Beam-wave Interaction Research Articles

Development of a novel overmoded sub-terahertz inclined coaxial clinotron with asymmetric mode suppressed

Overmoded coaxial high frequency structures can improve the power capacity of vacuum electron devices (VEDs) and make the fabrication and assembly of the devices easy in the terahertz band, but they also lead to mode competition and deleterious effect on the beam-wave interaction. The numerical investigation on the cold cavity characteristics indicates that the dispersion curve of the quasi-TEM mode almost overlaps with the dispersion curve of the high-order quasi-TE11 mode, and the 3-D numerical simulation results also illustrate that due to the mode competition, the quasi-TEM cannot be excited steadily in the VEDs with traditional coaxial SWSs with the available current density. In order to effectively suppress the mode competition of overmoded coaxial SWSs, novel inclined coaxial slow wave structures (SWSs) are proposed in this paper. As the overmoded ratio of the SWSs reaches 9.7, the 3D particle-in-cell simulation results indicate that this novel device can successfully produce the terahertz wave with the output power of 571 W at the operating frequency of 0.338 THz, and the frequency spectrum of the generated signal is very pure. The field distributions in the inclined coaxial SWSs illustrate that there is no asymmetric mode excited in the proposed device.

Nonlinear analysis of beam-wave interaction in a planar THz travelling-wave tube amplifier

Large-signal analysis is presented for analysing non-linear beam-wave interaction in a planar THz TWT with a sheet electron beam. The sheet beam within one RF beam wavelength is represented by uniform rectangular charged discs for confined flow of the beam, and the discs are tracked in small distance steps of forward integration. For a planar THz TWT, a staggered double-vane loaded rectangular waveguide slow-wave structure (SDVSWS) is used. Frequency-domain analysis is used to define the RF circuit field in the SDVSWS and the interaction of the RF circuit field with the electron beam is carried out for steady-state TWT operation. The RF circuit field is considered to be continuous as in a helix SWS and it is represented by transmission line theory at each integration plane. A large-signal analysis code of a helix TWT with pencil beam (SUNRAY-1D) was modified for a planar TWT with a sheet beam. The accuracy of the modified SUNRAY code was checked against the 3D e.m. field simulator (CST-PS) by comparing the output performance for a 0.22-THz planar TWT. The results for the gain and the output power over the operating band given by the two codes were found to be comparable. A significant advantage of the SUNRAY code is that it is very fast compared to the CST code. It takes less than a minute to simulate the RF performance of a typical THz TWT at a single frequency and drive power on a standard PC. The SUNRAY code can therefore, be used interactively for design a complete SWS with input/output couplers, sever, loss profiles and phase velocity taper for a high efficiency high gain THz TWT.

Tolerance Studies on an Inverse Magnetron Injection Gun for a 2-MW 170-GHz Coaxial-Cavity Gyrotron

The magnetron injection gun (MIG) is the most critical part of any gyrotron. Small tolerances in the manufacturing process and alignment of the subcomponents directly affect the electron beam quality and therefore the beam wave interaction. At the Karlsruhe Institute of Technology (KIT), an innovative new Inverse MIG (IMIG) is proposed for the European 2-MW 170-GHz coaxial-cavity gyrotronwhich is under the developmentand test at KIT. The design of the IMIG has been done under strict consideration of the gun design criteria published earlier. In order to find the maximum allowed tolerances of that IMIG which allows operation within the allowed gun design criteria, systematic theoretical studies have been done. Commonly used “conventional” MIGs have been shown that a small emitter, anode, and cathodemisalignment have a significant influence to the electron beam quality.

Effect of Cavity Ohmic Losses on Efficiency of Low-Power Terahertz Gyrotron

The problem of power degradation due to the dominant ohmic losses in gyrotron cavity is considered. As an example, the operating performances of the FU CW III gyrotron of Fukui University are investigated numerically. It is found that, for this gyrotron, there is a significant divergence between the classical and the self-consistent computations of the loss-induced power degradation. The reason is that the former computations ignore the effect of the cavity ohmic losses on beam-wave interaction efficiency.

