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Using a simple theory and finite-difference time domain (FDTD) simulations involving the Lorentz model, we demonstrate some problems in the determination of pulse waveform, amplitude, and peak power of high-power microwave pulses, when performed together with calorimetric measurement of the pulse energy, arising from the wave diffraction as well as from its dispersion in the absorbing liquid. Considering few typical pulse shapes and other experimental conditions, we outline the general limitations and recommendations for performing such measurements.

Coaxial transmission lines filled with saturated ferromagnetic material have been used for pulse sharpening purposes and high-frequency oscillation generation for a long time. These phenomena have been well studied experimentally, but have no exact analytical description; simplified models provide only qualitative agreement with the experiment. For quantitative description, it is necessary to solve Maxwell equations together with Landau-Lifshitz equation for the magnetization vector. The paper presents numerical simulation of high-voltage pulse sharpening process and excitation of high-frequency oscillations in coaxial lines filled with saturated ferrite. The modeling was performed using the KARAT code in three versions: axisymmetric (RZ) and three-dimensional Cartesian (XYZ) and cylindrical ( $\mathrm{R}\Theta\mathrm{Z}$ ). The results are compared with previous experiments.

In order to expand the range of mechanical tuning of generation frequency in a subgigawatt Cherenkov microwave oscillator, it is proposed to use а configuration of relativistic backward-wave oscillator (BWO) having no reflectors with the radiation output in the direction opposite to the electron beam. In a numerical experiment using KARAT electromagnetic PiC code, it is demonstrated that in this case, due to only variation of the slow-wave structure (SWS) period, it is possible to change the oscillation frequency by tens of percent preserving the TM01 operation mode. The frequency tuning range is thereby limited from below by the lower boundary of the SWS passband, near which the BWO oscillation regime is displaced by the orotron regime, and from above by high-frequency excitation of the competing TM02 mode. A moderately relativistic microwave oscillator is simulated with a tuning band between 2.73 and 4.15 GHz (frequency ratio 1.52) at the (−3 dB) level of the maximum microwave power (520 MW at 3.85 GHz). The feeding electron beam having 460 keV particle energy and 4.7 kA current is transported by the magnetic field with an induction of 1.0 T. Additional simulations demonstrate that the frequency tuning range is extendable to approximately an octave at the cost of lower generation efficiency.

Using numerical simulation, the operating mode of a relativistic Cherenkov microwave generator of the twistronic type has been demonstrated. The generator includes an electrodynamic system based on a backward-wave oscillator and modulating reflector with nonmonotonous, highly nonuniform energy exchange along the length of the system. The efficiency of power conversion from the electron beam to electromagnetic radiation is 56%, and the electronic efficiency is 66%. For an accelerating voltage of 340 kV and an electron beam current of 3.3 kA, the simulated generation power is 630 MW at a frequency of 9.7 GHz and a guiding magnetic field of 2.2 T.

The paper proposes a new scheme of high-power microwave oscillator of twistron type using a moderately relativistic high-current electron beam. In numerical experiment using axisymmetric version of the completely electromagnetic PiC code KARAT, a 56% conversion efficiency of electron beam power to electromagnetic radiation was demonstrated. With 340 kV accelerating voltage, 3.3 kA electron beam current, and 2.2 T guiding magnetic field strength, the simulated microwave power was 630 MW at 9.7 GHz. The “electronic efficiency” of the source reaches 66%.

Current characteristics and operating lifetime of the explosive emission cathode based on a carbon microfiber are investigated in the pulsed-periodic mode of operation with pulse duration of about 5 ns. Long-term (for up to 3.6 million pulses) tests of the cathode operating lifetime are carried out. Specific ablation of the fiber material equal to 2.4·10–4 g/C is obtained. Change in the morphology of the fiber surface during long-time operation caused by deposition of carbon from the cathode plasma is revealed. The microscopic electric field strength on the fiber surface is estimated taking into account the surface microrelief. The efficiency of microwave generation comparable with that of a velvet cathode in low (200 kV/cm) average electric field in the gap is obtained for the Cherenkov microwave generator with vacuum diode without external magnetic field of decimeter wavelength range based on the SINUS-7 pulsed-periodic high-current electron accelerator with current pulse duration of 50 ns. The operating lifetime no less than 105 pulses is demonstrated for the carbon fiber-based cathode of the microwave generator operating in the mode of pulse batch with duration of several seconds and pulse repetition frequency of 20–50 Hz.

