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Asymmetric Mode Competition in an X -Band Dual-Mode Relativistic Backward Wave Oscillator
Abstract
The initial experimental results of asymmetric mode competition in an
X
-band dual-mode relativistic backward wave oscillator (RBWO) operating at the low magnetic field are presented. In the experiment, the use of receiving antennas with different polarization directions allows direct determination of asymmetric mode, and the asymmetric mode with a frequency of 10.27 GHz is excited, which is in good agreement with the particle-in-cell (PIC) simulation results. Judging from that, the competition mode is asymmetric EH
mode. The asymmetric mode competition mechanism in the device is investigated theoretically. It is shown that the drift length plays a key role in the suppression and excitation of the asymmetric mode, validated by the experiment and the PIC simulation.
... Therefore, EH 31 mode also exhibits a large Q-value and a small starting oscillation current, which make it a potential asymmetric competition mode. 23 Compared to the hollow waveguide structure, this coaxial structure has a much smaller number of potential competition modes. The comparison of the mode number is shown in Fig. 4. As shown in Fig. 4, when D/λ exceeds 3, the number of modes in the coaxial waveguide is significantly lower than that in the hollow waveguide, and the gap between them increases with the increase in D/λ. ...
... It was found that drift2 and drift3 shown in Fig. 1 can be optimized to reduce the Q-value of EH 31 mode so that the competition of EH 31 mode is suppressed while maintaining the power efficiency. 23 Figure 10 illustrates the variation in the Q-value of EH 31 mode with drift2 and drift3. After optimization, pure TEM mode is generated in the output waveguide, as shown in Fig. 11. ...
Relativistic backward wave oscillators (RBWOs) have the characteristics of high power and high repetition rate. Reducing the magnetic field strength and the weight of the external permanent magnet (PM) is a significant development direction of RBWOs. In previous research, an X-band RBWO enclosed with a PM has achieved a power efficiency of 50%. However, the PM used in these papers requires a magnetic field magnitude of over 0.68 T, which leads to a weight exceeding 400 kg. The RBWO designed in this paper operates in coaxial TM01 mode, and the radial distance between its cathode and the surface of the slow wave structures reaches 7 mm. Under the condition of a low magnetic field, this design can provide a wide electron beam channel to avoid the rubbing of the electron beam envelope on the inner and outer conductors. Particle-in-cell simulation results have demonstrated that this RBWO achieves an output microwave power of 3 GW with a power efficiency of 50% enclosed in a PM with a magnetic field strength of 0.43 T and a weight of 74 kg.
... Preliminary 3-D simulation shows that during the extended time duration of the electron beam, other modes, including asymmetric modes, can be excited gradually [33], [34], and the output pulse is converted from the quasi-periodic sequence to chaotic radiation with time. The possible solutions to suppress other modes include adjusting the Q-factor of unnecessary modes [35], [36], cutting longitudinal slits in the wall of the SWS [37], adopting a 2-D SWS [38], [39], introducing part of the signal produced by the generator itself from the output waveguide to the end of the coaxial SWS region [11], [40], [41], or injecting an external sequence signal. This will be explored in the future. ...
A coaxial Cherenkov generator operating at a low magnetic field is proposed to generate ultrashort-pulse quasi-periodic sequences based on the effect of superradiance (SR). The device employs an extended slow wave structure (SWS) with combined variations in corrugation depth and period, resulting in enhanced spectral continuity and higher SR power. Most of the microwave power is coupled into the hollow waveguide formed by the inner conductor of the coaxial SWS. A portion of the SR pulse generated in the coaxial SWS is reflected back into the SWS, where it is primarily absorbed by disturbed electrons rather than seeding the next SR pulse. Particle-in-cell (PIC) simulations demonstrate that under a diode voltage of 720 kV, beam current of 19.2 kA, and guiding magnetic field of 0.54 T, six short pulses with instantaneous peak power of 15.5–28.5 GW and full width half maximum (FWHM) durations of 0.7–1.4 ns are produced within a 50 ns window. The inter-pulse intervals range from 4.4 to 8.8 ns. The frequency is in the X-band with an 800 MHz bandwidth at the -10 dB level. The variations in peak power and timing are thoroughly analyzed, and the physical mechanisms underlying the quasi-periodic ultrashort-pulse formation are elucidated. A new insight is provided that the critical factor for the sequence formation based on the SR effect lies in the recovery of the electron beam to an undisturbed state after each pulse.
