D. Kenneth Davies

Hybrid electronic fiber optic wavelength-multiplexed system for true time delay steering of phased array antennas

A hybrid electronic fiber optic true time-delay steering architecture is presented which retains the highly hardware compressive property of an earlier all-optical approach and introduces the economical advantages of electronics at every level possible without compromising overall performance. The architecture is particularly suited to large arrays where maximum advantage can be taken of the hardware compression. A detailed design based on this architecture is described for steering a linear, 16-element, L -band (0.7- to 1.4-GHz) array in transmit mode over a scan angle of ±20 deg with a delay resolution of 6 bits (0.63 deg). An analysis of the expected performance of the design is given together with progress toward the fabrication of the prototype system, which includes the first iteration electronic binary delay line subsystem and a single-segment, high-fidelity, directly modulated DFB laser diode fiber optic link. The experimental data from these modules is in agreement with the performance predicted from the analysis.

Mechanism of dc Electrical Breakdown between Extended Electrodes in Vacuum

A description is given of vacuum breakdown between extended copper electrodes in dc electric fields in terms of the relevant atomic‐collision processes. The theory is based on a model involving avalanche amplification of current in electrode vapor generated by the evaporation of an anode macroparticle during its transit to the cathode. Calculations are presented of the dynamics, heating, and evaporation of the macroparticle leading to the production of the vapor medium in the interelectrode gap. The inferred copper vapor density distribution accounts quantitatively for the absorption of resonance radiation measured just prior to current amplification in the gap. Calculations of electron avalanche multiplication in the vapor lead to predictions of breakdown conditions in agreement with our observations. The predicted size of the anode macroparticle which initiates breakdown is, on the average, of the order of 1 μ in diameter.

Properties of electric discharges sustained by a uniform source of ionization

A theoretical examination is made of the class of discharges which are sustained by a uniform source of electron and ion pairs produced between plane‐parallel electrodes. The calculations account for drift, diffusion, and recombination of the charge carriers, ionization of the gas by electrons, and distortion of the applied field due to space charges. It is found that the number and current‐density profiles of electrons and positive ions in the gaseous discharge are of three different types, where (i) the electron density is several orders of magnitude lower than the positive‐ion density at all interelectrode positions, (ii) a plasma region exists, but the electric field in the plasma is negligible compared with that in both electrode sheaths, and (iii) significant ionization occurs in the cathode‐sheath region. Examples of such discharges for which numerical solutions are given are as follows: for type (i), a γ‐ray photoionization chamber with a current density j<1 μA/cm2; for type (ii), a fission fragme...

Vacuum Electrical Breakdown between Plane-Parallel Copper Electrodes

Measurements have been made of prebreakdown currents between thoroughly outgassed plane-parallel copper electrodes in ultrahigh vacuum, for electrode separations in the range 0.03 cm to 0.2 cm. These currents are found to be in good agreement with the Fowler-Nordheim theory of field emission from the pure metal surface. From combined measurements of prebreakdown current and breakdown voltage at different electrode separations it is deduced that the cathode microscopic field at breakdown is constant and of magnitude (6±1)×107 V/cm.Spectroscopic measurements of both resonance line absorption and line fluorescence have revealed that the density of neutral copper vapor present in the interelectrode space during the application of electric fields only fractionally (<1%) less than the breakdown field is considerably less than that necessary for volume ionization to occur. For the same experimental conditions, infrared radiation observations have revealed no hot spots on the anode surface.From the measurements i...

The Effect of Electrode Temperature on Vacuum Electrical Breakdown between Plane‐Parallel Copper Electrodes

Combined measurements of current‐voltage, prebreakdown characteristics, and breakdown voltage have been made for plane‐parallel, copper electrodes in ultrahigh vacuum as a function of both cathode initial temperature and anode initial temperature. These measurements have been carried out for an electrode separation of 0.1 cm over the temperature range from 313° to 913°K. From the experimental data the temperatures of the hottest point of the cathode surface, (TC)s, and of the anode surface, (TA)s, at breakdown have been determined as a function of electrode initial temperature. Over the whole range of the investigation it is found that (TA)s is greater than (TC)s. Moreover, the calculated values of (TA)s at breakdown are essentially independent of anode initial temperature and have a mean value of (1100±150)°K. In contrast, the values of (TC)s at breakdown increase with increasing values of cathode initial temperature. The results suggest that a thermal instability of a point on the anode surface leads to...

