Index of content:
Volume 36, Issue 2, 01 February 1965
36(1965); http://dx.doi.org/10.1063/1.1713990View Description Hide Description
Three methods were used to obtain electron density as a function of radius in a magnetically confined, low‐pressure argon arc. The first utilized the phase shift of a reflected 35‐kMc/sec microwave, and the second determined density from the ion saturation current to a Langmuir probe. The third determination utilized the broadening of the H β line emitted by a small quantity of added hydrogen. Agreement among the three methods is satisfactory; the electron density at the axis of this discharge is of order 1014 cm−3, in agreement with results obtained by others for discharges of similar type.
36(1965); http://dx.doi.org/10.1063/1.1713991View Description Hide Description
A direct electrical method for measuring the velocity of a flowing plasma is based upon the polarization voltage induced when a plasma flows transversely to an applied magnetic field. A method has been developed to allow the estimation of certain background signals and the determination of allowable measurementcurrents arising from plasma and probe properties. Applications for this technique of ionized gas velocity measurements are indicated.
It was found that an ac magnetic field of the order of 5 G would give a sufficient signal to determine the gas velocity of a plasma jet as compared to a dc field of the order of 100 G. Gas stream velocities produced by a low‐power argon plasma jet were found to vary from 1000 to 3000 m/sec depending slightly on the mass flow rate and predominantly on the ambient pressure.
36(1965); http://dx.doi.org/10.1063/1.1713992View Description Hide Description
The performance of a ruby laser amplifier is described and compared with a theoretical model. The ruby laser amplifier was driven by a Q‐switched laser oscillator with a stable output, whose properties are due, in part, to a newly developed output coupler and a spinning roof‐top prism for Q‐spoiling. From the theoretical amplifier model, a relation between the population inversion and the energy stored in the system supplying the xenon flash lamps is derived and fitted to the particular laser configuration by a single parameter established by the laser threshold. Experimentally, the gain of the 15‐cm (6‐in.) ruby amplifier saturates at values between 5 and 7, apparently caused by complete pumping of the system rather than limited by the energy input to the flashlamp pump.
36(1965); http://dx.doi.org/10.1063/1.1713993View Description Hide Description
The edges of planar thin‐film strips of tin are shown to limit the supercurrent carrying capacity of the films in some cases. This edge effect is demonstrated by means of a special structure which provides two current flow‐paths through the same film. The film strip edges are included in only one of these two paths. Detailed results are given for a 4400‐Å film of tin in which dc critical currents near the Silsbee limit are observed when the edges are excluded. Including the edges reduces the critical current for this film by more than 30%. It is suggested that an evaporation process can probably be developed which leads to films which will not exhibit this edge effect. The special structure would provide a useful tool for this process development, and it can also be adapted to serve as an improved gate for thin‐film cryotrons.
36(1965); http://dx.doi.org/10.1063/1.1713994View Description Hide Description
The secondary electron yield from fission fragments has been measured directly using an ultrahigh‐vacuum diode containing uranium which was operated in a nuclear reactor. The current measured in this diode consisted of contributions from the positively charged fission fragments, secondary electron emission induced by fission fragments, and photoelectrons generated by gamma radiation. The secondary electron yield Δ was determined from the secondary electron current and the computed fission fragment emission rate. For a thick (>one range) uranium‐nickel (5.7 wt. % nickel) alloy the value of Δ is 207±10.
36(1965); http://dx.doi.org/10.1063/1.1713995View Description Hide Description
A fractional monolayerfilm of ions is formed by the adsorption of a gas onto a metal substrate. These ions form a discrete dipole layer, the potential across it being the change in electron work function of the surface. As additional ions are brought to the surface, they penetrate only a fraction f of the potential across the dipole layer.
In this work the penetration coefficient f valid for the entire range of coverages is derived for two limiting types of films, immobile and mobile. By considering thermal effects, a penetration coefficient for partially mobile films is also derived. The values obtained for the penetration coefficient are applicable to any substrate‐adsorbate system.
The theoretical expressions for the penetration coefficient are evaluated for cesium on tungsten and compared with the data of Taylor and Langmuir.
36(1965); http://dx.doi.org/10.1063/1.1713996View Description Hide Description
A description of the visual characteristics of some constricted discharges in the rare gases using dc,rf, and microwave excitation is given. These discharges require power inputs of from 20 to greater than 100 W/cc. The results of a series of spectroscopic and electrical measurements made on these discharges indicate the range of plasma parameters encountered. Gas temperatures were measured using the intensity distribution of the spectra of several diatomic molecules. The gas temperature falls in the range from 2000° to 3000°K, but atomic excitation levels are not in equilibrium at this temperature. The charge density is around 1014/cc from continuum intensity, line broadening, and electrical measurements. The electron energy distribution is Maxwellian with an average electron energy which falls between 1 and 2 eV. Thus, these plasmas lie between ``thermal'' and ``cold'' plasmas in their properties. The recombination process probably has contributions from both dissociative recombination and dielectronic recombination with an over‐all recombination coefficient of about 2×10−11 cm3/sec.
