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To explore the potential of orbital angular moment (OAM) imaging in underwater condition, a water-immersed antenna has been designed at 10GHz in this paper, the S 11 is optimized to -50dB, and the bandwidth is 4GHz. Based on the above antenna, an uniform circular array (UCA) with 16 elements are simulated to generate electromagnetic (EM) wave carrying OAM, the results of vortex EM fields of OAM modes 0-7 are obtained, high-qulity under water OAM modes can be generated at 10GHz efficiently.

A modified microstrip meander line (MML) slow-wave structure (SWS) embedded with dielectric rod for miniaturized traveling wave tubes (TWTs) is presented in this paper. High-frequency characteristi...

The staggered double vane was employed as the slow wave circuit and a transition structure and an output coupler were proposed to design a W-band backward wave oscillator (BWO) operating with sheet electron beam. The transition structure and the output coupler can effectively reduce the attenuation and reflection of such type of straight-waveguide device. Compared to conventional pencil beam device, the oscillator can provide much higher output power. The dynamic process between the electron beam and the electromagnetic wave was simulated by three dimensional particle-in-cell (PIC) code. The simulation results demonstrate that the BWO can provide a stable output power in the order of a few watts in the frequency range from 92 GHz to 98 GHz by adjusting the operation voltage, assuming a constant operation current of 12 mA. The output signal shows good monochromaticity, with the central frequency quite close to the design.

This paper investigates the properties of traveling wave-beam interaction in a rectangular helix traveling-wave-tube (TWT) for a solid sheet electron beam. The ‘hot’ dispersion equation is obtained by means of the self-consistent fleld theory. The small signal analysis, which includes the efiects of the beam parameters and slow-wave structure (SWS) parameters, is carried out by theoretical computation. The numerical results show that the bandwidth and the smallsignal gain of the rectangular helix TWT increase as the beam current increases; and the beam voltage not obviously in∞uences the small signal gain. Among difierent rectangular helix structures, the small-signal gain increases as the width of the rectangular helix SWS increases, however, the bandwidth decreases whether structure parameters a and

In this paper, relations between photonic band gaps and operating mode of slow-wave circuits constructed by photonic crystals are discussed, and methods for computing photonic band gaps and determining the operating band are given. As an example, a slow-wave circuit containing two-dimensional photonic crystals is analyzed. The results explain that complete photonic band gap is not necessary for traveling wave tubes, and more than one defect-mode maybe coexists inside one photonic band gap. If there have no complete photonic band gap, special input and output circuits capable of customizing modes are necessary. Compared with conventional traveling wave tubes, the numerical results do not suggest the bandwidth of the photonic crystal traveling wave tube is wider, and the interaction impedance is lower.

In this paper, we present a design where a bunched relativistic electron beam traversing inside the rectangular dielectric-loaded (DL) waveguide is used as a high power microwave generation device. Two kinds of methods of calculating the electromagnetic (EM) field excited by a bunched beam are introduced, and in the second method the calculation of EM pulse length is discussed in detail. The desired operating mode is the LSMn due to its strong interaction with the electron beam. For the designed 7.8 GHz operating frequency, with a 100 nC/bunch drive train of electron bunches separated by 0.769 ns, we find that high gradient (> 30 MV/m) and high power (> 160 MW) can be generated. An output coupler is also designed which is able to extract the generated power to standard waveguides with a 94% coupling efficiency.

A period-varying folded waveguide is formed by varying the period of a folded waveguide. It has the advantages of the space harmonic selectivity and the wide bandwidth. However, the regularities of the variety of these period-varying folded waveguides are unavailable from the published papers. In order to solve this problem, the principle of the space harmonic selectivity of a period-varying folded waveguide is analysed, and the conditions to select the space harmonic for this slow wave system are obtained. In addition, the space harmonic selectivities for a linear period-varying folded waveguide and a hyperbolic sine-varying period folded waveguide are also analysed as examples.

The periodic nonuniform folded waveguides are special structures, the physical dimension of which is between the periodic folded waveguide and the tapering period folded waveguide. Therefore, the synchronization between the microwave and the electron beam can be maintained in the whole interaction process and the periods are not tapered. In comparison with the tapering period folded waveguide, the theoretical analysis and the technological requirements for this structure are more convenient. In order to study this structure, the space harmonics are analysed, the conditions to make the m-th space harmonic synchronizing with the electron beam in the whole interaction process are present, and the dispersion curve and the coupling impedance curve are obtained by the simulation software HFSS.

A new type of printed rectangular helix slow-wave structure (SWS) is investigated using the field-matching method and the electromagnetic integral equations at the boundaries. The radio-frequency characteristics including the dispersion equation and the coupling impedance for transverse antisymmetric (odd) modes of this structure are analysed. The numerical results agree well with the results obtained by the EM simulation software HFSS. It is shown that the dispersion of the rectangular helix circuit is weakened, the phase velocity is reduced after filling the dielectric materials in the rectangular helix SWS. As a planar slow-wave structure, this structure has potential applications in compact TWTs.

