Ultra-low noise figure traveling-wave tube in X-band

Noise figures of 2.8 dB have been achieved in an X-band TWT by increasing the magnetic field, cooling to liquid nitrogen temperature, and inverting the second anode potential.     

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.     

Correction of the dispersion characteristic of a helix slow-wave structure in traveling-wave tubes

On the basis of a physical notions of the specific features of the slow-wave field distribution in the helix structure of a traveling-wave tube (TWT), various methods for controlling the amount of wave slowing and the slope of the dispersion characteristic are analyzed. The effect of the dielectric supports and correcting elements placed outside the helix is considered. In addition to the known methods for correcting dispersion through the use of a dielectric shield and longitudinally conducting elements, which weaken the interaction with the electron flow, new methods that provide both an efficient interaction with the electrons and the possibility of operation without backward-wave self-excitation at voltages higher than the voltages used with conventional helices are proposed and analyzed. With an analysis based on the equivalent-line method, it is shown that the slope of the helix dispersion characteristic can be decreased by the appropriate use of ring-shaped supports. Design solutions using the new methods of correction are considered. The history of development of ultrabroadband TWTs, including the work done in the Soviet Union on tubes with centrifugal electrostatic focusing, whose actual amplification band can be as wide as three octaves, is partially presented.     

The effect of various design parameters on the performance of medium-power traveling-wave tubes

In the design of traveling-wave tubes, the effect of changing such design parameters as beam current, frequency, or helix diameter is usually not immediately obvious. This paper makes the effect of some major design parameters more apparent by introducing certain approximations. Within the limits of the approximations, the gain-helix radius product is shown to depend only on beam perveance and a geometry-determined factor F. For a given geometry (defined as the ratios of all radial dimensions),CandQCare derived fromgamma aand beam perveance. Thus, universal graphs for gain per unit length,CandQC, are evolved. By keepinggamma a, beam perveance and tube geometry invariable, the RF behavior of traveling-wave tubes can be preserved. Consequently, scaling equations for traveling-wave tubes can be derived, and are presented in this paper.     

The effect of a space charge on amplification in traveling-wave tubes with periodic slow-wave systems

The effect of a space charge on solutions to the previously obtained universal characteristic equations and, accordingly, on amplification in traveling-wave tubes (TWTs) with periodic slow-wave structures (SWSs) is considered. The peculiarities of electron waves differing from the predictions of the Pierce theory, which is valid only for TWTs with smooth SWSs, are reported.     

The effect of circuit tapering on the efficiency bandwidth characteristics of dispersive traveling-wave tubes

The dispersive characteristics of the loaded-waveguide type interaction structures commonly employed in high-power traveling-wave tubes cause a large variation of efficiency with frequency. This efficiency variation can be reduced by period tapering of the output section of the tube. The physical reasons underlying the improvement in tube performance, calculated results using large-signal traveling-wave tube equations and experimental confirmation of the calculations are described.     

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.     

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.     

Some aspects of circuit power dissipation in high power CW helix traveling-wave tubes, part I: General theory

This paper describes the basic theory underlying calculations of the thermal behavior of helix-type slow wave circuits used in high power traveling-wave tubes. Typically, the helix is supported by means of three ceramic rods in a metal shell with the helix brazed to the rods. At the highest power levels the support rods are also brazed to the tube shell. The results of calculations on specific structures are presented which show the use of different support rod and helix materials, of beam interception, and of brazing the support rods to the shell. Of particular interest is the conclusion that the temperature difference between the hottest and coldest parts of the helix can be significantly larger than the temperature drop across the support rods when using high thermal conductivity ceramic materials brazed to the tube shell.     

The effect of gate leakage on the noise figure of AlGaN/GaN HEMTs

The effect of gate leakage on the noise figure of AlGaN/GaN high electron mobility transistor (HEMTs) is explored. It is shown that these devices have a sizable amount of gate leakage that cannot be ignored when measuring their noise performance. Measurements across a single sample have more than 1 dB of variation in minimum noise figure. We will show this variation is because of gate leakage. A modified van der Ziel model is used to predict this large variation and allows easy noise figure prediction of HEMT and MESFET devices.     

Some aspects of circuit power dissipation in high power CW helix traveling-wave tubes, part II: Scaling laws

This paper extends the basic theory of the thermal behavior or helix-type slow wave circuits described in Part I [1] to show the effect of scaling to higher frequencies. The differences in the scaling laws which result from the alternative assumptions of constant beam voltage or constant beam perveance are discussed, and a simplified formulation permitting the thermal behavior to be described in terms of universal curves is presented.     

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.     

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.     

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.