Volume 36, Issue 3, 01 March 1965

Saturation Ion Currents to Langmuir Probes
View Description Hide DescriptionThe parabolic variation of saturation ion current with probe potential observed in dense plasmas is fortuitous and is not directly related to the effects of orbital motion. Agreement between measured and computed saturation ion characteristics is illustrated. The discussion is in the framework of collisionless, magnetic‐field‐free theories; they apply to the experiments only if the ion Larmor radius is much larger than the probe radius.

Magnetization of Polycrystalline EuS as Determined by the Pyromagnetic Method
View Description Hide DescriptionMeasurements of magnetization of polycrystalline EuS powder are obtained by the pyromagnetic method between 4.2° and 60°K in fixed applied fields ranging from 4 to 14 kOe. For fields sufficient to saturate (H _{0}≥12kOe), the data, as analyzed by Weiss molecular field theory with a new graphical technique, specify T_{c} =16.5°±0.2°K, in good agreement with the specific heat anomaly at 16.2°K. The observed behavior of the approach to saturation suggests a preferred choice of mechanisms for the origin of the large saturation field.

Electroluminescence and Lasing Action in GaAs_{ x }P_{1−x }
View Description Hide DescriptionThe electroluminescence of Zn diffused diodes was studied in forward and reverse bias. Emission spectra obtained for forward biasing showed a near‐edge as well as a low‐energy line. In the composition range x>0.55, where the lowest conduction band minimum is at k=0, most photons are emitted in the near‐edge line. Lasing action could be observed in this range, and the shortest wavelength where stimulated emission was obtained at 77°K was 6380 Å. For x<0.55, where the lowest conduction band minimum is the (100) minimum, the low‐energy emission becomes dominant. The energy separation between near‐edge and low‐energy line varies between 0.4 and 0.47 eV at 77°K depending on composition. This suggests that the same defect is responsible for the low‐energy emission in all these diodes (including GaAs and GaP). In reverse bias, near‐edge emission and emission at higher energies than bandgap could be observed for x<0.55. The cutoff energy of the emission on the high‐energy side was measured as a function of composition. It was found to vary in a manner similar to the variation of the energy of the (000) conduction band minimum.
The external quantum efficiency of the forward bias emission drops sharply by two orders of magnitude at the composition x=0.55 when x is decreased. The efficiency of the reverse bias emission is independent of composition.

Transversely Pumped Magnetoelastic Instabilities
View Description Hide DescriptionA detailed theory is developed for first‐order transverse pumping mechanisms arising from terms in the magnetocrystalline energy which are linear in strain. A physical description of these mechanisms is given in terms of direct and indirect interactions among the particle displacement and magnetization variables, and the instabilities of the medium are classified as direct or indirect according to the mechanisms involved. Numerical values of the thresholds are calculated as a function of propagation angle for longitudinal magnetoelastic instabilities in YIG pumped at 1000 Mc/sec. It is shown that two magnetoelastic instabilities have thresholds which are substantially lower than the second‐order spin‐wave instability threshold. The implications of this result relative to the parametric amplification of elastic waves and the interpretation of certain anomalous saturation effects in ferrites are discussed.

X‐Ray Yield and Line/Background Ratios for Electron Excitation
View Description Hide DescriptionX‐ray yield in photons/sr/electron was measured for the Kα lines of Ti, Cr, Mn, Fe, Cu,Zn,Ge, Zr, and the Lα lines of Ta and Au. Incident electron energy was varied from 11 to 38 keV, and for some elements the take‐off angle for emerging x rays was varied from 6 to 45 deg. Theoretical yield values calculated after the method of Metchnik and Tomlin showed an average deviation of about 30% from the measured values, a very satisfactory agreement.
Line/background ratios were also measured for the same elements and conditions. Normalization to the natural x‐ray line breadths gave ratios between 100/1 and 26 000/1 depending on the element and experimental conditions. Line/background ratios are consistently larger at low take‐off angles indicating that the characteristic radiation must be generated relatively closer to the surface than the continuum of the same wavelength.

