Nikolay F. Kovalev

Scenario for output pulse shortening in microwave generators driven by relativistic electron beams

At the present time, microwave generators driven by high current relativistic electron beams are not baked and sealed, so their inner surfaces are densely covered with molecules of gas and oil. This allows the production of microwave pulses of 10/sup -8/ s to 10/sup -7/ s duration, but not longer. A microwave pulse termination scenario is speculated as follows: (1) Electrons oscillating in the strong RF field near the metallic surfaces multiply owing to the secondary emission (the multipactor effect); (2) the multipactor electron bombardment stimulates desorption of gas molecules from the metallic surfaces; (3) the gas undergoes avalanche RF breakdown; and (4) the resultant plasma stops microwave generation and, since electron-ion recombination is slow, does not allow the RF field to revive. At the gigawatt power level, the characteristic time of such a scenario is much shorter than that of the cathode and collector plasma expansion and electron beam instabilities. The energy output parameters of relativistic electron microwave generators can be (and usually are) improved at high pulse repetition rates. A more radical improvement is possible using the technology typical for high vacuum tubes, i.e., baking and sealing.

Mode conversion in a magnetron with axial extraction of radiation

We demonstrate the ability to form simple radiation patterns from a relativistic magnetron with axial extraction. This is achieved by tapering onto a conical antenna only those cavities of the anode block that correspond to the symmetry of the radiated modes. The efficiency of mode conversion of the operating pi-mode into a radiated mode using this method is demonstrated using computer simulations of a six-cavity magnetron

An X-band gigawatt amplifier

In an X-band Cerenkov amplifier driven by a 0.8-MeV 6-kA electron beam, a gigawatt-level of power radiated in a Gaussian pattern in a 70-ns pulse duration has been demonstrated. The coherence of the output radiation is provided by dividing the oversized interaction space into separate sections with different azimuthal symmetry that couples only with the electron beam. A large gain of 47 dB and an efficiency of 23% are obtained using a regenerative amplification of a backward-wave amplifier (BWA) that produce a modulation of the electron beam. The efficiency of this device is 27% when the BWA-modulator operates in the regime of auto oscillations.

Selective multichannel feedback

A simple and effective method of mode selection in oversized electrodynamic systems is described. The method is based on forming a solitary resonant combination of eigenmodes by a system of Bragg reflectors having a different azimuthal symmetry at the ends of the electrodynamic system. Application of this method to a resonant traveling-wave tube is considered.

Waveguide resonators with combined Bragg reflectors

We present a method to improve the selective properties of a waveguide resonator with Bragg reflectors on both ends, a passive device that is widely used in microwave engineering and optoelectronics. It is shown that additional mode conversions lead to a significantly rarefied spectrum of eigenoscillations with different transverse field structures. In particular, this can be achieved by using doubly periodic Bragg reflectors with different azimuthal symmetries forming four wave resonant field structures. The analysis of dispersion properties presented is applicable for resonators of arbitrary relation of lengths of the reflectors and waveguide.

Self-consistent theory of cyclotron phenomena in a relativistic BWO

We have developed a self-consistent theory for a relativistic backward wave oscillator (BWO) taking into account the interaction of fast cyclotron waves of the electron beam with both the operating contrary wave and the forward wave. The theory describes the extraction of electromagnetic energy from the interaction space, providing a correct description of the operation of a relativistic BWO in which the field structure is a product of the self- consistent solution. The theory suggests ways in which the requirement of a guide magnetic field in such devices can be reduced.

Suppression of parasitic self-excitation in Cherenkov amplifiers

A means for removing the parasitic feedback in microwave amplifiers, the main obstacle to achieving a high gain, is described here. The method is based on utilizing the resonant interaction between fast cyclotron waves on an electron beam and the electromagnetic waves that are propagating in the opposite direction. This is an effective method to prevent detrimental self-excitation in amplifiers whose operation is based upon the stimulated Cherenkov radiation of a forward-propagating electron beam in a guiding magnetic field. Conditions for the resonant interaction are provided by proper choice of the guiding magnetic field. At such resonances the counter-propagating waves are in stop-bands and, therefore, cannot propagate. Results of theoretical and experimental investigations of cyclotron absorption of counter-propagating waves in amplifiers are given in the present work. It is shown that the resonant cyclotron interaction leads to a complete suppression of the feedback and that the threshold of self- excitation becomes unachievable even for large reflections. Only a minor decrease in the amplification results in comparison with an ideal amplifier without reflections. It follows from these results that a spatially varying magnetic field can be applied along the axis of the amplifier to expand the zone of the cyclotron absorption and thereby exclude a re-tuning of the self-excitation frequency.

Experimental Study of a Relativistic Resonant Traveling-Wave Tube With Selective Feedback Provided by Bragg Reflectors

The realization of single-mode generation in a microwave oscillator is considered for a relativistic resonant traveling-wave tube (RTWT) with spiral Bragg reflectors at the ends of a multimode electrodynamic system. An oversized azimuthally symmetric slow-wave structure was chosen so that only two field structures are synchronous with a 1-MeV electron beam. Experiments using different means that allow us to separate these field structures are described. As a result, two radiation patterns were observed separately, which are a narrow Gaussian-like wave beam with a power of up to 1.5 GW and an efficiency of about 20% in 40-ns pulses and radiation corresponding to the E01 mode with four times weaker power. The dependences of RTWT operation on the parameters of the electron beam, the length of interaction space, reflection coefficients, and the amplitude of guide axial magnetic field are investigated in detail.

Measurement of high-power microwave pulses by resistive hot-electron sensors

Use of hot-electron semiconductor detectors for the measurement of high-power microwave pulses may lead to anomalously large errors. Methods of separation of the error signal and determination of the measurement error are considered.

X-band amplifier of gigawatt pulse power

In X-band Cherenkov amplifier driven by 0.8 MeV 6 kA electron beam, the gigawatt power in pulses with duration 70 ns is achieved. The coherence of output radiation is provided by division of the oversized interaction space on separate sections coupled with the electron beam only. The high gain of 47 dB (and accordingly narrow 1% bandwidth) are obtained with the regenerative regime of operation of BWA-modulator used as a pre-amplifier.