Relativistic Cherenkov Oscillators Research Articles

Research on Sub-Microsecond Compact L-Band Relativistic Cherenkov Oscillator

Prolonging the microwave pulsewidth is an important research direction to improve high-power microwaves’ energy and average power level. In this article, a sub-microsecond compact <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${L}$ </tex-math></inline-formula> -band relativistic Cherenkov oscillator (RCO) is studied. The device adopts a coaxial slow wave structure to reduce the radial size and a coaxial collector structure to improve the resonance characteristics of the slow wave structure to reduce the axial size. In particular, a coaxial outer corrugated slow wave structure is used instead of the coaxial double-corrugated slow wave structure to increase the power handling capacity (PHC) of the slow wave structure and reduce its surface electric field. Moreover, the collector structure has been improved to suppress the effects of plasma expansion. In the experiment, the device obtained a high-power microwave output with a microwave frequency of 1.496 GHz, a power of 2.0 GW, a pulsewidth of 317 ns (single pulse energy of 634 J), and a power conversion efficiency of 32%. The pulsewidth and the single pulse energy are the highest levels reported for the RCO.

Quasi-Optical Theory of Relativistic Cherenkov Oscillators and Amplifiers with Oversized Electrodynamic Structures

Using the quasi-optical approach, we investigate wave propagation along the periodically corrugated surfaces and their interaction with rectilinear relativistic electron beams (REBs). At the periodical structure, the field can be expanded into a series of spatial harmonics, which, in the case of shallow corrugations, represent paraxial wavebeams with mutual coupling described within the method of effective surface magnetic currents. We present the dispersion equation for the normal waves. Two limit cases can be recognized: in the first one, the frequency is far from the Bragg resonance and the wave propagation can be described within the impedance approximation with the field presented as a sum of the fundamental slow wave and its spatial harmonics. In the interaction with a rectilinear REB, this corresponds to the convective instability of particles’ synchronism with the fundamental (0th) or higher spatial harmonics (TWT regime), or the absolute instability in the case of synchronism with the −1st harmonic of the backward wave (BWO regime). In the latter case, at the frequencies close to the Bragg resonance, the field is presented as two antiparallel quasi-optical wavebeams, leading to the absolute instability used in the surface-wave oscillators operating in the π-mode regime. Based on the developed theory, we determine the main characteristics of relativistic Cherenkov amplifiers and oscillators with oversized electrodynamical systems. We demonstrate the prospects for the practical implementation of relativistic surface-wave devices in submillimeter wavebands.

A high-efficiency L-band coaxial three-period relativistic Cherenkov oscillator

Miniaturization is one of the important research directions of low frequency high power microwave sources. This paper presents a three-period coaxial slow-wave structure L-band high-power microwave source. Because the coaxial Quasi-TEM mode has no cut-off frequency, the radial size of the device can be reduced. At the same time, in order to reduce the transverse dimension, the coaxial extractor structure is introduced to realize the longitudinal mode selection and improve the conversion efficiency of the device. In simulation, the device obtains the microwave output with the central frequency of 1.53 GHz, the average power of 3.3 GW and the efficiency of 40%. By optimizing the scheme of electron beam collection, the phenomenon of pulse shortening is effectively suppressed. In the experiment, the device obtains the microwave output with the central frequency of 1.52 GHz, the average power of 3 GW, the efficiency of 33% and the pulse width of 40 ns.

Investigation of a cross-band relativistic Cherenkov oscillator based on the cathode adjustment

The mechanism and realization of a cross-band relativistic Cherenkov oscillator based on the cathode adjustment are presented. The device adopts coaxial dual-annular cathode to emit the inner and the outer electron beams, independently. When the inner annular cathode is extended and the outer annular cathode is retracted, the electron beams are emitted from the inner cathode and interact with the internal electromagnetic structure to generate Ka-band microwaves. On the contrary, the outer electron beams interact with the external electromagnetic structure to generate X-band microwaves. Thus, the cross-band microwave generation can be achieved only by adjusting the protrusion and retraction of dual-annular cathode. By the particle-in-cell (PIC) method, the physical process of beam-wave interaction in the device is studied, and the high efficiency internal-external double electromagnetic structures are designed. Simulation results show that the X-band microwave with a power of 1.50 GW and a frequency of 8.55 GHz is generated, corresponding to an efficiency of 32%. Meanwhile, the Ka-band microwave with power of 650 MW and frequency of 31.80 GHz is generated, corresponding to efficiency of 28%.

