Beam-wave Conversion Efficiency Research Articles

Toward 80% Efficiency in a Super Klystron-Like Relativistic Backward Wave Oscillator With Second and Third Harmonic Coaxial-Premodulation Cavity

To further increase the beam-wave conversion efficiency, a super klystron-like relativistic backward wave oscillator (RBWO) with coaxial-premodulation (CPM) cavity is proposed. Compared with previous dual-premodulation (DPM) cavities, the CPM cavity intensifies the second and the third harmonic components, resulting in a larger fundamental harmonic current both in the slow-wave structure (SWS) and the extraction cavity, and, consequently, leads to an enhanced microwave power generation by the Cherenkov radiation and transition radiation. In the particle-in-cell simulation, an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{X}$</tex-math> </inline-formula> -band microwave with a power of 3.44 GW and record efficiency of up to 80% has been achieved.

Experimental Demonstration of Dual-Mode Relativistic Backward Wave Oscillator With a Beam Filtering Ring Packaged With Permanent Magnet

It is reported on the design and experiment of a dual-mode (TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> -TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> ) relativistic backward wave oscillator (RBWO) with a beam filtering ring (BFR) packaged with permanent magnet. The BFR makes use of the large transverse oscillation of electrons at low magnetic field. By absorbing part of the electron beam periodically, the amount of the electrons that would absorb power from the microwave in the slow-wave structure (SWS) is greatly reduced, so the beam-wave conversion efficiency is improved. Experimental results demonstrate that a 3.2 GW mixed-modes microwave at 9.87 GHz with an efficiency of 40% is obtained with a 675 kV, 11.8 kA beam guided by a 0.68 T magnetic field. The BFR has a considerable lifetime and can effectively improve the efficiency of the device with stable operation.

An X-band high-power and high-efficiency coaxial relativistic klystron oscillator with four-gap buncher and three-gap extractor

Because of the scaling invariance, the over-mode ratio of the coaxial resonator can be increased to increase the power handling capability. However, as the over-mode ratio increases, the characteristic impedance and external quality factor decrease, which causes the modulation of the electron beam to be weakened. Moreover, when the output microwave power increases, the double-gap output cavity will suffer from severe radio frequency breakdown. Therefore, an X-band high-power and high-efficiency coaxial relativistic klystron oscillator with a four-gap modulation cavity and a three-gap extraction cavity is proposed. First, a four-gap modulation cavity can increase the modulation depth of the electron beam to improve the beam-wave conversion efficiency. The operating mode of the modulation cavity is the 3π/4 mode of the coaxial TM01 mode. Second, a three-gap extraction cavity is adopted to enhance the microwave extraction energy and reduce the RF field strength. The simulation results show that when the diode voltage is 650 kV, the beam current is 15.4 kA, and the guiding magnetic field is 0.48 T, the device outputs a microwave power of 4.2 GW, a frequency of 8.4 GHz, and an efficiency of 42%.

Theoretical calculation and particle-in-cell simulation of a multi-mode relativistic backward wave oscillator operating at low magnetic field

Increasing the dimensions of high power microwave devices is an efficient method to improve the power capacity. However, an overmoded structure usually results in mode competition and a low beam-wave conversion efficiency. In this paper, a multi-mode operation mechanism is used to avoid mode competition and increase the efficiency. The calculation results of nonlinear theory of beam-multimode interaction show that the optimized conversion efficiency is up to 48% when TM01 mode, TM02 mode, and TM03 mode are all considered. As only the TM01 mode, TM02 mode, or TM03 mode is taken into account independently, the corresponding efficiency is 38%, 22%, or 20%. Based on this, a multi-mode relativistic backward wave oscillator is proposed with the ratio of the mean diameter of the slow wave structure (SWS) to the wavelength of the output microwave to be 3.5. The non-uniform SWS is used to increase the beam-wave conversion efficiency, and a combined reflector is adopted to reflect partial of the mixed microwave modes and make the device compact. The particle-in-cell simulations show that as the diode voltage is 1.1 MV, the beam current is 22.8 kA, and the external magnetic field is 0.76 T, the conversion efficiency is 45%, and the output microwave of 11.3 GW is the mixed modes of TM01 mode, TM02 mode, and TM03 modes with the corresponding power ratio of 74%, 7%, and 19%, respectively.

