S. D. Korovin

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.

The RADAN series of compact pulsed power Generators and their applications

This paper presents results of development of a compact pulsed power high-voltage generators and high-current electron accelerators of the RADAN series. The basic high-voltage units of RADAN instruments are built around coaxial pulsed forming lines and efficient charging device represented by a Tesla transformer. The fields of applications in science and in practice are rather wide and include formation of nanosecond and subnanosecond voltage and ultrawideband RF pulses, high-power microwave generation, X-ray radiography, radiation physics, chemistry, and biology. The designed technique provided achievements of outstanding specific parameters of dense e-beams, microwaves, and ultrawideband pulses.

Efficiency enhancement of high power vacuum BWO's using nonuniform slow wave structures

The Sinus-6, a high-power relativistic repetitively-pulsed electron beam accelerator, is used to drive various slow wave structures in a BWO configuration in vacuum. Peak output power of about 550 MW at 9.45 GHz was radiated in an 8-ns pulse. We describe experiments which study the relative efficiencies of microwave generation from a two-stage nonuniform amplitude slow wave structure and its variations without an initial stage. Experimental results are compared with 2.5 D particle-in-cell computer simulations. Our results suggest that prebunching the electron beam in the initial section of the nonuniform BWO results in increased microwave generation efficiency, Furthermore, simulations reveal that, in addition to the backward propagating surface harmonic of the TM/sub 01/ mode, backward and forward propagating volume harmonics with phase velocity twice that of the surface harmonic play an important role in high-power microwave generation and radiation. >

Millimeter-Wave HF Relativistic Electron Oscillators

A review of the experimental study of single-mode oscillators based on stimulated bremsstrahlung and Cerenkov radiation of high-current relativistic electron beams is given. Three types of Cerenkov oscillators are investigated in detail: orotrons, surface wave oscillators and a flimatron (free electron maser (FEM) based on Smith-Purcell radiation). The bremsstrahlung oscillators studied are gyrotrons with TM modes, a ubitron operating at a quasi-critical frequency and cyclotron autoresonance masers. Electrodynamic and electron methods of mode selection provide stable radiation with a reproducible space structure of radiation in all oscillators under study. The radiation power attained 50-100 MW for long and 10-30 MW for short millimeter wavelengths at the efficiency up to 5-10 percent. Various types of oscillators are compared. Promising methods for increasing power and radiation frequency are discussed.

Repetitively pulsed high-current accelerators with transformer charging of forming lines

This article describes the principles of operation and the parameters of the SINUS setups designed at the Institute of High-Current Electronics, Siberian Division, Russian Academy of Science, over the period from 1990 to 2002. A characteristic feature of accelerators of the SINUS type is the use of coaxial forming lines (in particular, with a spiral central conductor) which are charged by a built-in Tesla transformer to produce the accelerating high-voltage pulses. This ensures a reasonable compactness and long lifetime of the setups. The range of parameters of the SINUS setups is as follows: ○ Voltage amplitude at the cathode: 200-2000 kV ○ Electron beam current: 2-20 kA ○ Equivalent load impedance: 30-180 Ω ○ Accelerating pulse duration: 4-130 ns ○ Pulse repetition rate: up to 400 Hz ○ Pulse amplitude instability (RMS): 0.7-2.5% A number of setups of this type use a three-electrode controllable gas gap switch. This has made possible on-line electronic control (from pulse to pulse) of the output voltage pulse amplitude. The control band width δ = ΔU/U max was up to 75%. Studies have been performed on the lifetime of explosive-emission cathodes. At current densities of 25-30 A/cm 2 , a pulse duration of ∼20 ns, and a pulse repetition rate of 100 Hz, the metal-dielectric cathode in a planar geometry retained its emissivity within 10 8 pulses. The SINUS accelerators are traditionally employed for producing high-power microwave radiation in various systems with a coaxial electron beam in a longitudinal magnetic field. For this purpose, magnetic systems with a solenoid powered from the bank of molecular capacitors have been designed. The duration of a quasistationary magnetic field was 1 s at a maximum solenoid power of 365 kW. The possibility has been shown to exist for a self-contained power supply of the accelerator from the bank of molecular capacitors in the batch mode. With an average power consumption of about 120 kW, the setup produces pulses in a batch of duration 2.5 s at a pulse repetition rate of 200 Hz.

Pulsewidth limitation in the relativistic backward wave oscillator

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.

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.

Enhanced frequency agility of high-power relativistic backward wave oscillators

This paper describes how finite length effects in high-power backward wave oscillators can be exploited in a controlled manner to achieve enhanced frequency agility. Experiments were performed using a Sinus-6 high-power relativistic repetitively pulsed electron beam accelerator. A uniform slow wave structure was used in these studies and its parameters were fixed. Sections of smooth-walled circular waveguide of varying lengths were inserted both before and after the slow wave structure. Variations in the length of smooth-walled waveguide on the order of a quarter-wavelength of the generated electromagnetic radiation were found to significantly affect both microwave frequency and radiation efficiency in a periodic-like manner. The experimental results were reproduced in TWOQUICK electromagnetic particle-in-cell simulations. A bandwidth of about 500 MHz centered around 9.5 GHz at hundreds of MW power levels has been achieved with constant beam and slow wave structure parameters.

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.

Compact 1000 pps high-voltage nanosecond pulse generator

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.