In the X-band relativistic klystron oscillator, mode hopping phenomena in the output microwave caused by low-order asymmetric TE11 mode competition are observed. Based on theoretical and simulation analyses of the electron conductance and external quality factors of the main mode TM013 and the TM113 mode, it is found that the electronic conductivity of both the main mode and the low-order TM113 mode is negative, and their external quality factors are close, which is the cause of the asymmetric mode competition. In particle simulations, by altering a pre-reflection cavity and altering the position of the cathode block, the reflection coefficient of the main mode TM013 is increased, thereby enhancing the resonance characteristics of the main mode and effectively suppressing the asymmetric mode competition. Simulation and experimental results indicate that by adjusting the depth of the pre-cavity and reducing the distance of the cathode block, the low-order asymmetric TE11 mode is effectively suppressed, enhancing the stability of the output microwave.
The suppression of asymmetric mode competition is one of the basic conditions for proper operation of high-power microwave (HPM) sources. This article presents experimental results on mode competition in a relativistic klystron oscillator (RKO) operating under low magnetic field conditions. A competing mode with a frequency of 6.27 GHz is observed in the experiments and is in good agreement with the particle-in-cell (PIC) simulation results. It is determined that the competing mode was the TM
mode. The asymmetric mode competition mechanism is found by theoretical and simulation studies to be that the asymmetric mode TM
mode and the symmetric mode TM
mode of the bunching cavity have the similar external quality factor
Q
. Upon modifying the bunching cavity of the RKO,
Q
increases from 1036 to 3421 for the symmetric mode and decreases from 1041 to 611 for the asymmetric mode, while other physical properties are less changed. The efficacy of this technique in suppressing asymmetric mode competition has been substantiated through both PIC simulations and experimental results.
Radio frequency (RF) breakdown in the slow-wave structures (SWSs) is a crucial bottleneck restricting relativistic backward wave oscillator (RBWO) to pursuing higher output power and longer pulse width. This paper has experimentally studied the influence of thermal accumulation during repetitive operation on RF breakdown in an X-band RBWO. A method for cooling the SWSs using water flow has been proposed to restrain the temperature rise to some extent. Under different heat dissipation conditions, the operating states of the RBWOs exhibit great differences. The greater the distance between the water-cooling heat transfer channel and the SWSs, the more serious the pulse shortening of high-power microwave. Moreover, the breakdown traces appearing in the SWSs become more obvious with the worse convective heat transfer capacity. The observed experimental phenomena provide a new guideline that helps to enrich the mechanism of RF breakdown and to explore corresponding suppression methods in RBWO.
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.
The relativistic backward wave oscillator (RBWO) is one of the most important high-power microwave generators. Its stability and reliability must be further improved to meet the demands of key applications. The factors that limit its performance are varied but include radio frequency breakdown and mode competition. Based on a C-band RBWO, the mode competition was carefully analyzed and the effects on the performance of the RBWO were revealed. As shown in the studies, for an axisymmetric system, asymmetric mode competition probably still exists. Therefore, a three-dimensional particle-in-cell model was configured to investigate the mode competition. The theoretical and experimental results confirmed that the competing mode for a C-band RBWO was EH21. Changing the gap between the cathode and anode was found to be effective for suppressing the asymmetric mode.
This paper describes a new design of a relativistic Cherenkov oscillator with strong modulation of the e-beam current at the input of two-sectioned slow-wave structure, as well as numerical and experimental results on generation of few gigawatt-level X -band pulses with pulse duration of up to 35 ns. Due to special phase shift between two sections and electrodynamic coupling between E₀₁ surface wave and E₀₂ volume mode, the first one is locked at the upper boundary of the passband and the second one gains more transparency. An electron beam can interact simultaneously with two waves with higher power conversion efficiencies, which in simulations amount up to 47% and 30% for high ( ~4 T) and low ( ~1 T) guiding magnetic fields, correspondingly. The tests of the geometry optimized in simulations were performed on the base of SINUS-7 high-current electron accelerator in the diode voltage range of 400-850 kV and current from 7 to 16 kA. The repeated and full-pulsewidth generation of about 1.5-GW microwave power is achieved only in the low magnetic field. The pulse shortening and the mode competition took place in the strong magnetic field. The results are considered as preliminary for creation of the efficient relativistic millimeter-wave oscillators at magnetic fields below the cyclotron resonance.
