Helical Slow-wave Circuit Research Articles

Microfabrication of diamond-based slow-wave circuits for mm-wave and THz vacuum electronic sources

Planar and helical slow-wave circuits for THz radiation sources have been made using novel microfabrication and assembly methods. A biplanar slow-wave circuit for a 650 GHz backward wave oscillator (BWO) was fabricated through the growth of diamond into high aspect ratio silicon molds and the selective metallization of the tops and sidewalls of 90 µm tall diamond features using lithographically created shadow masks. Helical slow-wave circuits for a 650 GHz BWO and a 95 GHz traveling wave tube were created through the patterning of trenches in thin film diamond, electroplating of gold half-helices, and high accuracy bonding of helix halves. The development of new techniques for the microfabrication of vacuum electronic components will help to facilitate compact and high-power sources for terahertz range radiation.

Effect of Thermal Strain in Helical Slow-Wave Circuit on TWT Cold-Test Characteristics

A simulated model has been established to calculate the temperature of helical slow-wave circuits accurately. The finite-element-method software ANSYS was used to analyze the thermal distortion of the traveling-wave-tube (TWT) helical slow-wave circuit. The thermal-analysis results show that the thermal stress deforms the helix primarily at the positions contacting the support rods and that the expansion of the helical tape width and thickness are very small and can be ignored. The effect of thermal strain on the helical slow-wave circuit cold-test characteristics was analyzed in detail. The periodic boundary conditions in the computer code MAFIA were employed to determine the effect on dispersion and on-axis interaction impedance. With increased helix temperature, the phase velocity decreases significantly, and the interaction impedance varies slightly.

Study of Novel Slow Wave Circuit for Miniaturized Millimeter Wave Helical Traveling Wave Tube

Two kinds of novel helical slow wave circuit, supported by Chemical Vapor Deposition (CVD) diamond, are presented. They are applying in miniaturized millimeter wave helical traveling wave tube. Cold test characteristic of these circuits are simulated by MAFIA code. Higher performances are achieved with smaller size, compared with conventional circuit supported by BeO rods. The nonlinear analysis is implemented by Beam and Wave Interaction (BWI) module, which is a part of TWTCAD Integrated Framework. Results have been found to be consistent with the expectation. It should be wider apply in microwave and millimeter wave vacuum electronic devices.

Investigation of fully three-dimensional helical RF field effects on TWT beam/circuit interaction

A fully three-dimensional (3D), time-dependent, helical traveling-wave tube (TWT) interaction model has been developed using the electromagnetic particle-in-cell code MAFIA. The model includes a short section of helical slow-wave circuit with excitation fed by RF input/output couplers, and electron beam contained by periodic permanent magnet focusing. All components of the model are simulated in three dimensions allowing the effects of the fully 3D helical fields on RF circuit/beam interaction to be investigated for the first time. The development of the interaction model is presented, and predicted TWT performance using 2.5D and 3D models is compared to investigate the effect of conventional approximations used in TWT analysis.

Effect of helical slow-wave circuit variations on TWT cold-test characteristics

Recent advances in the state of the art of computer modeling offer the possibility for the first time to evaluate the effect that slow-wave structure parameter variations, such as manufacturing tolerances, have on the cold-test characteristics of helical traveling-wave tubes (TWTs). This will enable manufacturers to determine the cost effectiveness of controlling the dimensions of the component parts of the TWT, which is almost impossible to do experimentally without building a large number of tubes and controlling several parameters simultaneously. The computer code MAFIA is used in this analysis to determine the effect on dispersion and on-axis interaction impedance of several helical slow-wave circuit parameter variations, including thickness and relative dielectric constant of the support rods, tape width, and height of the metallized films deposited on the dielectric rods. Previous computer analyzes required so many approximations that accurate determinations of the effect of many relevant dimensions on tube performance were practically impossible.

Backward-wave suppression in a very wide-band helix travelling-wave tube using a slow waveguide filter

In octave bandwidth travelling-wave tubes (TWT), employing helical slow-wave circuits, suppression of the backward-wave, which produces an unwanted oscillation just above the highest frequency of the normal operating band, is usually achieved by means of ‘lossy serpentine resonators’ deposited on the helix support rods. This technique is wholly satisfactory when the power levels involved are modest. However, when significant power levels are required, these deposited resonators are insufficiently robust and an alternative procedure is required. This paper describes an external slow-waveguide filter which extracts the backward-wave oscillation (BWO) energy in the output section of a travelling-wave tube through an array of apertures in the helix shield.

Optimum Design of a Potentially Dispersion-Free Helical Slow-Wave Circuit of a Broad-Band TWT (Short Paper)

The results of an equivalent circuit analysis are studied for a potentially dispersion-free slow-wave circuit of a TWT which consists of a dielectric-supported helix in a metal shell provided with vanes. The optimum vane dimensions are predicted, which should be helpful in broadbanding the performance of a TWT.

Design considerations and performance characteristics of microwave phototubes

Realization of the full communications potential of optical masers (lasers) requires the development of extremely broadband modulators and demodulators. Since general considerations for broadband light demodulators have been discussed previously, this paper presents specific design criteria and performance characteristics for traveling-wave microwave phototubes. Such a tube consists of a photoelectric cathode and a helical slow-wave circuit. Microwave-modulated light produces a current-modulated electron beam, which passes through the gun region and interacts with the helix. Hence, the tube acts like a traveling-wave tube amplifier in which the cathode is also the r-f input coupler. However, there are important differences between microwave phototubes and conventional TWT amplifiers. In particular, strong interaction can take place even at very low current levels since the beam can be almost purely current-modulated at the entrance to the helix. In addition, the output signal-to-noise ratio can be no better than its value at the cathode; hence, low-noise gun techniques cannot be used to suppress noise. The existence of several c-w lasers near one micron wavelength has stimulated the development of sensitive microwave phototubes for that spectral range. Particular processing techniques have been used to shift the spectral response of Ag-O-Cs photosurfaces toward longer wavelengths. Finally, performance data will be presented and compared with theoretical predictions.

Interaction of a Modulated Electron Beam with a Plasma

The results of a theoretical and experimental investigation of the high-frequency interaction of an electron beam with a plasma are reported. An electron beam, modulated at a microwave frequency, passes through a uniform region of a mercury arc discharge after which it is demodulated. Exponentially growing wave amplification along the electron beam was experimentally observed for the first time at a microwave frequency equal to the plasma frequency. Approximate theories of the effects of 1) plasma-electron collision frequencies, 2) plasma-electron thermal velocities and 3) finite beam diameter, are given. In a second experiment the interaction between a modulated electron beam and a slow electrostatic wave on a plasma column has been studied. A strong interaction occurs when the velocity of the electron beam is approximately equal to the velocity of the wave and the interaction is essentially the same as that which occurs in traveling-wave amplifiers, except that here the plasma colum replaces the usual helical slow-wave circuit. The theory predicting rates of growth is presented and compared with the experimental results.