Index of content:
Volume 35, Issue 8, 01 August 1964
35(1964); http://dx.doi.org/10.1063/1.1702847View Description Hide Description
Large‐amplitude solutions of the rate equations for the ruby laser have been obtained corresponding to the initial low value of photon density. The damping rate for these oscillations increases with optical pump rate in contrast to the decrease found experimentally by Walsh and Kemeny.
Variation of the damping rate with loss rate is also examined; the damping rate becomes the same order as that of the usual flash lamp (∼103 sec−1) for a loss rate corresponding to a rod.
35(1964); http://dx.doi.org/10.1063/1.1702848View Description Hide Description
Measurements have been made with a Geiger counter of photo‐exoemission from aluminum. The effects of time, temperature, and various adsorbates on the electron emission rate were measured.Adsorption reduces the electron emission rate; ion bombardment within the counter is shown to remove the effects of adsorbed gases. Adsorption of atmospheric gases can explain previously reported decay in emission and temperature effects.
35(1964); http://dx.doi.org/10.1063/1.1702849View Description Hide Description
This paper develops a procedure for the optimum detection of a deterministic, finite length signal obscured by Gaussian‐Markovian noise that approximates sea noise by employing the concept of a Likelihood Ratio. For the particular noise model, the optimum procedure is to use a cross‐correlation detector. The variables fed to the correlation device are additive combinations of each channel's signal and its derivative. This procedure is equivalent to filtering each channel to render the background noise white and then cross‐correlating the filter outputs.
Modification of the Threshold Current and Near‐Field Emission Pattern of a GaAs Laser by an Adsorbed Dielectric Layer35(1964); http://dx.doi.org/10.1063/1.1702850View Description Hide Description
We have observed the near‐field emission patterns of a number of GaAs diode lasers. The onset of laser action is accompanied by an abrupt increase in the intensity of light emission at one or several spots along the junction. The adsorption of a dielectric film at a constant rate on one or both surfaces of a laser causes the threshold current to vary periodically with time. The position of the threshold spot did not vary as long as the film was of uniform thickness along the junction; however, the emission pattern could be altered by a suitable nonuniform film. The variation of the threshold current with film thickness can be fitted by a simple model that assumes the gain in the active region is proportional to the current density. Calculated values for the loss and gain factors ranged from 10 to 60 cm−1 and 0.5 to 4×10−2 cm/A, respectively.
35(1964); http://dx.doi.org/10.1063/1.1702851View Description Hide Description
Beat‐frequency light generation in ammonium dihydrogen phosphate, arising from interaction of 3470‐Å rubymaser second harmonic and 7454‐Å rubystimulated Raman scattering from benzene, has been investigated. A single crystal can serve to efficiently produce both the ruby second harmonic and the beat note, since in each process the phase matching condition is satisfied at nearly the same crystal orientation. The phase matching effect is readily observable when the crystal is rotated.
35(1964); http://dx.doi.org/10.1063/1.1702852View Description Hide Description
Neutral mercury atoms, formed by charge exchange processes between positive mercury ions and mercury vapor atoms, were found to liberate a copious yield of secondary electrons from a metal surface. Values of γ as high as 0.40 were observed. The source of positive ions was the plasma of a mercury arc discharge. Since such a high yield of secondary electrons is not indicative of ground state neutral atoms, the atoms were assumed to be metastable. To test this hypothesis the neutrals were allowed to decay in their flight to the target. A second method of quenching the supposed metastable atoms consisted of admitting minute quantities of H2 and Ar gas into their flight path. In both cases the secondary emission was reduced by several orders of magnitude thus verifying the metastability hypothesis. The 4.66‐, 5.43‐, and 9.0‐eV metastable states were observed along with a long‐lived state having a mean lifetime of 6.0 μsec. As an experimental side light of great importance, it was observed that Hg − ions were formed in the collimated beam of excited neutral atoms.
35(1964); http://dx.doi.org/10.1063/1.1702853View Description Hide Description
Measured values of electron density and collision frequency in shock‐heated argon are presented and compared with theoretical equilibrium values. Both parameters are measured as a function of time during a single shock tube experiment. The measured density at 1 and 3 Torr forepressure equals or exceeds the predicted 1016−1017 electrons per cc at high Mach number (15–18); for lower Mach number,measured values fall far below the calculated curve, showing that loss mechanisms dominate thermal electron production over a significant interval of shock wave flow. The measured collision frequency compares well with the predicted value at 1 Torr but is lower by about an order of magnitude at 3 Torr. The experimental configuration comprises a theta pinch driven shock tube designed to drive a shock front into a coaxial waveguide. Two types of interference patterns are observed by transverse sampling of the longitudinal waves: the first, an anomalous pattern, is interpreted as a resonance caused by electrons ahead of the shock; the second, a well‐defined interference between incident and reflected waves, yields measurements of the plasma reflection coefficient magnitude and phase.
