Zhaoyun Duan

W-Band 1-kW Staggered Double-Vane Traveling-Wave Tube

A design study for a W-band traveling-wave tube (TWT) using a staggered double-vane slow-wave structure combined with a sheet electron beam shows that an output power of over 1 kW should be possible. Numerical eigenmode calculations indicated that the structure has a strong longitudinal component of electric field for interaction with the electron beam. A novel input and output coupler was proposed that can produce good input and output matches. Finally, a TWT model with moderate dimensions was established. The particle-in-cell simulation results revealed that the tube can be expected to produce over 1 kW of peak power in the range from 90 to 95 GHz, assuming an RF input signal with a peak power of 0.15 W and a beam power of 10.3 kW. The corresponding conversion efficiency values vary from 9.87% to 12.15%, and the maximum gain is 39.2 dB at 93 GHz.

Sine Waveguide for 0.22-THz Traveling-Wave Tube

A novel slow-wave structure called sine waveguide has been proposed to develop a wideband high-power terahertz radiation source. The sine waveguide evolves from a rectangular waveguide oscillating with sinusoid along its longitudinal direction. This letter reports the electromagnetic characteristics of the sine waveguide and its effective surface plasmon amplification mechanism. From our calculation, this circuit structure possesses low ohmic losses and reflection and can be applied to produce terahertz waves ranging from 0.2 to 0.25 THz with several hundreds of watts. Moreover, the maximum gain and interaction efficiency may reach 37.7 dB and 9.6%, respectively.

High-Power Millimeter-Wave BWO Driven by Sheet Electron Beam

The sheet beam vacuum electron device is an attractive choice for generating high-power high-frequency microwave radiation. A millimeter-wave sheet beam backward wave oscillator (BWO) is presented in this paper. The rectangular waveguide grating structure is used as its slow wave structure. The BWO is driven by a sheet beam with a cross-sectional area of 30 mm × 1 mm which is generated by a thin cathode. For a beam voltage of 167 kV and a beam current of 1.4 kA, the output power is 40 MW at 36.6 GHz. The beam-wave interaction efficiency is about 17%, which is higher than that of conventional hollow beam BWO. It is clear from the results presented in this paper that the sheet beam device is promising for producing high-efficiency high-power millimeter-wave radiation.

Enhanced reversed Cherenkov radiation in a waveguide with double-negative metamaterials

In order to enhance the radiation energy of reversed Cherenkov radiation (RCR), on the basis of the single charged particle model, we developed a theoretical method using a charged particle beam bunch to enhance RCR in a circular waveguide partially filled with anisotropic double-negative metamaterials (DNMs). In this case, the reversed radiation mechanism is further illustrated. Numerical example shows that the radiated energy can be effectively enhanced by increasing the charged particle number in a short bunch and thus be readily detectable. The method reported here offers a theoretical basis for directly observing RCR using a charged particle beam bunch.

Symmetric Double V-Shaped Microstrip Meander-Line Slow-Wave Structure for W-Band Traveling-Wave Tube

A design study for a low-voltage, high-efficiency, and wide-bandwidth W-band traveling-wave tube using a symmetric double V-shaped microstrip meander-line slow-wave structure combined with a sheet electron beam is described in this paper. The electromagnetic characteristics including the dispersion characteristics, interaction impedance, and transmission characteristics of this structure are presented, and the beam-wave interaction is calculated using particle-in-cell algorithms. Our study shows that, when the design voltage and current of the sheet electron beam are set to 4570 V and 100 mA, respectively, this miniature millimeter-wave power amplifier is capable of delivering several tens of watts output power, and the peak output power is about 110 W with a corresponding gain of 31.4 dB and an averaged electronic efficiency of 12% at 94 GHz.