Investigation of Hot Opaque Arc Plasmas by Microwave Cavity Techniques
View Description Hide DescriptionThe behavior of a steady‐state argon arc plasma flowing through a cold 1‐cm circular copper pipe at pressures from 0.1 to 1 atm and gas flow rate of 2600 ml/min is studied by microwave cavity techniques. The high electron density (n_{e} >10^{16} cm^{−3}) and large density gradients in the arc preclude the usual simple interpretation of data. At this density it is however possible to interpret the cavity resonant frequency as a measure of the ``diameter'' of the arc column. Experimental results show the dependence of this diameter on pressure, arc current, and the presence of contaminant gases in concentrations as small as 10 ppm. A simple explanation of the influence of diatomic contaminants is given which attributes column contraction to enhanced thermal conductivity brought about by diffusion of dissociation energy. Dilation of column caused by krypton is attributed to enhanced electron concentration in the cooler regions of the plasma due to krypton's lower ionization potential. Calculations of thermal conductivity of argon‐contaminant mixtures are shown to correlate well with the experimental results. The microwave technique appears in a new role as monitor and judge of the plasma behavior.

Deactivation of Neon Metastables by Hydrogenic Impurities
View Description Hide DescriptionMeasurements are reported of the total cross section for deactivation of neon ^{3} P _{2} and ^{3} P _{0}(2p ^{5}3s) metastables by inelastic collisions with H_{2}, D_{2}, CH_{4}, and NH_{3}. Observations were conducted by applying the methods of resonance radiation absorption and microwavediagnostics to the afterglow plasma following a weak pulsed discharge in a neon‐hydrogenic impurity mixture. The cross section for ^{3} P _{2} deactivation by H_{2} was found to increase from 0.7×10^{−15} cm^{2} at 300°K to 2.5×10^{−15} cm^{2} at 650°K. The cross section for ionization of H_{2} by Ne*^{3} P _{2}collision was found to represent only a fraction of the above. The reaction which dissociates H_{2} and partly converts the metastable energy into Hα(6563 Å) light has been observed in H_{2}, D_{2}, CH_{4}, and NH_{3} and may represent a significant contribution to the total deactivation in each case. No Hα light was produced as a result of the destruction of a helium ^{3} S _{1} metastable by collision with H_{2}.

Stabilization of the He–Ne Maser on the Atomic Line Center
View Description Hide DescriptionApplication of an automatic feedback control for long‐term frequency stabilization of a He–Ne optical maser at the center frequency of the atomic resonance is described. The length of the Fabry‐Perot resonator is modulated at a low frequency with small amplitude. The fundamental, second harmonic, and third harmonic components of this modulation frequency in the light output of the maser are used to control the tilt angles of the mirrors, the radio‐frequency excitation power input, and the separation of the mirrors, respectively. The frequency stability is determined by observing the photobeat between the outputs of two independently stabilized masers. Isotopically enriched samples of Ne are used in the two masers with one of them enriched in ^{20}Ne and the other in ^{22}Ne. The optical output of a maser stabilized in this manner is frequency modulated and the center frequency has been found to be stable to several parts in 10^{10} over very long periods.

Multiple Quantum Transition and Raman Action in Three‐Level Systems
View Description Hide DescriptionA theory is presented of induced Raman emission in a three‐level system whose parameters are appropriate for microwave to optical frequencies. Solutions of the time‐dependent Schrödinger equation are obtained for arbitrary intensity of the pump and signal radiations, which are assumed to be not too many line‐widths off resonance. It is shown how the quantum‐mechanical aspects of the problem may be separated from the statistical‐mechanical aspects, in contrast to the density matrix techniques. The frequency dependence of the Raman emission on the frequency and intensity of the pumping radiation obtained by this method is presented and compared with the results we have obtained by other methods. The intensity and line shape of the Raman emission when the pump frequency is far off resonance are also obtained. The case of three interacting fields is also considered.

