Beam-wave Interaction Efficiency Research Articles

Analysis of a Hollow Electron Beam Focusing and Transmission

For vacuum electronic devices (VEDs), the use of a hollow electron beam (HEB) increases the beam-wave interaction efficiency and permits a prominent decrease of the cutoff voltage in the focusing-electrode controlled electron gun. To obtain an HEB electron optics system (EOS) with stable propagation, lower ripple, high compression, and good laminarity, the issues related to the HEBs transmission characteristics in a uniform magnetic field are deeply investigated in this article. The studies on the Brillouin flow, which are different from the conclusion of the previous literature, are presented. Through combining the theoretical analysis and the simulation, the distinct motion ways of each layer in the HEB are clearly exhibited in both cases of the immersed flow and the partially shielding flow. This work can provide a meaningful reference for the design of a hollow beam EOS with high transmission and compression.

Using Phase Jumping Method to Enhance the Beam–Wave Interaction Efficiency in Terahertz Folded-Waveguide Traveling-Wave Tube

The folded-waveguide (FWG) traveling-wave tube (TWT) is a promising high-power terahertz (THz) amplifier. However, it is challenging to develop THz FWG-TWT with high beam–wave interaction efficiency. In this work, a phase jumping technology, which originates from free-electron lasers, is proposed to boost the beam–wave interaction efficiency of THz FWG-TWT. A modified slow wave structure acts as the phase shifter to adjust the phase gap between electron beam and electromagnetic wave. By jumping the phase gap to an appropriate value, the bunching center of electron beam keeps for a long time in the deceleration region. Thus, more energy is transferred to electromagnetic wave from bunching electron beam. To verify such proposition, a THz FWG-TWT example utilizing the phase jumping is presented in this article. Compared with conventional one, its efficiency within the operation bandwidth from 212 to 224 GHz is improved by 60%–140%.

Coherent Combination of Power in Space With Two X-Band Gigawatt Coaxial Multi-Beam Relativistic Klystron Amplifiers

The power capacity of an individual high power microwave (HPM) generator is severely limited by internal electric field breakdown, and the problem becomes more serious for long pulse and high frequency bands. One of the promising solutions to overcome this limitation is coherent combination of power in space. To realize coherent combination of power in space, the HPM generators must operate at the same frequency and stable phase differences. We presented an improved X-band gigawatt coaxial multi-beam relativistic klystron amplifier (CMB-RKA) with high phase stability. Two double gap buncher cavities were designed to improve the beam wave interaction efficiency, and decrease the distance for the modulated intense relativistic electron beam to reach its maximum, which significantly decrease the phase jitter due to the fluctuation of electron beam voltage and improve the phase stability of the CMB-RKA. Experimentally, the two X-band gigawatt CMB-RKA demonstrated the capability of coherent combination of power in space, typically of the combined efficiency of about 95% with the combined pulse width of 150 ns.

Investigation on High-Efficiency Beam-Wave Interaction for Coaxial Multi-Beam Relativistic Klystron Amplifier

To significantly improve the electronic efficiency of coaxial multi-beam relativistic klystron amplifier (CMB-RKA), the physical process of beam-wave interaction and parameters that affect efficiency was studied. First, the high efficiency of beam-wave interaction was discussed by simulating the efficiency versus the parameters (frequency of cavity, drift tube length between cavities, and external quality factor of output cavity), in the one-dimensional (1-D) large-signal simulation software. Moreover, the further physical process of beam-wave interaction was analyzed through simulating the current modulation factor and the number of particles at the entrance of the output cavity, in the three-dimensional (3-D) particle in cell simulation software. Last, with the optimal parameters in 3-D simulations, the CMB-RKA, which has 14 electron beams with a total current of 4.2 kA (14 × 300 A), can generate an output power of 1.02 GW with a saturation gain of 55.6 dB and an efficiency of 48.7%, when beam voltage is 500 kV, which indicated the CMB-RKA can achieve high efficiency for high-power microwave radiation.

Analysis of W-band traveling-wave tube based upon slotted sine waveguide slow wave structure

In this paper, a W-band traveling-wave tube (TWT) is proposed by combining the slotted sine waveguide (SSWG) slow-wave structure (SWS) with the cylindrical electron beam. The dispersion properties and interaction impedances of the SSWG SWS are analyzed by using the numerical simulation and compared to the conventional sine waveguide (SWG) SWS. The interaction impedances of the SSWG SWS are much higher than the SWG SWS over the entire operating frequency band. The whole high frequency system, which consists of the SSWG SWS, the coupler, and the attenuator, is designed suitably. From the particle-in-cell simulation results, the maximum output power of the SSWG TWT at the typical frequency of 94 GHz can reach 284.5 W with the cylindrical electron beam of 20.6 kV and 180 mA. Compared to the SWG TWT, the SSWG TWT possesses the higher beam–wave interaction efficiency and the shorter saturated tube length.

