ISSN:0021-8979    

DOI:10.1063/1.1721843

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The Cˇerenkov radiation emitted by a bunched electron beam passing along a dielectric material at a close distance is calculated for a flat beam near a plane surface, a flat beam between two plane surfaces and for a circularly cylindrical beam within a cylindrical hole.

ISSN:0021-8979    

DOI:10.1063/1.1721856

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

It is proposed that magnetization reversal in polycrystalline ferro‐ and ferrimagnetic materials is primarily due to the nucleation and growth of 180° Bloch walls. The origin of domains of reverse magnetization is discussed. The rate of growth of these domains is determined by a study of the elastic and frictional forces which retard the motion of their 180°‐Bloch‐wall boundaries. This theoretical model successfully explains the output‐voltage wave forms of polycrystalline materials. A figure of merit for the magnetization reversal of magnetic cores is defined as the switching coefficientSw=(Hm−H0)&tgr;, where &tgr; is the time required to reverse the magnetization,Hmis the applied magnetic field, andH0is the threshold field value at which the average domain‐wall velocity is zero.Swis composed of an eddy‐current contributionSweand a spin‐relaxation contributionSwr. The value ofSwis derived in terms of various fundamental parameters of the material. It is shown that in ferrites and ultra‐thin metal tapes,Swe«Swr. Theoretical relationships expressing the contributions of spin‐relaxation, eddy‐current, and hysteresis effects to energy losses are derived on the basis of this model. A study of the factors which affect the magnetization‐reversal time of materials with square hysteresis loops indicates that a better figure of merit will result if the proper hysteresis shape is obtained by grain alignment rather than by the techniques currently employed in ferrites. A number of experiments are presented in support of this model.

ISSN:0021-8979    

DOI:10.1063/1.1721867

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

Shadow pictures of turbulent flow regions give a particular kind of information about the three‐dimensional density field. If homogeneity and isotropy of the fluctuation field can be assumed, the statistical properties of the picture uniquely determine the correlation function or power spectrum of the three dimensional turbulent density field. Shadow pictures obtained in the wake of a supersonic projectile were analyzed in this manner and encouraging results were obtained. In order to check the validity of the method, the density fluctuations in a heated jet were measured both by the shadow method and by the more conventional hot‐wire technique.

ISSN:0021-8979    

DOI:10.1063/1.1721855

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The fundamental scheme of formulation of the amount of metal transfer and wear is based upon the theory suggested in the previous paper [J. Appl. Phys.23, 1011–1019 (1953)]. The basic terms used in formulating metal transfer and wear are (1) the potential amount of metal transferMt*; (2) the mechanical factor of the chance of metal transfer &phgr;m; (3) the thermal factor of the chance of metal transfer &psgr;&thgr;; and (4) the adhesion factor of the chance of metal transfer Xa. The expression for the amount of metal transferMtfrom one specimen to another isMt=Mt*&phgr;m&psgr;&thgr;, when &psgr;&thgr; is bigger than Xa, and isMt=Mt*&phgr;mXawhen Xais bigger than &psgr;&thgr;. The equation for the amount of wearMwrepresented by the weight loss of a specimen isMw=Mt*&phgr;m(1‐&psgr;&thgr;), when &psgr;&thgr; is bigger than Xa, and isMw=Mt*&phgr;m(1‐Xa), when Xais bigger than &psgr;&thgr;.

ISSN:0021-8979    

DOI:10.1063/1.1721857

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The discussion in this paper is limited to the case where the surface layer is thinner than the average depth of roughening. The surrounding atmosphere influences the adhesion factor Xathrough its effect on adhesion that depends on such factors as the nature and thickness of the surface layer and also the degree of plastic roughening. The influence of the surface layer on the thermal factor &psgr;&thgr; is similar to its influence on Xa. Because of the lack of the quantitative knowledge about its effect on adhesion and diffusion, the effect of surrounding atmosphere on metal transfer and wear is discussed qualitatively. The dependence upon such factors as the temperature, gas pressure, time of exposure, etc., of the effect of physically adsorbed and chemical‐compound surface layers on metal transfer and wear is also discussed in this paper.

ISSN:0021-8979    

DOI:10.1063/1.1721858

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

This paper describes a structure, namely, a cross‐wound twin helix which overcomes the disadvantages of a conventional helix for high‐voltage traveling‐wave tubes. The disadvantages of a single helix suitable for high voltages are: (1) the impedance for electron interaction is reduced because of the energy content of the noninteracting space harmonics, and (2) the high impedance of some of the space harmonics can result in backward‐wave oscillation. In a structure consisting of two helices wound in opposite directions, the symmetry of the fields results in most of the electric energy being stored in the fundamental component and most of the magnetic energy in the space harmonics. This results in a higher impedance for the fundamental component and a reduced impedance for the space harmonics. Typical numbers for dimensions suitable for 10‐kv operation are an increase of a factor of 2 in the fundamental impedance and a reduction of a factor of about 20 in the −1 space harmonic for the twin helix as compared to the single helix.

ISSN:0021-8979    

DOI:10.1063/1.1721859

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

Rates of evaporation of Ba and Sr from oxide cathodes in a practical tube structure were measured during life tests up to 20 000 hours. The effects on evaporation rates are shown for (a) impurity in the Ni support, (b) cathode temperature, and (c) space current. We conclude:1. The product evaporating from commercial type cathodes under normal conditions is largely Ba metal; less than 5 percent is Sr; less than 2 percent is BaO; and less than 0.01 percent is SrO.2. Chemical reducing agents in the support metal‡ are important in determining the rate of Ba evaporating during life.3. A factor not closely controlled in the present experiments—anode and grid composition—affects the rate markedly. This is especially true for the evolution of Ba during exhaust.4. Within the precision of these experiments, there is (a) no correlation between the rate of Ba evaporation and thermionic activity of individual cathodes, and (b) no effect of space current on the rate of Ba evaporation.

ISSN:0021-8979    

DOI:10.1063/1.1721860

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

A detailed treatment of reactor kinetics is given. The kinetic equations are derived from a diffusion‐Fermi age model. An iterative technique and a matrix method are discussed as possibilities for obtaining solutions. The relationship between the solutions with and without delayed neutrons is pointed out. The concept of stability is discussed for the system which includes the delayed neutrons. The similarity of the treatment here to that used in other branches of mathematical physics (especially quantum mechanics) is noted.

ISSN:0021-8979    

DOI:10.1063/1.1721861

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

Middleton and Sugar have shown that the scatter diagram of two random electrical quantities may be oscilloscopically displayed, and that a simple relation exists between the resulting pattern and the correlation coefficient, if a Gaussian distribution is assumed. The present paper shows that qualitative information concerning the correlation coefficient may be found from the scatter diagram, no matter what distribution is assumed.

ISSN:0021-8979    

DOI:10.1063/1.1721862

出版商:AIP    

年代:1955

数据来源: AIP