35(1964); http://dx.doi.org/10.1063/1.1702854View Description Hide Description
This paper proposes a novel method for dissociating high‐energy molecular ions, such as H2 +, which are injected into a magnetic mirror container. The dissociating medium is a highly ionized plasma consisting of protons and high‐energy electrons ejected in the decay of dense thermal neutrons which exist within a nuclear research reactor. The plasma is accumulated within a weak magnetic mirror container inside the reactor and then transferred magnetically to the site where the H2 + injection is to take place. Finally the dissociating plasma is compressed magnetically to a density of 1011/cm3 and injection is begun. The plasma provides an efficient means for dissociating the injected H2 + ions, but contributes negligibly to charge exchange losses of trapped high‐energy protons.
35(1964); http://dx.doi.org/10.1063/1.1702855View Description Hide Description
Microwave circuittheory is utilized to describe three devices which control coherent optical beams using birefringent crystals. A frequency demodulator uses the difference of the index of refraction along two axes of a birefringent crystal together with a Fabry‐Perot interferometer to obtain an ``s'' type frequency discriminator. An optical isolator consists of a crystal and a polarizer that function in such a way that the input of the device is isolated from the reflection which can occur at its output. An optical switch is based on the filter characteristics of a Fabry‐Perot interferometer which can be adjusted with an electrically activated birefringent crystal.
35(1964); http://dx.doi.org/10.1063/1.1702856View Description Hide Description
In this paper a simple derivation is given of the distribution of photons emitted when an energetic charged particle passes through a thin layer of dielectric material. This derivation emphasizes the individual microscopic events which occur during such a process. The distribution function is shown to agree with that obtainable from the transition radiation theory of Frank and Ginzburg and from the plasmon decay theory of Ferrell for the case of a thin foil. The special case of a double foil bounded by vacuum is considered in detail, and a general formula is found for the photondistribution function. An approximation analogous to that of Born's in quantum mechanics is developed to treat the case of a general composite foil whose thickness is small compared with λ but which may be large compared with vλ/c, where λ is the wavelength of the light emitted and v is the speed of the charged particle.
35(1964); http://dx.doi.org/10.1063/1.1702858View Description Hide Description
Experimental studies of field emission and breakdown with nickelcathodes in planar diodes and a cylindrical projection tube are presented. These demonstrate that the currents preceding electrical breakdown are due to field emission from protruding whiskers. An activation of these whiskers can proceed by desorption of gases, and very high electric fields are capable of inducing new intensely emitting whiskers to appear. At critical electrical fields the whiskers disrupt, leaving molten pools at the cathode even in the cylindrical geometry where the anode can play no part in the process. Image disappearance may be caused by the whisker's melting. An estimate of the whisker height can be made from the size of the images in the projection tube. Application of the Fowler‐Nordheim equation enables the current emitting area and the field magnification factor of a protuberance to be estimated. Whisker melting may be the origin of Cranberg's clumps for circuit limited breakdown.
35(1964); http://dx.doi.org/10.1063/1.1702859View Description Hide Description
An apparatus has been constructed to introduce known amounts of slip between a belt of an insulating material and a rotating, grounded metal roller over which the belt passes. This slip results in frictional generation of electrostatic charge on the belt. The charge generated has been measured at different belt speeds, tensions, angles of wrap of belt around the roller, and at various amounts of slip on the roller. The charge generated per unit area on the belt is correlated with the work per unit area done against the force of friction. A method is presented for obtaining a value of the coefficient of sliding friction between the belt and the roller by the measurement of the charge generated. In addition, evidence of increased charge transfer as a result of frictional heating is discussed.
35(1964); http://dx.doi.org/10.1063/1.1702860View Description Hide Description
Optical Faraday rotation of rare‐earth (III) borate glasses has been investigated at room temperature. The rotation is described in terms of an effective transition wavelength λ t which is an indicator of the spectral region of the 4fn ‐4f n−1 5d absorptions mainly responsible for the rotation. An experimental investigation of the magnitude of rotation as a function of rare‐earth (III) ion reveals the importance of the transition wavelengths and the electric dipole matrix elements as parameters affecting the rotation. The borate matrix is found to be more effective than many other environments in the realization of higher Verdet constants.
35(1964); http://dx.doi.org/10.1063/1.1702861View Description Hide Description
A procedure for uncoupling the charge transport and electric field equations under certain restrictions of dose‐rate limitation and no recombination in a parallel‐plate ion chamber is given. A solution to the first uncoupled transport equation for a flat‐topped ionization pulse, including diffusion, is obtained by finding the appropriate Green's function, and the subsequent current densities and external circuit current are derived. The usual assumption of neglecting diffusion is shown to be valid for most cases involving ion transport by an examination of the general form of the external chamber current. Variations of the external circuit current with the radiation pulse width and the ratio of negative to positive ion mobilities are given. An attempt to ``fit'' an observed record shows marked deviation of the decreasing portion of the signal from the calculated one. This is probably due to the cessation of an appreciable free electron current after irradiation not taken into account by the calculation.