Theoretical analysis of a 0.22 THz multistage confocal waveguide gyro-TWT with circle-sector-shaped electron beam

In order to improve the beam–wave interaction efficiency, a special circle-sector-shaped electron beam is designed and simulated in the confocal waveguide gyrotron traveling-wave tube (gyro-TWT). The linear and nonlinear characteristics of a 0.22 THz multistage confocal waveguide gyro-TWT are analyzed. Based on kinetic theory, the linear gain, critical current of absolute instability, and starting current of backward wave oscillation (BWO) are analyzed. A quasi-optical sever is utilized in the confocal waveguide to suppress the BWO while having a slight impact on the output power. Meanwhile, the impact of velocity spread on the output parameters is also taken into consideration. The nonlinear analyses indicate that the efficiency can be up to 34%, which is substantially higher than the traditional annular electron beam.

Research on optimized structure of a 220-GHz extended interaction oscillator

The extended interaction oscillator has high power and efficiency advantages for terahertz vacuum electronic devices. The ladder line is a crucial high-frequency structure with strong coupling ability and can interact with the sheet beam to significantly improve the beam-wave interaction efficiency and is widely studied and implemented. This paper presents an optimized structure, expanding the cavities on both sides based on the original ladder line structure. The TM11 mode is effectively suppressed in the optimized structure so that the sheet beam can interact with the TM31 mode, and the cavity characteristic impedance R/Q and output power are significantly improved. Based on the theoretical derivation of the original structure and the approximated analysis of the optimized structure, the change in dispersion with structure is discussed in this paper. The field distribution in the optimized structure based on the 0.22-THz frequency band is further studied and compared with the original structure by numerical calculation and particle simulation. Furthermore, the optimal operating voltage and output window parameters are analyzed using Particle-in-Cell software, and the surface loss simulation results are given. Finally, the cavity characteristic impedance R/Q is increased by 66.7%, the bandwidth is slightly increased compared with the original structure, and the output power is increased by 163.8%. The results of the preliminary processing and cold test are presented.

Study on a Depressed Collector for a 75 GHz Low-Voltage Compact Gyrotron for Industrial Application

The low-voltage compact gyrotron is suitable for industrial applications. However, the beam-wave interaction efficiency is low in conventional low-voltage gyrotron. To improve the whole tube efficiency, a compact depressed collector is introduced in developing a 75 GHz low-voltage compact gyrotron. The compact depressed collector is directly connected to the output waveguide. It is grounded and isolated with the cavity by a ceramic ring which is easy to be connected with the application system. The design of the original tube electron beam voltage and electron beam current are 10 kV and 1.2 A. In the particle-in-cell (PIC) simulation, the operating mode is TE0,1 and the generated power is 1.2 kW operated at the frequency of 75.5 GHz, which corresponds to an electron efficiency of 10%. When the depressed collector is performed and the electron reflux is under 5%, the efficiency of the whole tube can reach 30%, and when the reflux rate is controlled at about 15%, the efficiency of the whole tube can reach 50%. The dissipation power would be sufficiently reduced.

A Cerenkov microwave generator with cross-band frequency hopping based on magnetic field tuning

Frequency hopping is one of the most significant technological advancements in high power microwave (HPM) sources. Mechanical alteration is generally adopted in traditional cross-band microwave generation, but it has the detriments of a complex adjustment strategy. In this paper, a cross-band frequency hopping Cerenkov microwave generator with an advantageous adjustment strategy, high beam–wave interaction efficiency, and pure output mode is developed. Two-segment slow wave structures (SWSs), which work in C and X-bands separately, are adopted. Additionally, by utilizing the cyclotron resonance absorption phenomenon reasonably, only the relative band microwave is energized under different magnetic fields, which adequately represses mode competition and realizes cross-band frequency hopping. After the electron beam passes through the first SWS section, the bunching center is consistently at the acceleration or deceleration phase of the subsequent SWS section, which a guarantees high beam–wave interaction efficiency in the two bands. The preliminary results of the particle-in-cell (PIC) simulation are as follows: when the guiding magnetic field is 0.7 T, the microwave output comprising only the X-band is obtained, with an efficiency of 42%; when the guiding magnetic field is 1.5 T, the microwave output comprising only the C-band is obtained, with an efficiency of 30%.

