cross‐polarization intensities is accompanied by low‐frequency instabilities detected on the scattered light intensities and plasma emission. It is found that the particle radius and particle number density during the agglomeration phase can be well described by the Brownian free molecule coagulation model. Application of this neutral particle coagulation model is justified by calculation of the particle charge whereby it is shown that particles of a few tens of nanometer can be considered as neutral under our experimental conditions. The measured particle dispersion can be well described by a Brownian free molecule coagulation model including a log‐normal particle size distribution. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363118

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have used the three‐dimensional, particle‐in‐cell codeQUICKSILVER[J. P. Quintenz,etal., Lasers and Particle Beams12, 283 (1994)] to simulate radial applied‐Bion diodes on the particle beam fusion accelerator II at Sandia National Laboratories. The simulations agree well with experiments early in the beam pulse, but differ substantially as the ion‐beam current increases. This is attributed to the oversimplified ion emission model. We see the same instabilities seen in earlier simulations with idealized diode geometries; Early in time there is a diocotron instability, followed by a transition to an ‘‘ion mode’’ instability at much lower frequency. The instability‐induced beam divergence for the ∼10 MeV beam during the diocotron phase is <10 mrad, significantly less than the total beam divergence in experiments early in the pulse, but increases to ≳25 mrad after the transition. The ion mode has a distinct harmonic structure along the applied field lines, making the instability transition sensitive to the diode geometry. The ion mode instability in our latest simulations is consistent with evidence of instabilities from recent experiments. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363100

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A study of radiative properties of formed‐ferrite discharge driven by a new electrical circuit scheme with a prepulse initiation has been carried out. With the prepulse electrical initiation, we demonstrated that the jitter time of main discharge onset can be decreased from a few tens of microseconds to ∼0.1 &mgr;s allowing us to synchronize a number of formed‐ferrite discharges switched in parallel. Discharge plasma surface radiation characteristics in the spectral region of the XeF2photodissociation band (140–180 nm) were measured by the dynamic actinometry method. The plasma surface radiation intensity is 2.3×1023photons/cm2 s in 10 &mgr;s which corresponds to a brightness temperature of 22 000 K. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363116

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Twenty elements were codeposited with carbon in an arc discharge between graphite electrodes. The majority of them were evaporated from composite anodes that contained the elements or their oxides stuffed into central bores in the graphite rods. The deposits, found in the soot at the reactor walls or as slag at the cathode, were characterized using scanning and transmission electron microscopy, electron energy loss spectroscopy, and x‐ray diffraction. The products fall into four categories: (1) elements that can be encapsulated in the form of their carbides (B, V, Cr, Mn, Y, Zr, Nb, Mo); (2) elements that are not encapsulated but tolerate the formation of graphitic carbon cages (Cu, Zn, Pd, Ag, Pt); (3) elements that form stable carbides, competing with and pre‐empting the carbon supply for the graphitic cage formation (Al, Si, Ti, W); and (4) the iron‐group metals (Fe, Co, Ni) that stimulate the formation of single‐walled tubes and strings of nanobeads in the conventional arc discharge condition, and produce the nanometer‐size carbon‐coated ferromagnetic particles in a modified arc discharge in which metals are in molten form in graphite crucible anodes exposed to a helium jet stream. The criterion determining the formation according to one of the four categories is discussed on the basis of this extended study. It is apparent that the physical properties such as vapor pressure, melting and boiling points, the completeness of the electronic shells of the elements, or their heat of carbide formation are not sufficient to explain the selectivity of the encapsulation without exceptions. A hypothesis is advanced that emphasizes the existence of the carbide, interfacial compatibility with the graphitic network, as well as the transport and supply parameters in the reaction space. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363102

出版商:AIP    

年代:1996

数据来源: AIP