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This paper addresses the challenge of extending the pulse duration of high-power microwave (HPM) sources, a critical focus in HPM technology. Transit-time oscillators (TTOs) typically modulate electron beams by forming standing waves through the cutoff of the TM01 mode in the modulation cavity. This study proposes enhancing the power capacity of TTOs by widening the drift channel. To enhance modulation capability without compromising power capacity, front and rear reflectors are strategically introduced. In particle-in-cell simulations, the device exhibits an output power of 1.65 GW at a frequency of 12.6 GHz, with an efficiency of 33.6%. The saturation time for the output microwave is 30 ns, and the maximum internal electric field strength is 860 kV/cm. The presented TTO, with design improvements, offers a promising solution for applications requiring prolonged pulse durations in high-power microwave sources. [ABSTRACT FROM AUTHOR]

We have developed a method for calculation of the pulsed magnetic field in a sophisticated system of solenoids powered by time-shifted current pulses. The method served as the basis for analysis of the temporal dynamics of magnetic-field variations both in the region of formation of the helical electron beam, and in the space of its interaction with the microwave field in a relativistic gyrotron. It is shown that at some time instants, the degree of the field nonuniformity in the operating space of the gyrotron can reach 20%. The time interval is found, during which the magnetic field in the cavity is uniform. In the experimental process, this allows one to determine the time instant at which a high-voltage pulse is fed in for generation of microwave radiation. The calculated distributions of the magnetic field are compared with the experimental data. We have also developed a method for calculation of the time evolution of the parameters of a helical electron beam formed by a magnetron-injection gun with a pulsed magnetic field, calculated the velocity distribution in the electron beam, and found the time interval, during which a beam with the parameters acceptable for generation is formed. [ABSTRACT FROM AUTHOR]

In order to make the high-power microwave sources packaged with a permanent magnet more compact and miniaturized, we design a novel compact and lightweight permanent magnet package based on a C-band coaxial transit time oscillator. Before conducting the microwave generation experiment, research on the emission and transmission is carried out. In the beam transmission experiment, when the compact and lightweight permanent magnet is applied, the transmission efficiency in the diode is only 68.1%, while a phenomenon of side emission of electron beams is observed. When a solenoid is used, no side emission occurs. We theoretically analyze this experimental phenomenon and conclude that the radial magnetic field generated by the permanent magnet aggravates the side emission on the surface of a cathode holder. To solve this problem, a novel diode with a gradient cathode holder is proposed. In the beam transmission experiment using the compact and lightweight permanent magnet, this diode can suppress the side emission of electron beams, and the transmission efficiency in the diode is increased to 80.7%. The potential of the diode proved to work efficiently with this permanent magnet package, laying the foundation for the experimental research of microwave generation with this permanent magnet. [ABSTRACT FROM AUTHOR]

A powerful spatially extended planar Cherenkov maser project operating in ${W}$ -band is under development in collaboration between Budker Institute of Nuclear Physics of the Russian Academy of Sciences (BINP RAS) (Novosibirsk) and Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS) (Nizhny Novgorod). The ELMI accelerator (1 MeV/5–7 kA/ $3~\mu \text{s}$) forms a sheet electron beam with the transverse size (width) of up to 18 cm, which serves to drive the oscillator. The electrodynamic system of this Cherenkov maser is based on doubly periodical structure, which combines the properties of a slow wave system that realizes conditions for an effective Cherenkov interaction with a high-current rectilinear sheet electron beam and a high- ${Q}$ resonator that implements the mechanism of two-dimensional distributed feedback and provides selective excitation of the operating mode in the strongly oversized interaction space. In this article, the crucial elements and design parameters of the oscillator are discussed, and the results of simulations are presented to demonstrate the possibility of achieving a stable narrow-band generation regime for a transverse system size reaching about 50 wavelengths and a gigawatt output power level. To provide a single-directed output of radiation, a planar Bragg reflector is elaborated for installation at the cathode side of the interaction space. A mode converter formed by a slowly tapered waveguide section for transformation of the radiated wavebeam into a Gaussian-type wavebeam was designed to be installed at the oscillator output. In the electron-optical experiments, the formation of a large-width sheet electron beam with parameters acceptable to drive designed ${W}$ -band Cherenkov maser is demonstrated. [ABSTRACT FROM AUTHOR]

A super klystron-like relativistic backward wave oscillator (RBWO) is presented. In the device, the distributed Cherenkov radiation generated in the slow wave structure (SWS) is absorbed by the electron bunch in the last corrugation and re-accelerated, creating high-energy electrons that overcome the space-charge repulsion force to further compress the bunch length and maximize the fundamental harmonic current, contributing to the extremely efficient microwave generation in the extraction cavity through concentrated transition radiation. A novel elliptical extraction cavity is used to improve the power capacity. A soft magnet is inserted to precisely control the beam path, which provides a higher beam kinetic energy to be extracted and a stronger electric field that interacts with the tighter bunch. In the experiment, as the diode voltage was 585 kV and diode current was 7.5 kA, a microwave pulse with a power of 2.9 ± 0.3 GW measured in the radiation field, a frequency of 8.23 GHz, and a pulse duration of 22 ns was obtained. The corresponding efficiency is up to 66 ± 8%, which is an experimental record for GW-class relativistic vacuum electronic devices. [ABSTRACT FROM AUTHOR]