Study of virtual cathodes formation during beam-wave interaction in the reltron oscillator

In the present work, a high power microwave oscillator—reltron has been analyzed to investigate the virtual cathode formation mechanism during the beam-wave interaction. In reltron, a side coupled modulation cavity is used as its RF interaction structure containing three metal grids along the longitudinal direction. The space charge current responsible for the virtual cathode and its steady state electric field distribution has been analyzed. Space charge and beam impedance conditions for efficient device operation have been demonstrated. It has been shown that during the beam-wave interaction in the device, first a virtual cathode forms in the post-acceleration gap, and then the second virtual cathode develops between the first and second grids of the modulation cavity. These two virtual cathodes co-exist and cause the formation of a third virtual cathode between the second and third grids. At this instant, only the third virtual cathode remains, and for sustained device oscillation, this process repeats periodically in the device. The present study would be useful in understanding the beam-wave interaction mechanism as well as the design and development of efficient reltron devices.

Numerical design of a 100 W, 38 dB gain, W-band multi-section serpentine waveguide vacuum electronic TWT

The serpentine waveguide circuit is a robust beam-wave interaction circuit for W-band TWTs. Here presented the electromagnetic properties and design methodology for W-band multi-section SWG traveling wave tube. Cold-test (in absence of electron beam) numerical design performed theoretically and further optimized/validated with standard simulation code to predict the dispersion, interaction impedance, ohmic-loss and small-signal gain. Numerical simulation for the quarter wave transformer couplers with SWG circuit geometry shown the return-loss less than −20dB for the 5% frequency band. Later, in systematic manner, hot-test (in presence of electron beam) numerical design performed for multi-section TWT by using standard particle-in-cell 3-D simulation code. The three section, 60 periods SWG TWT predicted peak radiation power 130W at target frequency 94GHz, 39.5dB saturated gain, 5.3% instantaneous 3-dB frequency bandwidth, and 6.5% electronic efficiency.

A dielectric-embedded microstrip meander line slow-wave structure for miniaturized traveling wave tube

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 characteristics, fabrication processes, and beam-wave interaction of the modified MML SWS are analyzed. Compared with conventional MML SWSs supported by dielectric substrate, the modified MML SWS possesses a wider cold bandwidth (about 53.5% increased) and larger interaction impedances (about 21.4% increased). Moreover, this modified MML has better connection with the dielectric, which may improve the slow-wave structure’s stability and reliability under thermal shock, stress impact, and electron bombardment. The beam-wave interaction simulation results indicate that TWTs using the modified MML SWS is capable of delivering 75.85 W saturated output power, corresponding a gain of 28.8 dB and an electron efficiency of 6.32% at the frequency of 35 GHz, with the sheet electron beam of 6000 V and 200 mA.

Numerical Research on a 170-GHz Time-Dependent Multimode Coaxial Gyrotron With Outer Corrugation

In this paper, a coaxial gyrotron with outer corrugation is considered. Its multimode electron equation of motion is derived from the Lorentz equation, and its time-dependent amplitude and phase equation for the $\mathrm {TE}_{mn}$ mode is obtained from the wave equation. Using the time-dependent multimode beam–wave interaction equations, the time evolution processes of the time-dependent multimode coaxial cavity gyrotron with outer corrugation are investigated by numerical calculation. Results show that a 170-GHz coaxial gyrotron with outer corrugation can start up and operate stably at the $\mathrm {TE}_{34,19}$ mode and its competing modes are suppressed under different initial field amplitude conditions. By the calculation of the time-dependent multimode and single-mode coaxial gyrotron with outer corrugation, it is found that the output power and the output efficiency of the $\mathrm {TE}_{34,19}$ mode are 1.856 MW and 32.59%, respectively. These studies show the mode competition processes of the coaxial gyrotron with outer corrugation at the beginning start-up stage.

