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This paper describes two approaches which have been developed in the past few years for the creation of electronically controlled steering of the microwave beam formed by multichannel auto-oscillators. The first one is based on two- and four-channel gyromagnetic nonlinear transmission lines operating in the generation mode at subgigawatts power level and decimeter wavelengths. The second approach uses Cherenkov superradiance in two and four parallel backward-wave oscillators with centimeter wavelengths. We present the results of several recent experiments and analyze the physical and technical limitations to arrange new radiating arrays.

This paper presents the results of numerical and experimental study of a Ka-band relativistic Cherenkov oscillator with average diameter of the slow-wave structure ${D} \approx \text {2.6}\lambda $ . ( $\lambda $ is a wavelength). The combined mode selection including the diffraction loss in the electron diode was applied. The high-current accelerator SINUS-200 provided the thin-walled annular electron beam in the range of voltages 350–420 kV and currents of 2.9–3.8 kA at pulsewidth of 10 ns. The generation of 2.5–3-ns pulses at the central frequency 36.4–36.8-GHz and 400–600-MW output microwave peak power was realized. The corresponding efficiency of 42% ± 5% in power conversion has been obtained.

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.

The effective 1-s batch of 100-Hz pulse repetition rate mode of an X-band relativistic backward-wave oscillator with a resonance reflector (RBWO-RR) was realized using the “SINUS” pulse-periodic nanosecond accelerator allowing mechanically installed high voltage pulse length of 13 and 42 ns. Magnetic held about of 2 T was produced by two-section solenoid to transport the electron beam along RBWO-RR electrodynamic system. T M01 to T E11 mode converter connected to the oscillator output allowed Gaussian radiation pattern. The microwave oscillator allowed producing the microwave pulse length of 4.7 and 29 ns. The energy of the microwave pulse measured by the X-band aperture calorimeter was 1.4 or 6.3 J corresponding to the mentioned above microwave pulse length of 4.7 or 29 ns, respectively. That allowed estimations of the microwave peak power as high as 280 ± 30 MW and 210 ± 20 MW related to the pulse lengths. The efficiency of the oscillator was 23-25%. A possibility of the microwave pulse length fine tuning within 25 to 34 ns range in the “long pulse mode” by changing the solenoid magnetic held conhguration was shown.

A repetitively rated microwave oscillator whose frequency can be varied electronically from pulse to pulse in a predetermined manner is created for the first time. The microwave oscillator has a power on the order of 108 W and is based on the Cherenkov interaction of a high-current relativistic electron beam with a plasma preformed before each pulse. Electronic control over the plasma properties allows one to arbitrarily vary the microwave frequency from pulse to pulse at a pulse repetition rate of up to 50 Hz.

First version of relativistic BWO with TM 02 operating mode has been tested also in [1]. The cyclotron-resonance mode selection was used. In this case, because of requirement to the defined values of magnetic field and e-beam radius, the efficiency of oscillator was not very high (approximately 10%). Now, the different electrodynamic codes are available to optimize the geometry of Slow Wave Structure (SWS) and reflector for TM 02 mode with small re-emission to TM 01 . Then other techniques of mode selection such as the e-beam pre-modulation near the reflector [2], sectioning of SWS and diffractive losses at the coaxial magnetically-insolated diode can be used. So, refusing the cyclotron-type discrimination of TM 01 mode, we studied approaches of mode selection with higher magnetic field to increase the BWO efficiency, including the idea of self-modulated e-beam emission [3].

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.

A new method of converting a high-voltage video pulse into high-frequency oscillations using a nonlinear transmission line with temporal dispersion has been studied. The dispersion was provided by pulsed magnetization reversal in a ferrite, which was initially magnetized to saturation in an external magnetic field. For a 9-ns pulse, an average energy conversion efficiency of about 10% was achieved. It is demonstrated that oscillations at frequencies within 600 MHz-1.1 GHz with a spectral width of about 15% (at a −3 dB level) can be excited using voltage pulses with an amplitude of 110–290 kV. The optimum bias magnetization fields are within 20–40 kA/m.

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.

Summary form only given. Development of HPM sources based on gyromagnetic nonlinear transmission lines (NLTLs) includes the appropriate filtration of the high voltage video pulse and the irradiation antenna. We present experimental results of testing of band-pass filters designed for NLTLs. The influence of a band-pass filter bandwidth on the irradiated RF peak power is discussed. Two antenna systems have been developed to irradiate RF pulses with peak power up to 1.4 G W. To increase the directivity of the irradiation the 4 channel splitter into four 50 Ohm loads (antenna array) has been designed and tested. The coherent parallel excitation of several NLTLs is discovered to be possible due to the low jitter - about 5% of the period of oscillations. The possibility of the elongation of the RF pulses produced in the gyromagnetic NLTLs using coupled coaxial lines with spatial dispersion is discussed.

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.

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.

Report presents the results of calculations and numerical modeling of quasi-stationary, nanosecond relativistic X-band and Ka-band BWOs with peak power exceeding 100 MW where magnetic field guided electron beam (0.5–1.7 T) could be created by permanent magnets made of highly-coercivity materials. Magnetic field interpolation of the permanent magnets was used in numerical models of microwave devices (code KARAT). Particle-in-cell simulations have shown that in the frequency ranges of 8 GHz and 37 GHz, and for the B-field below cyclotron resonance value, the same parameters of HPM generation are available as it predicted and/or measured experimentally for prototype models with solenoids fed by DC or pulsed current.

Recent studies of the generation of GW-level nanosecond microwave pulses in extended BWO-type slow-wave structures have been showed an opportunity for stabilization of the RF field phase. Decline in the dependence of the output microwave power on the electron energy in this operation mode allows reaching radiation peak power spread not exceeding voltage variation on a vacuum diode. Superradiative microwave generators can produce high peak power with moderate beam energy and have good efficiency. This paper discusses the possible application of such systems.

We demonstrate electronically controlled beam steering by high power RF pulses produced by two gyromagnetic nonlinear transmission lines (NLTLs) connected to a one high voltage driver. Each NLTL is capable of producing several ns RF pulses with peak power from 50 to 700 MW (6% standard deviation) at frequencies from 0.5 to 1.7 GHz (1% standard deviation) with 100 Hz repetition rate. Using a helix antenna allows irradiating of RF pulses with almost circular polarization and 350 MW maximum peak power, which corresponds to 350 kV effective potential of radiation. At the installation of two identical channels, we demonstrate the possibility of beam steering within ±15° in the horizontal plane by coherent RF pulses with circular polarization at 1.0 GHz center frequency. Fourfold increase in the power flux density for in-phase irradiation of RF pulses is confirmed by comparison with one-channel operation.

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.

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. 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.

We have developed a relativistic Ka-band backward wave oscillator (BWO) capable of generating stable 250-ps pulses of peak power up to 300 MW. The BWO operates in a 1-s burst mode at a PRF of 1–3.5 kHz.

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.