Efficiency improvement by a beam filtering ring in a relativistic backward wave oscillator at low magnetic field

This paper presents a design method of the relativistic backward wave oscillator at low magnetic field, which can improve the efficiency by 29% in the particle in cell simulation. The core of this method is to introduce a beam filtering ring. The beam filtering ring takes the characteristic of the radial position change as the electron oscillates. The structure manipulates the axial current, so that a large proportion of the electrons expected to be in the accelerated phase in the slow-wave structure is absorbed by the structure. It greatly enhances the bunching of the beam in the RF field and improves the beam-wave conversion efficiency significantly. The particle in cell simulation results reveal that at a permanent magnet with a magnetic induction intensity of 0.68 T, the output microwave power of the relativistic backward wave oscillator with a beam filtering ring is 5.9 GW, and the conversion efficiency can be up to 54% when the diode voltage is 890 kV and the beam current is 12.2 kA.

Experimental Investigation of a Super Klystron-Like Relativistic Backward Wave Oscillator Operating With Low Magnetic Field

An investigation of a super klystron-like relativistic backward wave oscillator is presented in this article. To obtain high power and high efficiency, an additional anode cavity is used to promote electron-beam modulation. Additionally, two waveguide cavities at the end of the electron collector are employed to enhance the constructive resonance, which intensifies energy conversion from the modulated electron beam to the electromagnetic field. Because of the enhancement of centralized transition radiation, the power generated in the extractor constitutes over 95% of the total power. Consistent with the results from numerical simulations, microwaves with a power of 5.0 GW, a frequency of 4.4 GHz, and a pulsewidth of 36 ns were obtained in high-power microwave generation experiments with an external magnetic field of 0.42 T, in which the diode voltage and beam current were configured to be 815 kV and 15.5 kA, respectively, indicating a beam-wave conversion efficiency of 40%. The diode insulation was investigated experimentally, and an unfortunate shunting current of over 3 kA was observed when the working voltage was beyond 800 kV, which indicates an actual conversion efficiency of 33% in the diode region.

AV-Band Overmoded Coaxial Millimeter-Wave Oscillator Based on a New Method of Asymmetric Modes Suppression

In the design of the coaxial millimeter-wave Cerenkov devices due to the low starting current and high temporal growth, asymmetric quasi-TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v1</sub> modes are always preferentially excited. This article proposes a new method of hybrid modes coupling to suppress the asymmetric modes. The results show that when the quasi-TEM (N - 1)π/N mode couples with the TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> (N - 3)π/N mode, the asymmetric modes can be suppressed effectively. The external Q-factor, electric field distribution, and the beam conductance of the hybrid modes are analyzed theoretically to explain the mechanism of the asymmetric modes suppression. According to this method, a coaxial V-band millimeter-wave oscillator is proposed. The 3-D particle-in-cell simulation results show that the symmetric modes are generated and all the asymmetric modes are suppressed successfully. When the diode voltage is 355 kV and beam current is 3 kA, the dominant frequency is 62.6 GHz and the output power is 322 MW, corresponding to a beam-wave conversion efficiency of 30.2%.

A high power capacity Ka-band radial transit time oscillator with one-gap extraction cavity

The radial transit time oscillator (RTTO) is promising to realize high power output of millimeter-waves. Although the radial structure can enhance the power capacity, less cavities and small radial dimension make it difficult to improve the power capacity in RTTOs, especially in the extraction cavity. A one-gap extraction cavity in the Ka-band RTTO is proposed in this paper to improve the power capacity. Without electrons, taking the TM011 cavity as an example, the radial reversal resonant electric field can intersect with radial electrons. By choosing the sizes of the cavity, the synchronization of the electrons and the electric field can be realized to achieve effective energy exchange. In particle-in-cell simulation, the RTTO with the TM011 extraction cavity can output 1.0 GW high power microwaves (HPMs) at 31.2 GHz, and the beam-wave conversion efficiency is 31.6%. The maximum electric field in the TM011 cavity is only 800 kV/cm, which is less than one third that in the TM010 extraction cavities. In addition, the TM012 extraction cavity is employed to improve the efficiency to 35.4%. At the same time, because of the increase in the output power, the maximum radial electric field in the TM012 cavity increases to 850 kV/cm. Therefore, the one-gap extraction cavity can realize multiple energy exchanges to get high beam-wave conversion efficiency and enhance the power capacity in the extraction cavity significantly.