The competition of oscillations in a relativistic backward wave oscillator (RBWO) based on a coaxial waveguide has been studied by theoretical and experimental methods. It is established that the starting current for asymmetric magnetic TEm1 type waves is lower than that for the TEM wave. Predominant excita� tion of the former waves is confirmed by numerical simulations using the PICcode KARAT. In an experi� mental RBWO prototype, the excitation of asymmetric oscillations was suppressed by longitudinal cuts in the central conductor of the coaxial waveguide. As a result, stable generation of the TEM wave has been achieved at a frequency of 1.23 GHz, a peak power of 260 MW, and a 28% efficiency of electronbeampower conver� sion into radiation power for a microwavepulse duration of 33 ns.

A quasistationary generation mode was experimentally implemented in a moderately relativistic microwave generator of twistron type with an efficiency of energy conversion from the driving electron ...

An elementary theory of an annular high-current electron beam in a uniform transport channel and a coaxial magnetically insulated diode is generalized to the case of counterpropagating electron beams with a spread over kinetic energies. Expressions for the sum of the absolute values of the forward and backward currents in a uniform transport channel and for the flux of the longitudinal component of the generalized momentum in a coaxial magnetically insulated diode as functions of the maximum electron kinetic energy are derived for different values of the relative width of the energy distribution function. It is shown that, in a diode with an expanding transport channel and a virtual cathode limiting the extracted current, counterpropagating particle flows are established between the cathode and the virtual cathode within a certain time interval after the beginning of electron emission. The accumulation of electrons in these flows is accompanied by an increase in their spread over kinetic energies and the simultaneous decrease in the maximum kinetic energy. The developed model agrees with the results of particle-in-cell simulations performed using the KARAT and OOPIC-Pro codes.

The currents of 5-ns pulsed high-current electron beams produced in a planar vacuum diode with explosive-emission cathodes made of various materials with no external magnetic field at an average electric field strength in the gap of about 300 kV/cm have been measured and time-integrated observation of the optical luminescence of the cathode surface have been performed. Cathodes with a ceramic bushing and spring metal contacts, with ceramic plates set in a magnetic iron matrix, with blades made of stamped exfoliated graphite (Graflex), with blades made of foil fiberglass plastic, and a composite cathode made of crystalline boron and copper powders were tested. The current carried by one emission center has been estimated to range between 5 and 20 A for various cathodes. For the metal-dielectric cathode, the velocity of expansion of the cathode plasma over the ceramic surface has been estimated as 2·107 cm/s. The lifetimes of the cathodes at a pulse repetition rate of 50 Hz have been investigated.

One of the promising methods for generation of ultrashort electromagnetic pulses (with duration of about ten periods of high-frequency oscillations) is radiation from spatially localized electron ensembles (bunches), which can be considered a classical analog of Dicke superradiance known in quantum electronics. In classical electronics, superradiance can be related to various mechanisms of stimulated radiation. Until now, cyclotron, undulator, and Cerenkov (in the case of interaction with both copropagating and counterpropagating waves) superradiance of electron bunches as well as superradiance during stimulated scattering of a pump wave have been studied theoretically and experimentally. As a result of these studies based on high-current RADAN and SINUS accelerators and their modifications, a new class of oscillators producing pulsed electromagnetic radiation has been created. They have such unique characteristics as pulses of high peak power (up to 1 GW and 3 GW in the millimeter-and centimeter-wave ranges, respectively) and ultrashort duration (from 300 ps to 1 ns, respectively). In this case, regimes with a peak radiation power exceeding the electron-beam power are experimentally realized. Regimes with high (kilohertz) pulse repetition rate and high average power (up to 2.5 kW) are obtained.