Large overmoded coaxial mymargin slow wave structure was adopted to overcome the problems of low power capacity and narrow electron beam channel in V-band. Based on the simulation design of coaxial Cerenkov oscillator, the experimental research was carried out in this letter. The output microwave with power 226 MW, pulse width 18 ns, frequency 63.2 GHz can be measured, corresponding efficiency is 23%. The influence of voltage and magnetic field on the generation of microwave was also studied. The experimental principle is consistent with the simulation design. [ABSTRACT FROM AUTHOR]

Modulation and amplitude of transmitted electron beam current in a virtual cathode oscillator have been measured by using two beam detectors, large detector and small detector. The objective is to understand the mechanism of microwave oscillation in a virtual cathode oscillator. The large detector has given us information about amplitude of the transmitted beam current from which the condition of virtual cathode formation has been obtained. The small detector has been used to measure the current modulation in the transmitted electron beam. The results have provided direct evidence of the virtual cathode existence and oscillation. The time–frequency behavior of the beam modulation has matched with that of the output microwave. [ABSTRACT FROM AUTHOR]

A compact PFN (pulse forming network)-Marx generator, which is based on mica capacitors, is constructed to generate intense electron beams for the purpose of high-power microwave generation. The key components of the generator, including a PFN-Marx, a trigger source, a multi-stage gas switch, and a load, are introduced in this paper. The PFN-Marx is composed of 19 PFN modules, which are formed by four mica capacitors in parallel. The temperature characteristics and pulse characteristics are tested in this study. The mica capacitor is insensitive to the temperature, and its lifetime is tens of thousands. Considering the influence of the ultraviolet light pre-ionization, gas gap switches are assembled in a nylon house. The trigger source, which is a mini-Marx generator based on the integration into the magnetic transformer and the magnetic switch, is used for increasing the stability of the switches. The generator has been tested on a water load and a relative magnetron. The experimental results show that this generator can deliver electrical pulses with the pulse width of 120 ns and the amplitude of 400 kV on a 40-Ω water load at a repetition rate of 50 Hz. In addition, utilizing the generator to drive a Ku band magnetron at a voltage level of 330 kV, a 500 MW microwave pulse with the pulse width of 50 ns at the frequency of 14.4 GHz is obtained. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In anticipation of an Electron Cyclotron Resonance Heating system upgrade for the stellarator Wendelstein 7-X, a 1.5 MW – 140 GHz continuous-wave gyrotron is under development. In order to provide a first experimental verification of the scientific RF and electron beam optics design of the gyrotron with ms pulses, the Karlsruhe Intitule of Technology has developed a short-pulse pre-prototype gyrotron. In this work, we present details regarding the construction of the pre-prototype as well as measurements from the first experimental campaign delivering up to 1.6 MW in short pulses. [ABSTRACT FROM AUTHOR]

Surface-wave oscillators driven by intense relativistic sheet electron beams can produce sub-terahertz waves at sub-gigawatt level of output power; however, evanescent nature of the operating wave hampers further radiation transmission and application. For these devices, we propose a method of efficient conversion of the surface operating wave into the output Gaussian-like wavebeam radiated in transverse direction with respect to the grating by applying an additional corrugation with a period twice larger than the main period. The analysis based both on the quasi-optical model and on the PIC simulations shows feasibility of 150-GHz surface-wave oscillator with 35 % efficiency and 100 MW output power. The transverse energy extraction significantly reduces the Ohmic losses down to 10% of the radiated power, which is essential for sub-THz range. [ABSTRACT FROM AUTHOR]

The pulse-periodic operation regime of a plasma relativistic microwave generator is considered. The reasons for the increase of average frequency of the output microwave radiation in a pulse train created in the same conditions of initial plasma generation are analyzed. The effect of solid body erosion that occurs due to the interaction of the relativistic electron beam with the collector surface on the output microwave radiation is studied. The analysis of experimental data is accompanied by numerical simulations by the electromagnetic PIC code KARAT. It is shown that increasing the amount of mater extracted from the surface of the collector during erosion leads not only to an increase of the plasma density, but it can also lead to qualitative changes of the parameters of the output radiation, which are caused by the change of microwave generation regime. In particular, the generation of an ion background that is inhomogeneous in the radial direction caused a change of conditions of plasma–beam interaction during the second half of the pulse and a change of the spectral density of radiation at the main frequency. Numerical simulations were used to demonstrate this for two different admixtures, iron and graphite. [ABSTRACT FROM AUTHOR]

