Index of content:
Volume 28, Issue 9, 01 September 1957
28(1957); http://dx.doi.org/10.1063/1.1722916View Description Hide Description
Highly bunched electron beams in the 1 to 3 million electron volt range, having a sharply peaked velocity spectrum, represent a powerful attack on the submillimeter wave (100 to 3000 kmc/sec) generation problem. In this paper the theory of a rebatron for producing a high‐energy beam, wherein both the current wave form and the velocity distribution function approach delta functions, is presented. Computations indicate that the emergent 1.5‐Mev beam has an appreciable harmonic content up to harmonics as high as the 1000th and has a velocity spread of 0.07% for the electrons of interest. Pulsed beam powers of the order of 100 000 w should be possible using a 3 kmc/sec rebatron.
28(1957); http://dx.doi.org/10.1063/1.1722917View Description Hide Description
A practical design of an S‐band rebatron for producing a high power, bunched, megavolt electron beam, theoretically possessing harmonic current frequencies in the submillimeter range is presented. An indirect experimental method of evaluating the beam obtained from the rebatron, based on measuring momenta spectra, gives very satisfactory agreement with theory. The validity of this indirect method of beam analysis is strengthened by a measurement of the ratio of the 26th beam harmonic current amplitudes under two different sets of conditions determined experimentally by momenta spectra measurements. No highly critical tolerance conditions are encountered in the investigation.
28(1957); http://dx.doi.org/10.1063/1.1722918View Description Hide Description
A new method for the coherent generation of relatively high powers at submillimeter wavelengths through the electronic excitation at a single frequency of a selected higher order TM 0mn mode in a cylindrical cavity which is large compared to the wavelength is investigated. The theoretical relationship between the generated power, P, the harmonic current component, Is , and the wavelength, λ, is typically of the order of
28(1957); http://dx.doi.org/10.1063/1.1722919View Description Hide Description
Values of the photoelectric quantum efficiency of glass for different wavelengths have been determined and data have been obtained on the status of electrons placed on glasssurfaces. One of the borosilicate glasses commonly used in discharge tube manufacture and soda glass were studied in vacuum with 2537 A photons from a quartz Heraeus mercury arc and with light from a glow discharge operating in pure A and Ne from 10−1 to 10 mm pressure. Clean borosilicate glass in vacuum at 22°C with fields of some 1000 v/cm across it yields photoelectric current densities of the order of 10−12 amp/cm2 for the full light of a 110 volt arc focussed on 10 cm2 area through quartz optics. This current comes from the 3–4% Na impurity atoms on the surface. Soda glass yields currents in proportion to the density of Na atoms. The normal conductivity of the glass comes from Na+ ions of some 0.5 volt activation energy normally present. It is enhanced 10‐fold by Na+ ions liberated at the surface by light. Na+ ions next the cathode cannot leave the glass, but electrons enter the glass from the metal cathode backing of lower work function to neutralize the ion space charge as it builds up near the cathode. Electrons sprayed onto the glasssurface have a mobility of ∼5×10−5 cm2/volt sec in the glass in contrast to the value of ∼1×10−7 cm2/volt sec for the Na+ ions reported by Kraus. Photons above 3300 A do not contribute to the conductivity in any fashion. The photoelectric yield for 2537 A was obtained directly in absolute magnitude by use of a vacuum bolometer. For the light from glow discharges ≪1200 A, the efficiency was obtained indirectly by comparison with values for Pt and Au. The values obtained are tabulated below.
28(1957); http://dx.doi.org/10.1063/1.1722920View Description Hide Description
High frictional temperatures during the dry or imperfectly lubricated rubbing of steel produce gross structural changes and intense hardening of the surface layers. In consequence of these changes the wear time curve shows a marked inflexion, the wear rate of soft steel on soft steel diminishing by more than an order of magnitude as the hardened skin develops. The protective layer establishes itself more readily as the carbon content of the steel increases, a feature explained by the ease with which eutectoid pearlite transforms to austenite, and thence to martensite, during the rapid temperature cycle. By simulating the hot spots during wear by the thermal action of electric sparks, the hardness change is shown to be intensified by nitrogen or carbon absorbed from the atmosphere or lubricant.