Constricted Discharges in the Rare Gases. II. Analysis of the Macroscopic Properties of the Discharge36(1965); http://dx.doi.org/10.1063/1.1713997View Description Hide Description
The macroscopic properties of the rare gas constricted discharges are discussed. For the rf excited discharge an equivalent circuit for the electrode coupling has been found, comprising a capacitor with the electrode and plasma forming the plates and the discharge tube wall the dielectric. An explanation for the observed striations is advanced, based upon the existence of voltage standing waves at the plasma resonance frequency. A numerical analysis based on an energy balance indicates that these discharges should have a finite radius, in this case smaller than the discharge tube radius. It is necessary to require that the electron energy distribution be Maxwellian near the center of the discharge and that deviation from this energy distribution causes the sharp boundary observed for the heavier rare gases. For the microwave discharge it is pointed out that it is necessary to include the electromagnetic field equations in the analysis and that this requirement naturally leads to a maximum radius for a singledischarge channel which is much smaller than for the rf‐excited case. This, in turn leads to an explanation of the multicolumn effect observed.
36(1965); http://dx.doi.org/10.1063/1.1713998View Description Hide Description
The characteristics of the ``synchrotron light'' emitted by electrons in the NBS 180‐MeV electron synchrotron were studied in detail. The electron beam cross section was approximately 1×2 mm. The angular distribution and polarizationproperties were determined for the visible radiation; photoelectric techniques resulted in a more critical comparison of the experimental results to the theoretical predictions than heretofore possible. Doubts raised by previous work regarding the symmetry of the radiation with respect to the orbital plane have been allayed. Characteristics of the radiation in the extreme ultraviolet are calculated for the NBS machine, and applications of this source for physical experiments are discussed.
36(1965); http://dx.doi.org/10.1063/1.1713999View Description Hide Description
Multimode laser oscillation in the presence of internal resonator modulation is discussed. Modulation of the dielectric constant (ε) is shown to lead to frequency modulation of the laser light while modulation of the losses (or gain) leads to amplitude modulation. The results are discussed with particular reference to recent experiments and proposals involving modulation inside laser resonators. The mathematical formalism is that of normal‐mode parametric interactions.
36(1965); http://dx.doi.org/10.1063/1.1714000View Description Hide Description
Stopping cross sections were measured for H1 + in carbon and for He+ in carbon,aluminum, and chromium over the energy range of 65 to 180 keV. Layers of the stopping materials were evaporated in vacuo onto a gold‐plated quartz crystal and their areal densities were computed from the shift in the resonant frequency of the crystal. The energy lost by the ions in the stopping material was determined by measuring the energy of the scattered ions. Experimental results show a divergence from available theory above 80 keV.
36(1965); http://dx.doi.org/10.1063/1.1714001View Description Hide Description
Ruby lasers, when operated near threshold at 77°K with a low Qcavity (one end silvered for maximum reflectivity and the other end uncoated) oscillated at two frequencies. The frequency differences near threshold, measured with Fabry‐Perot etalons, were in good agreement with the splitting of the R 1 doublet, previously measured by microwave methods. As the power of the laser increased, the frequency difference between the oscillating components [2Ē → 4 A 2(±, ±½)] decreased. These results are accounted for on the basis of saturation broadening of overlapping Lorentzian lines. The observed variations in the splitting are in good accord with the theory. The rise in temperature of the ruby rods produced a negligible change in the splitting of the R 1 doublet. The same mechanism which produces the variation in the splitting should broaden the output spectrum of the laser.
36(1965); http://dx.doi.org/10.1063/1.1714002View Description Hide Description
In GaAs lasers with a homogeneously broadened spontaneous emission line, spatial inhomogeneity of the inverted population can lead to multimode oscillation. In this paper the behavior of higher‐order axial modes of Fabry‐Perot GaAs lasers at 4.2° and 77°K is shown to be in good agreement with the theory of Statz and Tang, strongly supporting the model of spatial inhomogeneity. Since diffusion plays a prominent role in smoothing out these spatial inhomogeneities, the theory calls for, and the experiment verifies, that GaAs lasers must be pumped harder at higher temperature to produce oscillations in a given number of modes.
36(1965); http://dx.doi.org/10.1063/1.1714003View Description Hide Description
A grain‐oriented polycrystalline thoriated tungsten emitter is described which demonstrates vastly improved emission characteristics over ordinary emitters of the same type.
The factor of four improvement in electron emission in high vacuum is almost entirely the result of an increase in A value from four to seventeen. The Richardson work function φ of the improved emitter is essentially the same as that of the ordinary one.
A comparison is made of the efficiencies of two cesium‐neutralized thermionic converters, identical in every respect except that one contains the improved emitter and the other an ordinary one. The measured improved factor of three in conversion efficiency is in close agreement with the predicted factor based on the difference in A values.
