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Study on two kinds of novel 220 GHz folded-waveguide traveling-wave tube

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2014 Jpn. J. Appl. Phys. 53 036201

(http://iopscience.iop.org/1347-4065/53/3/036201)

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Study on two kinds of novel 220GHz folded-waveguide traveling-wave tube

Minghao Zhang1, Yanyu Wei1, Guo Guo1, Lingna Yue1, Yuanyuan Wang1, Xianbao Shi1,Xianfeng Tang1, Yubin Gong1, Wenxiang Wang1, and Dazhi Li2

1National Key Laboratory of Science and Technology on Vacuum Electronics, School of Physical Electronics,University of Electronic Science and Technology of China, Chengdu 610054, China2Institute for Laser Technology, Suita, Osaka 565-0871, Japan

Received July 5, 2013; accepted December 17, 2013; published online February 21, 2014

Two kinds of novel 220GHz folded-waveguide (FWG) slow-wave structure (SWS) with different electron-beam tunnels are presented for producinga high power and considerable bandwidth. These structures, which have the potential to have a better performance than the conventional FWGSWS, are suitable for circle-beam electron guns and sheet-beam electron guns, respectively. In this study, the electromagnetic characteristics andnonlinear interaction between the electron beam and the electromagnetic field of the two kinds of novel FWG are investigated on the basis ofsimulation results. The influence of the beam tunnel with respect to its transverse shape and size on the circuit performance is investigated indetail. With different beam tunnels, the two novel FWGs exhibit similar radio-frequency characteristics and signal gain. Particle-in-cell simulationresults reveal that the novel tubes exhibit a gain greater than 32dB and a bandwidth of 10% with a 16.5 kVand 150mA electron beam and a 90mWpeak input power. Compared with the conventional FWG SWS, the novel FWGs have 32% higher output power under optimized conditions.

© 2014 The Japan Society of Applied Physics

1. Introduction

The terahertz (THz) range is generally regarded as beingfrom 100GHz to 10THz in the electromagnetic spectrum.1)

Many effective THz electron devices and componentshave been proposed.2–4) The folded-waveguide (FWG)slow-wave structure (SWS),5–9) as shown in Fig. 1, is anexcellent candidate for a traveling-wave tube (TWT)operating in the THz-wave band. Owing to its advantagesof a relatively wide bandwidth, large power capacity, simplecoupling structure, and easy fabrication with microfabrica-tion technologies, the FWG has attracted wide attention fordeveloping compact millimeter- and THz-wave radiationsources. Moreover, in relatively high power millimeter-or THz-wave vacuum electron devices, there is an increasingtrend to incorporate a sheet electron beam instead of a solidcircle beam.10) Owing to the benefits of lower space chargefields and a large current transport capacity, many SWSswith a planar configuration that are relatively easy tomake through microfabrication technologies have beenreported.11,12)

Recently, based on the earlier investigation of FWG TWTs,considerable effort has been devoted to searching for novelstructures with a circle-beam tunnel or sheet-beam tunnelto improve the performance of tubes.13–16) In this paper, twokinds of novel FWG structure with a circle-beam tunnel andsheet-beam tunnel are proposed for TWT application. Thethree models of the conventional FWG and novel FWGsare shown in Fig. 1. Three periods of the conventional FWGare shown in model 1 of Fig. 1. As depicted in model 2, thenovel FWG is suitable for a circle-beam tunnel, which isformed by adding a square tunnel in the middle of model 1and extending the square tunnel to the bends of the FWG.We conceived of this method to increase the interactionarea and change the electric field distribution to increase theinteraction impedances and subsequently increase the outputpower.17–20) Moreover, for the novel FWG with a sheet-beamtunnel, as shown in model 3 of Fig. 1, the fabrication processof the circle-beam tunnel can be omitted. In Fig. 1 andTable I, the primary structure parameters of the three modelsand the design values are given.

Fig. 1. (Color online) Three kinds of FWG structure discussed in thispaper. Model 1 shows a 3D view of the conventional FWG. Models 2 and 3show 3D views of the novel FWGs with a circle-beam tunnel and sheet-beamtunnel, respectively.

Table I. Parameters for the typical structure of SWS (in mm).

a 0.85

b 0.15

d 0.1

h 0.25

p 0.25

r 0.1

Japanese Journal of Applied Physics 53, 036201 (2014)

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Figure 2 shows the electric field distributions of models 1and 2 obtained by using the three-dimensional (3D) electro-magnetic (EM) simulation software CST Microwave Studio(CST MWS).21) As we can see from the figure, extendingthe square tunnel will affect the EM field distribution inthe beam tunnel section. Compared with model 1, the EMfield in the electron beam tunnel of model 2 is stronger.Moreover, the novel FWGs can adopt not only a circleelectron beam gun but also a sheet electron beam gun. Allthese features can give the novel FWSs a broader rangeof applications.