Standing Spin Wave Mode Spectra in Thin Permalloy Films Prepared in Vacuum of 10^{−7} Torr
View Description Hide DescriptionThe spacings between the standing spin wave modes excited in Permalloyfilms, prepared in a high‐vacuum system at pressures near 10^{−7} Torr, are found to obey a quadratic dependence on order number for the magnetic field normal to the plane of the film (H _{⊥}) as well as in the plane of the film (H _{II}). The spacings and intensities of the modes, as a function of the angle φ which the magnetic field makes with respect to the film normal, indicate that the modes are nearly unpinned for φ_{crit}>φ≥90° (H _{II}), are completely unpinned at φ_{crit}, and partially pinned for φ_{crit}>φ≥0° (H _{⊥}). This type of pinning behavior is explained qualitatively by assuming that the films are nearly uniformly magnetized except for a very thin surface layer, of the order of ten lattice spacings, which exhibits a lower magnetization than the bulk. In addition, the observed pinning for the modes suggests that the boundary condition imposed on the transverse component of the magnetization at an air‐ferromagnetic interface is that the normal derivative of the magnetization vanish.
It is concluded that ferromagneticfilms prepared in high vacuum should be suitable for the determination of physical parameters, such as A/M, the ratio of exchange constant to the saturationmagnetization, as their spectra are relatively free from the anomalous features observed in all of the previously reported SSWM investigations. Experimental evidence is presented that these anomalous features are probably due to contamination of the films by both residual gases in the evaporation chamber and by impurities introduced by the vacuum system itself.

Mechanism for the Shock Polarization of Dielectrics
View Description Hide DescriptionA new type of electromechanical coupling is postulated to explain the polarization signals resulting from the shock loading of nonpiezoelectric materials. The driving force is viewed as being proportional to the gradient of the stress. The electromechanical coupling constant is found for an ionic solid by deriving the effect for a linear ionic chain. A stress gradient of 10^{11} dyn/cm^{3} is found to give rise to a voltage gradient of 10 V/cm.

Resistivity of Xenon Plasma
View Description Hide DescriptionBy measuring voltage drop in xenon flashtubes the resistivity of intense xenon plasmas is found to bear an inverse square root relation to the current density. A weak dependence on pressure is discussed.

Ultraviolet Light Emission from Townsend Discharges in Hydrogen
View Description Hide DescriptionThe total ultraviolet radiation from self‐sustained Townsend discharges in hydrogen has been measured by means of a sodium salicylate wavelength shifter and a photomultiplier tube. The excitation coefficient per unit pressure δ_{0}/p has been determined for E/p=80−250 V/Torr‐cm. δ_{0}/p as a function of E/p passes through a maximum for E/p≈150 V/Torr‐cm. The pressure dependence of the radiation can be ascribed to quenching with a quenching pressure equal to 20 Torr. An attempt has been made to evaluate separately the excitation coefficient for the 2^{1}π_{ u }radiation by subtracting other uv contributions from the total light intensities observed.

Transient Motion of Space Charge through a Neutral Gas
View Description Hide DescriptionThe history of motion of charge carriersgenerated in a gap by ionizing radiation is investigated. The study is limited to cases where the field‐induced motion of these carriers overcomes their inertia and diffusion; the motion is thus completely collision‐dominated. The severity of the space‐charge effect is described by a single nondimensional parameter γ. When γ is not much smaller than unity, usual linearization procedures are inapplicable. The nonlinear problem is solved for two limiting cases. In the first, the electric current is carried by the free electrons and ions as originally created. In the second, the electric current is carried by positive and negative ions, the latter formed by electron attachment. The method of characteristics is employed to solve the equations of motion and exact analytical and numerical solutions are presented. It is shown that if γ>2 the nonlinear effects of the induced charges are important and depart appreciably from calculations based on linear approximations.

Modes of the Screw Instability in a Positive Column with Longitudinal Magnetic Field
View Description Hide DescriptionThe modes of screw instability in a positive column of a nitrogen gas discharge have been studied. Under a certain condition, the critical magnetic field for the m=2 mode has been found to be smaller than that for the m=1 mode. In addition, the regular oscillations have been observed at the magnetic field larger than the critical one. These phenomena can not be described by the results of Kadomtsev and Nedospasov which are calculated from the small‐amplitude theory. In this paper a theoretical explanation for these phenomena is proposed based on the conception that only the instability which has the maximum growth rate can grow while other instabilities are masked.