Effects of the Magnetic Field Taper on a Continuously Frequency-Tunable Gyrotron

Generally, the magnetic field profile in the gyrotron beam-wave interaction section is usually considered to be constant in the theoretical investigation, but actually it is approximately linearly tilted slightly due to the limitation of the superconducting magnet coils. The effects of the weak operating magnetic field taper on the operating frequency, the beam-wave interaction efficiency, and the frequency-tuning range for a continuously frequency-tunable gyrotron are investigated with a self-consistent nonlinear beam-wave interaction theory. It is found that the tapered operating magnetic field has a significant effect on the operating frequency, the frequency tuning bandwidth, and the beam-wave interaction efficiency. In addition, the effect of the weak operating magnetic field taper on the starting current is studied based on the linear theory. It is found that the values of not only the starting current but also the operating magnetic field are obviously different from those when the tapered operating magnetic field is taken into account. With the increase in the slope of the tapered operating magnetic field, the operating frequency and the frequency-tuning range are decreased, but the beam-wave interaction efficiency is increased. When the slope of the tapered operating magnetic field is negative, the variations in the operating frequency, the frequency-tuning range, and the beam-wave interaction efficiency are more obvious. The effects of the tapered magnetic field on different order axial modes are different, and the effect on a higher order axial mode is more notable.

Theoretical Investigations on Forward-Wave and Backward-Wave Operation of a Frequency-Tunable Gyrotron

Frequency-tunable gyrotron is a promising terahertz (THz) radiation source for many modern applications. Actually, a resonator-based gyrotron oscillator can play dual functions: gyromonotron and reflected-type gyro-backward-wave oscillator (BWO) corresponding to the forward-wave and backward-wave operation. The performances of the gyromonotron and the reflected-type gyro-BWO for a 0.5-THz frequency-tunable gyrotron have been theoretically investigated in detail. The calculated results indicate that the frequency tunability, the field profile of the axial mode, and the beam-wave interaction efficiency of the gyromonotron and the reflected-type gyro-BWO are dramatically different from each other. The gyromonotron state can switch to the reflected-type gyro-BWO state by means of increasing the operating magnetic field. In addition, we have systematically investigated the characteristics of the 0.5-THz frequency-tunable gyrotron with different beam-wave interaction structures, especially for the reflected-type gyro-BWO. It is found that the tapered angle of the output section has a significant effect on the frequency tunability of the reflected-type gyro-BWO, and the multisection cavity can improve the performance of the reflected-type gyro-BWO. The results of this study may be beneficial for getting a deeper insight into the physics of the forward-wave and backward-wave operation of a frequency-tunable gyrotron.

Simulation of Multimode Competition of a Coaxial Cavity Gyrotron With Multiple Electron Beams

In this article, the time-dependent multimode competition equations, the dependence of the starting current on the static magnetic field, and the current ratio of a coaxial cavity gyrotron with multiple electron beams are investigated according to the linear and nonlinear theories. By numerical calculation, the mode competition of a 170-GHz coaxial cavity gyrotron is studied when it has one, two, and three electron beams. Researches demonstrate that under the given magnetic field $B$ , electron beam voltage $U$ , and pitch factor ${\alpha }$ , a gyrotron with different electron beam numbers has different startup scenarios, which show that the gyrotron with three electron beams has potential in the selection of mode, the depression of mode competition, and the increase of output power. Results find that the coaxial cavity gyrotron with three electron beams can operate steadily in the ${\text {TE}_{38,18-}}$ mode at 170.39 GHz and it has an output power of 5.02 MW and a beam–wave interaction efficiency of 35.42% when $B=6.82\text{T}$ , $U=88$ kV, $\alpha =1.3$ , $I=158$ A and the three electron beams locate at 11.14, 12.93, and 14.32 mm. Three electron beams in the gyrotron have different beam–wave interaction efficiencies, and the closer the electron beam is to the inner conductor, the higher its beam–wave interaction efficiency is.