A novel L-band metamaterial relativistic Cherenkov oscillator with high conversion efficiency

A novel all-metal metamaterial slow wave structure (MSWS) which suits high-power microwave sources was proposed in our previous work. Recently, an experiment on the transmission characteristics of the MSWS was carried out. The experiment results show that the MSWS can transmit electromagnetic waves in the 1.40–1.60 GHz frequency band, while the same size circular waveguide TM01 mode has a cutoff frequency of 2.4 GHz. The experiment proved that the MSWS has the advantage of miniaturization. Then, based on the transmission characteristic experiment, a novel L-band relativistic Cherenkov oscillator with miniaturization and high conversion efficiency is proposed. By introducing a new coupled output structure and adjusting the MSWS period at both ends of the output structure, the electromagnetic field is better matched between the MSWS and the output coupler, and the conversion efficiency is increased from the previous 29.2% to 55.8%, with an average output power of 460 MW. Through the analysis of the electric field distribution at the main frequency of the generated microwave, it is determined that the working mechanism of the device is the Cherenkov mechanism.

Numerical and experimental investigation of 4 mm wavelength microwave oscillator based on high-current compact accelerator

The concept of two-wave relativistic Cherenkov oscillator for the generation of nanosecond microwave pulses at ∼72 GHz central frequency has been applied. A moderately oversized sectioned slow wave structure (average diameter ≈2.5 wavelength) provides the interaction of electrons simultaneously with a slow (or surface) TM01 wave and a fast (or volume) TM02 wave, which govern the microwave energy output. The generation of 85 MW, 1.3-ns long microwave pulses has been demonstrated in a single pulse operation with a pulsed guiding magnetic field of 3.8 T on the base of the desktop high-current accelerator RADAN-303.

Two-Wave Ka-Band Nanosecond Relativistic Cherenkov Oscillator

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 ≈ 2.6λ. (λ is a wavelength). The combined mode selection including the diffraction loss in the electron diode was applied. The high-current accelerator SINUS200 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.

Two-Wave Cherenkov Oscillator With Moderately Oversized Slow-Wave Structure

This paper describes a new design of a relativistic Cherenkov oscillator with strong modulation of the e-beam current at the input of two-sectioned slow-wave structure, as well as numerical and experimental results on generation of few gigawatt-level $X$ -band pulses with pulse duration of up to 35 ns. Due to special phase shift between two sections and electrodynamic coupling between $E_{01}$ surface wave and $E_{02}$ volume mode, the first one is locked at the upper boundary of the passband and the second one gains more transparency. An electron beam can interact simultaneously with two waves with higher power conversion efficiencies, which in simulations amount up to 47% and 30% for high (~4 T) and low (~1 T) guiding magnetic fields, correspondingly. The tests of the geometry optimized in simulations were performed on the base of SINUS-7 high-current electron accelerator in the diode voltage range of 400–850 kV and current from 7 to 16 kA. The repeated and full-pulsewidth generation of about 1.5-GW microwave power is achieved only in the low magnetic field. The pulse shortening and the mode competition took place in the strong magnetic field. The results are considered as preliminary for creation of the efficient relativistic millimeter-wave oscillators at magnetic fields below the cyclotron resonance.

Prospects of application of superconducting electrodynamic structures in electronic devices for their advancement to the terahertz range

It is shown that the application of superconducting electrodynamic structures in microwave electronic devices not only improves their characteristics, but also creates premises for implementation of devices like the autophase traveling-wave tube (TWT) and peniotron operating in the millimeter range with their further advancement to the terahertz range, which is impossible for conventional electrodynamic structures with Ohmic losses. Superconducting corrugated waveguides make it possible to suspend limitations imposed on the output power of pulsed relativistic Cherenkov oscillators, which are associated with thermal degradation of the working surface of conventional waveguides with Ohmic losses.