The issue of asymmetric modes output of a Ka-band super overmoded coaxial Cerenkov oscillator is analyzed in this paper. Due to serious passband overlapping in a super overmoded coaxial slow wave structure (SWS), the asymmetric competition mode EH11 can hardly be suppressed thoroughly by the methods adopted in moderately overmoded devices, especially in the startup of oscillation. If the output structures reflect the asymmetric modes, the asymmetric mode competition in SWS will be aggravated and the normal operation state will be destroyed. In order to solve this problem, a taper waveguide is inserted at a specific position to achieve the destructive interference of the reflected TM11, and a special support structure is designed to avoid reflection of TE11. With these methods, asymmetric mode competition can be successfully eliminated, and the oscillator is capable of achieving a steady fundamental mode operation performance.
For the first time, we demonstrate experimentally the possibility of Cherenkov superradiant generation with a phase imposed by an ultrashort seed microwave pulse. The phases of seed and initiated Ka-band microwave pulses were correlated with the accuracy of 0.5–0.7 rad for the power ratio down to −35 dB. Characteristics of such a process were determined in the frame of a basic theoretical model that describes both spontaneous and stimulated emission of an electron beam moving in corrugated waveguides. The obtained results open up opportunities of reaching extremely high radiation power density in phased arrays of short-pulse coherently operating microwave generators.
Most of the investigated overmoded relativistic backward wave oscillators (RBWOs) are azimuthally symmetric; thus, they are designed through two dimensional (2-D) particle-in-cell (PIC) simulations. However, 2-D PIC simulations cannot reveal the effect of asymmetric modes on beam-wave interaction. In order to investigate whether asymmetric mode competition needs to be considered in the design of overmoded RBWOs, a numerical method of determining the composition of both symmetric and asymmetric modes in three dimensional (3-D) PIC simulations is introduced in this paper. The 2-D and 3-D PIC simulation results of an X-band overmoded RBWO are analyzed. Our analysis indicates that the 2-D and 3-D PIC simulation results of our device are quite different due to asymmetric mode competition. In fact, asymmetric surface waves, especially EH11 mode, can lead to serious mode competition when electron beam propagates near the surface of slow wave structures (SWSs). Therefore, additional method of suppressing asymmetric mode competition, such as adjusting the reflections at both ends of SWSs to decrease the Q-factor of asymmetric modes, needs to be utilized in the design of overmoded RBWOs. Besides, 3-D PIC simulation and modes decomposition are essential for designing overmoded RBWOs. (C) 2014 AIP Publishing LLC.
A dual-mode operation mechanism in an overmoded relativistic backward wave oscillator is presented. The electron beam interacts with the −1st space harmonic of TM01 mode synchronously in the slow wave structure. Then the backward propagating TM01 mode is converted to the forward propagating TM02 mode. As the phase velocity of the volume harmonic of TM02 mode is about twice that of the surface harmonic of TM01 mode, the TM02 mode also plays an important role in the high-power microwave generation. Particle-in-cell simulation shows that an efficiency of 48% and a significant improvement of the power capacity have been obtained.
An over mode method for suppressing the RF breakdown on metal surface of resonant reflector cavity in powerful backward wave oscillator is investigated. It is found that the electric field is redistributed and electron emission is restrained with an over longitudinal mode cavity. Compared with the general device, a frequency band of about 5 times wider and a power capacity of at least 1.7 times greater are obtained. The results were verified in an X-band high power microwave generation experiment with the output power near 4 gigawatt.
This paper introduces a self-developed, three-dimensional parallel fully electromagnetic particle simulation code UNIPIC-3D. In this code, the electromagnetic fields are updated using the second-order, finite-difference time-domain method, and the particles are moved using the relativistic Newton–Lorentz force equation. The electromagnetic field and particles are coupled through the current term in Maxwell’s equations. Two numerical examples are used to verify the algorithms adopted in this code, numerical results agree well with theoretical ones. This code can be used to simulate the high-power microwave (HPM) devices, such as the relativistic backward wave oscillator, coaxial vircator, and magnetically insulated line oscillator, etc. UNIPIC-3D is written in the object-oriented C++ language and can be run on a variety of platforms including WINDOWS, LINUX, and UNIX. Users can use the graphical user’s interface to create the complex geometric structures of the simulated HPM devices, which can be automatically meshed by UNIPIC-3D code. This code has a powerful postprocessor which can display the electric field, magnetic field, current, voltage, power, spectrum, momentum of particles, etc. For the sake of comparison, the results computed by using the two-and-a-half-dimensional UNIPIC code are also provided for the same parameters of HPM devices, the numerical results computed from these two codes agree well with each other.