EXPERIMENTAL DEMONSTRATION OF DOUBLE- NEGATIVE METAMATERIALS PARTIALLY FILLED IN A CIRCULAR WAVEGUIDE

We have studied a new type of double-negative metamate- rials (DNMs) composed of split ring resonators (SRRs) and wire strips with substrate te∞on, suitable for generation of reversed Cherenkov radiation (RCR) which is TM radiation. We have experimentally ob- served a narrow pass band in a circular waveguide partially loaded with the DNMs and stop bands for SRRs-only with te∞on and for wire strips-only with te∞on, respectively. The experimental data show that the DNMs exhibit double-negative behavior over a frequency band of interest. This study provides a foundation for future experiment to observe RCR emitted by charged particles.

A 140-GHz Two-Beam Overmoded Folded-Waveguide Traveling-Wave Tube

A folded-waveguide (FW) traveling-wave tube (TWT) with two electron beams, which operates at the higher order mode, is simulated. Operating with two electron beams means that a larger beam current can be used for a higher output power. Meanwhile, the electric field density of the fundamental mode is not the strongest at the center of the beam channel, so the competition of the fundamental mode is significantly suppressed. A four-section two-beam FWTWT at 140 GHz is designed to get 100 W of average power. From the simulation results, we can see that this kind of tube is unstable if the gain per section is over 11 dB, because both fundamental and band-edge modes are strongly excited. Then, another way for a stable FWTWT operation with two electron beams by loading with dielectric is also presented in this paper. The results presented here can provide a new way to obtain high power radiation at the terahertz frequency.

Study on Wideband Sheet Beam Traveling Wave Tube Based on Staggered Double Vane Slow Wave Structure

In this paper, a wideband 220-GHz sheet-beam traveling-wave tube (TWT) based on staggered double vane slow-wave structure (SWS) is investigated. A novel method of loading the attenuator into the SWS for suppressing backward wave oscillation is proposed. In addition, a novel focusing electrode of the sheet beam gun is carried out in this paper, which is a whole structure but divided into two parts artificially, one is used to compress the electron beam in X-direction and the other is used to compress the electron beam in Y-direction. In addition, a novel anode is redesigned to reduce the defocusing effect caused by the equipotential surfaces. A nonuniform periodically cusped magnet is used for focusing the sheet electron beam, which is predicted to exhibit 100% beam transmission efficiency in a 75-mm length drift tube. The high-frequency characteristics of the SWS and the beam-wave interaction are also studied. The results reveal that the designed TWT is expected to generate over 78.125-W average power at 214 GHz, and the 3-dB bandwidth is 31.5 GHz, ranging from 203 to 234.5 GHz.

Accurate tape analysis of the attenuator-coated helical slow-wave structure

The tape-helix model is used to analyze the helical slow-wave structure considering the helix turns to be effectively shorted by the resistive attenuator coating on dielectric helix-support rods. An effective surface resistivity is calculated based on the resistive coating on discrete support rods. The results of the analysis are validated against reported experimental results in the special case of no attenuator coating. The attenuation constant and the phase propagation constant as well as the interaction impedance of the structure obtained by the present analysis in the tape model are compared with those obtained by the sheath-helix model reported elsewhere. The dependence of the attenuation constant, the phase propagation constant, and the interaction impedance on the effective surface resistivity is accurately predicted by our tape-helix model.

Stable Sheet-Beam Transport in Periodic Nonsymmetric Quadrupole Field

Stable sheet electron-beam transport is critical for sheet-beam microwave device which is attractive for high-power millimeter wave to terahertz-regime radiation. This paper studies the stable sheet-beam transport in periodic nonsymmetric quadrupole field. First, the conditions for stable round- and sheet-beam transport in periodic magnetic quadrupole field are deduced. In the deduction, we find that the symmetric quadrupole field and the space-charge field of sheet beam are not well matched. In order to settle this problem, we use periodic nonsymmetric quadrupole field instead of periodic symmetric quadrupole field to transport sheet beam. Finally, 3-D PIC simulations verify the conditions for stable sheet-beam transport and show that periodic nonsymmetric quadrupole field is intrinsically well suited for sheet-beam transport.