Detection of Electrode Vapor Between Plane Parallel Copper Electrodes Prior to Current Amplification and Breakdown in Vacuum

Simultaneous, time‐resolved measurements of current growth and resonance line absorption have been made for the time intervals 0.2 and 0.8 μsec, respectively, preceding breakdown between plane‐parallel copper electrodes in vacuum. These measurements have been carried out for an electrode separation of 0.1 cm and residual pressures in the low 10−9‐Torr range. The experimental data show that neutral copper vapor is present in the interelectrode volume before the current increases sufficiently to produce breakdown. It is found that the vapor is generated during times of the order of some microseconds prior to breakdown, and is highly localized to the region of the subsequent spark channel. Further, the vapor density decreases from cathode to anode along the path of the subsequent spark channel. The results are consistent with a model for vacuum breakdown proposed recently in which the transient production of vapor immediately prior to breakdown occurs by the evaporation of an anode macroparticle during its t...

An Interferometric Study of a High‐Intensity, Hollow‐Cathode Source

Measurements of the profile of the λ 3247.5‐A copper resonance line emitted from a high‐intensity, hollow‐cathode source of the Sullivan—Walsh type have been made using a pressure‐tuned, Fabry—Perot interferometer. It is found that the enhanced intensity from this source compared with that from a conventional hollow‐cathode source is accompanied by a considerable decrease in the self‐absorption of the resonance line within the source. The line‐profile measurements together with measurements of the decay of the resonance radiation on switching off either the hollow‐cathode discharge or the auxiliary discharge are in qualitative agreement with the hypothesis that the radiation is emitted partly from the hollow‐cathode discharge and partly from the auxiliary discharge.

Ionization and attachment coefficients in CO2 : N2 : He and pure CO2

Values of the ionization (α/N) and attachment (η/N) coefficients have been determined from spatial current growth measurements in uniform fields E for a CO2 : N2 : He (1 : 2 : 3) mixture and for pure CO2 over the ranges 4.66×10−16⩽E/N⩽8.48×10−16 V cm2 and 7.63×10−16⩽E/N⩽9.89×10−16 V cm2, respectively, where N is the gas density. The limiting values of E/N, at which α=η, are found to be (4.66±0.14) ×10−16 V cm2 and (8.20±0.16) ×10−16 V cm2 for the CO2 : N2 : He mixture and for CO2, respectively. The present values of the coefficients are in reasonably good agreement with those calculated from a solution of the Boltzmann equation using electron‐collision cross sections. Nevertheless, the present results indicate that even closer agreement would be obtained if the attachment cross section used in the calculations is reduced by about 15%.

Analysis of current growth measurements in attaching gases

An alternative procedure is presented for the analysis of spatial current growth measurements in attaching gases. The analysis gives unambiguous values of the net ionization coefficient for low values of Nd even when detachment and charge transfer of the primary negative ion occur simultaneously with ionization and attachment, provided secondary ionization processes are negligible. Examples are given of the application of the analysis to data in CO2, in a mixture of CO2, N2, and He, and in O2.

Net ionization coefficient in boron trifluoride

Values of the net ionization coefficient λ1 in BF3 have been determined from spatial current growth measurements in uniform electric fields E over the range 1.07×10−15⩽E/N⩽17.0 ×10−15 V cm2 for gas densities N in the range 4.07×1016 ⩽N⩽348×1016 cm−3. For the limited range 1.07×10−15⩽E/N⩽1.70×10−15 V cm2, values of the ionization and effective attachment coefficients have also been determined. Within the limits of experimental uncertainty all the coefficients are found to be functions of E/N only. The limiting value of E/N at which λl=0 is found to be 1.13×10−15 V cm2.