This article presents a design of the low-voltage Ku-range magnetron with two outputs of energy: an active with matched load and a passive that is matched with a load having reactive impedance and providing frequency tuning (or its adjustment). Studying the magnetron is carried out in continuous and pulsed modes under the following input operating parameters: an operating anode voltage varied in a range of 450–500 V, a magnetic field of 0.25 T, a pulse length of 5–20 μs, a pulse repetition rate between 100 and 300 Hz, and the magnetron weight ~0.85 kg subject to air cooling. The results of computer modeling the mode spectrum of a cold resonant anode block of the magnetron show that the separation between the operating N/2-mode and the closest low-voltage (N/2-1)-mode is ~1.6 GHz. Under these conditions, it is shown that the maximum frequency tuning of the cold resonant anode block is ~250 MHz. It is shown that the results of experimental studies of frequency tuning in the magnetron depend on the value of reactive loading in the second (passive) output. The limiting value of the frequency tuning range of the “hot” magnetron is ~110 MHz, providing an output average power of 15 W at ~36% efficiency. The available results of computer modeling are in good agreement with the experiment data. The implementation of the mode of frequency tuning enables to extend the functionality of the magnetron and a range of tasks that can be solved. [ABSTRACT FROM AUTHOR]

A high-power-microwave (HPM) source consisting of a backward wave oscillator (BWO) based on a metamaterial slow wave structure (MTM SWS) is investigated and tested. The MTM SWS is composed of a circular waveguide with periodic loading of two complementary electric split-ring resonator (CeSRR) disks and was designed at the University of California, Irvine, CA, USA (UCI). This SWS has been studied and simulated to quantify its cold structure properties. Moreover, a BWO using the proposed SWS has been simulated using a particle-in-cell solver to evaluate its performance. In the simulations, this BWO has generated 88-MW peak power in a pulse duration on the order of 15 ns with frequency about 2.9 GHz. This SWS was fabricated using 3-D printing and copper plating technology and was tested using the SINUS-6 pulsed electron beam accelerator at the University of New Mexico, Albuquerque, NM, USA (UNM). An output of 22 MW at 3.0 GHz was observed in experiments. [ABSTRACT FROM AUTHOR]

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%. [ABSTRACT FROM AUTHOR]

A method is proposed for producing microwave generation in a relativistic magnetically insulated vircator in the under-limit mode of an electron beam. The method is implemented if the repeated or multiple passage of the electron beam of the drift tube is organized. In this case, a virtual cathode is formed in the vircator and the powerful microwave generation occurs. Using the particle-in-cell (PiC) simulation, the microwave producing characteristics of the radiation are calculated upon the re-introduction of the beam into the drift tube and the nonlinear dynamics of the electron beam is studied. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

High power microwave (HPM) generation studies were carried out in KALI-5000 pulse power system. The intense relativistic electron beam was utilized to generate HPMs using a coaxial virtual cathode oscillator. The typical electron beam parameters were 350 kV, 25 kA, and 100 ns, with a few hundreds of ampere per centimeter square current density. Microwaves were generated with graphite and polymer velvet cathode at various diode voltage, current, and accelerating gaps. A horn antenna setup with diode detector and attenuators was used to measure the microwave power. It was observed that the microwave power increases with the diode voltage and current and reduces with the accelerating gap. It was found that both the peak power and width of the microwave pulse is larger for the velvet cathode compared to the graphite cathode. In a coaxial vircator, velvet cathode is superior to the graphite cathode due to its shorter turn on time and better electron beam uniformity. [ABSTRACT FROM AUTHOR]

We suggest method of short microwave pulse generation by means of compression of chirped pulses obtained in the process of Stimulate Raman Backscattering (SRB) on electron beam with variable energy. We assume that in frequency modulator the pump wave, co-propagating with the electrons is scattered into counter propagating signal wave with frequency downshift. The absolute instability, developing in this system, allows for the smooth tuning of the signal frequency by varying the energy of electrons. The SRB modulator can produce a frequency-tunable signal with a bandwidth up to 10% and conversion efficiency up to 70%, when using a 50 kW, 35 GHz gyrotron as a source of pump wave. Simulation of subsequent compression with multi-fold helically corrugated waveguide (HCW) as dispersive line showed that optimization of energy temporal dependence in modulator together with compressor parameters provides the possibility to produce pulses with the peak power up to 3 MW which exceeds the power of driving gyrotron by the factor of 60. [ABSTRACT FROM AUTHOR]