36(1965); http://dx.doi.org/10.1063/1.1714004View Description Hide Description
We present the results of numerical calculations of the potential, space‐charge, and current distributions in linear magnetrons, taking into account the three‐dimensional spread of the initial electron velocities. For most practical configurations, the potential distribution is nearly parabolic within the electron cloud, with a slight dip in front of the cathode and the space‐charge distribution is remarkably constant and approximately equal to that predicted by the single stream flow, although the electron trajectories are now anything but linear. For other choices of the parameters, the space‐charge distribution can be ``triangular'' or may resemble that associated with the so‐called double‐stream flow. In all such cases the potential, space charge, and current distributions cannot be predicted on the basis of a Brillouin flow and have to be calculated separately.
We have also shown that in most practical cases the systems seem to operate in what is called the magnetic‐field‐limited, rather than space‐charge‐limited regime, so that the anode current depends on the values of the external parameters only and is independent of the amount of space charge or the potential distribution between the electrodes. In ballistic terms this means that the force due to the magnetic field acting on an electron at the peak of its trajectory is greater than the corresponding force due to the electric field. Finally, we have shown that contrary to our expectations, there seems to be a great deal of agreement between the operations of systems using finite and infinite cathodes, even when the initial electron velocities are taken into account.
36(1965); http://dx.doi.org/10.1063/1.1714005View Description Hide Description
The phenomenological theory of Wiseman and Feaster, based on the two‐charge theory of Gross, expresses the polarization response of an absorptive dielectric as the superposition of responses to changes in the internal field, using a time‐dependent specific polarization function. This theory is extended to explain quantitatively the charge decay of unshielded dielectrics of finite resistivity. From measurements of the net surface charge on unshielded Carnauba wax electrets at 40°C and 10−5 Torr, it is concluded on the basis of this theory: (i) that the internal field is the main decay agent, governing both polarization growth and real surface decay, which together account for the decay of the net surface charge; (ii) that there is no net local charge within the volume of the dielectric; (iii) that the volume resistivity is 9.0±0.5×1017 Ω‐cm, and the dielectric constant at a frequency of the order of 1 cps is 3.08±0.12.
36(1965); http://dx.doi.org/10.1063/1.1714006View Description Hide Description
A method utilizing reflection measurements on a liquid‐filled line section terminated in a short circuit is described. Its chief feature is the use of line lengths such that the impedance at the liquid surface is real, which simplifies the calculation of* from the measurements. Additionally it reduces systematic errors due to the window used to constrain the liquid surface, and the nonzero impedance of the sliding plunger used to adjust the length. These factors give errors in′ about an order of magnitude below those in″. This makes the method particularly suitable for determination of the dielectric relaxation time of solutions of polar in nonpolar liquids. Limits on the window and plunger parameters to give an accuracy of about 5% in″ and 0.5% in′ are derived. Results of measurements with equipment satisfying these requirements are given, and it is seen that relaxation times can be found to about 5% accuracy for solutions 1–10 Molar % nitro‐benzene in benzene.
36(1965); http://dx.doi.org/10.1063/1.1714007View Description Hide Description
Probe curves and their second derivatives are automatically plotted for a small spherical probe going through a double layer (DL) at the opening of a hollow hot oxide cathode in a discharge in pure Ne of about 1 Torr. The cathode emission electrons are accelerated toward the DL in its fringe fields. Between cathodesurface and DL, the electrons behave approximately according to the ``accelerated half‐Maxwellian''; in the DL they are accelerated to energies equivalent to the metastable Ne levels near 16.5 V. A region of strong excitation and ionization (``ball of fire'') forms just outside the cathode opening. The cathode emission electrons enter the ball of fire as a ``fast group,'' superimposed on the approximately Maxwellian slow group of the ball of fire plasma. The development of potential, density, and e spectrum is discussed.
36(1965); http://dx.doi.org/10.1063/1.1714008View Description Hide Description
Continuous self‐sustained oscillations at 6834 Mc/sec were obtained with an optically pumped rubidium maser amplifier in a regenerative feedback loop. Some additional gain introduced by a parametric amplifier was required. The use of pulsed light for optical pumping permits the study of the oscillation build‐up under various experimental conditions.
36(1965); http://dx.doi.org/10.1063/1.1714009View Description Hide Description
The quasistatic analysis is used to examine propagation and instabilities of waves in a plasma in which the transverse bounds and the temperature are important. A dispersion equation is derived by use of a dielectrictensor, which is correct within the quasistatic assumption and the assumption of longitudinal velocity, only. The dispersion curves have a smooth transition from the zero temperature case at long wavelengths to the unbounded finite temperature case at short wavelengths. The stop‐bands which appear in the zero temperatureanalysis become propagating regions in the more general case. Landau‐type damping is calculated; the cyclotron wave is strongly damped. The effect of the boundaries and the magnetic field on a double‐humped velocity distribution is examined. The transverse boundaries are stabilizing with respect to space‐charge waves, but the cyclotron interactions are strengthened. The temperature always exerts a stabilizing influence which is particularly marked for the interaction between the cyclotron waves. Curves of the limiting regions of stability, with respect to various parameters, are given, illustrating these effects. By the use of an example it is demonstrated that a moderate temperature can suppress the cyclotron instability. This result is in agreement with the experimental observation that a plasma, predicted to be unstable with respect to the cyclotron‐wave interaction, was actually stable.