X-band Backward Wave Oscillator Research Articles

Numerical Analysis of High-Power X-Band Sources, at Low Magnetic Confinement, for Use in a Multisource Array

High-power microwave sources are typically relativistic in nature, employing multi-kilo-ampere electron beams that require significant magnetic confinement for efficient operation. As the desired output power increases, so does the complexity, and overall energy requirements, of the source. It can, therefore, be advantageous to consider the use of several, moderate-power, sources operating as a phased array; for an array of N sources, the far-field peak intensity scales as N², and the peak-of-field may be steered electronically by varying the relative phases of the different output signals. In this article, we present the numerical analysis of a short-pulse (~1 ns) X-band backward-wave oscillator, driven by a 210-keV, 1.4-kA electron beam, suitable for use as the radiative element in such an array. The investigation of the required magnetic confinement showed two peaks in performance, with the highest efficiency, of 43%, predicted at the low magnetic confinement peak at 0.3 T, corresponding to 125-MW peak output power. The magnitude and timing of the peak in the output pulse were functions of the rise time of the electron beam energy, with longer rise times resulting in delayed peak-of-field and lower peak output power. When operating in an array, to maintain effective output in the region of N², it was determined that the beam rise times, across all sources, should be ≤150 ps with the adjustment of the relative timing between output's being ±30 ps.

Linear analysis of an X-band backward wave oscillator with a circular-edge disk-loaded cylindrical waveguide driven by an annular electron beam

An X-band backward wave oscillator (BWO) with a circular-edge disk-loaded periodic metallic slow wave structure (CDSWS) is proposed and studied numerically. The structure is the modified version of our previously modeled semi-circularly corrugated slow wave structure (SCCSWS). The CDSWS is energized by an intense relativistic electron beam (IREB) which is directed by a strong magnetic field. The electromagnetic (EM) wave of the slow wave structure (SWS) merges with the space charge wave of the beam under the guidance of the strong axial magnetic field. The inner wall contour of CDSWS is modeled by a finite Fourier series and the dispersion characteristics of different TM modes are solved by utilizing the linear Rayleigh-Fourier (R-F) technique, which is verified by a commercial EM solver. To study the temporal growth rate (TGR) for the fundamental TM01 mode, the dispersion equation is solved for the beam current of 0.1-1.0kA and the beam energy of 205-665kV. For the TM01 mode, the TGR that occurs at the unstable region, which provides a qualitative index of the strength of the microwave generation, is compared with those of the BWOs with sinusoidally corrugated SWS (SCSWS), disk-loaded SWS (DLSWS) and triangularly corrugated SWS (TrCSWS) for different beam parameters. The dimension of the CDSWS is determined by comparing the dispersion characteristics of fundamental TM01 mode with DLSWS and SCSWS. For the same set of beam parameters, an average of 3.5%, 7%, 1.5% and more than 50% higher TGR have been obtained with the proposed CDSWS than that of SCSWS, DLSWS, TrCSWS and SCCSWS respectively. Moreover, the presented structure also provides an advantage in the fabrication process and is less prone to RF breakdown since it has no sharp edges in the inner wall where the electric field intensity can be infinitely high.

Numerical Study of the Dispersion Properties of an X-band Backward Wave Oscillator with Rectangularly Rippled Wall Resonator

Backward Wave Oscillators (BWOs) are devices that transform the electron beam energy into electromagnetic radiation at microwave frequencies. O-type BWOs consist of an axial electron beam propagating through a resonant cavity comprising of a slow wave structure (SWS). The electron beam is guided by a strong magnetic field while propagating through the cavity. The SWSs are used in BWOs in order to slow down the phase velocity of the electromagnetic wave so that the electron beam can resonantly interact with the wave which is the prerequisite of microwave generation. Dielectric loaded periodic structures were used as SWSs during the early stage of development of BWOs. However, this method of slowing down the wave suffers from dielectric breakdown as it cannot support high electric field. In order to prevent the dielectric breakdown, periodically corrugated metallic hollow waveguides are now being used as SWSs in BWOs. The sinusoidally corrugated SWS (SCSWS) has received the greatest attention among different corrugation profile. However, the fabrication process of this profile is complex and requires sophisticated tools. In an attempt to mitigate the fabrication problem, SWSs with simpler geometry have been proposed by the researchers. In this work, an earlier work on rectangularly corrugated SWS (RCSWS), which was investigated for non-relativistic electron beam has been extended for relativistic BWO in the X-band frequency range. The dispersion properties and the temporal growth rate of the axisymmetric transverse magnetic (TM) modes of have been analyzed numerically. In order to avoid the complicated boundary condition at the discontinuous boundaries of the rectangular radial profile, Fourier series has been used to approximate the axial profile of the SWS and the linear Rayleigh-Fourier theory has been utilized to determine the dispersion properties. The study shows that the growth rate of microwave for the RCSWS is somewhat lower than that for the case of SCSWS; however, a design tradeoff can be made to obtain a comparable performance.

