Pulsed Tube Research Articles

Constructional and design difficulties of wide-band high-power pulsed travelling-wave tubes in the 10 Gc/s region

The paper describes some of the difficulties met in the development of a pulsed travelling-wave amplifier having a maximum power in the region of 1kW and gain greater than 30dB over the frequency band 7.5-11.5Gc/s. This is a considerably higher power level than has been previously attempted in this country with a helix-type tube, and the parts of the development which are described are, in general, only those in which difficulty was met due to the particular combination of high power, high frequency and broad bandwidth.Broad bandwidth in travelling-wave tubes necessitates the use of a helical circuit. This imposes design limitations which restrict the operating voltage in order to avoid backward-wave oscillation. The maximum power that can be achieved is thus limited, even under pulsed conditions, and becomes less as the frequency increases.In a practical design of a waveguide-coupled high-power pulsed travelling-wave tube for the X-band, it was found necessary to maintain a small external diameter. This led to a large dielectric loading factor, which was reduced by a suitable choice of wire diameter and by control of the envelope wall thickness.A simple coupling was developed which gives a voltage standing-wave ratio of less than 1.5 over the range 7.5-11.0Gc/s. An attenuator which is well matched and has substantially constant loss over the same band has also been provided.In operation, the onset of backward-wave oscillation is decreased by the use of an operating voltage considerably higher than the synchronous voltage.From the final valve, pulsed output powers of 1kW are obtained at a gain of 30dB. Small-signal gains of up to 50dB have also been observed.

Discussion on “Constructional and design difficulties of wide-band high-power pulsed travelling-wave tubes in the 10 Gc/s region”

Discussion on “Constructional and design difficulties of wide-band high-power pulsed travelling-wave tubes in the 10 Gc/s region”

General design procedure for high-efficiency helix-type traveling-wave amplifiers

Heretofore traveling-wave tubes have been designed using Pierce's small signal theory, and the design then modified according to previously obtained experimental results in order to achieve high saturation output and a reasonable efficiency. A procedure has been developed that makes use of the large-signal calculations on traveling-wave amplifiers to design tubes to operate at high saturution efficiency. The design procedure is particularly useful in the design of tubes for high power output and may be applied to the design of either cw or pulsed tubes. A procedure has been developed which enables the design of a traveling-wave tube to be completed with the use of a minimum number of curves and requires a minimum of calculation on the part of the designer. The design curves for helix-type tubes were calculated using the sheath helix impedance modified by the appropriate impedance reduction factors.

Apparatus for Precision Flash Radiography of Shock and Detonation Waves in Gases

An apparatus is described which is based on the technique introduced by Kistiakowsky utilizing the absorption of soft x-rays to measure densities behind gaseous shock and detonation waves. Experimental conditions leading to the smallest absorption statistical uncertainty consistent with maximum sensitivity are defined. These may be approximated at reasonable pressures and tube diameters by adding a strongly absorbing rare gas diluent to the experimental gas mixture. Under such conditions measured densities are accurate to ±1%, as demonstrated by comparison of experimental and calculated densities for shock waves in krypton. At some sacrifice in accuracy, space resolution of 1 mm and time resolution of 1 μsec may be achieved. A continuously pumped, laboratory-built, demountable, pulsed x-ray tube with an L-cathode and tungsten target is used at an accelerating voltage of 20–-30 kv to obtain currents up to 0.2 amp for durations of the order of 1 msec. Copper and chromium targets were also investigated. For calibration purposes the tube is operated at a steady current of ≈0.2 ma and detector signals are measured potentiometrically to obtain transmission as a function of gas density.

Development of traveling-wave tubes for radar operation

The design of a pulsed traveling-wave tube for use as a power driver for megawatt klystrons will be described. The tube will provide more than 5 kw of peak power output over a frequency range of 1.5:l with 30-db gain in the ultrahigh frequency region. The ease in adapting the same construction to other frequency ranges will be described. An analysis of the beam focusing, rf coupling, and attenuators used in this tube will be described including the procedures followed in order to eliminate spurious oscillations. The constructional details of the tube will be shown and the assembly techniques for small production quantities will be described.

