Helix Traveling-wave Tube Research Articles

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.

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.

Theoretical analysis of log-periodic circuits in microwave amplifiers

The feasibility of applying the log-periodic concept to microwave amplifiers has been investigated. This approach, successfully utilized in antenna design, offers the potential of greater bandwidth at a given power level than attainable from existing uniform devices. Log-periodic devices analogous to klystron, traveling-wave, and hybrid amplifiers have been studied. The dimensions of each section, including the beam diameter, in a log-periodic amplifier are a constant factor times those of the preceding section. Thus the general outline of such devices is conical. These devices have been simulated on a digital computer using a model in which the coupling between periodic sections is provided by one space-charge mode and one circuit mode. The accuracy of this model has been verified by simulation of existing traveling-wave tubes. Computer results on a 100kW log-periodic "klystorn," using a backward-wave circuit with a total taper ratio of 4.6 to 1, indicate that a useful bandwidth of approximately 3 to 1 can be obtained. Increasing the mechanical taper increases the bandwidth in direct proportion. Computer results on log-periodic versions of helix traveling-wave tubes are also given.

Experimental 1 kW c.w. helix travelling-wave tube for 5–10 Gc/s

A helix travelling-wave tube which was built to demonstrate the advantage of using beryllium-oxide ceramic as the supporting medium for wideband slow-wave structures is described. This experimental tube, designed to operate over the 5–10 Gc/s octave, has given a power output greater than 1 kW c.w.

An investigation of the magnetic transverse waves on an electron beam

An electron beam in the presence of an axial magnetic field supports four transverse waves; two cyclotron waves, and two synchronous waves. A general coupling theory is developed from an electronic equation and a circuit equation to describe how these waves can be coupled to traveling-wave circuits. The polarization and power flow characteristics of the waves are derived. The theory is applied to. the bifilar helix, a circuit which can couple selectively to each of the four modes. A bifilar helix traveling-wave tube was used to investigate experimentally the four beam modes and provide a quantitative check of the coupling theory.

Power limitations in helix travelling-wave tubes and the application of fluid cooling

Scaling equations are developed for broad-band helix travelling-wave amplifiers and are used to study the limitations due to cathode current density, beam interception and attenuation with cooling by radiation and conduction. The latter is more efficient but must be treated with reserve at the shorter wavelengths, owing to the added attenuation. By using the form of a theoretical equation to obtain correct dimensional behaviour and an experimentally determined factor, the attenuation calculations should be more reliable than hitherto. Limiting power/wavelength relationships for the various cases all show the allowable beam power to be proportional to a high power of the wavelength. Attenuation dissipation in particular imposes a rapid decrease in power level at wavelengths below the S-band. The possibilities of fluid-cooling small helices would from tube are then explored, a peculiarity of which is that higher-power valves at a given wavelength are favoured. The power in the beam must exceed a given level which rises as the wavelength is reduced, because the increases in helix size brought about by higher voltages, at a fixed wavelength, lower the flow impedance more rapidly than that required to cope with the increased power. Eventually, at 1-2 cm wavelength, the helix size cannot be increased without backward-wave oscillation. This method of cooling thus makes possible c.w. broad-band amplifiers with outputs of carcinotron magnitude using beam powers of the order of kilowatts at wavelengths of a few centimetres.

Discussion on “Power limitations in helix travelling-wave tubes and the application of fluid cooling”

Discussion on “Power limitations in helix travelling-wave tubes and the application of fluid cooling”

Wave Propagation Along a Magnetically-Focused Cylindrical Electron Beam

This paper analyses the nature of wave propagation along a cylindrical electron beam, focused in Brillouin flow by means of a finite axial magnetic field. Two different types of conducting boundaries external to the beam are treated: (1) the concentric cylindrical tube, forming a drift region; and (2) the sheath helix, forming a model of the helix traveling-wave tube. The field solution of the helix problem is used to evaluate the normal-mode parameters of an equivalent circuit seen by a thin beam, thereby permitting computation of the gain constant of growing waves. The gain constant of the cylindrical beam with Brillouin flow is found to exceed that of a similar beam with rectilinear flow, presumably because of the transverse component of electron motion in the former

Traveling-Wave Amplification by Means of Coupled Transmission Lines

The theory of interaction between coupled transmission lines moving relative to each other is developed, and conditions yielding waves increasing exponentially with distance (amplification) are found. The case of lossless lines, one moving and one stationary, is analyzed in considerable detail, and the results are shown to be applicable to the helix traveling-wave tube. The theory also suggests the possibility of traveling-wave amplification in systems other than electrical, and a mechanical traveling-wave oscillator has been built.