The article presents a review of works (mainly, of experimental ones) on production of subgigawatt and gigawatt microwave pulses of extremely short duration (5–7 RF periods) using backward-wave systems fed with nanosecond and subnanosecond high-current electron beams produced by compact accelerators. Theoretical approaches to the generation process (which is essentially non-steady-state) are briefly summarized. Using the effect of spatial accumulation of energy in a short running microwave pulse allows production of pulses with peak power notably higher then the driving electron beam power. Compact microwave sources developed for operation in the Ka-band and X-band are described. Special attention is given to the issue of high pulse repetition frequency operation of the sources.

The advent of pulsed power technology in the 1960s has enabled the development of very high peak power sources of electromagnetic radiation in the microwave and millimeter wave bands of the electromagnetic spectrum. Such sources have applications in plasma physics, particle acceleration techniques, fusion energy research, high-power radars, and communications, to name just a few. This article describes recent ongoing activity in this field in both Russia and the United States. The overview of research in Russia focuses on high-power microwave (HPM) sources that are powered using SINUS accelerators, which were developed at the Institute of High Current Electronics. The overview of research in the United States focuses more broadly on recent accomplishments of a multidisciplinary university research initiative on HPM sources, which also involved close interactions with Department of Defense laboratories and industry. HPM sources described in this article have generated peak powers exceeding several gigawatts in pulse durations typically on the order of 100 ns in frequencies ranging from about 1 GHz to many tens of gigahertz.

This paper presents a review of recent studies of superradiant (SR) regime of high-power microwave generation of relativistic backward-wave tubes. The fundamental possibility of producing microwave pulses with peak power substantially exceeding the driving electron beam power was demonstrated in theory and simulation. In experiments, beam-to-microwaves power conversion factors of 1.5 in the Ka band and up to 1.8 in the X band were achieved. High-efficiency Ka band generation was obtained at a low B field (2 T) below the cyclotron resonance. With the use of a dc solenoid, SR pulses were generated in a batch mode at a pulse repetition frequency of 1000-3500 Hz.

In this paper, we present the results of numerical and experimental studies of a resonant relativistic backward-wave oscillator. Using reflections of the operating TM01 mode from the ends of an electrodynamic system, optimal conditions for the interaction of an electron beam with both the backward (-1)st-order spatial harmonic and the forward fundamental harmonic are reached. Single-mode oscillations at 3.6 GHz with a peak pulse power of 5 GW and an efficiency of 30% were obtained in the experiment. We show the possibility of a 15% oscillation-frequency change corresponding to the FWHM power level by varying the period of a slow-wave structure at constant electron-beam parameters.

This article describes S-band sources of high-power microwave (HPM) pulses: a resonant backward wave oscillator (BWO) producing ∼5-GW, 100-J pulses, based on the SINUS-7 electron accelerator, and a double-section vircator with a peak power of ∼1 GW and a pulse width of 20–50 ns, powered from either the SINUS-7 accelerator or the MARINA inductive-store pulse driver with a fuse opening switch.

This article presents results of theoretical and experimental studies on the production of ultrashort (a few RF cycles duration) microwave pulses of gigawatt peak powers based on superradiance from high-current electron beams. With the Cherenkov backward-wave–electron-beam interaction in a low-dispersion slow-wave structure, microwave pulses with a peak power greater than the peak power of the driving electron beam have been produced for the first time. In an experiment using the SINUS-150 compact high-current electron accelerator, with a 2.6-kA injected beam current and a 330-kV electron energy, microwave pulses of 1.2 GW peak power and ∼0.5 ns duration (FWHM) were generated in the X-band. Production of superradiance pulses in a repetitive regime (3500 Hz) in the Ka-band has been demonstrated using a compact hybrid SOS-modulator. The effect of spatial accumulation of microwave energy in extended slow-wave structures with substantially nonuniform coupling has been demonstrated. In an experiment using the SINUS-200 compact accelerator, X-band pulses of ∼3 GW peak power and 0.6–0.7 ns width (FWHM) were produced with a power conversion efficiency of 150–180% and an energy efficiency of ∼15%.

The results of theoretical and experimental studies allowing the use of Cherenkov superradiance for the generation of electromagnetic pulses with a peak power higher than electron flow power are reported. For an injection current of 2.6 kA and a particle energy of 330 keV, the power of a microwave pulse with a carrier frequency of 9.3 GHz and a duration of 0.5 ns attained 1.2 GW.