The mechanism and realization of a cross-band relativistic Cherenkov oscillator based on the cathode adjustment are presented. The device adopts coaxial dual-annular cathode to emit the inner and the outer electron beams, independently. When the inner annular cathode is extended and the outer annular cathode is retracted, the electron beams are emitted from the inner cathode and interact with the internal electromagnetic structure to generate Ka-band microwaves. On the contrary, the outer electron beams interact with the external electromagnetic structure to generate X-band microwaves. Thus, the cross-band microwave generation can be achieved only by adjusting the protrusion and retraction of dual-annular cathode. By the particle-in-cell (PIC) method, the physical process of beam-wave interaction in the device is studied, and the high efficiency internal-external double electromagnetic structures are designed. Simulation results show that the X-band microwave with a power of 1.50 GW and a frequency of 8.55 GHz is generated, corresponding to an efficiency of 32%. Meanwhile, the Ka-band microwave with power of 650 MW and frequency of 31.80 GHz is generated, corresponding to efficiency of 28%. [ABSTRACT FROM AUTHOR]

Evolution of low-frequency instabilities degrades electron beams in beam transportation channels and, not less importantly, deteriorates the coherence of the output radiation produced by microwave generators based on the use of such beams. These instabilities also determine the beam dynamics, including that associated with transitions of the beams from one stationary state to another. In this work, we use the method of coupled waves to study the stability of stationary states of the beams with high electron densities in weakly irregular vacuum beam transportation channels, specifically, in channels with shallow periodic wall corrugations. It is shown that in this case, the Bursian instability of the absolute type can evolve, which leads to a jump-wise transition of the beam to a state with a lower electron density. [ABSTRACT FROM AUTHOR]

The particle-in-cell simulation of a reflex triode with a radially divergent magnetized electron beam was carried out. The dynamics of the electron beam and generation characteristics of the reflex triode was studied. It was shown that two virtual cathodes formed in the triode. It was calculated that, when the current of the beam was I0 = 5 kA and the cathode potential was Uc = –100 kV, the peak power of the microwave radiation attained P = 19 MW. There was a frequency peak on the spectrum of the microwave frequency radiation ν = 5.2 GHz. [ABSTRACT FROM AUTHOR]

This paper describes two sets of experiments. In the first set, the ultraviolet radiation produced by the spark channel creates the photoionized plasma in air between two parallel plates. The spatial resonator formed between the two plates generates many microwave lines that are recorded by the E-field probe. Comparison is made between these results and the data obtained in the experiments done in vacuum using the electron beam. In the second set of experiments, some details of the microwave generation in atmospheric air and argon are presented. The frequency lines arising from the simple spatial resonator and from the resonant cavity are observed and recorded. The electrons are generated by the electric field and by the photoionization. Comparison between the data leads to the suggestion of the possibility to enhance the microwave power level in the plasma-assisted slow-wave oscillator. [ABSTRACT FROM PUBLISHER]

Using measured waveforms from a synchronous-wave ferrite nonlinear transmission line (NLTL), particle-in-cell simulations demonstrate the potential to emit GW-class electron beams (hundreds of kilovolts, multiple kiloamperes) with current modulations as great as 30% of the average beam current, with modulations occurring at a peak frequency equal to that of the peak NLTL output frequency. The NLTL-modulated beam is then coupled to disk-on-rod slow wave structure (SWS) simulations in which it is shown that the extractable RF generated through interaction of the modulated beam with the SWS is up to ten times greater than that directly extractable from the NLTL itself. [ABSTRACT FROM PUBLISHER]

A regime of quasi-stationary microwave generation in moderately relativistic microwave oscillator of the twistron type with 50 ± 20% efficiency of electron beam power conversion into electromagnetic radiation has been obtained in experiment through optimization of the electron–wave interaction. For the selected electron beam parameters (diode voltage, 210 kV; beam current. 1.36 kA) the output microwave power at 10.63 GHz frequency was 140 ± 40 MW in a guiding magnetic field of about 1.9 T. The duration of microwave pulses was about 16 ns. [ABSTRACT FROM AUTHOR]

Multicavity resonators for an axial vircator are developed and experimentally investigated. The frequency and radiation power are analyzed for different cathodes and anode meshes and varied cathode–anode gaps. [ABSTRACT FROM PUBLISHER]

In relativistic backward wave oscillators (RBWOs), although the slow wave structure (SWS) and electron beam determine the main characteristics of beam-wave interaction, many other factors can also significantly affect the microwave generation process. This paper investigates the influence of voltage rise time on beam-wave interaction in RBWOs. Preliminary analysis and PIC simulations demonstrate if the voltage rise time is moderately long, the microwave frequency will gradually increase during the startup process until the voltage reaches its amplitude, which can be explained by the dispersion relation. However, if the voltage rise time is long enough, the longitudinal resonance of the finitely-long SWS will force the RBWO to work with unwanted longitudinal modes for a while and then gradually hop to the wanted longitudinal mode, and this will lead to an impure microwave frequency spectrum. Besides, a longer voltage rise time will delay the startup process and thus lead to a longer microwave saturation time. And if unwanted longitudinal modes are excited due to long voltage rise time, the microwave saturation time will be further lengthened. Therefore, the voltage rise time of accelerators adopted in high power microwave technology should not be too long in case unwanted longitudinal modes are excited. [ABSTRACT FROM AUTHOR]

