Index of content:
Volume 34, Issue 10, 01 October 1963
34(1963); http://dx.doi.org/10.1063/1.1729097View Description Hide Description
A simplified charge control type of analysis is made of the response of photoconducting imaging devices with a floating electrode. The change of electrode potential with discharge of the device is found to have an importance not previously recognized. The results are relevant for a variety of electrostatic photographic devices and show the dependence of the performance on certain details of the image development process.
The floating electrode limits the gain during exposure to light to one charge pair per quantum absorbed. For the Vidicon TV camera picture tube, a detailed study of the process of reading the stored information demonstrates that net gain exceeding unity can be achieved. The upper limit to the gain is the ratio of the time for the scanning beam to traverse a picture element to the transit time of a free carrier through the layer. This conclusion holds when the time required to trap a free carrier exceeds its recombination lifetime. None of the gain‐bandwidth relations yet deduced for photoconductors is applicable to this case. Earlier studies which led to a maximum gain of approximately unity for the Vidicon assumed fast trapping of free carriers.
For the Vidicon, the conditions for gains greater than unity with full frame storage and small lag are developed, including the characteristics of the scanning beam and of the trapping and recombination centers. Suitable targets with only one free carrier should have a low sensitivity photoconductor with a significant density of slow trapping states. Discussion of the similarity between the performance of a homogeneous target with one free carrier and an npn structure shows that the slow trapping states in the uniform layer play the role of the donors and acceptors in the junction device.
34(1963); http://dx.doi.org/10.1063/1.1729098View Description Hide Description
Masers exhibit interesting transient behavior that cannot be completely understood on the basis of the rate equations. The use of the rate equations in most transient analyses is usually justified on a more or less intuitive basis and the implied assumptions are not always clear. In this paper, the macroscopic maser rate equations are derived systematically from the Boltzmann equation for the density matrix of the atomic systems and Maxwell's equations for the radiation fields. When the coherencelinewidth (T 2 −1) of the atomic systems is much larger than the cavity linewidth and the natural linewidth (T 1 −1) of the atomic emission, and with a WKB approximation, in the lowest order of approximation one obtains the two widely used, coupled first‐order nonlinear rate equations of Statz and deMars. On the other hand, if the cavity linewidth is much larger than the atomic linewidths (T 1 −1 and T 2 −1), one can use the so‐called ``reaction‐field principle'' of Anderson and obtain, again, two coupled first‐order rate equations; however, only one of the equations is nonlinear. The ranges of validity of both approaches are discussed in some detail.
34(1963); http://dx.doi.org/10.1063/1.1729099View Description Hide Description
The propagation of finite amplitude electromagnetic waves in a gyromagnetic medium is discussed. It is shown, using the method of characteristics, that in the quasistatic or low frequency limit a wave propagating parallel to the external dc magnetic field distorts continuously until a discontinuity or electromagneticshock wave appears in the wave profile. When higher‐order effects are taken into account, the shock front is found to have a finite width which is reached by a balance between the nonlinear wave‐steepening torque term and the wave‐broadening loss term. The calculated shock‐front width is found to be independent of crystalline anisotropy and exchange and magnetoelasticeffects. The theoretical results are compared with numerical calculations and the possibility of using electromagneticshock waves in pulse sharpening networks and microwave devices is discussed.
34(1963); http://dx.doi.org/10.1063/1.1729100View Description Hide Description
A detailed analysis of space‐charge instabilities in diodes is presented. The oscillation when the input current exceeds the ``limiting input current'' (from an earlier analysis for a short circuited diode with a zero temperature stream) persists with the addition of a diode resistance, or with a two‐velocity input or a Maxwellian velocity distribution input, or with streams of finite diameter. The time‐averaged value of the minimum potential is nearly zero, as in the classical solution; however, the time‐averaged minimum position and transmitted current differ considerably from the classical values. The recovery of the stable state is by a jump rather than gradually as in the classical solution. A small‐signal prediction of time growth of all first‐order variables (potential, current, velocity) at the point of limiting is given. An energy argument using the zero‐order variables is presented showing the onset of instability. Experiments agreed with some parts of the analysis but did not show oscillations with amplitudes as large as expected.