The design and results of tests of a vacuum feedthrough insulator for a pulsed electron accelerator with a voltage of ∼1 MV, a current of ∼20 kA, and a half-height voltage-pulse duration of ≥ 400 ns are presented. A method for distributing the voltage over the sections of the insulator due to an electron emission from specially created surfaces is proposed.

This paper describes an S-band vircator system with electron beam premodulation, built on the base of the MARINA compact high-voltage pulser with inductive energy storage. The peak microwave output was /spl sim/1 GW with /spl sim/5% power efficiency and 50-ns full-width at half-maximum. The microwave frequency was constant during the pulse as determined by the resonator. Substantial spontaneous shortening of the microwave pulse was observed. Possible mechanisms for this phenomenon are discussed and supported by three-dimensional KARAT simulations.

Theoretical, numerical, and experimental studies of a vircator with the premodulation of a dm-wave electron beam are performed. Possible oscillation mechanisms in virtual-cathode systems (modulation of passing current, effect of reflex klystron, and inertial bunching of particles reflected) are analyzed. The microwave efficiency of the vircator using a two-gap electrodynamic system is shown to be significantly higher than that of the vircator with a one-gap system. Based on the results of the numerical experiment, a prototype of a two-stage vircator in the absence of an external magnetic field is designed. In experiments using a high-current pulsed-periodic accelerator, single-mode oscillations with a power of up to 1 GW, a duration of ≈25 ns, and an efficiency of ≈5% are generated in the dm-wave range. The oscillation frequency is demonstrated to be stable during a pulse and from pulse to pulse, which suggests the decisive effect of the electrodynamic system. It is shown that the oscillation frequency can continuously be tuned in a half-power bandwidth of ≈15% by varying the parameters of the resonator.

The paper describes theoretical, numerical, and experimental studies of a decimeter wavelength vircator with electron beam premodulation. Possible mechanisms for the excitation of electromagnetic oscillations in virtual cathode systems are analyzed (modulation of current, reflex klystron effect, and inertial bunching of reflected particles). It is demonstrated that the microwave efficiency in a double-gap virtual-cathode system may be substantially higher than that in a single-gap one. Based on one- and three-dimensional numerical simulations, an experimental mockup of double-gap vircator without external magnetic field has been developed. In experiments employing the SINUS-7 high-current repetitively-pulsed electron accelerator, single-mode microwave generation in the S-band was obtained with /spl sim/1 GW peak power and /spl sim/25 ns pulse width at about 5% efficiency. The generator showed frequency stability from pulse to pulse and throughout each pulse, which proves the dominant role of the electrodynamic system used in the vircator construction. Varying the cavity parameters allowed continuous frequency tuning within about 15% at half power. Sample batch operation of the system at 20 and 50 p.p.s. was demonstrated.

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.

We present results of the investigation of different types of cathodes operating in an electron diode powered by a high-voltage generator (300 kV, 250 ns, 84 Ω, ⩽5 Hz). The cathodes which have the same emitting area of 100 cm2 are made of metal–ceramic, carbon fibers, carbon fabric, velvet, or corduroy. We also tested carbon fibers and carbon fabric cathodes coated by CsI. It was shown that for all types of cathodes the electron emission occurs from the plasma which is formed as a result of a flashover of separate emitting centers. The amount of the emitting centers and the time delay in the electron emission were found to depend strongly on the accelerating electric field growth rate. Experimental data concerning the uniformity of the light emission from the cathode surface and divergence of the generated electron beams are presented. Data related to the general parameters of the diode, namely its impedance, power, and energy are given as well. For all the cathodes investigated the observed diode impedance indicated the existence of a quasistationary cathode plasma boundary for electron current density ⩽20 A/cm2. We present the dependencies of the average emitted electron current density and of the time delay in the electron emission on the number of generator shots. We also present data of the vacuum deterioration as a result of the tested cathodes operation. The obtained data are discussed within the framework of plasma formation as a result of cathode surface flashover.