Efficient generation regime with a high power output has been experimentally realized in a klystron-like relativistic backward wave oscillator, in which a modulation cavity is inserted between the slow wave structure to decrease the energy spread of modulated beam electrons, and an extraction cavity is employed at the end of the slow wave structure to further recover energy from the electron beam. At a guiding magnetic field of 2.2 T, a microwave pulse with power of 6.5 GW, frequency of 4.26 GHz, pulse duration of 38 ns, and efficiency of 36% was generated when the diode voltage was 1.1 MV, and diode current was 16.4 kA. When the diode voltage was 820 kV, efficiency up to 47% with microwave power 4.4 GW was also realized experimentally.
A high-power relativistic microwave oscillator with low magnetic field is created based on a backward wave oscillator (BWO) with resonance reflector. For fixed parameters of the slow-wave structure (SWS) and the electron beam, the oscillation frequency of this BWO can be mechanically tuned within a 12% band by moving the resonance reflector relative to the SWS. A maximum output pulse radiation power of 4 +/- 1 GW at a working frequency of 3.6 GHz is achieved with a magnetic field of 4.6 kOe. (C) 2004 MAIK "Nauka/Interperiodica".
This paper presents the results of investigations of the mode of nonsteady-state oscillations with a short-time power burst which is characteristic of the initial stage of the transient process in a backward wave oscillator when the beam operating current is far in excess of the starting current. Numerical simulations have yielded the conditions under which the efficiency of the power transfer from an electron beam with a particle energy of 300 keV, a current of 2 kA, and a duration of 1 ns into a microwave pulse containing 8-10 high-frequency field periods approaches 90%. Experimentally, it has been demonstrated that the production of pulses like these with a duration of 200-250 ps, a power of up to ∼400 MW, and a central frequency of about 38 GHz is feasible.
A three-dimensional (3-D) model has been developed for the investigation of the coupling of the lowest symmetric and asymmetric modes in a high-power, high-efficiency traveling-wave amplifier. We show that in a uniform structure and for an initially nonbunched beam, the interaction efficiency of the asymmetric mode may be much higher than that of the symmetric mode. It is also shown that the coupling between these two modes is determined by a single parameter that depends on the beam characteristics; its value varies between zero when no coupling exists and unity in case of maximum coupling. For a beam that is uniform at the input end, this parameter varies linearly with the guiding magnetic field. In case of a bunched beam, it decreases linearly with the increasing phase-spread of the bunch. Because of the interaction, the radius of the beam increases linearly with the power associated with the asymmetric mode at the input end; it increases rapidly in the case of an initially uniform beam relative to the case of a prebunched beam. Selective damping to suppress the asymmetric mode is described and analyzed.
Results of theoretical and experimental studies of a GW-class,
large diameter microwave oscillator are presented. The device consists
of a large cross-section (overmoded), slow-wave structure with a unique
profile of wall radius specifically designed to support surface waves
and to provide a strong beam-wave coupling at moderate voltage (500 kV),
an internal adjustable microwave reflector, a coaxial microwave
extraction section, and a coaxial magnetically insulated field emission
electron gun. In preliminary experiments carried out at 8.3 GHz, the
power level exceeding 0.5 GW and efficiency of 15% have been measured
calorimetrically
An L-band high-power relativistic backward wave oscillator is designed. In the simulation, microwaves centered at 1.6 GHz are generated, with the power of 3.6 GW and the efficiency of 40%. In the preliminary experiment, the pulse duration of the device was only 45 ns, presenting a pulse-shortening phenomenon. Through the 3D particle-in-cell simulation analysis, it was found that the accelerating diode resonances significantly impact the operation of the L-band high power relativistic backward wave oscillator, and the resonance of a TE 11 mode in the accelerating diode played the primary role in the pulse shortening. Moreover, we found that choosing the appropriate distance between the cathode baffle and the end of the annular cathode is beneficial to effectively suppress the starting oscillation of the parasitic TE 11 mode. In the improved experiment, we changed the distance between the cathode baffle and the end of the annular cathode from previous 5.4 to 4.6 cm. Eventually, when the diode voltage is 650 kV and the diode current is 14 kA, microwaves centered at 1.58 GHz are generated with the power of 3.3 GW, the efficiency of 36%, and the pulse duration above 104 ns.