Theoretical analysis and design of G-band extended interaction klystron amplifier

Extended interaction klystron is a very important millimeter-wave and sub-millimeter-wave vacuum electron device with many actual and potential applications, such as space-borne cloud profiling radar, communication, imaging system, precision guided missiles, etc. Kinematical theory and space charge wave theory are extensively used to analyze the bunching process of electrons. Kinematical theory is precise when electron beam is especially small because the influence of space charge effect is ignored, while space charge wave theory is accurate when the modulation of electron beam is small since it is based on the premise of small amplitude. Based on kinematical theory, law of induce current, principle of charge conservation in a one-dimensioanl mode and small signal condition, the influence of electron beam on standing wave electric field in multiple-gap cavity is analyzed, and the expression of beam-loading conductance and beam-loading susceptance in multiple-gap cavity are derived. The influence of the direct current transmit angle of single gap, the number of multiple gaps and the direct current transmit angle of between center of adjacent gaps on beam-loading conductance and beam-loading susceptance are analyzed. The results show that the beam-loading conductance and beam-loading susceptance of multiple-gap cavity can change to a bigger extent when the number of cavity gaps is bigger, which means that the maximum beam-wave conversion efficiency and the range of loaded frequency increase with the number of cavity gaps increasing. The results also show that the direct current transmit angle between centers of adjacent gaps is the most important parameter for the beam-wave interaction effect. Based on the above analysis, a G-band extended interaction klystron amplifier consisting of three five-gap cavities is designed by an three-dimensional PIC code. An output power of 225.5 W at 217.94 GHz with an efficiency of 6.26%, whose gain and 3 dB bandwidth are 30.5 dB and 470 MHz respectively, is obtained by simulation. This study is of great significance for the physical design and process in engineering the G-band extended interaction klystron amplifier.

A novel Ku-band relativistic transit-time oscillator with three-cavity extractor and distance-tunable reflector

A novel Ku-band relativistic transit-time oscillator with a three-cavity extractor and a distance-tunable reflector is proposed. The physical mechanism of the three-cavity extractor and distance-tunable reflector is investigated by theory and simulation. It is shown that by introducing a three-cavity extractor instead of the dual-cavity one, the novel device has higher power capacity. Meanwhile, the three-cavity extractor is more favorable for increasing the beam-wave conversion efficiency of the device. In addition, analysis indicates that the distance-tunable reflector employed in the device plays important roles in the enhancement of conversion efficiency. Particle-in-cell simulations show that with the three-cavity extractor and distance-tunable reflector, this proposed device generates a 12.4 GHz, 2.2 GW microwave, while the diode voltage is 570 kV, the beam current is 10.5 kA, and the guiding magnetic field is 0.7 T. The corresponding power efficiency reaches 37%.

Particle-in-cell simulation of a new X-band low-impedance high power microwave source

High power microwave (HPM) source is attractive in generating gigawatt (GW) class microwaves based on the beam-wave interaction. Generally, HPM source with a high beam-wave conversion efficiency has a higher impedance. To improve the single-tube output power of HPM source, reducing the impedance of the device and increasing its power capacity are necessary. In this paper, a new low-impedance HPM source is proposed and proved to be capable of generating two phase-locked high power microwaves, which makes it promising to realize a higher combined power in a single HPM device.The new low-impedance HPM device consists of a two-cavity TKA (denoting the outer sub-source in the following) and a multiwave Cerenkov generator (referring to the inner sub-source below) inserted in the TKA inner conductor. These two sub-sources are connected in parallel and share a common magnetic field. A dual-concentric annular cathode is used in this microwave source, which is capable of emitting two concentric annular electron beams and driving the internal and external sub-source simultaneously. The advantages of this device are reducing the impedance and improving the injection electric power. When a voltage pulse is applied to the diode, part of microwaves generated in the inner subsource will leak into the outer sub-source (i.e., TKA) through the A-K gap. By amplifying the leakage microwaves, the TKA will be easily locked by the inner sub-source. Considering the fact that the microwave source consists of two sub-sources, the power capacity will also be greatly improved.As a result, particle-in-cell simulation indicates that when the diode voltage is 687 kV and the axial magnetic field is 0.8 T, two microwave beams that have a nearly identical frequency of 9.72 GHz and output powers of 1.20 GW and 2.58 GW respectively, are generated. The corresponding power conversion efficiencies are 28% and 30%, respectively. The frequency difference between these two microwaves fluctuates within 3 MHz and their phase difference is not in excess of 3. When the diode voltage changes from 665 kV to 709 kV, frequency difference between the two sub-sources fluctuates within 3 MHz and their phase difference fluctuation is within 5 in one voltage burst; the phase difference changes 10 in this voltage range. The impedance of this HPM source is as low as 36 .To sum up, the new HPM source proposed in this paper has a lower impedance and higher power capacity. The phase difference between the inner sub-source and the outer sub-source is very stable and favorable for the coherent power combination, which indicates that the new HPM source promises to realize a higher output power in a single-tube device.