For the purpose of coherent high power microwave combining at high frequency band, an X-band phase-locked relativistic backward wave oscillator is presented and investigated. The phaselocking of the oscillator is accomplished by modulation of the electron beam before it reaches the oscillator. To produce a bunched beam with an acceptable injected RF power requirement, an overmoded input cavity is employed to provide initial density modulation. And a buncher cavity is introduced to further increase the modulation depth. When the beam enters the oscillator, the modulation depth is enough to lock the frequency and phase of the output microwave generated by the oscillator. Particle-in-cell simulation shows that an input power of 90kW is sufficient to lock the frequency and phase of 1.5GW output microwave with locking bandwidth of 60MHz. [ABSTRACT FROM AUTHOR]

The resistive wall amplifier (RWA), a vacuum electron device, has been theoretically predicted to provide extremely high gain. However, the properties of practical, naturally available bulk materials for the resistive wall support structure significantly reduce achievable gain. This paper investigates the capabilities of a metamaterial-enhanced RWA (ME-RWA). We predict that using epsilon-negative metamaterials can yield high gains that closely approach the idealized performance limit. ME-RWAs may exhibit bandwidths and gains much higher than many commercially available vacuum electron devices. [ABSTRACT FROM PUBLISHER]

We study the behavior of the generation efficiency and radiation spectrum of a three-cavity axial vircator as a function of the injected electron-beam radius, impedance, and energy homogeneity. We show that for each geometry of a three-cavity resonator, there exist optimal (maximizing the generation efficiency) values of these parameters and determine the range over which they may vary without losing the efficiency of generation. [ABSTRACT FROM PUBLISHER]

The study of the microwave generation regimes with intense higher harmonics taking place in a high-power vircator consisting of a relativistic electron beam with a virtual cathode has been made. The characteristics of these regimes, in particular, the typical spectra and their variations with the change of the system parameters (beam current, the induction of external magnetic field) as well as physical processes occurring in the system have been analyzed by means of 3D electromagnetic simulation. It has been shown that the system under study demonstrates the tendency to the sufficient growth of the amplitudes of higher harmonics in the spectrum of current oscillations in the VC region with the increase of beam current. The obtained results allow us to consider virtual cathode oscillators as promising high power mmw-to-THz sources. [ABSTRACT FROM AUTHOR]

A long-pulse generator TPG700L based on a Tesla transformer and a series pulse forming network (PFN) is constructed to generate intense electron beams for the purpose of high power microwave (HPM) generation. The TPG700L mainly consists of a 12-stage PFN, a built-in Tesla transformer in a pulse forming line, a three-electrode gas switch, a transmission line with a trigger, and a load. The Tesla transformer and the compact PFN are the key technologies for the development of the TPG700L. This generator can output electrical pulses with a width as long as 200 ns at a level of 8 GW and a repetition rate of 50 Hz. When used to drive a relative backward wave oscillator for HPM generation, the electrical pulse width is about 100 ns on a voltage level of 520 kV. Factors affecting the pulse waveform of the TPG700L are also discussed. At present, the TPG700L performs well for long-pulse HPM generation in our laboratory. [ABSTRACT FROM AUTHOR]

In this paper, we present the design of a microwave generator using metamaterials (MTMs) in a negative index waveguide interacting with a high-power electron beam. The microwave structure is formed by inserting two MTM plates loaded with complementary split-ring-resonators (CSRRs) into a rectangular waveguide. Electromagnetic simulations using the high-frequency structure simulator code confirm the presence of a negative index TM-like mode suitable for use in a backward-wave oscillator (BWO). Particle-in-cell (PIC) simulations using the computer simulation technology (CST) Particle Studio code are performed to evaluate the efficiency of an S-Band MTM-based BWO (MTMBWO) excited by a 500 keV, 80-A electron beam. After about 250 ns, the MTMBWO reaches a saturated output power of 5.75 MW with an efficiency of 14% at a frequency near 2.6 GHz. The MTMBWO is also modeled by representing the MTM plates, which consist of CSRRs, as dielectric slabs whose effective permittivity is given by a Lorentzian model. The dielectric slab model is also simulated with the CST PIC code and shows good qualitative agreement with the simulations including the CSRR loaded plates. A cold test structure was fabricated from brass to test the theoretical predictions of the microwave transmission versus frequency of the negative index waveguide. Test results using a vector network analyzer showed very good agreement with the simulations for the excitation of the negative index TM-like mode near 2.6 GHz. The proposed structure appears to be promising for use in a MTMBWO high-power microwave generator. [ABSTRACT FROM PUBLISHER]