Operation of explosive-emission cold cathodes made from various materials was studied at a large number of pulses at current densities of /spl sim/1.0/sup 4/ A/cm/sup 2/. The cathode voltage and the beam current were /spl sim/500 kV and 5 kA, respectively, with a pulsewidth of /spl sim/20 ns. At a small number of pulses (/spl les/10/sup 3/), cathodes of like geometry (even made from different materials) demonstrated similar emission properties. For most of the materials tested, with a large number of pulses (/spl ges/10/sup 3/), the current risetime increased to the fullwidth of the voltage pulse and the maximum current of the vacuum diode decreased. When using a graphite cathode, the maximum current remained invariant until 10/sup 8/ pulses. Mass losses were measured for a series of cathode materials. The results obtained offered the possibility to realize long-lived operation of an X-band relativistic backward-wave oscillator with an almost invariant output power of 350-400 MW during 10/sup 8/ pulses at a pulse repetition rate of 100-150 p.p.s.

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 /spl sim/80 J, thereby demonstrating pulse lengthening by a factor of four.

The dependence of the current on emitter size is obtained for a high-current planar diode with a discrete emitting surface. It is shown that if the distance between the emitters appreciably exceeds their size, the dependence of the current on the ratio of the emitter size to the diode gap is a power dependence with an exponent of 3/2. The voltage dependence of the current obeys the “three-halves” law up to higher voltages than that for a planar diode with a homogeneous emitting surface.

It is shown that reduction in the pulse length in a relativistic BWT, is accompanied by the appearance of explosive-emission plasma on the surface of the ripples of the slow-wave system. Generation stops because electrons emitted by the plasma absorb the electromagnetic wave. This absorption is sharply enhanced by the presence of ions emitted from the plasma.

Formulas for the space-charge-limited electron current in a planar diode with a small hemispherical or hemi-cylindrical plasma emitter have been obtained theoretically and by a numerical simulation. For a periodic cathode structure of hemispherical emitters, an approximate expression for current is derived. Based on these results, the increase in current with time in high current explosive-emission diodes is explained. A condition has been found under which the expanding cathode plasma has no effect on the diode impedance. This is related to the electric field pressure on the cathode. A configuration of the cathode surface has been proposed which provides realization of this condition.

Results from a study of a relativistic X-band backward-wave oscillator (RBWO) with 3-GW output microwave power are presented. The RBWO was driven by the high-current electron accelerator SINUS-7. The dependence of radiated microwave pulse duration on microwave power was obtained. Pulse shortening occurring at the higher power levels is probably attributable to explosive electron emission from the slow wave structure (SWS). An increase in the cross section of the electrodynamic structure seems to be a way to increase the microwave pulse duration. Experimental results from a moderately oversized X-band RBWO using a resonance reflector are described. This tube can be operated with a low external magnetic field.

A compact high-voltage nanosecond generator is described with pulse repetition rate of up to 1000 pps. The generator includes a 30-/spl Omega/ coaxial forming line charged by a built-in Tesla transformer with high coupling coefficient, and a high voltage (N/sub 2/) gas gap switch with gas circulating between the electrodes. The maximum forming line charge voltage is 450 kV, the pulse duration is /spl sim/4 ns, and its amplitude for a matched load is up to 200 kV. The generator has been applied to create powerful sources of ultrawide-band electromagnetic radiation and nanosecond microwave pulses.

In this paper, we present the results of investigations of a uniform three-centimeter relativistic backward-wave tube (BWT) with circular waveguide operating mode E01 with pulse power 3 GW and efficiency about 20%. We have obtained a dependence of the radiation pulse duration on the generated power. We discuss the problems involved in shortening the duration of the microwave pulses in oscillators with high microwave field intensity on the surface of the slow-wave structure.

The results of numerical modeling and experimental investigation of a high-power, resonance relativistic backward wave tube are presented. By using the working TM01 mode reflections from the ends of the electrodynamic system, optimum conditions for the electron beam interaction with both the (−1)st harmonic of the backward electromagnetic wave and the main harmonic of the concurrent wave are achieved. A single mode generation with 5 GW output power and a 30% efficiency was obtained in experiments at a frequency of 3.6 GHz. The working frequency can be controlled within 15% (at the half maximum power level) by changing the slow-wave structure period at the constant electron beam parameters.