A high output power and high-efficiency L-band inverted relativistic magnetron (IRM) driven by a virtual cathode (VC) is proposed. By switching the configuration of cathode and anode, a greater magnitude of current could be generated from an external annular emission surface. In order to excite the desired mode more efficiently and enhance the mode stability, the emitting region is divided into eight azimuthally periodic parts. The prolonged VC is formed in the interaction region by dc electric field directly. The particle-in-cell (PIC) simulation demonstrates that the proposed IRM could generate an output a TM₀₁ mode with a microwave power of 1.97 GW for a diode voltage of 600 kV and a uniform guiding magnetic field of 0.18 T. Comparing with IRM with an emission surface inside the interaction region, the power conversion efficiency is improved from 18% to 86%.
Spatial coherent combination of multiple high power microwave (HPM) sources is considered as a promising scheme to improve the equivalent radiation power of the HPM system dramatically. Relativistic klystron amplifier (RKA) is one of the most suitable sources for the coherent power combination owing to its specific capabilities of stable microwave frequency and controllable phase. However, the RKAs operating at high frequency-band are severely limited by the problems of the intense space-charge effect and radio frequency breakdown. The radial-line HPM sources driven by the disk-shape electron beam may provide the potential to alleviate this issue due to its attractive features of the weak space-charge effect, the high power handling capacity, and the strong electron collection ability. In this paper, a disk-beam relativistic klystron amplifier (DB-RKA) is proposed and physically designed aiming to generate long-pulse HPM radiation at Ku-band. The physical idea, design principles, and simulation results are presented in detail. In a preliminary experiment, the disk-shape intense electron beam is well focused with an axial-width of 1.2 mm by an improved magnetic-excited method. Furthermore, the DB-RKA is demonstrated to be capable of generating Ku-band HPMs typically with peak power of 320 MW, pulse duration of 100 ns, and gain of 42 dB.
In the design of the coaxial millimeter-wave Cerenkov devices due to the low starting current and high temporal growth, asymmetric quasi-TE
v1
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
01
(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%.
This article presents time-dependent numerical modeling of V-band relativistic orotron: a high-power microwave generator with a TM03 oscillation mode in an oversized (D/λ ∼ 2.7) electrodynamic structure and diffractive output in the TM02 mode. Single-mode operation of the oscillator is ensured by the cyclotron selection of the working axisymmetric mode, with simultaneous depression of competing non-axisymmetric modes by means of cutting longitudinal slits in the wall of the slow-wave structure. The simulated output microwave power is 350 MW with a power conversion efficiency of 31% when using a 3.6 kA, 310 keV electron beam transported in the 3.9 T magnetic field. The simulation employed 2.5D axisymmetric and 3D Cartesian versions of the KARAT code.
Asymmetric mode competitions are observed in the design of an X-band triaxial klystron amplifier with an asymmetric input cavity, and the generation mechanism of the asymmetric mode competition is analyzed in the paper. The results indicate that the asymmetric modes are excited in the buncher cavity. The asymmetric mode (coaxial TM612 mode) in the buncher cavity with the highest shunt impedance can start up first among the asymmetric modes with negative beam loading conductance. The coupling of the corresponding coaxial TE mode between the buncher and input cavity exacerbates the start oscillation of the asymmetric mode competition. The rationality of the analysis is demonstrated by cutting off the propagation of the corresponding coaxial TE modes between the buncher cavity and the input cavity, and the asymmetric mode competitions are thoroughly suppressed by specially designed reflectors and lossy material. In simulation, a microwave with a power of 1.28 GW and a frequency of 9.375 GHz is generated, and the extraction efficiency and the gain are 34.5% and 41.5 dB, respectively.
A relativistic Ku-band coaxial transit-time oscillator has been proposed in our previous work. In the experiments, we find that the asymmetric competition mode in the device limits the microwave power with the increase of the input electric power. For solving such a problem, the methods for analysis and suppression of the asymmetric competition mode in the device are investigated theoretically and experimentally. It is shown that the structure and the material of the collector, the concentricity, and the electron emission uniformity play an important part in the suppression of the asymmetric competition mode in the relativistic Ku-band transit-time oscillator. In the subsequent experiments, the asymmetric mode was suppressed effectively. At a low guiding magnetic field of 0.7 T, a microwave pulse with power of 1GW, frequency of 14.3 GHz close to the simulation one, and efficiency of 20% was generated. (C) 2014 AIP Publishing LLC.
