摘要:

We have analyzed the migration of spherical and cylindrical inclusions under a direct current and a thermal gradient in an infinite medium and near a planar boundary. Our study of inclusion motion in an infinite medium is concerned primarily with the application of the marker‐motion technique to measure mass transport in the lattice. It is shown that, in addition to the lattice motion, there are contributions to marker movement coming from mass transport by surface and volume diffusion, which is induced by the change of potential gradient on the marker surface. To measure the true lattice velocity, the marker used must be a good insulator compared to the matrix material, and its size should exceed a few microns. The study of inclusion motion near a boundary is for analyzing pore migration by electromigration in thin films. Three cases have been considered, which include a spherical pore near a plane and between two planes and a cylindrical pore connected to two planes. We concluded that the pore migration in the lattice is due to surface electromigration and the planar boundary generally increases pore velocity and distorts its shape during migration.

ISSN:0021-8979    

DOI:10.1063/1.1658379

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Prior treatments of the scattered x‐ray intensity from diffusion zones in two component crystalline systems lack a formal basis and tend to be intuitive in approach. Relatively simple and useful results can be obtained if the continuous composition changes are approximated by either a series of steps or by a system of linear segments. This paper develops equations for the coherent scattering from both approximations. Summing the scattering from all segments leads to the formation of intensity bands. Equations, containing an absorption correction, are given for planar diffusion zones in the micron and submicron range.

ISSN:0021-8979    

DOI:10.1063/1.1658380

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

A composite structure consisting of an array of parallel 2 &mgr; wires was produced by directionally solidifying the Ni&sngbnd;W eutectic. A multineedle field‐emission cathode was then made by selectively etching and pointing the filament tips. The voltage characteristics of a simple diode constructed from this cold cathode were measured and found to follow the Fowler‐Nordheim field‐emission equation. At a macroscopic current density of 2×10−4A/cm2and an electric field of 4×104V/cm a vacuum arc occurred that damaged the multineedle cathode. A field‐emission microscope was constructed using a multineedle cathode to observe the emission distribution and the pattern was compared with scanning electron micrographs of the cathode before and after emission. It was concluded that the current was not uniformly distributed over the filaments and that failure was accompanied by local melting.

ISSN:0021-8979    

DOI:10.1063/1.1658381

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Electron‐beam loading of a single‐cavity standing‐wave accelerator has been investigated experimentally and theoretically for the case of continuous injection over ten cycles. The particular cavity used was a 2.4‐m long right‐circular cylinder excited at 50 MHz in its TM010mode with an unloaded Q of 1.2×105. Beam currents were varied from 40 to 210 A, field strengths from 0.2 to 6 MV/m, and the energy of the ejected beam from 1 to 10 MeV. During beam injection the energy supplied per cycle to the accelerator by its power amplifiers was negligibly small compared to the energy absorbed per cycle by the injected electron beam. The energy extracted by the electrons was determined from cycle‐to‐cycle oscilloscopic observations of the decay of the accelerating field. Energy extraction per cycle was as great as 8% of that residing in the electromagnetic field at the time of beam injection. The results agree with predictions over most of the range of the experimental parameters when assuming paraxial trajectories. Disagreement with calculations, when using the same assumption, arose when there were serious departures from paraxial flow. The special case of injection of a sharply bunched relativistic electron beam is also reported. Here each 18‐MeV bunch contained about 1.7 &mgr;C and was about 6‐nsec long. When in an accelerative mode the electric field was degraded and energy flowed from the field into the beam bunches at a rate of about 0.6 BW. However, when the phase of the field was reversed to establish a decelerative configuration the electric field was enhanced, energy flowed coherently from the beam bunches into the field at a rate of about 0.6 BW. Within experimental uncertainties, these values agree with predictions.