An experimental S-band vircator with premodulated electron beam based on a compact generator with inductive energy storage is described. The vircator radiation power was 300 MW at a power efficiency of about 5% and a pulse duration of 50 ns. The oscillation frequency, determined by the electrodynamic system parameters, remained unchanged during the pulse.

A new method of insulator surface treatment, which allows the electric strength of the material to be increased, is proposed and verified. Using this method for treating the slow-wave structure of a relativistic backward wave tube (BWT), the microwave pulse duration was increased from ∼6 to ∼30 ns at an output BWT radiation power of 3 GW.

The results of experimental studies of different types of cathodes—carbon-epoxy rods, carbon-epoxy capillary, edged graphite, and metal-dielectric—under the application of high-voltage pulses with an amplitude of several hundreds of kV and pulse duration of several nanoseconds are presented. The best diode performance was achieved with the edged graphite and carbon-epoxy-based cathodes characterized by uniform and fast (

The conditions required to achieve a constant current from an explosive cathode during the pulse in a high-current planar diode are calculated.

Experiments involving backward wave oscillators having nonuniform amplitude slow wave structures (NBWOs) operating in the x-band frequency range have produced 500 MW of rf power with 20% efficiency. To elucidate the processes occurring in the NBWO, and to ascertain the possible advantage of using a nonuniform amplitude sow wave structure over one that is uniform, the authors have performed a simulation study involving several different tubes, nonuniform and uniform, using the 2.5D electromagnetic particle-in-cell code TWOQUICK. The simulations model experiments performed on the SINUS-6 electron beam accelerator at the University of New Mexico. Results of the simulation study are presented. They show that the slow phase velocity surface wave that interacts with the electron beam coexists with an ordinary waveguide mode, which forms a standing wave in both nonuniform and uniform BWOs. Analysis of BWOs using a single particle phase model that accounts for both fast and slow TM waves shows that it may be possible to increase the output power of these tubes in general by choosing the length of the slow wave structure to simultaneously optimize the coupling of the beam to the slow wave, while forcing a transit time interaction between the standing wave and the electronmore » beam, which pumps beam energy directly into the ordinary waveguide mode. The authors discuss the possible advantages of using a NBWO to accomplish this. In addition, comparisons with SINUS-6 experiments are made.« less

A recent numerical calculation predicts efficiencies of 25--30% for uniform BWOs over a wider range of beam parameters than for nonuniform BWOs. This new model includes the interaction between the electron beam and the reflected forward traveling wave. For efficient BWO operation, it is important that the backward traveling wave and the reflected forward traveling wave are properly matched at the cutoff neck and the adiabatic transition into the output waveguide. A uniform BWO based on the principles described in the above model was investigated using the Sinus-6 relativistic electron beam accelerator. Cathode voltages from 400 kV to 650 kV and beam currents from 2.5 kA to 5 kA were obtained by varying the pressure in the Sinus-6 spark gap switch. The RF pulse was 10 ns long at 9.6 GHz. A peak power of 250 MW was measured in the far field with a corresponding efficiency of 20%. At higher beam parameters significant air breakdown was observed in front of the 15 cm diameter conical horn aperture. The TM{sub 01} mode pattern was observed and photographed in the plasma formed by the air breakdown. A mode converter was used to convert the TM{sub 01} mode into a TE{sub 11}more » mode with the corresponding air breakdown pattern observed. A standing wave was created by placing a metal mirror 1.5 meter from the aperture. PIC code simulations of this BWO were made with KARAT and TWOQUICK. Power and frequency measurements are compared with the experiment.« less

The paper describes S-band HPM sources developed at IHCE: the resonant BWO producing /spl sim/5 GW, 100 J pulses, based on the SINUS-7 electron accelerator, and a double-section vircator with peak power of /spl sim/1 GW and pulse width of 20-50 ns, fed from either SINUS-7 accelerator or MARINA inductive-store pulse driver with fuse opening switch.

