On-axis Interaction Impedance Research Articles

Experimental Validation of Slow-Wave Phenomena in Curved Ring-Bar Slow-Wave Structure

Curved ring-bar (CRB) slow-wave structure (SWS) has been presented before in order to design an SWS for high-power and wideband traveling-wave tube in S-band. It was analyzed using the coupled transmission line theory and predicted to deliver 1-MW output power across 1.8–2.4-GHz bandwidth. However, the slow-wave characteristics were not experimentally validated through measurements. In this paper, we present $\omega -\beta $ measurement results to experimentally demonstrate that the CRB structure is providing the predicted slow-wave characteristics at the S-band. A novel synthetic technique is used to determine the $\omega -\beta $ relation using a six-period fabricated CRB structure. The measurement results exhibit a phase velocity of $0.7c$ – $0.75c$ across 2–2.5 GHz with a maximum error of less than 5%. In addition, the measured on-axis interaction impedance was $>43\Omega $ across the specified frequencies implying satisfactory agreement with theoretical predictions.

Investigation of Microstrip Meander-Line Traveling-Wave Tube Using EBG Ground Plane

A novel electromagnetic bandgap meander-line (EBG-ML) slow-wave structure (SWS) is proposed for improvement of the interaction impedance and reduction of circuit attenuation by in-phase reflection of EBG for traveling-wave tube applications. With high on-axis interaction impedance, the EBG-ML SWS can interact with a low-current-density electron beam. The backward-wave mode of EBG-ML SWS can be suppressed by the transmission stopband of EBG. Particle-in-cell simulations are carried out for the proposed structure with a sheet beam source. The simulation results show that a 50% improved interaction impedance and a 20% reduced circuit attenuation of EBG-ML SWS can be achieved, in comparison with those of traditional ML SWS. With a small input signal, the RF efficiency of EBG-ML is improved by 137.5% over that of the ML SWS.

Full-Scale Three-Dimensional Electromagnetic Simulations of a Terahertz Folded-Waveguide Traveling-Wave Tube Using ICEPIC

This paper discusses simulation and modeling of the slow wave structure of a folded-waveguide terahertz traveling wave tube (TWT) using the Improved Concurrent Electromagnetic Particle In Cell (ICEPIC) software. This is the first time ICEPIC has been used for simulation of a TWT amplifier. Cold test simulations compare favorably with analytical models; at 368 GHz, the on-axis interaction impedance is 7.8 Ω. Hot test (beam included) ICEPIC simulations were used to determine the effects of space charge on the gain calculations. At 368 GHz, the normalized beam plasma frequency from ICEPIC simulations is Ωp = 0.56. Analysis of our ICEPIC simulations at 368 GHz indicates a normalized beam plasma frequency 75% larger than an analytical model we improvised from a sheath helix model taken from the literature. The hot test ICEPIC simulations at 368 GHz for a 64 periods long slow wave structure and a 10 mA, 25 kV electron beam produce small signal gain of 27 dB. The small-signal fractional 3-dB bandwidth of the TWT is 2.9%. The saturated fractional 3-dB bandwidth is 3.2%. Large signal simulations indicate that the saturated power at 368 GHz is 39.4 dBm and the saturated gain is 21.8 dB. A snapshot of a cross section of the electron beam shows bunching in space and a corresponding modulation in the velocity of the electron beam.

Design and Development of Helix Slow-Wave Structure for Ku-Band TWT

A helix slow-wave structure (SWS) for a high-efficiency (electronic efficiency >; 25%) Ku-band 140-W space traveling-wave tube (TWT) has been designed. The cold circuit parameters of helix SWS, like propagation constant and on-axis interaction impedance, were determined using 3-D electromagnetic field simulators ANSOFT-HFSS and CST-MWS, and those were validated with the experimental results. The in-house developed large-signal code SUNRAY-1D was used to design a complete helix SWS in two sections with sever and taper to achieve desired output power, gain, efficiency, and other linearity parameters like phase shift, AM/PM factor, and I/M components. The simulated RF performance of the Ku-band 140-W helix TWT was validated with the experimental results. A close agreement between the simulated and experimental results has been found.