This paper studies the motion of a virtual cathode in a two-section drift tube with the formation and breakup of the "compressed" state of an electron beam. Experimental arrangements to intercept part of the injected current during the voltage pulse and to provide virtual cathode motion toward the collector are proposed. The arrangements were implemented on the SINUS-7 high-current electron accelerator. Theoretical and experimental dependences of the virtual cathode velocity on the injected current and cathode voltage are presented. The experimental data on virtual cathode motion agree with its theoretical model based on analytical solutions of equations assisted by computer simulation with the PIC code KARAT. The results of the work demonstrate the feasibility of controlling the virtual cathode motion which can be used in collective ion acceleration and microwave generation. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive understanding of the influences of the modulation cavity and extraction cavity on microwave generation and starting oscillation in a klystron-like relativistic backward wave oscillator. Near the region of the modulation cavity, the axial electric field is small, which is favorable to the reduction of the energy spread of the modulated beam electrons. In addition, the presence of the modulation cavity increases the fundamental harmonic current in the extraction cavity, where the beam-wave interaction is predominant due to the larger axial electric field. The introduction of the extraction cavity leads to a large increase in the amplitude and a sudden change in the phase of the axial electric field, which is beneficial for decelerating the bunched electron beam that has fallen into the acceleration phase. Furthermore, the peak electric field interval in the second sectional slow wave structure (SWS) and extraction cavity is decrescent, making them similar to the role of nonuniform SWS. The extraction cavity enhances the fundamental harmonic current significantly. The modulation cavity acts like a localized wave absorber during the process of starting oscillation and enhances the start current, and the extraction cavity decreases the start current due to the increased coupling impedance. [ABSTRACT FROM PUBLISHER]

A method for 2-D simulation of an axial vircator with a resonance cavity is described. It uses a self-consistent solution of relativistic motion equations and Maxwell equations for scalar and vector potentials in the Coulomb gauge. The vector potential is represented as a superposition of resonator eigenmodes, where each eigenmode corresponds to a damping oscillator. Damping enables considering radiative energy losses from the open end of the resonator. Compliance of the simulation results with both published experimental studies and those performed by our team is demonstrated. [ABSTRACT FROM AUTHOR]

The results of investigations of a double-gap vircator driven by a 20 \Omega and 500-ns generator operating in the output voltage range 400–600 kV are presented. The vircator generated microwave pulses with a peak power of up to 200 MW at \sim5% efficiency and the frequency varied from 2.0 to 2.3 GHz depending on the cavity geometry. The limitations on the microwave pulse duration not related to the cathode plasma expansion are addressed. On the one hand, the microwave generation is terminated because of the plasma formation at the foil separating the cavity sections, so that the virtual cathode (VC) electron space charge is neutralized by the plasma ion flux. On the other hand, if the electron beam energy deposition into the foil is reduced, a substantial delay in the start time of the microwave generation appears, which has been studied in detail. With these limiting factors, the microwave pulse full duration varied from 100 to 350 ns; the maximal full width at half maximum duration achieved in the experiments was \sim180 ns. Measurements of the current transmitted through the vircator cavity indicated the existence of a VC in spite of the absence of microwave generation during the delay. The experimental dependence of the microwave generation starting current on the diode voltage is presented, and possible mechanisms behind the generation delay are discussed. Simplified numerical simulations emphasize the role of the portion of electrons that are reflected from the VC, the number of which must be sufficient for the microwave generation to occur. [ABSTRACT FROM PUBLISHER]

This paper presents the design and development of a repetitive Marx generator rated at 1 kJ, 300 kV, 12 kA, and 10 Hz and which is suitable to drive the load directly. Bipolar charging of a Marx generator scheme has been adopted to get the following: 1) faster rise time and 2) relatively low charging voltage. The faster rise time is due to the reduced number of spark gaps down to six for 12 in-series capacitors (0.15 \mu\F and 50 kV) of a six-stage bipolar-charging Marx generator. Interstage inductive charging further enhances the efficiency of the overall system as compared to resistive charging. The generator is characterized by an aqueous resistive load for both polarities at the output voltage. Output voltages of the same magnitude have been achieved for both polarities. The matched impedance is found to be 25 \Omega for which a suitable reflex triode is designed. Intense electron beams and high-power microwaves have been generated using this Marx generator and reflex triode system. Experimental results demonstrating the Marx generator and the reflex triode operation are presented and discussed. [ABSTRACT FROM AUTHOR]

High-power microwave generation via a relativistic electron beam propagating through a periodic focusing quadrupole transport (PFQT) system is studied. Transverse confinement of e-beam in a PFQT system is provided by segmented cylindrical electrodes with applied oscillatory voltages Vosc(t) over 90^\circ segments. With regard to transverse confinement, there is no applied axial guide magnetic field. An analytic theory of laser gain in the low-gain-per-pass limit is developed. Numerical computations of electron trajectories and gain are also presented. It is shown that the presence of transverse oscillation frequency due to PFQT leads to significant gain enhancement under appropriate conditions. [ABSTRACT FROM AUTHOR]