Some factors that influence the emission uniformity of explosive emission cathodes (EECs) in foilless diodes such as the guiding magnetic field strength and the rise rate of electric field have been researched in former literatures. This paper is concentrated on another factor that has been overlooked previously, i.e., the defects with dimensions of tens of micrometers on cathode surfaces, especially at blade edges. It is shown that these defects will significantly worsen the emission uniformity of EECs by introducing extraordinarily large microscopic field enhancement factors, and thus they should be eliminated. The micrometer-scale irregularities, however, should be maintained to provide effective emission micropoints. Meanwhile, the outer edge of an annular cathode blade should be rounded off to improve the emission uniformity. After being disposed like this, the emission uniformity of annular graphite cathodes in a foilless diode is obviously improved, which leads to an increase of energy efficiency by more than 20% by broadening the microwave pulse duration under a power level of 2.8 GW for an X-band relativistic backward wave oscillator.
Recent experimental results of three kinds of long-pulse high-power microwave (HPM) sources operating in S-, C-, and X-bands are reported. The difficulties in producing a long-pulse HPM for the O-type Cerenkov HPM source were analyzed theoretically. In S- and C-bands, single-mode relativistic backward-wave oscillators were designed to achieve long-pulse HPM outputs; in X-band, because of its shorter wavelength, an O-type Cerenkov HPM source with overmoded slow-wave systems (D/λ ≈ 3) was designed to increase power capacity. In experi- ments, driven by a repetitive long-pulse accelerator, both S- and C-band sources generated HPMs with power of about 2 GW and pulse duration of about 100 ns in single-shot mode, and the S-band source operated stably with output power of 1.2 GW in 20-Hz repetition mode. The X-band source generated 2 GW microwaves power with pulse duration of 80 ns in the single-shot mode and 1.2 GW microwave power with pulse duration of about 100 ns in the 20-Hz repetition mode. The experiments show good per- formances of the O-type Cerenkov HPM source in generating repetitive long-pulse HPMs, especially in S- and C-bands. It was suggested that explosive emissions on surfaces of designed eletrodynamic structures restrained pulse duration and operation stability.
The initial experimental results of an L-band relativistic backward wave oscillator with a coaxial slow-wave structure are presented. The asymmetric-mode-competition mechanism in the device is investigated theoretically and experimentally. It is shown that the diode voltage, guiding-magnetic field, and concentricity play a key role in the suppression and excitation of the asymmetric-mode (coaxial quasi-TE11 mode). In the experiments, the asymmetric-mode with a frequency of 2.05 GHz is suppressed and excited, which is in good agreement with the theoretical results.
A compact P-band coaxial relativistic backward wave oscillator (BWO) with only three periods slow wave structure (SWS) is investigated both theoretically and numerically. The characteristics of the coaxial SWS are analyzed when the SWS is changed from the structure with only outer conductor ripple to the structure with both inner and outer conductor ripples. It is found that the existence of the inner conductor ripple can reduce the period length of coaxial SWS to maintain the same operating frequency of the BWO and can largely increase the temporal growth rate and the spatial growth rate of the device. Then, the effects of SWS period numbers on the generation of the microwave in the P-band relativistic BWO are studied by PIC simulations. The results show that three periods SWS cannot only make the device more compact but also has a wide region of single-frequency operation and relatively large efficiency and output power in a wide range of the diode voltage. Typical simulation results show that, with a 585 kV and 7.85 kA electron beam guided by a 0.8 T solenoidal field, the microwave of 1.65 GW is generated at the frequency of 900 MHz, and the interaction efficiency is about 36%. Compared with the conventional P-band coaxial relativistic BWO with five periods SWS, the axial length of the SWS is reduced by about one half, which is only 38.4 cm, and the saturation time of the microwave signal is reduced by about 10 ns.
