Edward B. Abubakirov

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.

Impact of Spent Electrons on BWO Operation

As a result of their interaction with an operating wave in a backward-wave oscillator (BWO), electrons give up their energy to the electromagnetic field and acquire a large energy spread. The energy spread decreases the space-charge-limiting current, which can result in the formation of a virtual cathode (VC) for low-energy electrons in the output channel. We analyze the influence of electrons reflected from the VC on BWO operation, which manifests as a decrease in the generated power and in the appearance of modulated power. In the region of cyclotron absorption, the reflected electrons cause high-frequency generation with a high noise level.

Peculiarities of Backward-Wave Amplification by Relativistic High-Current Electron Beams

Microwave amplification in relativistic backward wave oscillator operating below its self-excitation threshold is considered. The conditions that allow to achieve high gain provided with stable operation of the backward wave amplifier (BWA) were found. Several schemes of use of the relativistic BWA providing high power microwave amplification were studied. It is shown that employing a BWA in regenerative amplification regime allows to obtain more than 40 dB gain with efficiency over 20%.

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.

AMPLIFICATION AND GENERATION OF HIGHPOWER MICROWAVE BY RELATIVISTIC ELECTRON BEAMS IN SECTIONED SYSTEMS

The highest powers of microwave radiation produced with devices driven by high-current relativistic electron beams amount to 10 9 -10 10 W in pulses of 1- 100 ns duration. As the high-frequency electric fields in the devices are limited with microwave breakdowns, further enhancement of microwave pulse energy requires broader interaction space. The main problem with such oversized and, accordingly, multi-mode structures, is keeping coherence of output radiation. Because of this problem, special means for mode selection must be applied. In the paper, methods of mode selection based on sectioning of the operating space are discussed and examples of their experimental realization are presented.

Effects in a high power BWO induced by the exhausted electron beam

Summary form only given, as follows. As a result of the interaction with the operating wave in a relativistic BWO, electrons obtain a great velocity spread for practically any efficiency of beam-to-microwave energy conversion. Calculations show that the distribution of electrons is almost symmetric with respect to the average electron energy. The main portion of the electrons is found in two fractions: one of them has energy almost twice that of the average value, whereas the electrons of the second fraction have very low energy that lead to a decrease in the value of the space-charge-limiting current. When the electron collector is placed far from the interaction space, local virtual cathodes appear in the output channel, even when the electron beam propagates close to the wall. Part of the reflected electrons reach the cathode and result in a decrease of the beam current and the subsequent disappearance of virtual cathodes. As a result, modulation of the output radiation appears, the depth of modulation depending on the collector conditions. In experiments performed at the Institute of Applied Physics using a Sinus accelerator, a 0.5 MV, 17 ns X-band relativistic BWO radiated microwave pulses of 10 ns duration. The placement of a massive copper ring directly at the end of the electrodynamic system resulted in the microwave pulse duration equaling the voltage pulse duration. In the case without the massive ring, pulse shortening could be attributed to a decrease in electron emission owing to electrons reflected from the collector; the electron beam current would fall below the start oscillation current, thereby preventing microwave generation. Therefore, conditions at the collector of a relativistic BWO can interrupt the regime of stationary microwave generation.

Influence of Spent Electrons on BWO Operation

Summary form only given. One of the important problems for high power microwave sources is to provide removal of spent electrons to avoid overheating and erosion of the output channel, part of which is used as the electron dump. An effective solution of this problem is achieved by spreading the electrons along the surface of the output channel. However, it is noted that changing the location of the electron dump leads to a change in the output characteristics of the BWO (such as radiation power P and pulse duration). Evidently, spent electrons can exert an influence on BWO operation owing to electrons reflected from an electron dump or a virtual cathode. The appearance of a virtual cathode is associated with a large spread of electron energies after passing through the interaction space. Computer simulations show that the distribution of spent electrons over energies becomes almost symmetrical with respect to their average energy gamma macr with maximal values of current I on the ends gammamin, max ap gamma macrplusmnDeltagamma of the distribution I(gamma), where Deltagamma ap gamma macr. Thus, there is a large fraction of electrons with very low energy, and even an insignificant deceleration of the spent electron beam in the output channel can lead to the appearance of a virtual cathode. Two factors can lead to the degradation of output characteristics owing to reflected electrons. One of them is an increase of space charge near a cathode suppressing its electron emission, which leads to degradation of radiation when the BWO operates in a regime of short pulses or modulation of radiated power. Another reason is the influence of cyclotron absorption. Simultaneously with double (Cherenkov and cyclotron) interaction of initial monoenergetic electron beam with the operating wave, the same double interaction of reflected electrons with the reflected operating wave takes place, which significantly extends the zone of cyclotron absorption due to the wide energy distribution. As a result, a very strong guide magnetic field is required to achieve the maximum radiated power. Computer simulations of the experimental relativistic BWO confirming the aforementioned factors are presented.

Generation of a Periodic Sequence of High-Power Ultrashort Pulses in a Chain of Coupled Backward-Wave and Traveling-Wave Tubes Operating in the Regimes of Amplification and Nonlinear Kompfner Suppression

We have studied the dynamics of an electronic microwave generator with backward-wave and traveling-wave tubes connected in the feedback circuit and operating in the regimes of amplification and nonlinear Kompfner absorption, respectively. It is shown that radiation generated in such a system can have the form of a periodic sequence of ultrashort pulses. The studied mechanism of pulse generation is analogous to the method of passive mode locking, which is widely used in laser physics.

Developing a high-current relativistic millimeter-wave gyrotron

A relativistic gyrotron fed by a 500 keV, 2 kA helical electron flow in the TE and TM modes is simulated on the basis of a self-consistent system of time-dependent equations with a non-fixed field structure and using a three-dimensional version of the KARAT PIC code. The possibility of reaching an output power 200–250 MW at a wavelength of 10 mm is shown.

Dynamics of transient processes in relativistic backward wave tube driven with an external electromagnetic signal

The paper is devoted to theoretical and experimental study of relativistic backward wave tube driven with input electromagnetic signal. It was observed that input electromagnetic signal of “traditional” electronic devices can produce essential effect on operation of relativistic backward-wave tube despite its low level compared with the power of relativistic electron beam. It can reduce start-up time or even by impact for auto-oscillations in hard regime. There are parameter regions where frequency control can be realized: amplification and phase locking.