New helix-support method for traveling-wave tubes

Traveling-wave tube helices can be supported at every turn by shrinking a nonprecision glass envelope directly on to the wires. Contact of glass and wire can occur continuously or at discrete flutes. The glass-shrinking procedure and the equipment used are described in detail.     

Strapped bifilar helices for high-peak-power traveling-wave tubes

A method of increasing the peak-pulsed power output of broad-band traveling-wave tubes is described. The method involves the use of a modified bifilar helix for the slow-wave structure. The modification employs 1) special straps or 2) mode-selective attenuation to prevent backward-wave oscillation in the anti-symmetric mode. This results in the possibility of using helices in the symmetric mode at values of ka (circumference-to-free-space wavelength ratio) as large as 0.6 at the highest amplification frequency. This in turn makes possible an increase in peak-pulsed beam power of a factor of approximately sixteen times that possible with a single helix. Both analytical and experimental results regarding the behavior of the structures are presented showing the propagation characteristics. The experimental results include cold measurements to determine ω-β diagrams and measurements with an electron beam which yield experimental values of interaction impedance.     

Light-weight aluminum foil solenoids for traveling-wave tubes

Most traveling-wave tubes use solenoids to constrain their electron beams. Each tube requires a certain diameter, length, and strength of magnetic field, which are the basic specifications for the solenoid. In addition, for airborne applications it becomes important to minimize weight and power, and to provide suitable cooling. Methods have been developed for winding coils of aluminum foil, together with a suitable insulation. The ends of the coils can be machined, then bonded to a heat-conducting plate or cooled by an air stream. There is only one insulating boundary; thus heat is readily removed from the winding. This design permits operation at high current densities without exceeding safe hot-spot temperatures, so that aluminum coils can be made about the same size, for given characteristics, as copper wire coils. The saving in weight is thus proportional to the densities of the two materials. A further interesting feature of the aluminium foil solenoids is that the ends of the coils can be machined to almost any desired shape. Slots or holes can be cut in the coils to permit the introduction of power leads or mechanical devices.     

Analysis of coupled-structure traveling-wave tubes

Until now there has been no general method available for taking into account the effect of the electron beam on coupled-structure couplers and attenuators for traveling-wave tubes. This paper presents an analytical procedure for this purpose based on the coupled mode approach of S. E. Miller and J. R. Pierce. It is valid for loosely coupled structures with small loss, only one of which is coupled directly to the beam. In addition to other relations, a general root equation is developed which takes into account loss in either or both of the coupled structures, space charge in the beam, and the possibility of different phase velocities in each structure. Typical plots of the incremental wave parameters are shown. An experimental verification of the theory is presented that shows that predictions made by means of it will be fair to good.     

High-power pulse millimeter traveling-wave tubes

A line of traveling-wave tubes operating in the 8-mm wave band with an output pulse power of 0.2–6 kW, which are suitable for application in transmitters of radar stations, is designed.     

Modified contra-wound helix circuits for high-power traveling-wave tubes

Experiments performed on modified forms of the Chodorow-Chu contra-wound helix circuit are presented with emphasis on those properties useful in the design of high voltage, high power traveling-wave tubes. Velocity and impedance measurements are shown for a fair range of pitches, crossover angles, wire widths, and wire thicknesses. Impedances are compared with those of a circuit having ideal fields (and the same phase and group velocities) and are found to be very good. Loading effects of glass and metal cylinders (envelopes), current paths, a second mode, and transitions from helix to waveguide are described. It is shown that periodic supports (stubs), used to make the structure much more rugged mechanically and capable of large heat dissipation, also increase the (already high) impedance almost as much as they decrease the group velocity, implying that the stubs add but little stored energy.     

Design and calculation procedures for low-noise traveling-wave tubes

A step-by-step procedure for designing low-noise traveling-wave tubes is presented in this paper. The procedure permits the rapid calculation of the significant design parameters corresponding to the conditions for minimum noise figure. The procedure includes the use of a special transmission-line type chart which facilitates the calculations and aids in understanding the principles involved in achieving low-noise behavior. By using the chart in connection with a second procedure, noise figure variation with frequency and with the tube parameters can be predicted for arbitrary electron gun design. Measured noise figures for two X-band low-noise tubes designed essentially by these procedures agree with the theoretically predicted noise figures to within 1.5 db.     

A gun and focusing system for crossed-field traveling-wave tubes

A problem encountered in the design of crossed-field traveling-wave tubes (particularly M-type amplifiers or backward-wave oscillators) is a limitation on current due to the restricted cathode size in the usual gun systems. The scheme considered here incorporates into a crossed-field device the techniques commonly used in O-type devices for designing converging Pierce-type strip-beam guns, which can increase the effective cathode area by perhaps ten times. A design procedure is presented for getting a well-formed beam from such a gun, which is magnetically shielded, through a fringing crossed-field region into the uniform-field region of interaction. An analysis of electron flow through the fringing-field region, including the effect of space charge, is presented. A trajectory equation, solved on a digital computer, yields trajectories for the beam and design curves for various values of the important parameters. The results show the scheme to be feasible. Results are also presented from tests on an experimental, demountable tube used to test the focusing scheme. About 90 to 95 per cent of the current entering the crossed-field region could be focused to the collector, and the system behaved generally as the design predicted. An evaluation of the experimental data showed the scheme to be useful.     