35(1964); http://dx.doi.org/10.1063/1.1702862View Description Hide Description
A solution of cryptocyanine in methanol has been used as a self‐synchronizing, nondestructive, passive Q‐switch in a ruby laser. A symmetric giant pulse of ∼10 nsec width and 5–10 MW peak power is produced with a standard ruby laser system. This pulse is comparable with pulses generated by other Q‐switching techniques.
35(1964); http://dx.doi.org/10.1063/1.1702863View Description Hide Description
Initial measurements on the plasma produced in the Scylla IV theta‐pinch of its electron density, soft x‐ray emission, neutron emission, plasma shape and motion have been carried out as a function of the initial deuterium pressure and bias magnetic field. Interferograms establish electron densities in the range 4 to 7×1016 cm−3 for the plasma at peak compression and give azimuthally symmetric plasma shapes, which show the absence of the ``rotating flute''instability as well as the absence of large trapped magnetic fields at peak compression. Streak photographs also indicate gross plasma stability. Shadowgraphs in conjunction with deflection mapping of a grid pattern show sharp plasma boundaries. Soft x‐ray absorption measurements yield plasma electron temperatures in the range 400 to 1200 eV, which are dependent on the magnitude of the reversed bias field and have a gross correlation with the neutron emission. Neutron emissions in the range 107 to 2×109 per discharge are observed and depend upon the magnitude of the reversed bias field. Neutron collimation experiments yield a plasma length of approximately 70 cm in the 1‐m mirrorless compression coil. The observed electron temperature,neutron emission, and electron density as functions of the bias magnetic field and filling pressure are consistent with the product of the plasma particle energy and the plasma density being a constant, as required for a β=1 plasma.
35(1964); http://dx.doi.org/10.1063/1.1702864View Description Hide Description
Ag ion conductioncurrents in α quartz were measured at temperatures between 300° and 550°C with 2.5 V dc applied between Ag electrodes on the inner and outer surfaces of a natural quartz tube. The activation energyH for this conduction process depends upon the average electric field intensity according to the equation H=H 0‐βE ½, where H 0=38 kcal/mole and β=0.32 kcal/[mole (V/cm)½]. A theory which appears to explain this field dependence postulates the formation of a lattice‐bound negative space charge near the Ag anode. This space charge is formed by the migration of mobile impurity cations away from the anode, leaving behind a net charge due to the presence of substitutional ions such as Al+3. This negative space charge creates electrical field intensities of the order of 105 to 107 V/cm at the anode surface. This field lowers the height of the 38‐kcal/mole barrier near the anode. Graphs of the available data for Li+ and Na+ migration indicate that the rate‐limiting step for these processes might also be the surmounting of a potential barrier near the anode‐quartz interface.
35(1964); http://dx.doi.org/10.1063/1.1702865View Description Hide Description
The dielectric constant and loss factor of natural quartzsingle crystals have been measured in the temperature range from room temperature to 573°C, and in the frequency range from 50 cps to 100 kc/sec. Parallel to the c axis a large dispersion in the dielectric constant of the order of ε s ‐ε∞ = 500 was observed. Associated with this dispersion is a peak in the loss factor of the order of ε″max = 250. The data were analyzed in terms of the Debye equations with a single thermally activated relaxation time. The activation energy is about 16.7 kcal/mole. Perpendicular to the c axis no peak in ε″ was observed, but both ε′ and ε″ begin to increase at low frequencies and high temperatures.
The loss peak parallel to the c axis was substantially reduced when impurities were removed by electrolysis. The introduction of Ag, Cu, or Na gave rise to new peaks while Li tended to reproduce the original peak. It is concluded that the original loss was due to the motion of Li+ ions along structural channels parallel to the c axis. From the charge transported in electrolysis experiments, it can be inferred that the number of ions involved was about 5×1017 per cm3. The observed activation energy is believed to be the Coulomb binding energy between a substitutional Al3+ ion and an interstitial Li+ ion.
35(1964); http://dx.doi.org/10.1063/1.1702866View Description Hide Description
Ferromagneticdomain walls tend to move in phase with an alternating stress or magnetic field. This movement is opposed by directionally ordered interstitial atoms. When the time associated with the relaxation process is the order of the period of the moving walls, a phase lag between the applied field and the domain‐wall movement occurs. This phase lag, coupled with normal hysteresis losses, causes a variety of magnetic and magnetoelastic phenomena, among them a strong temperature dependence of the ac magnetic permeability and a broad internal‐friction peak in the temperature range of the Snoek peaks. These phenomena can be interpreted in terms of viscoelastic theory, with a relaxation process (interstitial directional ordering) having a unique activation energy.