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The paper presents research results on enhancing the peak power of microwave pulses with sub- and nanosecond length using a backward-wave oscillator (BWO) operating at 29 GHz frequency and possessing a reproducible phase structure. Experiments are conducted in two modes on a high-current electron accelerator with the required electron beam power. In the first (superradiation) mode, which utilizes the elongated slow-wave structure, the BWO peak power is 3 GW at 180 ns pulse duration (full width at halfmaximum, FWHM). In the second (quasi-stationary) mode, the BWO peak power reaches 600 MW at ~2 ns pulse duration (FWHM). The phase spread from pulse to pulse can vary from units to several tens of percent in a nanosecond pulse mode. The experiments do not show any influence of microwave breakdown on the BWO power generation and radiation pulse duration.

The paper describes a study of a new relativistic microwave Cherenkov generator of millimeter wavelength range with the transverse dimension of the electrodynamic system D>2.5λ, where D is the average diameter of the slow wave structure, and λ is the radiation wavelength. The investigations were carried out with a high-current SINUS-200 electron accelerator at a voltage of 473 kV and an electron beam current of 3.8 kA. The use of several, complementing each other, mechanisms of suppression of parasitic waves provides stable microwave generation at a specified frequency of 36.4 GHz. The microwave power in the experiment 804 MW with 45% beam-to microwave power conversion.

An efficient microwave oscillator (320 MW and 7.9 GHz) that generates microwave pulses with a duration of 90 ns is developed using optimization of an electron-wave system and decompression of the longitudinal magnetic field with a maximum induction of 0.62 T in the region of an explosive-emission cathode and a lower field (0.36 T) with respect to cyclotron resonance in the slow-wave structure. In a packet (up to 10 ns) repetitively-pulsed (100 Hz) regime, the maximum conversion efficiency of the electron-beam power to microwave radiation is 27%. The mean energy of the radiation pulse (23 J) is about 18% of the pulse energy of high-voltage oscillator.

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.

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.

The operation of cold explosive-emission cathodes having a current density of ∼104 A/cm2, fabricated using various materials, was investigated under a large number of switching cycles. The cathode voltage was ∼500 kV, the maximum current ∼5 kA, and the pulse duration ∼20 ns. It is shown that when the number of switchings is small (⩽103 pulses), cathodes having similar geometry exhibit similar emission properties. For most of the materials studied, as the number of switching cycles increases (⩾103 pulses), the current rise time increases (as far as the pulse duration) and the maximum vacuum diode current decreases. When a graphite cathode was used, the maximum current remained unchanged up to 108 switching cycles. The mass removed from the cathode was determined for various materials. The results were used to achieve continuous operation of a relativistic 3 cm backward-wave tube having an output power of 350–400MW and an almost constant power level during 108 pulses at a repetition frequency of 100–150 Hz.

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.

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.

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 (

Electron beam parameters and material consumption were measured for edge-type explosive-emission cathodes made from different materials, working in a BWO-based HPM source, at a large number of pulses. Most of the materials demonstrated several degradation of emission properties and decrease in microwave emission after 10/sup 4/-10/sup 6/ pulses. A graphite cathode demonstrated stabilization of its emission parameters after 10/sup 7/ pulses and successful operation during 10/sup 8/ pulses at a pulse repetition rate of 100-150 pps.

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.

The paper presents the results of an experimental study of the cold cathodes used in the Sinus-type repetitive high-current accelerators. The accelerating voltage for this accelerator is on a level of 100-1000 keV, the electron beam current is 1-10 kA, the pulse duration is 10-40 ns, and the pulse repetition rate is up to 200 pps. The emissive properties of the cathodes were investigated for both planar and coaxial magnetically insulated vacuum diodes. To produce high-current electron beams in coaxial diodes, a strong magnetic field of strength up to 30 kOe created by a superconducting solenoid was used. Investigated in the experiment were explosive-emission cold cathodes of varied geometry made of various materials such as copper, steel, stainless steel, bronze, graphite, and carbon-graphite cloth as well as metal-dielectric cathodes depending for their operation on an incomplete electrical discharge over the dielectric surface. The longest lifetime has been attained for a metal-dielectric cathode in a planar vacuum diode; it is over 10/sup 8/ shots. The cathode provided the current density at the anode 20-25 A/cm/sup 2/ for the accelerating voltage 600 kV, pulse duration 20 ns, and pulse repetition rate up to 200 pps.

Summary form only given, as follows. Producing HPM emission at high pulse repetition rates requires strong magnetic fields, which commonly employs superconductive systems. Realizing batch high rep-rate regimes allows utilization of quasi steady-state magnetic fields using capacitive energy stores. Molecular capacitors storing as much as /spl sim/3.8 J/cm/sup 3/ are most suitable for this purpose. The paper presents a fully controllable power source for a one-second solenoid storing 650 kJ. The store consists of 12 molecular capacitors connected serially in four stages. The total store capacity is 3.45 F and its output voltage is 600 V. The bank charge is made by four independent 10-A DC sources and it takes less than four minutes. When forming the magnetic field pulse, the solenoid is switched onto the energy store via a pulsed stabilizer of a current based on isolated gate bipolar transistors (IGBTs). Controlling the transistors from a PWM-controller with solenoid current feedback allows a decrease in the magnetic field less than 5% while the capacitor bank voltage drops from 600 to 300 V. The maximum current of the solenoid is 1.2 kA. Thus, using the pulsed current stabilizer allows useful consumption of up to 75% of the capacitor bank stored energy.

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.

This paper presents a nanosecond periodically-pulsed generator based on a spiral line charged by means of a high-coupling Tesla transformer. At repetition rate of 100 p.p.s., 130 ns, 500-700 kV pulses were produced in a 100-150 /spl Omega/ load.

Results of study of a relativistic X-Band BWO with 3 GW peak microwave power are presented. The BWO was realized on the basis of the high-current electron accelerator SINUS-7. Dependence of microwave pulse duration on microwave power was obtained. Pulse shortening occurring at high powers is probably bound on explosive electron emission from the slow wave structure. Increase of electrodynamic structure cross-section seems to be a way to increase microwave pulse duration. Experimental results for moderately oversized X-band BWO with resonance reflector are described. This tube can be operated at low external magnetic field.