Study of the beam–wave interaction of a 220 GHz coaxial-cavity gyrotron with two electron beams

In this paper, the mode selection, ohmic loss quality factor, mode competition, and beam–wave interaction of a 220 GHz coaxial-cavity gyrotron with two electron beams are studied by using the linear and nonlinear theories of a gyrotron. Research shows that a coaxial-cavity gyrotron with two electron beams has more advantages than a coaxial-cavity gyrotron with one electron beam in mode selection, suppressing mode competition and increasing output power, and the eigenvalue and the ohmic loss quality factor of modes have similar monotonicity in the direction of the gyrotron axis. Results find that (1) the current ratio K of the gyrotron has a greater influence on the beam–wave interaction efficiency of the two electron beams, (2) under B = 8.90 T, I = 136 A, α = 1.40, and K = 0.45, the coaxial-cavity gyrotron with two electron beams can operate steadily in the TE51,32− mode, which has an efficiency of 37.67% and an output power of 4.53 MW at f = 220.04 GHz, and (3) the total ohmic loss power of the gyrotron accounts for 1.3% of the output power.

Numerical investigation on beam–wave interaction of W-band extended interaction oscillator

Based on the basic principle of energy conservation and electron disk beam–wave interaction equation, combined with the characteristic impedance and quality factor of the oscillator, a self-consistent nonlinear theoretical model of extended interaction oscillators (EIOs) is obtained. Taking the W-band EIO as an example, the effects of operating parameters and structural parameters on the oscillation, output power, and electron conversion efficiency of the extended interaction oscillator are analyzed. The numerical implementation of this model shows that the electron beam–wave synchronization interaction is a basic prerequisite for the oscillator to start, the positive feedback to energy in the resonator is a necessary condition for spontaneous oscillations, the distribution of electric field amplitude on each gap directly affects the beam–wave interaction efficiency and output power, and the electron trajectories in all gaps at steady state illustrate the conversion efficiency. At an operating voltage of 18 kV and a direct current of 0.5 A, a seven-gap EIO model with an output power of 2629 W and electron efficiency over 29% is predicted for gradually increased electric field amplitude distribution on each cavity.

Efficient generation of multi-gigawatt power by an X-band dual-mode relativistic backward wave oscillator operating at low magnetic field

An X-band dual-mode relativistic backward wave oscillator (RBWO) operating at low magnetic field is presented in this paper. Three new design principles are introduced in the device. First, the electron beam interacts with TM01 mode and TM02 mode simultaneously, rather than with a fixed single mode. Second, the device outputs with mixed modes, rather than with a pure mode. Third, an internal reflector inserted into the annular cathode, rather than a long resonant reflector before the slow-wave structure, is adopted to reflect part of the backward wave. Accordingly, the beam–wave interaction efficiency is increased significantly and the whole device is very compact. The particle in cell simulation results reveal that at a magnetic field of 0.64 T, the output microwave power is 4.8 GW and the conversion efficiency is up to 44%. In the experiment, at a guiding magnetic field of 0.66 T, a microwave pulse with power of 4.6 GW, frequency of 9.96 GHz, pulse duration of 42 ns, and efficiency of 42% was generated when the diode voltage was 880 kV and beam current was 12.5 kA, which agree well with the simulation results. Furthermore, as the diode voltage was 1.17 MV, a highest microwave power of 7.6 GW was achieved. This is a record of high efficiency and high power of microwave generation in an X-band RBWO operating at low magnetic field.

Investigations on the Effect of the Electron Beam Misalignments on a Continuously Frequency-Tunable Gyrotron

The effect of the electron beam misalignments, including the parallel shifted electron beam and the tilted electron beam on the operating frequency, the beam-wave interaction efficiency, and the frequency-tunable range for a continuously frequency-tunable gyrotron has been studied based on a self-consistent nonlinear beam–wave interaction theory. And the effect of the parallel shifted electron beam on the starting current has also been investigated based on linear theory. It is found that the misaligned electron beam has a little effect on the operating frequency and the frequency-tunable range but a significant effect on the beam-wave interaction efficiency. The misaligned electron beam has a different effect on different axial modes, the influences on the operating frequency, the frequency-tunable range, and the beam-wave interaction efficiency of a higher order axial mode are more obvious. Meanwhile, the influence of the parallel shifted electron beam on the starting current is significant; not only the order of magnitude of starting current increases, but the operating magnetic field is also changed. These results present that the effect of the misaligned electron beam is significant; the electron beam misalignments should be taken into account and avoided in the design and fabrication of a continuously frequency-tunable gyrotron.