An Improved Helical Resonator Design for Rubidium Atomic Frequency Standards

The slow-wave structure (SWS) can be used as the resonant cavity for rubidium atomic frequency standards (RAFSs), in which the longitudinal component of the magnetic field can enable the rubidium atoms' participation in the stimulated transitions. In this paper, an improved helix SWS based on good propagation characteristics as a helical resonator for RAFS is designed. The theoretical analysis is performed together with the simulations of the propagation characteristics. Using finite integration technique (FIT), the simulated results, including phase velocity, on-axis interaction impedance, and distribution of the magnetic field, are implemented and compared with the experimented results. The effects of the different helical pitch and dielectric material in the insulator cylinder on the propagation characteristics are analyzed in detail. The analyzed results show that smaller helical pitch and proper usage of dielectric material in the insulator cylinder can make the propagation characteristics of the helix SWS superior. The improved helix SWS for RAFS adapts a smaller helical pitch and is loaded with the dielectric material of Teflon in the insulator cylinder. This structure can reduce the cavity volume to 14.7 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . The short-term stability of RAFS is up to 1.28 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> /¿ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> , and the frequency stability is 2.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> /°C at the temperature range of -20°C to +55°C.

Investigation of the Dielectric-Loaded Folded Waveguide Traveling-Wave Tube Amplifier

The cold-test characteristics of the dielectric-loaded folded waveguide traveling-wave tube (FWTWT) amplifier are investigated theoretically and the Pierce small-signal theory is employed to confirm the results. For the purpose of analysis, the discussions are separated into symmetric case and unsymmetrical case according to the loading form of the dielectrics. The calculation results indicate that both of them, especially the unsymmetrical form, can significantly reduce the phase velocity and flatten the dispersion curve. However, loading dielectric in an unsymmetrical form will lead to a large decrease in the on-axis interaction impedance, thus it is recommended to use the symmetrical form. It is further found that when the thickness of the dielectric is small, i.e. h1/a ≤ 0.1, the interaction impedance is slightly affected by the variations on dielectric constant. The Pierce linear theory for a Q-band dielectric-loaded FWTWT amplifier shows that the bandwidth is increased from 15.08% to 27.03% and the operating voltage is decreased from 20.5 kv to 18.6 kv.

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.

Traveling-wave tube cold-test circuit optimization using cst MICROWAVE STUDIO

The internal optimizer of CST MICROWAVE STUDIO (MWS) was used along with an application-specific Visual Basic for Applications (VBA) script to develop a method to optimize traveling-wave tube (TWT) cold-test circuit performance. The optimization procedure allows simultaneous optimization of circuit specifications including on-axis interaction impedance, bandwidth or geometric limitations. The application of MWS to TWT cold-test circuit optimization is described below.

Exact treatment of the dispersion and beam interaction impedance of a thin tape helix surrounded by a radially stratified dielectric

An exact dispersion relation is obtained for electromagnetic waves propagating on a thin metallic tape helix of arbitrary width, supported by a radially stratified dielectric layer and enclosed by a metallic shell. By expanding the surface currents on the tape in a series of Chebyshev polynomials, the unquantifiable assumptions made in all previously published analyzes of the tape helix regarding the forms of the surface current on the tape, or the electric fields at the radius of the tape, are avoided. The power flow and interaction impedance are exactly computed. The dispersion relation is solved numerically for slow waves and the resulting phase velocity and interaction impedance are compared to those computed using the frequently made assumptions of constant current along the tape and zero current across the tape. It is found that for wide tapes significant errors are made in both the phase velocity and interaction impedance when neglecting the transverse variation of the longitudinal current and neglecting the transverse current. For narrow tapes, the two approaches agree to good accuracy. Plots of the surface currents for wide and narrow tapes are presented. The longitudinal current shows a significant variation across the tape. An example is given showing the existence of an optimum tape width, at which the on-axis interaction impedance is maximized. It is separately shown how an approximate, but useful model of metallic vanes may be incorporated in the analysis by the modification of certain boundary conditions. In all cases, computations of phase velocity and impedance across a wide frequency band take well under a minute on a modern workstation.

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.

Novel high-gain, improved-bandwidth, finned-ladder V-band traveling-wave tube slow-wave circuit design

In this paper, the three-dimensional electrodynamic simulation code MAFIA (Solution of MAxwell's Equations by the Finite-Integration-Algorithm) is used to investigate methods of increasing the bandwidth and lowering the operating voltage of the ring-plane circuit. Calculations of frequency-phase dispersion, beam on-axis interaction impedance, attenuation, and small-signal gain per wavelength were performed for various geometric variations and loading distributions of the ring-plane TWT slow-wave circuit. Based on the results of the variations, a circuit termed the finned-ladder TWT slow-wave circuit was designed and is compared here to the scaled prototype ring-plane and a conventional ferruled coupled-cavity TWT circuit over the V-band frequency range. The simulation results indicate that this circuit has a much higher gain, significantly wider bandwidth, and a much lower voltage requirement than the scaled ring-plane prototype circuit, while retaining its excellent thermal dissipation properties. The finned-ladder circuit has a much larger small-signal gain per wavelength than the ferruled coupled-cavity circuit, but with a moderate sacrifice in bandwidth.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>