2. Analysis of the radio-frequency characteristics

The radio-frequency characteristics of the two novel FWGsand conventional FWG are analyzed by CST MWS. In orderto make the comparison meaningful, the parameters of thethree models are set to be the same. The typical parameters

of the FWGs are shown in Table I. The width d of the squaretunnel is investigated because it has a significant effect onthe radio-frequency characteristics. The novel FWGs havepotential to improve the output power, which is shown by thefollowing results.

A comparison of the dispersion relation between the con-ventional FWG and novel FWGs for the fundamental mode isshown in Fig. 3. As can be seen, the center frequency of thenovel FWGs is slightly higher than that of the conventionalFWG, and the normalized phase velocity of the novel FWGsis also higher than that of the conventional FWG. Figure 4shows the average interaction impedance of the first spatialharmonic of the three FWGs calculated at the beam tunnel. Itis shown that the average interaction impedance of the novelFWGs is about 1.5³ larger than that of the conventionalFWG over the entire bandwidth, which suggests strongerbeam-wave interaction and higher efficiency. Moreover, the

Fig. 2. (Color online) Comparison of electric field distributions of the models 1 and 2.

Fig. 3. (Color online) Dispersion property of the three models. Fig. 4. (Color online) Average interaction impedance of the three models.

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average coupling impedance over the cross section of theelectron beam is calculated. As many as 13 points on thecircle beam cross section and 15 points on the sheet beamcross section are involved in the calculation to evaluate theaverage interaction impedance as shown in Fig. 5. When weinvestigate the effects of changing the value of d, the crosssection of the electron beam should be changed in the sameproportion. The ohmic loss of each model is analyzed withthe circuit material set to copper with an electric conductivityof 5.8 © 107 S/m without considering surface roughness.22)

As can be seen in Fig. 6, the loss of the novel FWGs is lowerthan that of the conventional FWG over the entire bandwidth,and the loss of model 2 is higher than that of model 3. Theresults show that the novel FWGs have lower ohmic loss thanthe conventional FWG.

The square tunnel width d is an important parameterrelated to SWS fabrication and electron transmission in thenovel FWGs. Figures 7–9 respectively show the influencesof the square tunnel size d on the dispersion relation, interac-tion impedance, and ohmic loss of model 2. As d increases,the center frequency, the normalized phase velocity, andthe interaction impedance increase while the ohmic loss ofthe FWG decreases. Figure 10 shows the transmission char-acteristic curves for different values of d for model 2. Thesimulation results indicate that as the width of the squaretunnel increases, S11 becomes larger. When the width d isgreater than 0.3mm, S11 exceeds ¹10 dB, which means thatthe SWS is impractical. In order to make novel FWGs withsuitable radio-frequency and transmission characteristics, asuitable value of d must be selected.

3. Analysis of beam-wave interaction (large signal)

The nonlinear interaction between the electron beam andEM field in the novel FWGs is simulated by using the 3Dparticle-in-cell (PIC) code CST Particle Studio. All the calcu-lations are conducted with the assumption that the struc-ture material is copper without the roughness of the struc-ture surface taken into consideration. Two novel interactioncircuit models with different beam tunnels are illustrated inFig. 11. An electron beam gun, a serpentine waveguide for

Fig. 5. Sketch of electron beam tunnel and beam cross section.

Fig. 6. (Color online) Ohmic loss of the three models.

Fig. 7. (Color online) Dispersion property versus square tunnel width d ofmodel 2.

Fig. 9. (Color online) Ohmic loss versus square tunnel width d ofmodel 2.

Fig. 8. (Color online) Average interaction impedance versus squaretunnel width d of model 2.

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slowing down the EM wave, a tunnel for the electron beamto pass through, and a sever within the serpentine circuit areconstructed in two PIC simulation models. Figure 11 alsoshows the input/output couples in the novel FWGs, whichare well matched to the signal source and to the load. Withthe input/output couples of model 2 [Fig. 11(a)], we choosethe first few periods at both ends of the SWS and decreasethe width of the original square tunnel in the couplerslinearly. For model 3 [Fig. 11(b)], the couplers are rotated90°. This design enables the electron beam to be injected.The optimized electrical parameters of the FWGs are shownin Table II. The magnetic fields used for focusing for models1, 2, and 3 are 0.3, 0.4, and 0.5 T, respectively. These valuesare optimized by electron beam trajectory analysis. Note thatthe lengths of the three models are different because of thedifference in the average interaction impedance.

Although the SWS can be well matched to the signalsource and to the load, there will normally be reflectionsat the input and output terminals because of the difficulty

in making impedance transitions from the RF structure.To prevent the occurrence of oscillations, a sever is used inTWTs. In our PIC simulation models, the sever is employedto cut off the circuit between the input and output portsin order to suppress backward waves. The sever is modeledby removing the bending waveguide at the sever position,and the two straight sections are extended outward. The openboundary condition is applied to the port of the straightwaveguides. Thus, the sever model is an ideal one.