Rapid startup in relativistic backward wave oscillator by injecting external backward signal

Investigation on accelerating the building up of oscillation and achieving a rapid startup in powerful relativistic backward wave oscillator by injecting a weak external backward signal is carried out in this paper. Synchronizing the signal with the backward wave excited by intense electron beam extracting with slow wave structure, the initial noise is greatly reduced and mode competition is restrained. The analysis is demonstrated by high power X-band backward wave oscillator experiment, in which a plasma switch is designed to realize the backward signal injection. The results show that the significant reduction of microwave output delay is attained and the start time of oscillation is ahead of 10 ns with the energy conversion efficiency increases about 62%.

Design of X-band backward wave oscillator with 10 GW output power

An efficient high power relativistic backward wave oscillator (RBWO) is designed. To enhance the beam-wave conversion efficiency and output power, an overmoded slow wave structure (SWS) is used and an inner conductor is inserted into the SWS. When electron energy is 1.3 MeV while the beam current is 17 kA, the simulated output microwave power is 10.4 GW at 8.8 GHz frequency for the RBWO.

Generation of 3 GW microwave pulses in X-band from a combination of a relativistic backward-wave oscillator and a helical-waveguide compressor

The phenomenon of passive compression of frequency-modulated (FM) pulses in a dispersive media (DM) was used to increase the peak microwave power up to the multigigawatt level. A helically corrugated waveguide was used as the DM, while a relativistic X-band backward-wave oscillator (RBWO) with a descending-during-the-pulse accelerating voltage served as a source of FM pulses. Compression of pulses down to a halfwidth of 2.2 ns accompanied by a 4.5-fold power increase up to a value of about 3.2 GW has been demonstrated.

Observation of chaotic dynamics in a powerful backward-wave oscillator.

Self-modulation regimes of generation in a powerful 10-micros X-band backward-wave oscillator were studied theoretically and experimentally. The sequence of the self-modulation patterns and corresponding bifurcation values observed as the current was increased were in good agreement with the results of simulations. It was found that at a current of 120 A chaotic self-modulation set in at a power of 2 MW and a relative spectral width of 4%.

Pulsewidth limitation in the relativistic backward wave oscillator

Spontaneous pulse shortening occurring in a relativistic backward wave oscillator (BWO) at gigawatt power levels is studied in experiment and theory. It is experimentally demonstrated that this phenomenon is accompanied by formation of an explosive-emission plasma at the surface of the corrugated slow-wave structure (SWS). Termination of microwave emission is explained by the increase of the BWO starting current from the absorption of the operating electromagnetic wave by electrons emitted from the plasma, whereas the intensity of the absorption radically increases offing to the presence of positive ions emitted from the plasma. Application of oil-free vacuum and electrochemical polishing of the SWS surface in an X-band BWO allowed generation of 3-GW, 26-ns microwave pulses with an energy of /spl sim/80 J, thereby demonstrating pulse lengthening by a factor of four.

Relativistic X-band BWO with 3-GW output power

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.

Cyclotron effects in relativistic backward-wave oscillators operating at low magnetic fields

A small-signal theory of a resonant relativistic backward-wave oscillator (BWO) operating at a very low-guiding magnetic field is developed. The theoretical approach is based on the successive iteration method of analytical solution of the exact three dimensional (3-D) equations of electrons motion. It was shown that cyclotron resonance effects in relativistic BWO (400 kV) operating at low-focusing magnetic field can lead to three different regimes of combined Cerenkov-cyclotron interaction: 1) cyclotron absorption due to cyclotron resonance between an electron beam and a fast spatial harmonic of forward electromagnetic wave; 2) complete compensation of cyclotron absorption under certain conditions; and 3) efficiency enhancement at low-focusing magnetic field. The theoretical results are compared with measurements of the output microwave power and frequency of the Maryland X-band BWO at very low magnetic field (0.8-4 kG). The measured dependence of the microwave output power on the strength of the guiding magnetic field is consistent with this model. The measured dip in the output power at around 2 kG can be explained as a cyclotron absorption by the forward wave. Design of a BWO operating at low magnetic field has definite advantages compared with a traditional BWO because it provides considerable reduction of volume, weight, and energy consumption of BWO.