A two-section coaxial vircator with radial electron beam injection and electrodynamic feedback was developed. The use of electron beam premodulation in the vircator allows an increase in efficiency and ensures single-mode oscillation. The vircator operates with no external magnetic field, features a comparatively low operating impedance of the vacuum diode (10–15 \Omega), and makes feasible wide-band frequency tuning through varying the resonator parameters. The vircator was simulated using the PIC-code KARAT. It is shown that, in a wide range of electron beam power (10–25 GW), the oscillation efficiency is 12%–15% and the frequency tuning bandwidth is 15%. In the experiments on a compact linear transformer driver at a cathode voltage of 300 kV, diode current of 20 kA, and driving pulse duration of 250 ns, the single H11 mode oscillation at 2.2 GHz with a 300-MW microwave peak power and 130-ns pulse duration at half the power level was realized. [ABSTRACT FROM PUBLISHER]

The KALI-1000 pulse power system has been used to generate single-pulse nanosecond-duration high-power microwaves from an axial virtual cathode oscillator. The typical electron beam parameters were 250 kV, 15 kA, and 100 ns, with a current density of a few hundreds of amperes per square centimeter. The shot-to-shot variation of the microwave pulsewidth was studied for various anode-cathode gaps and two different cathode materials (velvet and graphite). Results indicated that the average microwave pulsewidth for a diode with a graphite cathode increased as the anode-cathode gap was increased. The one-way analysis of variance was employed to examine the statistical correlation between the diode voltage, current, perveance, and the microwave pulsewidth for various diode gaps and for two different cathode materials. It was shown that the microwave pulsewidth variations are not statistically correlated with the diode voltage, current, and perveance. [ABSTRACT FROM AUTHOR]

KALI-1000 pulse power system has been used to generate single pulse nanosecond duration high-power microwaves (HPM) from a virtual cathode oscillator (VIRCATOR) device. HPM power measurements were carried out using a transmitting{ receiving system in the presence of intense high frequency (a few MHz) electromagnetic noise. Initially, the diode detector output signal could not be recorded due to the high noise level persisting in the ambiance. It was found that the HPM pulse can be successfully detected using wide band antenna, RF cable and diode detector set-up in the presence of significant electromagnetic noise. Estimated microwave peak power was ∼59.8 dBm (∼1 kW) at 7 m distance from the VIRCATOR window. Peak amplitude of the HPM signal varies on shot-to-shot basis. Duration of the HPM pulse (FWHM) also varies from 52 ns to 94 ns for different shots. [ABSTRACT FROM AUTHOR]

Recently, a nontrivial space-charge limit for off-axis bunched electron beams in a coaxial conducting structure was derived theoretically. The space-charge limit describes the minimum strength of an external uniform magnetic focusing field which is needed to stabilize the beam's center-of-mass motion in the presence of induced surface charges on the coaxial structure. In this paper, we perform a parametric study of the space-charge limit to numerically determine its dependence on the conducting structure geometry, i.e., the ratio of the inner and outer conductor radii, as well as its dependence on the transverse and longitudinal bunch distributions. [ABSTRACT FROM AUTHOR]

This paper analyzes parasitic backward-wave oscillation power as a function of interaction length and focusing magnetic field parameters in a generic helix traveling-wave tube (TWT) amplifier in the absence of amplified signal. The permanent periodic magnetic (PPM) focusing of the electron beam, the relatively narrowband (less than one octave) TWT, the electron gun with no control grid, the pulsed full beam current cutoff operating mode of the TWT, and the accelerating voltage pulse wider than the amplified RF pulse are considered. Under such conditions, the parasitic backward-wave oscillation can build up at the leading and trailing edge and at the top of the accelerating pulse before and after the input RF signal is applied when the instantaneous accelerating voltage provides for the synchronism condition. The parasitic backward-wave oscillation, although non-desirable in general, can be tolerated if its power does not exceed some allowable level. In this paper, the nonlinear (large-signal) theory of beam-wave interaction in the TWT in the specified case is developed. The theory accounts for the interaction of the multiple harmonics of the backward wave of the slow-wave circuit with the electron beam that alternatively changes the direction of its rotation on each half period of the focusing magnetic field. A system of equations, which makes accessible the start oscillation length and the starting Pierce relative velocity parameter as a function of the electrical parameters of the TWT, the PPM focusing field period, and the magnetic flux density distribution in the large-signal regime, is obtained. A particular numerical example reveals the relation between interaction length, PPM focusing field period, and parasitic backward-wave oscillation power. The approach permits one to design a TWT having the maximum possible interaction length under the allowable parasitic backward-wave oscillation power. Also, the results demonstrate that the focusing magnetic field parameters have a significant effect on the interaction of the rotating electron beam with the backward wave in the nonlinear regime, as they have in the linear regime. [ABSTRACT FROM AUTHOR]