Three-dimensional particle-in-cell simulation studies on a new radial three-cavity coaxial virtual cathode oscillator

A new radial three-cavity structure of the coaxial virtual cathode oscillator is proposed and studied numerically in this paper. Using the radial three-cavity structure, the beam-wave conversion efficiency is enhanced by modulating the electric field in the beam-wave interaction area, while the resonator composed of the radial three-cavity configuration and the mesh anode helps restrain mode competition effectively . And then the coaxial extraction structure benefits the energy extraction, and it can also absorb the used electrons entering into the drifting tube. Therefore, this new kind of virtual cathode oscillator can achieve a high output power. With an electron beam of 50 kA at 400 kV, a peak power of about 6 GW is achieved by simulation at 4.5 GHz. The mean power reaches 3.1 GW and the beam-wave conversion efficiency is about 15%.

Design of X-band backward wave oscillator with 10 GW output power

An efficient high power relativistic backward wave oscillator (RBWO) is designed. To enhance the beam-wave conversion efficiency and output power, an overmoded slow wave structure (SWS) is used and an inner conductor is inserted into the SWS. When electron energy is 1.3 MeV while the beam current is 17 kA, the simulated output microwave power is 10.4 GW at 8.8 GHz frequency for the RBWO.

HPM Generation by Tri-Anode Coaxial Vircator

A novel type of coaxial vircator with 3 anodes is proposed. The advantages on high power microwave generation is tested by particle-in-cell simulation and theoretical analysis model is set up as well. The tri-anode coaxial vircator behaves a very stable frequency in the pulse duration. In the tri-anode coaxial vircator, the injecting electron beam is well premodulated in the first anode cavity and forms an inerratic bunch-dispersion mechanism in the second anode cavity; the energy-loss electrons penetrate through the third anode and absorbed by the dumping cavity locates in the centre electrode, which helps keeping the bunch-dispersion cycle undisturbed. A 3.8 GHz tri-anode coaxial vircator is theoretically designed and optimized by particle-in-cell simulation, which gives beam-wave conversion efficiency of over 20% with a stable predominant frequency, in accordance with the particle-in-cell simulation and analytic model.

Investigation on an S-band relativistic klystron oscillator

The physics of modulation and rf extraction of an S-band relativistic klystron oscillator is studied in this paper via experiment, theory, and simulation. It is found that the intense relativistic electron beams (IREBs) can be intensely current-modulated when the IREBs drift through three pillboxes with high coupling. After bunching in the downstream the modulated IREBs can excite high-power microwave in the triaxial cavity. These properties, which have short oscillating time, compact geometry and high beam-wave conversion efficiency, were encouraging. Using a 1 MV, 13 kA, 40 ns electron beam and a 0.9 kGs leading magnetic field, 3.5 GW radiated power was extracted in 20 ns FWHM pulses at 2.86 GHz. The efficiency was 27%, and the instantaneous bandwidth was 2%. The radiated power was 3.4 GW when the repetition rate IREBs was 20 Hz. The experimental results agree well with the simulations.

Study of a coaxial vircator with a three-mirror quasi-optical resonant cavity

A new kind of coaxial vircator with a three-mirror quasi-optical resonate cavity is proposed and studied in this paper. Theoretical study and particle-in-cell (PIC) simulation are both included. Through two-and-a-half-dimensional PIC code MAGIC, using an input voltage pulse with peak value of 600 kV the average voltage obtained on the diode is 460 kV and the diode current is 50 kA, the instantaneous output power with peak value over 4 GW and the average power 2 GW are obtained with frequency 7.0 GHz. The beam-wave conversion efficiency is reaching 8.7%.