摘要:

A method for the exact determination of orientation from electron diffraction patterns by diffracted beam intensity measurements is presented. The method is applicable to strain‐free specimens and results indicate that the accuracy obtained is considerably better than previous techniques.

ISSN:0021-8979    

DOI:10.1063/1.1702963

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Initial investigations on the room‐temperature acoustic attenuation in sapphire and rutile atSband, which were reported earlier, showed the attenuation in both of these materials to be relatively low. These measurements have now been extended to other frequencies, and curves of attenuation are shown for rutile shear waves and sapphire longitudinal waves covering the frequency range fromLband toXband, and for sapphire shear waves fromLband toSband. The attenuation has a nearly linear variation with frequency at the lower frequencies, and increases monotonically as frequency increases. This behavior may be attributable to Rayleigh scattering from submicron scattering centers within the crystals. Nickel‐film magnetostrictive transducers, coupled to dielectric resonators, were used to generate the acoustic waves in the samples under study, and the acoustic attenuation was determined from echo measurements. Conversion efficiencies and certain other observed properties of nickel‐film transducers over the above frequency ranges are also discussed.

ISSN:0021-8979    

DOI:10.1063/1.1702964

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

An electron beam at a few kilovolts is used to generate a plasma in helium or argon at 10−4to 10−2Torr in a glass tube aligned with an axial uniform magnetic field of 0.3 to 2 kG. At and above a critical beam current and gas pressure, the discharge changes from a glow region the size of the beam to a glow discharge filling or nearly filling the glass tube (total glow). Measurements of the plasma density, the velocity distribution of the beam, and the spectrum of the rf noise generated in the plasma as functions of beam current, voltage, and gas pressure have been made. These data are consistent with an explanation of the effect involving a convective forward‐wave beam‐plasma oscillation which has a starting current equal to the measured critical current for the transition to total glow and which is determined by the net gain of the system including reflections at the ends. The oscillation, once started, is not a coherent oscillation, but can have a very wide noise bandwidth extending essentially from zero frequency to near the electron plasma frequency.

ISSN:0021-8979    

DOI:10.1063/1.1702965

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Rutile crystals of controlled composition have been grown from Na2O·3B2O3and K2O·3B2O3fluxes. The compositions were controlled by the use of various atmospheres of known oxygen activity as the equilibrium vapor phase. The crystals are prisms 2 to 10 mm in length and 0.1 to 0.3 mm in thickness. The morphology of the crystals and their response to changes in temperature, oxygen activity, and flux composition are described. The average dislocation density is 4×104cm−2, but dislocation‐free areas can be found.

ISSN:0021-8979    

DOI:10.1063/1.1702966

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The theory of second‐harmonic generation of light was examined with particular emphasis on the saturation effects occurring at high laser power levels. The nonlinear coupled Maxwell's equations were solved approximately to yield solutions of simple form. The effect of beam divergence was taken into account by averaging the solution over the entire angular spread of the laser beam. The second harmonic thus generated is seen to be proportional to the32power of the laser power independently of the length of the crystal in long crystals. Experimental data in good agreement with the theory are presented.

ISSN:0021-8979    

DOI:10.1063/1.1702967

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The distribution of column lengths,p(n)/Nc, in a strain‐free powder sample is given by the relationshipp(n)/Nc=k(An+1−2An+An−1)/(A1−Ak+1)among the Fourier coefficients of the x‐ray powder diffraction line shape under the assumption that the total number of columns of length 1, 2, …,kunit cells is negligibly small. This eliminates the necessity for use of the experimentally imperfect coefficientA0and is equivalent to Warren's method for elimination of the ``hook effect.'' It is also shown that the technique of Warren and Averbach is valid for either narrow or broad diffraction lines if the crystallite size distribution is treated as a function of a discrete variable.

ISSN:0021-8979    

DOI:10.1063/1.1702968

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

A theory of diffusion‐controlled dislocation damping is developed by considering the motion of pinning points within the dislocation core when an oscillating driving stress is applied to a crystal. At low temperatures, the pinning points are immobile and the result is equivalent to that of the Granato‐Lu¨cke theory of resonant dislocation damping. At higher temperatures, however, the pinning points migrate under the influence of an externally applied stress &sgr;, if &sgr; exceeds a critical value &sgr;c, and the resonant‐type loss becomes time dependent. When &sgr; is reduced below &sgr;c, the pinning points thermally diffuse to their original equilibrium positions and the time‐dependent damping, characterized by a relaxation time &tgr;(0), nearly decays exponentially. In addition, a relaxation‐type loss is predicted when the applied frequency approximately equals &tgr;−1(0). The analysis relates &tgr;(0) to the ``pipe'' diffusion coefficient of the pinning speciesDpby the expression &tgr;(0)=2l02/Dp, wherel0is the average dislocation loop length, and procedures are suggested that allow the determination ofDpfrom dislocation damping measurements.

ISSN:0021-8979    

DOI:10.1063/1.1702969

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

A regular hydromagnetic oscillation in a simple toroidal discharge has been found for the first time. The externally applied longitudinal magnetic field and high degree of ionization are essential to generation of the oscillation. The observation by magnetic probes and streak camera shows that a plasma column of them=1 helix rotates in the direction of right‐handed screw to the applied magnetic field. The wavelength of helix and rotational frequency depend on the pressure, ion mass, and applied magnetic field. The phase velocity of the plasma helix coincides with the Alfve´n velocity.

ISSN:0021-8979    

DOI:10.1063/1.1702970

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The electrical resistivity of carbon resistor material has been experimentally determined as a function of temperature and has been used to calculate the static response characteristics of a particular carbon bolometer element without making use of the usual isothermality and constant thermal conductivity assumptions.

ISSN:0021-8979    

DOI:10.1063/1.1702971

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Excitation of ruby lasers by means of double‐pumped high‐intensity helical flashtubes of approximately 10−4sec flash duration is described. Electrical discharge parameters for double‐pulse operation of the helical flashtubes were correlated experimentally and theoretically with maximum loading possible for such tubes. Time‐resolved spectra of such discharges were obtained and plasma temperatures were computed from the xenon continuum. Temperatures thus computed varied from 1 to 2 eV and were correlated with peak discharge currents. The laser energy obtained from such excitation of rubies was found to be the same as that from normal (1 msec or longer) discharges having the same pump energy, but the power in the laser beam was proportionately increased with the decrease in the excitation time.

ISSN:0021-8979    

DOI:10.1063/1.1702972

出版商:AIP    

年代:1965

数据来源: AIP