ISSN:0021-8979    

DOI:10.1063/1.1658382

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Simultaneous, time‐resolved measurements of current growth and resonance line absorption have been made for the time intervals 0.2 and 0.8 &mgr;sec, respectively, preceding breakdown between plane‐parallel copper electrodes in vacuum. These measurements have been carried out for an electrode separation of 0.1 cm and residual pressures in the low 10−9‐Torr range. The experimental data show that neutral copper vapor is present in the interelectrode volumebeforethe current increases sufficiently to produce breakdown. It is found that the vapor is generated during times of the order of some microseconds prior to breakdown, and is highly localized to the region of the subsequent spark channel. Further, the vapor density decreases from cathode to anode along the path of the subsequent spark channel. The results are consistent with a model for vacuum breakdown proposed recently in which the transient production of vapor immediately prior to breakdown occurs by the evaporation of an anode macroparticle during its transit to the cathode; amplification of the prebreakdown current in this vapor then leads to breakdown. From the data obtained in the present experiment, it appears that breakdown occurs primarily while the macroparticle is located between mid‐gap and the cathode.

ISSN:0021-8979    

DOI:10.1063/1.1658383

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The imprisonment time of Doppler‐broadened resonance radiation in a planar gas discharge is calculated numerically and compared with the analytic approximation of Walsh. This comparison shows that Walsh's results are higher than the numerical results by as much as 17% for no electronic collisional deexcitation, and by as much as 63% for copious electronic collisional deexcitation (at an optical thickness of 10). In addition, the effect of diffusely reflecting walls on the imprisonment time is calculated numerically and compared with the analytic approximation of Weinstein where possible. The numerical results are rigorously valid in the optically thick limit and approximately valid in the optically thin limit.

ISSN:0021-8979    

DOI:10.1063/1.1658384

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

A method is presented for extending the theory of wall stabilized arcs to free burning arcs. It is based on accounting for the finite length of the arc column and utilizing the fact that such arcs are normally very thin compared to this length. Detailed calculations are performed for an arc column with constant properties, and an outline is given for incorporating more realistic properties. An interesting result of the analysis is that the axis temperature must increase greatly in front of the electrodes.

ISSN:0021-8979    

DOI:10.1063/1.1658307

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Measurements have been made of the ion species present in the positive column of dc discharges of 0.05% and 0.5% argon in helium, by sampling through a 35‐&mgr;‐diam orifice in the wall of the discharge tube. Data for 0.05% argon has been obtained over a current range of 15–90 mA and for pressures from 3 to 30 Torr. The main ion species observed were Ar+, He+, and He2+. Theoretical analysis combined with measurements of the variation of Ar+with distance from the cathode, enable estimates to be made of the degree of ionization of Ar, and also an upper limit for the helium metastable particle densities present in the discharge. Metastable density values are, in general, an order‐of‐magnitude larger than the electron densities.

ISSN:0021-8979    

DOI:10.1063/1.1658308

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

For planewave propagation in a random continuum, the changes in mean‐square amplitude and phase variations are derived due to the presence in the medium of a large spherical object arbitrary in density and refractive index, and positioned on the transmitter‐to‐receiver line‐of‐sight. The random continuum is taken as statistically homogeneous and isotropic in space with weak, large‐scale refractive‐index variations representable by a Gaussian autocorrelation function. Calculations are performed for far‐field observation of the forward‐scattered fluctuating signal.

ISSN:0021-8979    

DOI:10.1063/1.1658309

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The partial separation of helium and argon in a sample withdrawn through a tube at right angles to a capillary through which the mixture is flowing at high velocity is measured. With a helical capillary the effect is greater by at least a factor of 4 than can be accounted for by the centrifugal field due to the average velocity along the helix, even if no mixing or turbulence is assumed. The effect in a straight capillary is somewhat smaller and is about that expected to be produced by a stable eddy near the entrance to the sampling tube. The helix results depend in sign as well as magnitude on the flow conditions. Separations up to 0.7% are observed. The effect vanishes at small withdrawal rates, indicating an interaction between any possible effect of the helix and the demonstrated effect at the withdrawal hole.

ISSN:0021-8979    

DOI:10.1063/1.1658311

出版商:AIP    

年代:1970

数据来源: AIP