28(1957); http://dx.doi.org/10.1063/1.1722921View Description Hide Description
Using perturbation theory, expressions are derived for the frequency shift of a circularly polarized resonant microwave cavity upon insertion of a ferrite sample small compared with the wavelength. Retardation effects are considered in deriving the rf field h 2 internal to the sample. By integrating h 1·h 2 dv (where h 1 is the applied rf field) over the sample volume, a closed‐form expression is obtained for the cavity frequency shift. The theory is applied to the magnetic resonance in ferrites. The calculated magnetic resonance line shape is found to remain symmetrical as the sample size is increased. Retardation effect, thus, does not explain the asymmetry of the resonance lines which have been observed for polycrystallineferrite. Results of the theory are also compared with measurements on a single crystal of ferrite having the rather large maximum permeability loss term of 100. Theoretical and measured values are in close agreement.
The theory is also compared with the somewhat different results of other recent theoretical treatments, in which an approximate expression for an effective magnetic dipole moment for the sample was computed, rather than integrating the internal rf fields over the volume of the sample.
28(1957); http://dx.doi.org/10.1063/1.1722922View Description Hide Description
Electron mirrormicroscopy is a rather novel research instrument. It has recently been demonstrated that it can provide micrographs not only of the relief structure of a specimen but also of such purely electrical properties as contact potentials, conductivities, surface charges, and even space charges. This paper deals with the feasibility of electron mirrormicroscopy for pictorial representation of magnetic properties in general, and of magnetic domain patterns in particular. It is shown that electron mirrormicroscopy is actually a feasible and independent method for electronically depicting magnetic patterns. The section of the paper dealing with experimental work begins with magnetic specimens of artificial nature, e.g., a recorder head, containing one or more linear magnetic dipole stray fields which are controllable. After establishing the basic facts of magnetic contrast formation with this type specimen, the paper reports on experiments conducted with specimens of barium ferrite and nickel ferrite containing actual magnetic domain patterns. Samples of electron mirror micrographs of domain patterns of these materials are shown and compared with domain patterns of the same specimen areas obtained by the conventional powder technique. The identical nature of the configurations on both micrographs provided final proof of the feasibility of electron mirrormicroscopy in depicting magnetic patterns. Criteria required to distinguish magnetic patterns from those of electrical or relief origin are derived from the experimental findings and are verified by elementary theoretical considerations about magnetic contrast formation in electron mirrormicroscopy.
28(1957); http://dx.doi.org/10.1063/1.1722923View Description Hide Description
Two identical linear circuits are excited by generators furnishing equal and opposite voltages. Homologous portions of the two circuits are interchanged at random times with, on the average, r interchanges per unit time. Because of these interchanges, the currents in the remaining portions are irregular. A theorem is derived that permits one to calculate the statistical averages of these currents. It states that one may disregard the interchanging; instead, one merely replaces the complex frequency s by s+2r in the circuit functions [e.g., the impedances Z(s)] of the portions that were being interchanged. On the basis of this theorem one may design counting rate meters with nonlinear (e.g., logarithmic) scale, useful in reactor instrumentation, and function generators for functions of practical importance, such as the logarithmic and exponential functions, and powers with arbitrary exponent. The nature of the function depends only on linear passive circuit elements, such as resistors and capacitors.