RF Behavior of Cylindrical Cavity Based 240 GHz, 1 MW Gyrotron for Future Tokamak System

In this paper, we present the RF behavior of conventional cylindrical interaction cavity for 240 GHz, 1 MW gyrotron for futuristic plasma fusion reactors. Very high-order TE mode is searched for this gyrotron to minimize the Ohmic wall loading at the interaction cavity. The mode selection process is carried out rigorously to analyze the mode competition and design feasibility. The cold cavity analysis and beam-wave interaction computation are carried out to finalize the cavity design. The detail parametric analyses for interaction cavity are performed in terms of mode stability, interaction efficiency and frequency. In addition, the design of triode type magnetron injection gun is also discussed. The electron beam parameters such as velocity ratio and velocity spread are optimized as per the requirement at interaction cavity. The design studies presented here confirm the realization of CW, 1 MW power at 240 GHz frequency at TE46,17 mode.

Study on Ka-band sheet-beam, three-slot-staggered-ladder coupled-cavity traveling-wave tube in a small tunable periodic cusped magnet

A Ka-band sheet electron beam (SEB) traveling-wave tube (TWT) based on three-slot staggered-ladder (TSTL) coupled-cavity slow-wave structure (CC-SWS) is proposed. The high-frequency characteristics of TSTL SWS were analyzed in detail. A well-matched input/output coupling system that solves the problem of energy coupling between the SWS and the external circuit was designed. A new method of loading the distributed attenuator for TSTL SWS was investigated for suppressing backward wave oscillation effectively. In addition, a SEB gun which can produce the SEB conveniently was optimized. A small tunable periodic cusped magnet (STPCM) system was designed to focus the low voltage, high-current SEB, which decreases the whole dimension of the TWT sufficiently. Moreover, the special STPCM system is predicted to exhibit 100% beam transmission efficiency. Finally, the beam-wave interaction was studied. The results obtained by CST simulation show that the SEB TWT can produce output power over 5 kW within the bandwidth ranging from 31.5 to 35.5 GHz, and the maximum output power is 10.15 kW at 33 GHz, corresponding electron efficiency of 10.36%. The fabrication of the device is under way.

A High-Efficiency Magnetically Insulated Transmission Line Oscillator

A novel magnetically insulated transmission line oscillator (MILO) with high efficiency has been proposed, based on a kind of conventional MILO. The new MILO has two typical characteristics. First, the width of the anode vanes decreases gradually, and especially, the extraction vane decreases to be the minimal. Therefore, the proportion of the electrons, which have little or no contribution to the beam–wave interaction, is decreased. As a result, the efficiency is enhanced. In addition, there is an extra extraction cavity designed behind the extraction vane. So the beam–wave interaction process is lengthened, and the extraction efficiency is enhanced. The results of the particle-in-cell simulation show that the power efficiency of this MILO can be elevated to be 20.5%, while it is only 14.4% for the conventional MILO.

Development of a Subterahertz Folded-Waveguide Extended Interaction Oscillator

A novel type of two-section folded wave- guide (TSFW) slow wave structure for the development of a subterahertz (sub-THz) extended interaction oscillator (EIO) is proposed in this paper. We focus on the development of sub-THz EIO, in which the key parts, including the electron optics system, the high frequency structure for beam-wave interaction, and output window, are fabricated and tested. Based on the key parts, the prototype of EIO is developed. The experimental results show that over 12-W output power at the frequency of 107.9 GHz at 2% duty cycle is achieved. This paper demonstrates that the feasibility of a high power THz oscillator can be obtained by the use of a prebunching electron beam produced by TSFW.