34(1963); http://dx.doi.org/10.1063/1.1729101View Description Hide Description
The pressure shift of both the coherent and the incoherent emission of GaAs junction lasers has been measured at about 200°K. The peak of the spontaneous emission shifts by +1.09×10−5 eV/atm, which is in agreement with the pressure coefficient of the band gap in GaAs determined by experiments based on the change of resistance under pressure. The shift of the coherent modes is much smaller, namely, +2.96×10−6 eV/atm. The effect of the compressibility on the latter shift is shown to be negligible. It is concluded from considerations of a simple model that the shift of the coherent radiation is primarily due to a change of the dielectric constant with pressure.
34(1963); http://dx.doi.org/10.1063/1.1729102View Description Hide Description
Measurements have been made of the electrical conductivities of gaseous mixtures formed by the addition of small fractions of cesium vapor to nitrogen, helium, neon, or argon. The mixtures were maintained near thermal equilibrium at temperatures in the 1500°–2000°K range and a total pressure of one atmosphere. The cesium vapor pressures ranged over two decades, from 0.1 to 10 Torr.
The apparatus consists, in essence, of two heated zones connected by a slow flow. The first zone is a low‐temperature (200°–400°C) oven where the body‐gas flow picks up the cesium vapor. The second zone is a small electrically heated furnace (1250°–1850°C) containing a diode test section.
The principal measurements taken were the seeding temperature,furnacetemperature, and voltages and currents in the test section.
The results exhibit variations with temperature, seeding pressure, and gas species that correlate reasonably well with simple theory and values for electron collision frequencies and cross sections taken from the literature.
34(1963); http://dx.doi.org/10.1063/1.1729103View Description Hide Description
Small differences between the emission and absorption spectra of three‐level maser centers can have a pronounced effect on maser operation. This paper considers the effects of a Stokes shift, the simplest and most probable difference. If there is a Stokes shift which is large compared to the linewidth, the operation of the maser will be essentially four level. If there is a small Stokes shift (some fraction of the linewidth) the operation will still be three level, but the Stokes shift will significantly affect the operation of the maser. The principal effects will be a shift of the maser line from the peak in the fluorescent emission line (away from the absorption peak) and a reduction in the upper level population required to reach the threshold of oscillation.
The evidence is not conclusive as to whether there is a Stokes shift in the rubyR 1 line, but if there is a shift it is at most a small fraction of the fluorescent linewidth at room temperature. Shifts of the rubymaser line to the red have been observed by D'Haenens and Asawa and have been attributed to large temperature changes. It is likely that the temperature shift, while important, will not explain the shift which has been observed. Gires and Mayer have found the lower state population of an operating rubymaser to be approximately 75% of the unexcited population rather than approximately 50% and have suggested a Stokes shift as a possible cause of the reduction in the upper state population at threshold. This paper considers the effect of such a small Stokes shift on the operation of an otherwise ideal three‐level maser.
34(1963); http://dx.doi.org/10.1063/1.1729104View Description Hide Description
The disk model of an electron beam, generalized to allow a distribution of dc velocities, leads to a set of equations that include space charge and accurately describe the nonlinear behavior of a multivelocity electron beam in a drift region. Difference equations are obtained which are suitable for solution on a digital computer. Excellent agreement is obtained at small‐signal levels with linearized theory. Available for the first time as a result of the validity of the present analysis for nonlinear conditions are: (1) harmonic behavior at small‐signal levels and (2) fundamental and harmonic behavior at large‐signal levels.
Nonlinear rf Behavior of Electron Beams with Velocity Distribution. II. Application to Rectangular Velocity Distribution34(1963); http://dx.doi.org/10.1063/1.1729105View Description Hide Description
The nonlinear disk electron analysis in Part I is applied to a practical example. Two changes are imposed on the analysis to decrease the computer running time. Electron trajectories and the first three components of current are calculated for a beam with a rectangular velocity distribution that is simulated by eight dc velocity classes. Excellent agreement is obtained with a linear theory for small values of initial density modulation. Periodic variation of the space‐charge parameter leads to the equivalent of scalloped beam amplification in a multivelocity beam.
A linear force‐taper correction is derived and its stabilizing effect is shown. Its use is suggested for improving the accuracy of monovelocity disk electron calculations.
The interpretation of space‐charge wave decay in multivelocity electron beams as spatial Landau damping of the fast wave is questioned. It is suggested that damping is due to multidimensional effects not taken into account by present linear and nonlinear one‐dimensional theories.