Pure Modes for Elastic Waves in Crystals
View Description Hide DescriptionIn an anisotropic medium there are only certain directions along which elastic waves can propagate in pure longitudinal and transverse modes. For the determination of third‐order elastic coefficients from sound speed measurements in stressed crystals it is desirable to know these modes. Using a method due to Borgnis the pure mode directions are determined for all crystal point groups belonging to the orthorhombic, tetragonal, cubic, rhombohedral, and hexagonal systems. The eigenvalue problem is solved for each of these directions, and the polarization vectors and the wave speeds are tabulated.

Determination of Third‐Order Elastic Coefficients in Crystals
View Description Hide DescriptionThird‐order elastic coefficients can be derived from sound speedmeasurements in statically stressed crystals. The necessary relations between experimental data and elastic coefficients are here presented for orthorhombic, tetragonal, cubic, rhombohedral, and hexagonal crystal classes. For each symmetry a sufficient number of wave modes and stress systems is selected to determine all third‐order coefficients and to allow experimental cross checks. All the modes considered are pure longitudinal or transverse. They are selected so as to minimize the number of specimen orientations required, and following experimental procedures for uniaxial static stress, only wave modes propagating at right angles to the stress direction are considered.

Evaluation of Kossel Microdiffraction Procedures: The Cubic Case
View Description Hide DescriptionA complete theory of the lens‐shaped intersection of Kossel lines in the transmission pseudo‐Kossel method is developed. The general expression gives the lattice parameter with respect to the characteristics of the interaction of two arbitrary conics, i.e., conics corresponding to arbitrary diffracting planes and arbitrary wavelengths. Geometrical considerations involving the conics are developed and from these, the errors involved in lattice parameter measurement are evaluated. Lattice parameter variations of 10^{−5} can be easily measured with suitably chosen conic intersections. The precision to be expected as a function of measurement errors is expressed as a sensitivity ratio and tabulated for an Fe‐3wt.%Si alloy. Several methods of reducing measured lengths to lattice parameter data are evaluated.
The IBM 7090 computer has been used to find suitable intersections for precise lattice parameter measurement. The computer supplies the wavelengths and conics to be used in the transmission pseudo‐Kossel method. Consequently, a long and tedious experimental investigation is avoided. Examples using Fe‐3wt.% Si are given; the lattice parameter for this alloy is found to be 2.86268±0.00003 Å. Proper radiations for investigation of Ge and diamond are indicated.

Thermal Phase Transitions in Superconducting Nb‐Zr Alloys
View Description Hide DescriptionExperimental data are presented for the thermal propagation of a normal‐superconducting boundary along wires of Nb‐25%Zr, Nb‐37%Zr, and Nb‐51%Zr. The thermal propagation velocity was measured as a function of current at 4.2°K in constant longitudinal and transverse magnetic fields up to the upper critical field H _{c2} as well as in a zero magnetic field for temperatures between 2.5° and 11°K. Previous models which describe thermal propagation phenomena neglected the temperature dependences of the specific heat, thermal conductivity, and electrical resistivity. A more exact model is proposed here which takes into account the temperature dependences of these quantities, and all the present experimental data are in good agreement with this model. From a measurement at 4.2°K of the dependence of the thermal velocity on magnetic field, the high‐field transition temperatures as a function of magnetic field are calculated for temperatures above 4.2°K. In conjunction with the theory, the linear dependence of the thermal velocity on current implies a weak current dependence for both the transition temperature and superconducting‐state specific heat in Nb‐Zr alloys up to 3×10^{5} A/cm^{2}. A method is discussed for using thermal propagation techniques to determine the superconducting‐state specific heat in the presence of known transport currents and magnetic fields.

Elastic Constants of Cubic Lead Fluoride at Room Temperature
View Description Hide DescriptionUsing an ultrasonic pulse echo technique operating at frequencies ∼10 Mc/sec, we have obtained the following elastic constants for cubic PbF_{2}: C_{11}=8.88×10^{11} dyn/cm^{2}, C _{12}=4.72×10^{11} dyn/cm^{2}, C _{44}=2.454×10^{11} dyn/cm^{2} at 27°C. From these data a Debye θ of 221.2°K was calculated.