On the basis of a rigorous field theory, two different physical models of attenuator and sever have been proposed. One is named High attenuation (HATT) model in which both attenuator and sever are considered as a unified attenuator, but the sever is regarded as an area of very high loss; the other is called Sever and attenuator (SATT) model in which the sever is modelled as a drift area in which the electric and magnetic fields both vanish. A complex function is derived and potential sinking effect is also considered. Thus, a set of more practical self-consistent equations of nonlinear beam-wave interaction is formulated. Simulations are carried out under the conditions of the two different physical models, and the simulation results are compared with the experimental data. The results show that in the case of single signal drive, the unknown second harmonic should be included for predicting the saturated output power. It is also evident that the SATT model and the HATT model predict the same physical nature, whereas the results predicted by the HATT model are much closer to the experimental data than those obtained from the SATT model. Therefore, these results provide a strong theoretical basis for designing broadband and high gain helix travelling wave tubes.

The output section of a helix travelling wave tube usually contains a helix pitch taper for high rf electron efficiency. By keeping the rf field as synchronous as possible with the decelerating electron beam bunches, the rf field can extract much more energy from the beam, and thus the maximum electron efficiency can be realized. Recently, a global simulated annealing algorithm has been employed to design the helix pitch profile so as to improve the electron efficiency as much as possible. From the numerical results, it is concluded that the electron efficiency can be enhanced by about 4%–8%.

We present a novel helix slow wave structure with an elliptical cross section shielded by an elliptical waveguide. The rf characteristics including dispersion properties, interaction impedance of zero mode in this structure have been studied in detail. The theoretical results reveal that weaker dispersion even abnormal dispersion characteristics is obtained with the increasing eccentricity of the elliptical waveguide, while the interaction impedance is enhanced by enlarging the eccentricity of elliptical helix.

In this paper, five modified Tunneladder slow-wave structures (SWS), the ridge-loaded stub-supported meander line with circular electron tunnel (CET-RSML), ring-plane frame line (RPFL), the ridge-loaded thick ladder line (RLTLL), the ridge-loaded pole-piece folded-waveguide circuit (RLPFWL), and the double-period Tunneladder line (DPTL) were discussed. The dispersion characteristics and the interaction impedances of the five modified Tunneladder SWS were calculated by simulation and the corresponding analytic models. The calculation results explain that the CET-RSML and the RLTLL hold wider bandwidth in comparison with the Tunneladder, when the volume dimensional lengths are same. When the bandwidth is definite, the interaction impedance of RLTLL can be higher than Tunneladder, but that of CET-RSML can’t. The dispersion curves of the RLPFWL and the DPTL are more linear in comparison with Tunneladder, but the interaction impedances are very low.

A slow-wave structure (SWS) with two opposite gratings inside a rectangular waveguide is presented and analysed. As an all-metal slow-wave circuit, this structure is especially suited for use in millimetre-wave travelling wave tubes (TWTs) due to its advantages of large size, high manufacturing precision and good heat dissipation. The first part of this paper concerns the wave properties of this structure in vacuum. The influence of the geometrical dimensions on dispersion characteristics and coupling impedance is investigated. The theoretical results show that this structure has a very strong dispersion and the coupling impedance for the fundamental wave is several tens of ohms, but the coupling impedance for –1 space harmonic wave is much lower than that for the fundamental wave, so the risk of backward wave oscillation is reduced. Besides these, the CST microwave studio is also used to simulate the dispersion property of the SWS. The simulation results from CST and the theoretical results agree well with each other, which supports the theory. In the second part, a small-signal analysis of a double rectangular waveguide grating TWT is presented. The typical small-signal gain per period is about 0.45 dB, and the 3-dB small-signal gain bandwidth is only 4%.

The rectangular waveguide grating slow-wave structure (SWS) with arbitrary shaped grooves is presented and analyzed in this paper. As an all-metal slow-wave circuit, it has properties that can be used in high-power millimeter-wave or sub-millimeter wave traveling wave tube (TWT). The unified dispersion equation and the expression of coupling impedance are obtained in this paper by means of an approximate field-theory analysis, in which the profile of the groove is approximately replaced by a series of steps and the field continuity at the interface of two neighboring steps together with the field matching conditions at the interface between the groove region and the interaction region are employed. A rectangular groove SWS was manufactured and the cold measurement was made. The experimental data are in good agreement with the numerical calculation. The derived transcendental equations are resolved numerically for four classical structures such as rectangular, dovetail, ladder and cosine. Finally, taking the rectangular waveguide grating SWS with rectangular grooves for example, the influences of physical dimensions on dispersion relation and coupling impedance are discussed.