28(1957); http://dx.doi.org/10.1063/1.1722924View Description Hide Description
The condensation zone of a condensing vapor flowing in a channel is idealized as an interface separating vapor and liquid phases of the fluid with a concentrated friction, heat removal, and cold liquid injector mechanism at the interface. Rankine‐Hugoniot relations are obtained including effects of the mass, momentum, and energy sources at the interface. For a stationary interface, an explicit approximate solution is obtained giving the final liquid state in terms of the other parameters. In addition, a rapidly converging iterative procedure is presented with this solution as a first approximation and utilizing directly thermodynamic tables of the vapor and liquid. Examples corresponding to steam flow in a pipe and to an underwater steam jet are considered and their Hugoniot curves are computed and compared to similar curves obtained in detonation theory. The thermodynamic requirements of the existence of a steam‐water interface limit these curves in extent and result in upper and lower bounds on the values of the cooling parameters.
28(1957); http://dx.doi.org/10.1063/1.1722925View Description Hide Description
A dynamical theory is developed for the explosive production of multiple shocks in metals which undergo a phase transition. In addition to the formation of two compressive shocks, it is found that a rarefaction shock is also produced which will overtake one of the compressive shocks.
28(1957); http://dx.doi.org/10.1063/1.1722895View Description Hide Description
The dielectric strength of polyethylene at 3300 megacycles per second was determined as being 16.8 kv/mil, 13.1 kv/mil, and 6.0 kv/mil at 26°C, 57°C, and 95°C, respectively. These values are the same as those reported for measurements using a dcelectric field. The electrode system, a high Q resonator, and the experimental procedures were designed to reduce the effects of secondary factors such as heating and corona.
28(1957); http://dx.doi.org/10.1063/1.1722896View Description Hide Description
A new method of visualizing low‐density flows is described which employs the absorption by oxygen of radiation in the wavelength region 1400 A to 1500 A. It was developed for use in the low‐density supersonic continuous flowwind tunnels at Berkeley because the conventional methods (shadowgraph, schlieren, and interferometer) were predicted to have inadequate sensitivity at the unusually low densities in the test section. A xenon discharge is used for the source of radiation, its 1470 A resonance line being isolated by a calcium fluoride vacuum monochromator. A 26 mm diameter beam of parallel radiation is passed through the test section where it is absorbed more or less depending on the oxygen density along the radiation path. The center of the test section is then focused by a calcium fluoride vacuum camera on an ultraviolet‐sensitive photographic plate. Pictures of shock waves from cylinders ⅛ and ¼ inch in diameter and from a 60° ⅜ inch wedge were obtained with exposure times of about 4 minutes. Development of a stronger source would reduce the exposure time appreciably. This oxygen absorption method is shown to be suitable, in principal, for general wind tunnel work where the absolute pressures are below about 1 mm Hg.
28(1957); http://dx.doi.org/10.1063/1.1722897View Description Hide Description
The present paper analyzes a rotational model of flux reversal in ferromagnetic materials. A brief review of the experimentally established details of the flux reversal process in square loop ferrites is given. The discrepancies between the experimental results and the predictions of the domain wall motion theory are discussed. The switching coefficient Sw is examined in detail.
The rotational model is based on the solution of the modified Landau and Lifshitz equation and predicts the shape of the output voltage pulse, the relationship between the flux reversal time and the applied field, and the minimum switching coefficient that can be obtained for a given ferrite. The switching coefficients determined experimentally are within a factor of three of the minimum value of Sw predicted by this model.
28(1957); http://dx.doi.org/10.1063/1.1722898View Description Hide Description
Krishnan's relation is deduced rigorously from Maxwell's equations when scattering by a single body is considered and an average overa‐ll orientation of the body is taken. When scattering by a large number of randomly‐oriented particles is considered, Krishnan's relation holds true either if multiple scattering and the effect of the container walls are neglected, or if a certain symmetry condition is satisfied. Where this condition is not satisfied, deviations from Krishnan's relation may be expected at high concentrations. The desirability of extrapolating experimental results to zero concentration is emphasized. Relations analogous to Krishnan's are also deduced which do not require any averaging over orientations, but involve instead the interchange of the dielectric constant and magnetic permeability, relative to the surrounding medium, at each point of the scattering system.