The Discharge Mechanism in the High-Vacuum Cold-Cathode Pulsed X-ray Tube

The existing theory of the discharge mechanism in the cold-cathode pulsed X-ray tube based on ionization of residual gas and plasma formation is reviewed and is shown to fail for a high-vacuum tube. A new theory is presented which is based on the production at the cathode of a vapour jet having a velocity of approximately 106 cm sec-1. This results in an expanding region of low voltage discharge, and leaves most of the applied voltage across a rapidly contracting vacuum region adjacent to the anode. It is shown that the tube current and voltage as functions of time and the X-ray pulse duration can be predicted from a knowledge of the voltage supply and tube parameters, and the results are verified experimentally.

"High-power S-band pulsed traveling-wave tube"

This paper describes a pulsed traveling-wave tube operating in the frequency range of 2400 to 3500 megacycles. The tube delivers a r-f power output of 2 kilowatts peak at 30 db minimum gain. Operating in the small signal region, with a slight readjustment of voltages, gains in excess of 45 db are realized at power outputs in the order of 50 watts. If the tube is driven into saturation as much as 4 kilowatts of output power is possible at a reduction in gain. Beam conversion efficiencies of 40 percent have been observed on a number of these tubes.

Direct direction measurements of noise in CW magnetrons

For some time the subject of noise generation in magnetrons has been under study in our laboratory. The unmodulated CW magnetron was chosen as the first subject for investigation, because some of the complications of pulsed or modulated tubes are thus avoided. Since any heterodyning method introduces the noise of another microwave oscillator, a method of direct detection has been employed. This method involves the use of a crystal-video receiver for both AM and FM detection.

Image-Converter Camera for Studies of Explosive Phenomena

A single-frame camera is described which uses a pulsed image-converter tube as a shutter to give 10−7-second exposures. The image-converter tube converts an optical image into an electron image and reconverts it into an optical image; gating or deflection in the electron phase is easily accomplished. Circuits and methods for application to single-frame photography of high explosive phenomena are given and representative photographs are shown.

The use of discharge tubes in electric circuits

Experience in the manufacture of high power microwave tubes (multi-MW range) has shown that the materials which constitute the tube are as much important as the materials which constitute the thermionic cathode itself. Indeed, where the oxide coated cathode is concerned, it has been recognized since the time of Wehnelt that the residual gas and the cathode performance are related. For a long time, the influence of the gas evolved of the electrodes and the tube envelope during operation, on cathode emission, has been recognized, too. All these phenomena have been designated “ambient phenomena” where “ambient” means the gases or vapours in the vicinity of the cathode and, in a wider sense, the sources of these gases and vapours which present the constructional parts of the tube. Since in the tube practice it is not always convenient to point out the nature of the ambient gases in function of the materials and the treatments they undergo, it has been found useful to examine the influence of the materials on an oxide coated cathode in a test diode, a technique which is in principle well known for controlling the cathode and tube materials of receiving tubes (“standard diodes”). The diode in question is adapted to the needs of pulsed tubes and allows—thanks to this fact and contrary to the usual standard diodes—to measure the cathode performance under the operating conditions except the anode voltage which is lowered. After a rapid description of this diode, it is shown that the influence of the ambients reveals three different behaviours of a I23 plot, where the available useful current (under space charge limitation) is limited either by apparition of the sparking phenomenon or by saturation. The influence of the operating conditions (d.c. or pulsed operation, pulse length, repetition frequency) on the current where the sparking occurs are demonstrated and the results obtained with certain anode materials are indicated; that is: Iron, Kovar, Titanium, Molybdenum, Copper, Platinum and Nickel. The best anode materials were platinum and nickel; in this latter case, in pulsed operation a current density of more than100Acm2 has been obtained, the limitation occuring not by saturation, but by sparking. Finally, the influence of the glass used as the diode envelope is shown. In conclusion, the diode reveals as a relatively simple control device. Nevertheless, when the nature of the ambients must be pointed out, more improved devices should be applied, such as the mass-spectrometer, a subject of the next paper.