The ordinary, or O-type, TWT employs a magneticallyfocused electron beam. The electron beam velocity isadjusted to be approximately equal to the phase velocityfor an EM wave propagating along the SWS. When anelectron beam is injected along the axis of the SWS, the axialelectric field components accelerate some electrons anddecelerate others. Under these conditions, electron bunchingcan occur in the SWS. The energy extracted from the electronbeam in slowing the electrons to the bunch velocity istransferred to the circuit field, thereby amplifying the field.Figure 12 shows electron beam bunching and electric fieldpatterns near the output port (left side) of model 2, indicatinga strong beam-wave interaction. The sever prevents reflec-tions from the load and the output terminal from reaching theinput terminal. Although the forward growing wave in theinput section of the SWS is lost at the sever, current andvelocity modulation remain on the electron beam enablingthe signal to across the sever region for further amplificationin the output section.

Since the PIC simulation results of model 3 are similar tothose of model 2, here we mainly give the typical simulation

(a)

(b)

Fig. 11. (Color online) Interaction models of model 2 (a) and model 3 (b)with input and output couples.

Table II. Operating parameters.

Model 1 Model 2 Model 3

Beam voltage (kV) 15.9 16.5 16.5

Beam current (mA) 150 150 150

Focusing magnetic field (T) 0.3 0.4 0.5

Length of first section (mm) 14 12.5 12.5

Length of second section (mm) 20 18 16

Input signal power (mW) 90 90 90

Fig. 10. (Color online) S11 parameter curves with different square tunnelwidths d of model 2.

Fig. 12. (Color online) Electron beam bunching and electric field patternsnear the output port of model 2.

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results of model 2 when the electron dynamic system hasbeen in the steady state. Figure 13 shows the PIC simulationresults of the input and output signal amplitudes of model 2.It can be seen that after the process of beam-wave interaction,the input signal of 0.3V is amplified to 11.5V with anelectron efficiency of 5.3%. Because the termination ismatched well, the reflected power remains low and oscillationwas not observed. Figure 14 shows a phase momentum plotof the bunched electron beam along the axial direction at 3 ns.As the electron beam propagates along the circuit, most of theelectrons are decelerated at the end of the circuit. The energyof the electron beam is converted to electromagnetic energy.

Figure 15 plots the average electromagnetic power versusthe axial distance of models 2 and 3. In the inset plot, it isobserved that the electromagnetic power increases slightly(about 14 dB) in the input section and decreases dramaticallyat the position of the sever (12.5mm). Because of themodulated electron beam, the power continues to increaseuntil the RF peak power is 130W at the end of this circuit.Figure 16 shows that the frequency spectrum of the outputsignal is very stable and pure.

Comparisons of the interaction performance from 194 to250GHz for the three FWG TWTs are shown in Figs. 17 and18. The simulations are carried out by CST Particle Studiowith nearly the same number of mesh cells and particles.These figures show the amplitude versus frequency and gainversus frequency characteristics of the output signals of the

Fig. 13. (Color online) Input and output signals of model 2 at 220GHz.

Fig. 14. (Color online) Phase momentum plot of the bunched electron beam of model 2 at 220GHz.

Fig. 15. (Color online) Power growth in the circuit along the axialdirection of models 2 and 3.

Fig. 16. Frequency spectrum of output RF signal at 220GHz for model 2.

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three models with a fixed input power of 90mW. Becauseof the difference in phase velocity between the sheet-beamtunnel and the circular-beam one, the frequencies with themaximum gain are different for the two novel structures.With the same size parameters, the comparison of the outputpower between the conventional FWG and novel FWGs isillustrated. From the figures, it can be seen that the novelFWGs can work at higher frequencies and produce largeroutput power. Although the bandwidth of the novel FWGs isslightly narrower than that of the conventional FWG, theycan be used in many applications.

4. Conclusions

Two novel FWGs have been investigated for a TWToperating in the THz band. The performances of the THzband FWGs were studied by PIC simulation. The outputpower and gain of the novel FWGs were compared withthose of the conventional one under the same working con-ditions except for the beam voltage. The simulation results

indicate that the novel FWGs have better “cold” and “hot”performances than the conventional FWG. The novel FWGscan produce a higher output power and work in a higherfrequency band. More importantly, the novel FWG with thesheet-beam tunnel is easier to fabricate than the conventionalFWG, while the novel FWG with the circle-beam tunnel canadopt the relatively mature focusing and transmission elec-tron optical systems of a circle electron beam, which meansthat most of the existing design of the conventional FWG canbe redesigned by this method. All these characteristics of thenovel FWGs reveal their potential applications in high-powermillimeter- and THz-wave TWTs.

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Fig. 17. (Color online) Plot of signal output peak power versus frequency.

Fig. 18. (Color online) Plot of signal gain versus frequency.

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