28(1957); http://dx.doi.org/10.1063/1.1722899View Description Hide Description
Dcbreakdown studies on Mylar film show that defects cause a decrease in dielectric strength with decreasing film thickness at room temperature but not at −180°C. The breakdown at defects is associated with moisture content. Using self‐healing electrodes to eliminate weak areas, a dcdielectric strength of 6 Mv/cm is found, independent of film thickness. Prebreakdown current measurements indicate ionic currents which are fairly uniform throughout a sample, and field emission currents localized at incipient breakdown points. Breakdown probably occurs as a result of local heating by field emission currents.
28(1957); http://dx.doi.org/10.1063/1.1722900View Description Hide Description
By rapidly reducing the stress after yielding, it was possible to study the plastic deformation of copper and silverwhiskers. The whiskers exhibit extremely sharp yield points and extensive easy ``glide'' regions which are due to the propagation of Lüders bands. It is unlikely that the sharp yield points are caused by dislocation pinning. The propagation of the Lüders bands often becomes obstructed giving rise to repeated yielding in the plastic region. The nature of these obstructions and the mechanism of Lüders band propagation are not clear.
28(1957); http://dx.doi.org/10.1063/1.1722901View Description Hide Description
A mass spectrometer of the omegatron type is described by which gases up to mass number 44 can be detected and analyzed at pressures as low as 10−10 mm. Performance characteristics of this spectrometer are given together with a description of auxiliary vacuum equipment with which it was used for studies of high vacuum phenomena. Results are given for analyses of: 1. the gas evolving from a number of materials used in high vacuum work; 2. the residual gas existing above getters of the flash type; and 3. the residual gas in television picture tubes. The gases found in the residual gas mainly are argon, methane, and water vapor.
28(1957); http://dx.doi.org/10.1063/1.1722902View Description Hide Description
Thermionic emission measurements have been carried out on [Ag]–Cs2O, Ag, Cs layers in a state of near maximum thermionic yield. The data have been plotted according to the Fowler equation to determine thermionic work functions; an average value of 0.83 ev has been obtained. However, cooling the layers, below ambient, to temperatures of −40°C, results in an increase of work function of approximately 0.3 ev. Photoelectric emission measurements on these same layers yield an average photoelectric work function of 1.06 ev.
Conductivity measurements on Cs2O, Ag, Cs layers are linear between 100 and −70°C; an activation energy of ∼0.1 ev has been determined from data in this temperature range. Conductivity measurements on Cs2O layers containing no reduced silver exhibit changes in slope similar to the thermionic data. Activation energies computed from above the break in conductivity data are ∼0.25 ev. Cooling the layers to −70°C and below results in increases of activation energy of ∼0.1 ev.
28(1957); http://dx.doi.org/10.1063/1.1722903View Description Hide Description
An experimental Peltier refrigerator utilizing thermoelements of annular configuration has been designed, constructed, and operated. The thermoelectric materials used are p‐ and n‐type Bi2Te3 with ρ p =2×10−3 ohm‐cm, ep =180 μvolts/°C, ρ n =8.5×10−4 ohm‐cm, and en =−150 μvolts/°C. The device has been operated under ``no‐load'' conditions over a range of power inputs and has exhibited a maximum ΔT of 49°C. A simple theory relating to the performance of Peltier thermocouples is derived. Experimental performance of the device is shown to agree with theoretical predictions within experimental error.
28(1957); http://dx.doi.org/10.1063/1.1722904View Description Hide Description
Adhesive breaks (separation at the interface) have been obtained between high polymers and glass that require no external mechanical equipment to induce break. Visible light is emitted during these breaks. The polymers are contained in Pyrex vials that are immersed in a 125°C oil bath. After a nominal polymerization period, the polymers are removed from the bath. The adhesive breaks occur while the polymer‐glass system is cooling toward room temperature. The properties of the light due to these breaks have been studied with a photomultiplier, pulse‐height discriminator combination. The wavelength of the light was measured spectrographically and the internal temperature of the polymers was monitored during the cooling process.