The paper presents the results from theoretical and experimental studies of a backward wave oscillator with a resonant reflector incorporated in electromagnetic system. The reflector serves simultaneously as a short modulating gap and this provides the selection of the TM/sub 01/ operating mode among parasitic transversal modes in an extended approximately twice slow-wave structure. In course of the experiment 0.8 GW microwave power in the X-band was obtained with an efficiency of 25%. The electron beam pre-modulation and the extension of BWO transversal size allowed for increasing its efficiency in a low magnetic field. A periodically-pulsed regime for BWO operation with a 20% efficiency utilizing a DC magnet with a 0.6 T field was realized.

The paper considers the effect of the space charge of emitted electrons on the electric field at the tip of a field emitter. To analyze this phenomenon, the Poisson equation was solved numerically for a spherical vacuum diode with a field emission cathode. The dependence of the actual electric field on the geometric electric field has been compared with the known approximate dependencies. Strong screening of the electric field at the cathode by the space charge of emitted electrons at fields over 5/spl times/10/sup 7/ V/cm is noted. Based on the obtained dependencies of the actual field on the geometric one and using the SuperSAM code, the field emission occurring in a Muller projector has been simulated. It has been shown that the "ring effect" appearing in field emission patterns can well be explained by the field emission from the peripheral regions of the field emitter. The current density distributions over the surface and in the bulk of the emitter have been analyzed.

Summary form only given, as follows. At high powers of generation, as the electric field strength on the surface of electrodynamic structure rises, limitation of pulse duration is observed in microwave sources. One of the reasons of this phenomenon is that the generation is affected by explosive electron emission excited on the surface of electrodynamic structure by intense high frequency electric fields. In this paper, the results of study of a 3 cm relativistic BWO with lowest electric wave TM/sub 01/ of circular waveguide are presented. The experiments have been conducted using the high current repetitively-pulsed electron accelerator SINUS-7 constructed at HCEI. The maximum parameters of the accelerator are as follows: electron energy up to 2.0 MeV and the beam current up to 20 kA in single-shot regime and 1.5 MeV and 15 kA at a repetition rate of 100 p.p.s. The pulse duration is 45 ns. In experiments, the maximum microwave power 3 GW was obtained, with an efficiency of generation about 20% and the pulse duration about 6 ns. It was observed that the duration of the microwave pulse depends on the power, rising at lower powers. Typically, it is closed in order to the estimated time of appearance of explosive emission centers on the surface of BWO slow wave structure.

Summary form only given. On the base of fully electromagnetic 3D computer simulation using KARAT code, an oscillator was designed to operate at 1.5 GHz. The device possesses the features of both the vircator and the split-cavity oscillator. The construction uses rectangular waveguides and includes the main running-wave cavity, the modulating cavity, a feedback loop, and the coaxial diode with explosive emission. No external magnetic field is used. The operating wave is TE/sub 10/ with possible further conversion into a Gaussian beam. The simulated efficiency of the vircator is /spl sim/10%. The device is resonant, the time of generation establishment is 15-18 ns. The microwave power is /spl sim/500 MW at a beam current of /spl sim/9 kA and diode voltage of /spl sim/600 kV. The oscillator is efficient in a relatively wide range of beam powers. Optimizations of various parameters of the generator were made in the simulation.

Results of 3D time-dependent numerical simulations of a powerful microwave device with a virtual cathode and electron beam premodulation are presented. The source does not require external magnetic field and is capable of frequency tuning of /spl sim/20%.

Summary form only given, as follows. Presents recent results of theoretical and experimental studies of HPM production in virtual cathode systems with electron beam premodulation. The phase separation of electron current in these devices is due to strong energy dependence of electron current passage just above the critical current level and develops at distances on the range of the wave length or shorter. Thus, the generator is efficient even with its self length is not over the wavelength where the kinematic bunching is inefficient. This provides working capacity of the device without external magnetic field. In theory, conditions for electromagnetic wave excitation in single- or double-gap vircator systems are anal, zed. Optimum amplitudes and phasing of RF fields in the gaps and the corresponding maximum microwave efficiencies are estimated in theory, supported with 1D-3D PIC simulations. The efficiency of double-gap system was found to multiply exceed the efficiency of single-gap system. The maximum efficiency for a double-gap system demonstrated in 3D electromagnetic simulation with KARAT code approached 20% at gigawatt power level. Experiments were performed using high-current periodically-pulsed electron accelerator SINUS-7 (pulse width 50 ns, maximum 2 MeV, 20 kA). L-band and S-band vircator systems were produced with output power of up to 1 GW and efficiency of up to 6% at 20-30 ns microwave pulse pulse width. The range for continuous frequency tuning reached 15% at half power level. Batch regime of periodically-pulsed operation of the vircator was realized (batch length 1 sec at 50 p.p.s. or 20 sec at 20 p.p.s.).