Harmonic generation in octave bandwidth traveling-wave tubes

This paper describes the investigation of harmonic generation in traveling-wave tubes (TWT's) via large-signal analysis and digital computer techniques. Efficiency degradation and harmonic power content are shown to be importantly related to such TWT design considerations as circuit dispersion, harmonic coupling impedance ratio, and gain level. Also described is the phenomenon of second harmonic interference in the beam bunching process and how it leads to substantial efficiency reduction in TWT's employing relatively nondispersive structures.     

Behavior of traveling-wave tubes near circuit cutoff

A theory is developed which explains the operation of a traveling-wave tube when operated near the cutoff frequency of the slow-wave circuit, including the effect of two circuit waves instead of the usual one. The theory is normalized in a manner analogous to that used in more conventional analyses and a first-order expansion about the cutoff frequency is used, making a relatively small number of curves applicable to a large number of cases. The relationship between this theory and the three-wave theory usually used in traveling-wave-tube analysis is shown, and they are in agreement when the system is operated far from the cutoff frequency. Numerical results are given for a range of parameters which might be useful in traveling-wave-tube design, and an excellent agreement with published experimental results is shown.     

Characteristics of traveling-wave tubes with periodic circuits

An analysis of an electron beam which interacts with a chain of coupled resonators is presented. Several important characteristics of traveling-wave tubes which employ periodic slow-wave circuits are described. It is found that, even for a lossless circuit, the gain does not become large near either pass band edge although the interaction impedance does become very large. Furthermore, useful amplification is found to occur outside the normal circuit pass band, particularly when the frequency is below the low-frequency cutoff where the circuit presents an inductive reactance to the beam. The problem of matching uniform transmission lines to the periodic circuit is discussed from the equivalent circuit point of view and it is shown that the terminating impedance which produces no reflection from the output end of the circuit when the beam is present may be appreciably different from that required when the beam is absent. The method of analysis applies to spatial harmonic operation, including backward spatial harmonics, as well as to synchronously tuned multicavity klystrons.     

Investigation of improved magnetic materials for PPM-focused low-noise traveling-wave tubes

Low-noise traveling-wave tubes focused by a combination of a uniform field in the gun region and periodic field (PPM field) over the remainder of the tube usually have a noise figure from 3 to 5 dB higher than those of tubes focused by a uniform magnetic field (PM field). This paper describes how this disadvantage of higher noise has been largely overcome as a result of improvements in magnetic materials and PPM-focusing circuit design. The electrostatic lens effects were reduced by reducing the transverse magnetic field with more uniform magnets and by reducing the period for improved transmission. The magnetic lens effect in the drift region was minimized by extending the length of the uniform gun field so that at least 3 dB of gain occurred in this region. The final tube showed improved electrical and environmental results. A maximum noise figure of 7.9 dB was achieved from 6.4 to 12.0 Gc/s. Noise figures of 6.7 to 6.9 dB were obtained over the 9.2 to 10.2 Gc/s band. The final 5.3 lb low-noise amplifier offers a substantial weight and volume reduction for most low-noise applications and demonstrated compliance with MIL-E-5400F Class 2 equipment specifications.     

Effects of high-velocity secondary electrons in traveling-wave tubes

The presence of a beam of high-speed secondary electrons in a traveling-wave tube affects the amplification and cross-modulation characteristics of the tube. When primary electrons strike the collector electrode, secondary electrons are produced. The high-speed secondary electrons have a velocity approximately equal to the primary electrons and can form a reverse beam which serves as a feedback mechanism within the traveling-wave tube. Coupling of the signal to the reverse beam and the influence of the reverse beam on the forward amplification system are investigated. The principal effects of this secondary beam are that a second-order fluctuation is superimposed on the usual variation of gain as the helix voltage is varied, and that the cross-modulation characteristics of the tube are materially altered, with the possible production of "negative" cross modulation. Experimental results demonstrate the existence of these effects and indicate that they are absent when the secondary beam is eliminated.     

An improved procedure for the design of periodic-permanent-magnet assemblies for traveling-wave tubes

A detailed analysis of the magnetic-field configurations in periodic focusing structures and new knowledge related to the design of permanent magnets which are to be used in the presence of fields of other magnets have led to the development of a new procedure for the design of periodic-permanent-magnet (ppm) focusing structures for traveling-wave tubes. With this new procedure, the ppm focusing structures can be designed very quickly, and the parameters of the structures produced will approximate the design predictions very closely. This paper describes the derivation and use of a set of design curves which have been used in the design of ppm focusing structures by the RCA Microwave Tube Operations Department for six years. It has been found that the peak axial fields of the structures produced on the basis of these design data are within five percent of the predicted values. Frequently, agreement between actual and predicted values within one percent has been observed. The design procedure requires only a few minutes time, and is applicable to any magnet material whose demagnetization curve is known. A section on the design of the soft-magnetic pole pieces ("shims") is also included.     

A large-signal analysis of beam-type crossed-field traveling-wave tubes

The equations describing a two-dimensional model of a beam-type crossed-field device are presented in a form adaptable to numerical computations. A method of computing the space-charge forces similar to that used by Tien, Walker, and Wolontis in describing the ordinary traveling-wave tube is outlined and the difficulties associated with this method are pointed out. Numerical results covering the interaction of a thin beam with a backward or a forward wave are presented for a variety of space-charge conditions. Space-charge effects reduce the large-signal gain of a backward-wave amplifier; it appears that 70 per cent conversion of available potential energy to RF energy would be excellent efficiency. Space-charge fields appear to have little influence on forward-wave interaction. For either forward- or backward-wave interaction, the computations indicate that a large fraction of the beam current is collected on a very short length of circuit, thus placing a limitation on the average power capabilities of such a crossed-field device.