An X-band magnetically insulated transmission line oscillator (MILO) is designed and investigated numerically and experimentally for the first time. The X-band MILO is optimized in detail with KARAT code. In simulation, the X-band MILO, driven by a 720 kV, 53kA electron beam, comes to a nonlinear steady state in 4.0 ns. High-power microwaves (HPM) of TEM mode is generated with an average power of 4.1 GW, a frequency of 9.3 GHz, and power conversion efficiency of 10.8% in durations of 0–40 ns. The device is fabricated according to the simulation results. In experiments, when the voltage is 400kV and the current is 50kA, the radiated microwave power reaches about 110 MW and the dominating frequency is 9.7 GHz. Because the surfaces of the cathode end and the beam dump are destroyed, the diode voltage cannot increase continuously. However, when the diode voltage is 400kV, the average power output is obtained to be 700 MW in simulation. The impedance of the device is clearly smaller than the simulation prediction. Moreover, the duration of the microwave pulse is obviously shorter than that of the current pulse. The experimental results are greatly different from the simulation predictions. The preliminary analyses show that the generations of the anode plasma, the cathode flare and the anode flare are the essential cause for the remarkable deviation of the experimental results from the simulation predictions.
We present the design and experimental results of a novel overmoded slow-wave high-power microwave (HPM) generator that is featured by its compactness, low-operation magnetic field, and potentially high power and high efficiency. The device includes two slow-wave structure (SWS) sections, a resonant cavity, and a tapered waveguide. The resonant cavity was well designed and was used to achieve the axial mode selection and to decrease the length of the SWS sections. The radial mode selection is achieved using the property of "surface wave" of the device to excite the TM01 mode while making the higher TM0n modes unexcited. The physical mechanisms of axial and radial mode selections ensure that the microwave is produced with a single mode and a narrow band. The feasibility of low magnetic field operation is also investigated based on the characteristics of the overmoded slow-wave devices. Experiments were carried out at the Spark-2 accelerator. At diode voltage of 474 kV, beam current of 5.2 kA, and guiding magnetic field strength of 0.6 T, a microwave was generated with power of 510 MW, mode of TM01, and frequency of 9.54 GHz. The relative half width of the frequency spectrum is Δf/f= 0.6%, and the beam-to-microwave efficiency is about 21% in our experiment.
A repetitive X-band relativistic backward-wave oscillator (BWO) driven by a SINUS-881 accelerator is described. Relativistic electron beams with peak current of 5.4 kA and voltage of 610 kV at a repetition rate of 100 Hz were generated by the SINUS-881 and then guided through the corrugated waveguide by an axial magnetic field of 3.0 T produced by a superconducting magnet. An electron collector was used to collect the electron beams in order to mitigate the effect of secondary emission electrons and to prevent ionization and breakdown near the electron beam dump. This BWO produces a microwave pulse power of 1.1 GW at a 100-Hz repetition rate, a frequency of 9.38 GHz, a pulse duration of 23 ns, and a power transforming efficiency of 33%.
Spontaneous pulse shortening occurring in a relativistic backward
wave oscillator (BWO) at gigawatt power levels is studied in experiment
and theory. It is experimentally demonstrated that this phenomenon is
accompanied by formation of an explosive-emission plasma at the surface
of the corrugated slow-wave structure (SWS). Termination of microwave
emission is explained by the increase of the BWO starting current from
the absorption of the operating electromagnetic wave by electrons
emitted from the plasma, whereas the intensity of the absorption
radically increases offing to the presence of positive ions emitted from
the plasma. Application of oil-free vacuum and electrochemical polishing
of the SWS surface in an X-band BWO allowed generation of 3-GW, 26-ns
microwave pulses with an energy of ~80 J, thereby demonstrating pulse
lengthening by a factor of four
The achievements resulting from the application of advanced pulsed
power to the generation of high power microwaves (HPM) have included the
generation of multi-gigawatt pulses of RF energy. The power achievable
is orders of magnitude greater than conventional microwave sources can
generate. However, the introduction of the HPM technology into logical
applications has been limited to date due to the phenomenon of pulse
shortening in which the RF pulse terminates before the pulse power
source used to produce it. Conventional microwave tubes can generate a
few to 10 MW of power with pulsewidths of many microseconds when
required. High power microwave sources can produce gigawatts of power,
but only for relatively short pulsewidths, typically tens to hundreds of
nanoseconds. An international effort during the past few years has
generated important new discoveries toward the elimination of pulse
shortening. Some of the new techniques have the potential for helping
the conventional tube industry as well as being practical for high power
microwave sources. This paper reviews the pulse shortening problem, its
causes, and the worldwide scope and direction of research conducted to
date to resolve it. The paper also discusses the potential remedies for
the problem and recommends a course of research to further progress on
the issue