Plasma density measurement in a gas-filled X-band backward wave oscillator with a double conversion heterodyne microwave interferometer

Plasma density was measured with a heterodyne microwave interferometer in both a gas-filled X-band backward wave oscillator (BWO) and in a smooth tube. Plasma is generated by impact ionization of a 650 kV, 2 kA electron beam. For fixed gas pressure we found that the plasma density rise in the operating BWO was much faster than in a smooth tube, indicating that Trivelpiece-Gould modes, or high power microwaves, increase plasma generation. Additional plasma enhanced BWO microwave output power. Measured plasma density at optimum power levels was n cr ≈ 6 × 10 12cm −3 at onset of emitted microwaves.

Electric field measurement in a plasma-filled X-band backward wave oscillator

The electric field in a plasma-filled X-band backward wave oscillator (BWO) was measured by the Stark-effect method. Field strengths were as high as 110 kV/cm when the BWO power level was ∼75 MW. Observed electric fields lasted throughout the ∼60 ns high power microwave pulse. After this pulse, fields 30 to 40 kV/cm persisted until beam shutoff, presumably arising from beam-plasma turbulence and low power level electromagnetic radiation.

Experimental study of a plasma-filled backward wave oscillator

Experimental studies of a plasma-filled X-band backward-wave oscillator (BWO) are presented. Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement by a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from <or=0.5 GHz to 2 GHz when the BWO was working at the RF power enhancement pressure region. The RF power enhancement appeared over a much wider pressure range in a high beam current case (10-100 mT for 3 kA) than in a lower beam case (80-115 mT for 1.6 kA). The plasma-filled BWO has higher power output than the vacuum BWO over a broader region of magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) are observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the T-G modes and the X-band Tm/sub 01/ mode prevailed when the background plasma density was below 6*10/sup 11/ cm/sup -3/. At a critical background plasma density of approximately=8*10/sup 11/ cm/sup -3/ power enhancement appeared in both X-band and the T-G modes. Power enhancement of the S-band in this mode collaboration region reached up to 8 dB. Electric fields measured by the Stark-effect method were as high as 34 kV/cm while the BWO power level was 80 MW. These electric fields lasted throughout the high-power microwave pulse.<<ETX>>

Observation of Trivelpiece-Gould modes in a plasma-filled backward wave oscillator.

Trivelpiece-Gould modes (TG modes) have been observed with frequencies up to the background plasma frequency in a plasma-filled X-band backward wave oscillator. The TG mode emission was broadband with observed frequencies ranging from 2.6 GHz, the lower limit of our frequency diagnostic, to ${\mathit{f}}_{\mathit{p}}$, the plasma frequency. Mode competition between the TG modes and the X-band ${\mathrm{TM}}_{01}$ mode prevailed when the background plasma density was below 6\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ (${\mathit{f}}_{\mathit{p}}$\ensuremath{\sim}7 GHz). At a critical background plasma density of ${\mathit{n}}_{\mathrm{cr}}$\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ (${\mathit{f}}_{\mathit{p}}$\ensuremath{\sim}8 GHz) a sudden power enhancement appeared simultaneously in both X band and the TG modes. Power enhancements in S and J bands were up to 8 and 3 dB, respectively, with the X band enhanced up to a factor of 6.

A single control electrostatic focus backward wave oscillator for X-band operation

This paper describes an X-band backward wave oscillator using the electrostatic focus principle applied to an interdigital line as described by Hergenrother and Bowers last year. The tube operates over the frequency range of 8.5 to 9.6 kmc with a minimum power output of 20 milliwatts and can be voltage tuned by altering a single electrode voltage. The tube is of metal and ceramic construction and has a waveguide output projecting from its side which may be used for supporting the tube. The use of electrostatic focus has permitted a weight reduction of 75% over a corresponding magnetic focus tube resulting in a net weight of 1\frac{1}{2} lbs. The design of the interdigital line will be described and performance data curves for the tube will be presented.

A new type of slow-wave structure for millimetre wavelengths

A new type of narrow-band slow-wave structure, consisting of two adjacent planar arrays of conductors, is described. An approximate dispersion equation based on a simplified model is derived, and the numerical results are shown to be in good agreement with experiment. The circuit has been used in an X-band backward-wave oscillator, covering a 10%frequency band. The geometrical arrangement of the circuits is favourable for assembly in a multiple structure.