The effect of X-band HPM pulses from a SINUS-500 relativistic HPM source on Drosophila melanogaster embryos, larvae, and pupae was studied. The test objects were irradiated with 10-ns pulses during 5-min batches at a pulse rate of 3-29 p.p.s. The peak microwave power was 500 MW, the electric field in the irradiation zone was /spl sim/1.5/spl times/10/sup 4/ V/cm and the X-ray dose was 1-3 /spl mu/Rad per pulse. To understand the mechanisms of the effects and to estimate the comparative contribution of microwave and X-ray components of the SINUS-500 radiation, the objects were also exposed to (a) X-ray-free microwave pulses from a magnetron (wavelength 3 cm, pulse width 730 ns, pulse power up to 40 kW, electric field at the object /spl sim/1.5/spl times/10/sup 3/ V/cm), (b) pure X-ray pulses produced in the periodically-pulsed regime by the SINUS-500 facility where the microwave component was cut by a metal screen, and (c) continuous X-rays with nearly the same dose and exposition time (2-30 mRad during 5 min). The irradiation caused notable disturbance in the individual development of Drosophilas in the form of increased PID, teratism of imagoes, delay of imago fly etc. The strength of the effects was dependent upon the stage of development of the organism at the moment of irradiation, and the pulse repetition rate. Repeated extra exposures did not increase the effect. The pulses from the SINUS-500 facility produced a stronger effect than magnetron pulses. The strength of X-ray pulse action depended on the pulse rate while the effect from continuous X-rays was negligibly small. The paper discusses possible mechanisms of action of nanosecond HPM pulses.

Summary form only given. We present a comparison between the parameters of a metal-ceramic cathode and the parameters of a velvet cathode which were studied at the same conditions. The cathodes had an emitting area of 100 cm/sup 2/. Experiments were performed using of PFN generator having the following output parameters: accelerating voltage of 250 keV, impedance of 80 /spl Omega/ and 140 /spl Omega/, and pulse duration of 250 ns and 450 ns at FWHM. The time delay of the electron emission was measured versus the amplitude and rise time of the accelerating voltage. The uniformity of the cathode plasma formation and its expansion towards the anode was studied by a fast framing camera. The electron beam uniformity with spatial and temporal resolution and its micro-divergence was obtained using the X-ray image produced by the electron beam interaction with a Ta-made anode and by a multi pinhole camera, respectively. In addition, array of collimated Faraday cups was used to obtain electron beam uniformity. The changes of the emission properties of the metal-ceramic cathode were studied at a repetition rate of 1 Hz during 10,000 shots of the PFN operation. Compatibility of the cathodes with vacuum requirements is presented.

Summary form only given. Spontaneous pulse shortening occurring in the relativistic BWO at a gigawatt power level is studied by experiment and theory. In the first experiments with a 3-GW relativistic BWO, spontaneous microwave pulse width limitation a 6 ns was observed. It is now experimentally demonstrated that this phenomenon is accompanied by formation of an explosive-emission plasma at the surface of a corrugated slow wave structure and by the appearance of intense electron flows between the SWS ripples. The termination of microwave emission is explained in terms of increase of the BWO starting current due to partial absorption of the operating electromagnetic wave by electrons emitted from the plasma, while the intensity of the absorption radically increases owing to the presence of positive ions emitted from the plasma. To improve the electric strength of the BWO electrodynamic system, treatment of its surface with low-energy, high-current electron beam (LEHCEB) is proposed. Effectively smoothing the metal surface and removing dielectric inclusions from the surface layer, such treatment allows suppression of the explosive emission.