34(1963); http://dx.doi.org/10.1063/1.1729106View Description Hide Description
The ultrasonic‐diffraction effect was utilized as a shutter to gate and amplitude modulate the output of a ruby optical maser (laser) by regulating the optical feedback within the Fabry‐Perot interferometer. In addition, an oscillation resembling limit cycle operation was induced in the ruby laser by an ultrasonic‐progressive wave that periodically changed the optical path length between the interferometer reflectors by varying the refractive index.
34(1963); http://dx.doi.org/10.1063/1.1729107View Description Hide Description
Time‐resolved optical studies were made of the cathode‐fall region of a wall‐bounded pulsed discharge in hydrogen. The temporal variation and spatial configuration of the electric fields within the cathode‐fall region are presented as a function of the experimental parameters. The short cathode‐fall region formed within 0.1 μsec and remained stable for the duration of the applied‐voltage pulse. This rapid formation requires that the positive‐ion space‐charge cloud near the cathode face be formed locally. It is speculated that a fast initial breakdown wave leaves behind a low‐density plasma, which then goes into the equilibrium configuration by an over‐damped plasma oscillation mode.
34(1963); http://dx.doi.org/10.1063/1.1729108View Description Hide Description
The stability of one of the low‐frequency plasma oscillation modes is examined from the standpoint of two‐fluid theory. It is found that when the scattering time is short enough, a density perturbation may actually grow at the frequency of one of the acoustic modes. In this short scattering time regime, the phase difference between the electron and hole perturbation velocities can lead to a net growth of the perturbation. If the damping effects of diffusion and recombination of excess carriers can be overcome by the growing mechanism, the net result would be a growing instability. The results presented are applicable to both gaseous and solid‐state plasmas. Some discussion is given on which solid‐state materials might exhibit this new instability. It is found that InSb is best, but that bismuth might also be suitable.
34(1963); http://dx.doi.org/10.1063/1.1729109View Description Hide Description
A derivation is given for the linewidth of an optical maser at steady power, taking into account its peculiar physical configuration and mode structure. Two limiting cases are considered: one in which the number of back and forth reflections between the end plates is not sufficient to establish a mode in the sense of Fox and Li, the other in which such a mode is established. The linewidth for the latter case may be interpreted as the ``theoretical lower limit.'' It is pointed out, however, that this limit could not be attained even if all other sources of noise in the maser system could be eliminated. General conditions are also developed for the oscillation of a system consisting of any combination of laser and mirror elements.
A Correlation of Mechanical Properties and Microstructure of Polytetrafluoroethylene at Various Temperatures34(1963); http://dx.doi.org/10.1063/1.1729110View Description Hide Description
Mechanical properties and microstructure of PTFE at various temperatures were correlated through the effects of temperature and band size on the flow stress and microstructure deformation. Electron microscopic examination of deformed microstructure showed that at low temperatures the crystalline deformation modes (kinking and bowing) were favored, whereas noncrystalline modes (sliding and rotation) were predominant at high temperatures. At low temperatures PTFE showed a crystalline size effect and stress drop phenomenon in its flow‐stress relations not manifested at high temperatures.
34(1963); http://dx.doi.org/10.1063/1.1729111View Description Hide Description
34(1963); http://dx.doi.org/10.1063/1.1729112View Description Hide Description
Radioactive, cylindrical, steel projectiles were explosively accelerated into plates of stainless steel and aluminum. The amounts of radioisotopes Fe59 and Cr51 in the projectile prior to acceleration and in the crater in plate were measured in order to determine the fraction of the original projectile remaining in the crater.
Impact experiments with steel plates showed that 3.4 to 7.8% of the original projectile remained in the crater of the steel plate; 3.6 to 7.5% of the original projectile remained in the crater of the aluminum plate. Data on any given crater, obtained by the measurement of both Fe59 and Cr51, are in good concordance. The fraction of the projectile remaining in the crater is independent of the impact velocity over the range of 12 000 to 14 900 ft/sec (3600 to 4540 m/sec).