Design of second harmonic terahertz gyrotron cavity based on double confocal waveguide

Quasi-optical confocal cylindrical waveguide possesses a lot of good characteristics, such as big power capacity and low mode density, which can suppress the mode competition in beam-wave interaction. So quasi-optical waveguide has a great advantage in designing high harmonic terahertz gyrotrons. For the reason that part of electron beams located in a region of weak field intensity play a limited role in beam-wave interactions, the beam-wave interaction is not efficient in confocal cavity. Motivated by enhancing the beam-wave interaction efficiency of quasi-optical gyrotron, we propose a novel terahertz harmonic gyrotron cavity with double confocal waveguide in this paper. The transverse field distribution and the mode spectrum in double confocal waveguide are analyzed and presented. A 330 GHz second harmonic gyrotron with double confocal cavity is designed, theoretically analyzed and simulated by using a particle-in-cell (PIC) code. The results obtained for double confocal cavity are compared with the results for single confocal cavity, and the physical mechanism of beam-wave interaction enhancement in double confocal cavity is discussed. Theoretical results show that the double confocal cavity is able to increase the coupling strength of beam-wave interaction, thus, to improve the output power and the interaction efficiency of quasi-optical gyrotron. The PIC simulation results suggest that a high-order waveguide mode in double confocal cavity can steadily interact with the high harmonic cyclotron mode of electron beam without mode competition. Driven by a 40 kV, 2 A electron beam with a guiding center radius of 1.65 mm and velocity ratio equal to 1.5, output power of 9.9 kW at 328.93 GHz can be generated in the designed double confocal cavity. The beam-wave interaction efficiency increases from 5.3% in single confocal cavity to 12.4% in dual confocal cavity under the same operation parameters. The double confocal cavity has great potential applications in terahertz band. Moreover, this study indicates that the eigen mode in double confocal waveguide is a kind of hybrid mode superimposed by two independent single confocal waveguide modes. This mode characteristic will be beneficial to designing a multifrequency gyrotron oscillator operated in two modes and two cyclotron harmonics, simultaneously, with a single electron beam used, which provides a new possibility to develop the novel terahertz radiation source.

Design and Simulation of a Multi-Sheet Beam Terahertz Radiation Source Based on Carbon-Nanotube Cold Cathode.

Carbon nanotube (CNT) cold cathodes are proving to be compelling candidates for miniaturized terahertz (THz) vacuum electronic devices (VEDs) owning to their superior field-emission (FE) characteristics. Here, we report on the development of a multi-sheet beam CNT cold cathode electron optical system with concurrently high beam current and high current density. The microscopic FE characteristics of the CNT film emitter is captured through the development of an empirically derived macroscopic simulation model which is used to provide representative emission performance. Through parametrically optimized macroscale simulations, a five-sheet-beam triode electron gun has been designed, and has been shown to emit up to 95 mA at 3.2 kV. Through careful engineering of the electron gun geometric parameters, a low-voltage compact THz radiation source operating in high-order mode is investigated to improve output power and suppress mode competition. Particle in cell (PIC) simulations show the average output power is 33 W at 0.1 THz, and the beam–wave interaction efficiency is approximately 10%.

High-Efficiency Study of an SBEIO Based on the SAA Optimization

The improvement of the beam–wave interaction efficiency of the sheet beam extended interaction oscillator (SBEIO) is studied and presented in this article. A novel multi-parameter optimization scheme is developed, which is based on the simulated annealing global optimization algorithm, the nonlinear theory, and 3-D simulations, including the axial electric field distribution and the distances between each gap. The 3-D particle-in-cell (PIC) simulations are performed to verify the scheme. A $\textit {Ka}$ -band SBEIO with nine gaps is optimized with a 36.25-kV and 3.5-A electron beam. The final optimized efficiency obtained from the developed code is 31%, which shows a good agreement with the 32.6% results of the PIC simulation. Compared with the uniform axial electric field distribution, the optimized design increased the efficiency from 15.8% to 32.6%.

A high-efficiency dual-band relativistic Cerenkov oscillator based on dual electron beams

High efficiency and high frequency are key research topics for the development of high power microwave devices. There are, however, serious issues with mode competition in relativistic Cerenkov devices based on a single electron beam, which can lead to low efficiency in the beam-wave interaction process. In this paper, a high-efficiency relativistic Cerenkov oscillator based on a dual electron beam is proposed. The hollow slow wave structure is embedded in the inner conductor of the coaxial waveguide to obtain a double coaxial electromagnetic structure, which can suppress mode competition and improve the beam-wave interaction efficiency of the dual-band radiation. Moreover, each band of radiation is independent of the other, such that high frequency output radiation is generated. Using the particle-in-cell method, the physical processes involving the beam-wave interaction in the oscillator are studied with the aim of designing a high-efficiency double-electromagnetic structure. Simulation results indicated that the output power levels for the X-band and Ka-band were 1.50 GW and 710 MW, respectively, and the efficiencies were 34.7% and 30.7%, respectively, for a diode voltage of 550 kV and a guiding magnetic field of 0.85 T.