We present the formulation of elliptical corrugated waveguides with a concentric circular hole using the field-matching method and the addition theorem for Mathieu functions. The dispersion equation and the mean interaction impedance of this structure are derived separately. The numerical results, which are generally based on the current approach, agree well with the results obtained by the commercial software package CST. As a slow-wave structure, this structure has potential applications in high power microwave amplifiers and possibly filtering structures.

A model for studying the ionization effects in a microwave tube has been developed. This model is simulated by a two-dimensional particle-in-cell code with the Mont Carlo collision model for the electron-neutral ionization process. The transient-state process of ion noise and ion focusing effects are observed. A simple theory about ion motion is given for interpreting the phenomenon of the ion moving to the wall of the tube when the beam is not neutralized. The computed result agrees with the experiment and simulation result.

A time-dependent non-linear theory is developed for ion channel electron cyclotron maser (ICECM), and the corresponding numerical calculation results are given for the first time. The theoretical analysis shows that the longitudinal plasma wave tends to enhance the energy exchange between the beam and wave. Using the numerical calculation of the self-consistent time evolution of scattering wave growth, the non-linear effect of ion density on the beam-wave interaction efficiency has been studied. The time norm, which must be satisfied in the operation of ICECM, is verified. Taking 6.8 × 1013 cm−3 ion density, 1 MV accelerated voltage and 1.5 KA beam current, the present numeric calculation results show that the pulse scattering wave power and the frequency can achieve about 200 MW and 284 G rad/s respectively.

We present a new periodic all-metal slow wave structure, a coaxial step-disc-loaded system and the dispersion characteristics of the structure. By using the field-matching method, the dispersion equation and the coupling impedance of this structure are obtained. The coaxial structure makes the bandwidth broader than that of the non-coaxial one. Compared with the coaxial disc-loaded and ridged-disc-loaded structures, the pass-band of coaxial step-disc-loaded structure is the broadest. The calculation results show that increasing the step width and decreasing the step thickness can improve the bandwidth.

A rhombus-shaped microstrip meander-line slow-wave structure is proposed for use in a low voltage, wide bandwidth, moderate power and high efficiency millimeter-wave traveling-wave tube. The structure is evolved from the original V-shaped microstrip meander-line slow-wave structure. Compared with the conventional slow-wave structure, it is a kind of planar structure, whose fabrication can be easily realized by utilizing the technology of 2-D micro-fabrication. Whats more, the structure can realize beam-wave interaction with sheet electron beam, and no additional electron beam channel is needed. The electromagnetic characteristics and the interaction between the sheet electron beam and slow wave in the structure are obtained by utilizing the electromagnetic simulation software HFSS and the particle-in-cell code in CST Particle Studio, respectively. Our calculations indicate that, at a beam voltage of 7 kV and a beam current of 90 mA, the traveling wave tube based on the structure is capable of delivering several tens of watts output power with an interaction efficiency of 14.3％ and a transient 3 dB bandwidth of 18 GHz(ranging from 132 GHz to 150 GHz)

A novel sine-shape ridge waveguide (SSRWG) is put forward to develop the ultra-broadband high power terahertz (THz) backward-wave oscillator (BWO). It is found that this kind of slow-wave structure (SWS) possesses very wide 'cold' bandwidth and quite low transmission losses from the analyses of the electromagnetic characteristics. Moreover, the beam-wave interaction results indicate that the BWO based upon the SSRWG SWS can produce the output power exceeding 0.625 W in the frequency range from 0.617 THz to 0.99 THz. In particular, the output power at the typical operating frequency of 0.853 THz can reach 1.186 W by using the sheet electron beam of 16 kV and 8 mA. Therefore, this device has great potential to be an ultra-broadband watt-level THz radiation source.

A slow-wave structure (SWS) named micro-strip angular log-periodic meander-line is proposed for a low operating voltage vacuum amplifier. It evolves from the logarithmic spiral by cutting off a fan-shaped part and connecting the ends alternately. This letter reports the electromagnetic characteristics of this kind of SWS, and the beam-wave interaction is calculated by means of the particle-in-cell (PIC) method. From our calculation, this kind of traveling-wave tube can give 26 W output power with the 3 dB frequency range of 27-46 GHz, while the operating voltage is 809 V only. Moreover, the maximum gain and interaction efficiency are about 26.5 dB and 21.7%, respectively. Through power combination of SWSs of this kind, power of hundreds of watts can be output.

By field matching method, we have derived strictly out the dispersion equation and coupling impedance of ridge-loaded helical groove. Through numerical calculation of these formulas, we could get the relative curves of dispersion and coupling impedance with geometrical dimensions. The analysis in this paper has provided theoretic basis of the designing and calculating for ridge-loaded helical groove millimeter TWT.