34(1963); http://dx.doi.org/10.1063/1.1729113View Description Hide Description
The electrical conductance was measured during vacuum deposition of tin and leadfilms to gain information concerning the scattering of conduction electrons at the film boundaries and the diffusion of atoms on the substrate surface during the initial stages of growth. Deposition rates varied from hundredths of angstroms per second to a hundred per second on substrates held at 300°, 255°, 195°, and 77°K and at pressures no greater than 10−6 Torr. Generally, the initial portions of the conductance curves suggest formation of larger grains at higher substrate temperatures or at lower deposition rates. In the latter case, grains again become small when the rates of arrival of residual gas atoms and vapor atoms are comparable. From the thickness for onset of electrical conduction, activation energies for the agglomeration of tin and leadfilms were calculated. From earlier measurements by other investigators the same quantity was calculated for mercury and copper. There is good correlation between this activation energy and heats of vaporization. Although no detailed theoretical model can be offered, the experimental values appear to be compatible with activation energies for surface diffusion. For films much thicker than the electron mean free path, and for filmsdeposited at rates greater than a few angstroms per second so that residual gases do not play a predominant role, surface scattering of the conduction electrons is found to be more spectral the higher the substrate temperature during deposition or the lower the deposition rate. For filmsdeposited or annealed at 300°K the electron scattering is almost entirely spectral. Interpretation is aided by electron microscope examination of the films.
34(1963); http://dx.doi.org/10.1063/1.1729114View Description Hide Description
A calculation is presented of the generation of microwaveelastic waves by the uniform precession resonance in ferromagnets. The excitation of the thickness shear modes in a normally magnetizedferromagnetic disk is treated. For uniform precession the elastic modes are driven only on the surfaces of the disk. The effects of long wavelength spin wavesmagnetostrictively driven by the elastic waves can be included in the analysis by a change in the elastic modulus. The coupling out of the elastic energy by bonding the disk to a non‐magnetic delay medium is considered for arbitrary values of the elastic impedances of the two media. An equivalent electrical circuit for transducers is presented; the efficiency and bandwidth of disk transducers are calculated for a disk placed in a matched one port resonant cavity. A figure of merit for such transducers is defined, and calculated for yttrium iron garnet. Due to interference between the elastic waves generated on the two disk surfaces, the realizable bandwidth of the transducer is restricted by the thickness of the disk. By bonding two disks together or by using a rod, this restriction may be reduced.
34(1963); http://dx.doi.org/10.1063/1.1729115View Description Hide Description
The effects of precipitation from solid solution on the superconducting properties of lead‐tin and lead‐cadmium alloys have been studied by magnetization measurements at 4.2°K. Microstructural changes have been followed by optical and electron microscopy. These alloys as‐quenched are superconductors of the second kind, their upper critical fields enhanced by quenched‐in solute. At various stages in the precipitation process, the upper critical field continues to be determined primarily by the remanent solute. Magnetic hysteresis and trapped flux result from the interaction between precipitates and the flux filaments of the mixed state, a given precipitate distribution causing much more flux trapping if the matrix is a superconductor of the second kind than if it is a superconductor of the first kind. Several instances are seen where superconductingmagnetization measurements yield useful metallurgical information.
34(1963); http://dx.doi.org/10.1063/1.1729116View Description Hide Description
The generation of second harmonics has been observed in a low‐field nuclear‐induction experiment with nearly coincident, constant and alternating, magnetic fields. A pair of Helmholtz coils furnishes both a constant (H 0) and an alternating (amplitude ηH 0) magnetic field, and a second Helmholtz pair furnishes a constant, small magnitude (hH 0) field perpendicular to the first pair. Steady‐state solutions of the modified Bloch equations (relaxation to the instantaneous field) that utilize the indicated field configuration (obtained by both analytic and numerical methods) indicate first, the existence of resonance when ω R , the alternating field frequency, is a subharmonic of the gyromagnetic frequency ω0(≡γH 0), and, second, relatively slow variation of the magnetic‐moment components as functions of η. Analysis and experiment indicate saturation of the signal with increasing h. The nuclear sample, water with a heavy concentration of Cu(NO3)2·3H2O, about 3 cm in diameter, was closely surrounded by a 1000‐turn pickup coil the output of which was fed to a narrowband wave analyzer tuned to 2ω R . With H 0=6.11 G, η=2, and h=0.013, the predicted and observed second‐harmonic (52 kc/sec) voltage was 0.1 μV (rms).