Development of a 140-GHz folded-waveguide traveling-wave tube in a relatively larger circular electron beam tunnel

A 140-GHz folded-waveguide traveling wave tube (FWG-TWT) with a relatively larger circular electron beam tunnel was designed and developed. Using simulation software, the high frequency characteristics, the transfer characteristics, and the beam-wave interaction of the TWT were studied in detail. The simulation predicted up to 98 W output power and about 43 dB gain of FWG-TWT at the center frequency. The input and output couplers compatible with the FWG-SWS and the periodic permanent magnet focusing structure were designed. The FWG-TWT thus designed was fabricated and tested. The experimental result shows the output power exceeding 1.6 W and the gain exceeding 25 dB at 140 GHz when the input power is 5 mW. The reasons for the departure of the experimental from the simulated results have been explained.

U-shaped meander-line slow-wave structure with stub-loading

In this paper, a U-shaped meander-line slow-wave structure (SWS) with stub-loading is proposed for applications in Ka-band traveling-wave tube (TWT). This new slow-wave structure, loaded with a stub at the center of the U-turn section, has higher interaction impedance and lower phase velocity compared with conventional U-shaped meander-line SWSs, indicating that the devices based on this structure may have a lower operating voltage and higher output power. The dispersion characteristic, interaction impedance, transmission characteristics, and beam-wave interaction are simulated by utilizing simulation tool. The simulation result predicts that the millimeter-wave traveling-wave tube design based on this slow-wave structure is capable of delivering over 200 W with a gain of 33 dB and interaction efficiency 14.5% at the center frequency 34 GHz. This design, more compact and powerful in comparison with those based on more conventional vacuum electronic mechanisms, is demonstrated as a prospective option for integrated millimeter-wave power modules (MMPMs) empowering a broad spectrum of fields, from target detection, to imaging and telecommunications, among others.

Design and simulation of a sub-terahertz folded-waveguide extended interaction oscillator

In this paper, an interesting type of a two-section folded wave-guide (TSFW) slow wave structure (SWS) for the development of sub-Terahertz (sub-THz) extended interaction oscillator (EIO) is proposed. In this sub-THz device, the prebunching electron beam is produced by the TSFW SWS, which results in the enhancement of the output power. To verify this concept, the TSFW for sub-THz EIO is developed, which includes the design, simulation, and some fabrications. A small size of electron optics system (EOS), the TSFW SWS for beam-wave interactions, and the output structure are studied with simulations. Through the codes Egun and Superfish, the EOS is designed and optimized. With a help of CST studio and 3D particle-in-cell (PIC) simulation CHIPIC, the characteristics of beam-wave interaction generated by the TSFW are studied. The results of PIC simulation show that the output power is remarkably enhanced by a factor of 3, which exceeds 200 W at the frequency of 108 GHz. Based on the optimum parameters, the TSFW is manufactured with a high speed numerical mill, and the test transmission characteristic |S21| is 13 dB. At last, the output structure with a pill-box window is optimized, fabricated, integrated, and tested, and the result shows that the voltage standing-wave ratio of the window is about 2.2 at an operating frequency of 108 GHz. This design and simulation can provide an effective method to develop high power THz sources.

The influences of beam quality and Ohmic loss on the beam-wave interaction in a 420 GHz second-harmonic complex-cavity gyrotron

Recently, a 0.42 THz second-harmonic gyrotron with complex cavity has been investigated and measured in the Terahertz Research Center. The experimental results show that the designed gyrotron can operate stably at the candidate mode TE−17.4 with the pulsed power of 19.3 kW, corresponding to an efficiency of 8.6%. However, the measured output power and efficiency are much lower than the theoretical values. In this paper, the influences of beam quality and Ohmic loss on the beam-wave interaction in the designed complex-cavity gyrotron are analyzed explicitly by a nonlinear, time-dependent, multi-mode code. The numerical simulations show that in the low-beam-voltage region, the calculated powers perfectly fit with the experimental results when the electron beam width is 0.195 mm, the electron velocity spread is 6% and the Ohmic quality factor of TE−17.4 is about 75000. Meanwhile it is also found that competing modes are well suppressed although considering the beam quality and Ohmic loss, which reproduces the experimental observation of the high-power single-mode oscillation of the TE−17.4 mode as the design mode.

