Ivan K. Kurkan

Relativistic X-band BWO with 3-GW output power

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 vircator with electron beam premodulation based on high-current repetitively pulsed accelerator

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.

Decimeter-band frequency-tunable sources of high-power microwave pulses

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.

Repetitive sub-gigawatt rf source based on gyromagnetic nonlinear transmission line

We demonstrate a high power repetitive rf source using gyromagnetic nonlinear transmission line to produce rf oscillations. Saturated NiZn ferrites act as active nonlinear medium first sharpening the pumping high voltage nanosecond pulse and then radiating at central frequency of about 1 GHz: shock rise time excites gyromagnetic precession in ferrites forming damping rf oscillations. The optimal length of nonlinear transmission line was found to be of about 1 m. SINUS-200 high voltage driver with Tesla transformer incorporated into pulse forming line has been designed and fabricated to produce bursts of 1000 pulses with 200 Hz repetition rate. A band-pass filter and mode-converter have been designed to extract rf pulse from low-frequency component and to form TE(11) mode of circular waveguide with linear polarization. A wide-band horn antenna has been fabricated to form Gaussian distribution of radiation pattern. The peak value of electric field strength of a radiated pulse at the distance of 3.5 m away from antenna is measured to be 160 kV/m. The corresponding rf peak power of 260 MW was achieved.

A multigigawatt X-Band relativistic backward wave oscillator with a modulating resonant reflector

Effective generation regime with a high output pulse power has been experimentally realized in a relativistic backward wave oscillator (RBWO) with a resonant reflector and a slow-wave system having a diameter 1.6 times the radiation wavelength. At a guiding magnetic field of 4.5 T, the maximum peak power amounted to 4.3 GW at a frequency of 9.4 GHz, an efficiency of 31%, and a microwave pulse duration of 22 ns.

Relativistic backward wave oscillator with a discrete resonance reflector

We study, both theoretically and experimentally, a relativistic backward wave oscillator BWO with a discrete resonance reflector. It is shown that premodulation of electrons in the reflector region leads to considerable changes in the oscillator characteristics. If a homogeneous slowingdown system (SS) is considered and the space charge is neglected, then the calculated maximum efficiency of oscillation increases from 15% without modulation to 58% in the case of optimal modulation. A pulse-periodic mode of microwave oscillation with pulsed power 0.5 GW at wave-lengths λ≈3 cm was reached in the experiment.

S-band vircator with electron beam premodulation based on compact pulse driver with inductive energy storage

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.

Vircator with electron beam premodulation built around a high-current pulsed-periodic accelerator

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.

Effects of Different Cathode Materials on Submicrosecond Double-Gap Vircator Operation

The operation of a double-gap S-band vircator has been investigated at submicrosecond duration of a high-current electron beam generated in a planar diode. The experiments were performed at accelerating voltages of les550 kV and diode currents of up to 17 kA using a radio-frequency cavity with a wide coupling window between its two sections. Three types of cathodes have been studied, namely, metal-dielectric, carbon fiber, and velvet cathodes. The main features of the operation of the vircator using each cathode are analyzed. The microwave pulse duration with the metal-dielectric and carbon fiber cathodes reached ~250 ns at the peak power level of ~100 MW; with the velvet cathode, a duration of ~400 ns was achieved. It has been found that, in addition to the common limitations of the microwave pulse duration related to the dynamics of the diode impedance governed by the cathode plasma expansion, there is another factor, namely, the anode-cathode gap, which determines the delay at the beginning of the microwave generation. The latter effect is explained by the role of electrons oscillating between the virtual and real cathodes in the generation process. The issue of radiated microwave frequency behavior is discussed as well.

Relativistic BWO with electron beam pre-modulation

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.