To study the performance of compact systems for microwave generation, a series of experiments have been performed with a microwave source powered directly by a Marx generator. The system consists of a 20-stage 400-kV/400-J Marx generator, a powerful 40-kV charger, a reflex triode, and a vircator-type microwave source. Different parametric studies were performed such as variation of the anode-cathode distances and the emitting area of the cathode. The results have been analyzed and compared to an equivalent electric circuit model of the system. The experiments, generating microwave frequencies between 3 and 5 GHz, can be fairly accurately reproduced by the model both in terms of discharge currents and microwave frequencies. [ABSTRACT FROM AUTHOR]

In its classical embodiment-a circular cylindrical cavity driven by a rectilinear electron beam-the simplest of the transit-time microwave tubes, the monotron, has its conversion efficiency limited to 20.0%. However, we demonstrate here that on considering a stepped electric-field axial profile described by a tanh(z) function, the monotron efficiency can attain the theoretical maximum value of 57.0%, almost three times as high as that obtained from the conventional uniform field distribution. This is accomplished by using two cavities electromagnetically coupled by an annular slit crossed by an electron stream. From a one-dimensional analysis, a two-cavity monotron is developed to operate in the TM010 mode at 4.0 GHz when self-excited by a 20-keV, 10-A hollow electron beam. The device operation is examined through a 2.5-dimensional particle-in-cell (PlC) simulation giving a 47.5% conversion efficiency. Departure from the theoretically predicted efficiency is explained by beam-thickness effects. [ABSTRACT FROM AUTHOR]

Plasma-assisted slow-wave oscillators (pasotrons) are unique sources of microwave radiation, in which the beam propagation in the absence of external magnetic fields is provided by the ion focusing and the electron interaction with the RF field is essentially two-dimensional. This paper overviews the results obtained in the last several years and presents recent accomplishments in the theory and experimental performance of the pasotron. In particular, the 50% efficient operation at the 0.5-MW power level and more than 30% efficient operation at more than a 1-MW level are reported. Theoretical results of the analysis of nonstationary self-consistent processes in the beam focusing and transport agree with PlC simulations. The paper also contains the first results of the noise measurements in the pasotron. The paper is focused at describing the progress in the field, and explaining the interesting physical effects and improvements in the device performance without going into details of either the theoretical formalism or experimental parameters. [ABSTRACT FROM AUTHOR]

We investigated the attainable stability of the monofilar-helix traveling wave tube (TWT) with respect to the parasitic backward-wave oscillation under periodic magnetic focusing. We accounted for the effect of the electron beam rotation in the nonhomogeneous focusing magnetic field, magnetic field amplitude and periodicity, and the specific coupling of the electron beam with the synchronous harmonics of the electromagnetic wave existing on the slow-wave circuit. This specific coupling takes place in the monofilar-helix TWT only and decreases its stability with respect o the parasitic backward-wave oscillation in comparison to other types of helix TWT. For the purpose of this analysis, we developed further the linear two-dimensional (2-D) multiwave theory of the helix TWT described in our previous work [1]. In this theory, we account for space charge fields by means of waveguide excitation model based on the direct solution of Maxwell equations. Differently from [1], the present version of our theory allows for accounting of the coupling of the synchronous 0th spatial harmonic (forward employed for the amplification) and -1st (parasitic backward) harmonic of the circuit wave with the electron beam in the monofilar-helix TWT. By numerical modeling, we obtainet quantitative data on the starting conditions for the parasitic backward-wave oscillation in the typical monofilar-helix TWT in the intersection point of the dispersion curves of these two harmonics. The described coupling mechanism does not exist in the bifilar-helix TWT. Therefore, the TWT with the bifilar helix is more stable with respect to the parasitic backward-wave oscillation than the TWT having the monofilar helix. These theoretical results correlate with the existing experimental evidence. The results of the present study, in particular, explain why the backward-wave oscillation suppression by tailoring of the focusing magnetic field [1] has a significant effect in the bifilar-helix TWT but does not work in the monolilar-helix TWT. [ABSTRACT FROM AUTHOR]

Microwave emission, in the x-band frequency range (8.2-12.4 GHz), from a thin, large, rectangular sheet plasma has been measured. The plasma electron density was such that the plasma frequency was within or just above this frequency range. The plasma was immersed in an external magnetic field from a set of Helmholz coils. The magnetic field was oriented parallel to the electric field between the anode ground plane and a cylindrical, hollow cathode. The spectrum of the emitted noise was measured for both ordinary mode (P to B) and extraordinary mode (⊥ to B) polarization in the x-band. The emission was strongest at high harmonics of the electron cyclotron frequency. Mechanisms that might produce this noise and its potential use as a diagnostic tool are discussed [ABSTRACT FROM PUBLISHER]