Improved Design and Microfabrication of ${H}$ -Plane and ${E}$ -Plane Loaded Rectangular Slow-Wave Structure for THz TWT Amplifier

This paper reports further investigation on a recently proposed ${H}$ -plane and ${E}$ -plane loaded slow-wave structure (SWS) for terahertz traveling-wave tube (TWT) amplifier. An improved input–output coupler is designed to enable easy fabrication by microfabrication technology, UV-lithography, electroplating, and molding (LIGA), and deep reactive-ion etching. The coupler shows very low reflection coefficient, ${S}_{11} dB over a frequency range 360–450 GHz. To improve the beam-wave interaction and enhance the saturated output power of the TWT, the SWS with tapered design is implemented. By tuning the period, the wave is resynchronized with the beam at the end of the SWS, resulting in more than 60% increase in the saturated output power across the 80-GHz bandwidth. In addition, the sensitivity of the output power of the TWT to the fabrication tolerance of the individual geometrical parameters is also studied in detail. It is found that the output power reduces by 80% for approximately 2% variation in the synchronized beam voltage. The KMPR-based UV-LIGA technique is adopted to fabricate the ${H}$ -plane and ${E}$ -plane loaded SWS. The measured lateral dimensional accuracy within $\sim 2~\mu \text{m}$ and rms roughness of metal sidewall surface less than 80 nm is obtained.

Theoretical studies on stability and feasibility of 0.34 THz EIK

The stability of the circuit and the tolerance during the manufacture process are theoretically studied in the design of 0.34 THz extended interaction klystron, which are helpful to increase the feasibility of the device. By using the small signal theory, the beam-loading conductance is studied to increase the efficiency of the beam-wave interaction. Combined with the study of start current for oscillation modes, the analysis of stability in multi-gap cavity is proposed, leading to the optimization of cavity. As a crucial factor affecting the ultimate performance of device, the inaccuracy during the fabrication process is researched. The acceptable tolerance is summarized through discussion of various geometrical dimensions' influences on cavity's characteristics. The study of power loss in the conductive wall is presented and the copper is believed to be adapted in making the device practicable with low attenuation. The physical design is simulated and verified by the particle-in-cell (PIC) method, and the results show that the output power of 142 W can be reached steadily at the frequency of 347.7 GHz, approaching the gain of 37.9 dB.

Coherent radiation at the fundamental frequency by a Smith-Purcell free-electron laser with dielectric substrate

This letter reports an approach to realize coherent radiation at the fundamental frequency of the wave that is resonant with the electron beam in a Smith-Purcell (SP) free-electron laser (FEL). We found that a two-dimensional grating structure equipped with a dielectric substrate could provide an electromagnetic mode which shows the property of a surface wave in the vacuum region where the beam-wave interaction occurs, and operates as a radiative wave in the dielectric region. Thus, an electron beam holding medium energy resonant with the mode could radiate at the fundamental frequency, which is different from the typical Smith-Purcell free-electron laser that radiates only at harmonics of the resonant frequency.

A 170 GHz time-dependent multi-mode coaxial gyrotron with inner corrugation

In this paper, a time-dependent multi-mode coaxial gyrotron with inner corrugation is introduced. The time-dependent amplitude and phase equation of the mode TEmn and the multi-mode electron motion equation are derived from the wave equation and the Lorentz equation, respectively. Using these equations, the time evolution of the time-dependent multi-mode coaxial cavity gyrotron with inner corrugation is studied by numerical calculations. Researches show that a 170 GHz coaxial gyrotron with inner corrugation can start up and stably operate in the mode TE32,17 and its competition modes are depressed under the given operation parameters. It is found that the output power and the output efficiency of the coaxial gyrotron with inner corrugation are 2.205 MW and 40%, respectively. These studies display the mode competition process of the time-dependent multi-mode coaxial gyrotron with inner corrugation. It is helpful to further research on the beam-wave interaction of the gyrotron.