A. Bromborsky

Theory of relativistic backward-wave oscillators with end reflectors

Microwave sources based on backward-wave oscillators (BWOs) with relativistic electron beams are capable of producing high-power coherent radiation in the centimeter- and millimeter-wavelength regimes. Although there have been a number of experiments reported over the last decade on this topic, there are only a few publications providing a theoretical description of these devices. Thus, there is a need for theoretical models which can be compared in detail with the experimental data. This work is devoted to filling this need. The linear and nonlinear theory if BWOs is developed taking into account reflection of the electromagnetic wave at the boundaries of the slow-wave structure. It is found that owing to end reflections the start oscillation current and the efficiency are sensitive functions of the operating parameters. Regions of stable single-frequency operation in these devices are determined numerically. The effects of finite duration and rise time of the electron beam pulse on device operation are discussed. >

A High-Power Backward-Wave Oscillator Driven by a Relativistic Electron Beam

A high-power backward-wave oscillator (BWO) has been constructed that is driven by a relativistic electron beam (REB). A typical electron beam of 2-4 kA is accelerated across a diode potential of 650-800 kV and then guided through a section of corrugated transmission line by an axial magnetic field of 5-15 kG. Peak microwave powers of 100-200 MW have been observed in the circular TM01 mode at the predicted frequency of 8.4 GHz. The basic principles and parameters of operation will be discussed, along with the dependence of peak microwave power on applied axial magnetic field.

A novel highly accurate synthetic technique for determination of the dispersive characteristics in periodic slow wave circuits

A highly accurate (0.1-0.5%) synthetic technique for determining the complete dispersive characteristics of electromagnetic modes in a spatially periodic structure is presented. It was successfully applied for the cases of the fundamental (TM/sub 0(1)/) as well as higher-order (TM/sub 0(2)/, TM/sub 0(3)/) passband modes in a corrugated waveguide. This structure is commonly used in relativistic backward wave oscillators, traveling wave tubes, extended interaction oscillators, and a variety of multiwave Cerenkov generators. An appropriately shorted periodic structure resonates at specific frequencies. To measure these frequencies accurately and unambiguously, the authors used unique antenna radiators to excite pure modes in the circuit under test. An analytical technique for deriving the complete dispersion relation using the experimentally measured resonances is presented. This technique, which is based on the intrinsic characteristics of spatially periodic structures, is applicable to slow wave structures of arbitrary geometry. >

Experimental studies of overmoded relativistic backward-wave oscillators

Internal field-emission breakdown in the electrodynamic structures of high-power microwave (HPM) devices can seriously limit the devices output power and pulse duration. Increasing the diameter of the electrodynamic structure to several times an electromagnetic wavelength can reduce these internal fields to below critical breakdown levels, but may introduce mode competition as an unwanted side effect. This paper presents the design and results of experiments with overmoded (D//spl lambda//spl sim/3), sinusoidally corrugated backward-wave oscillators (BWOs) that successfully produced TM/sub 01//sup /spl odot//, high-power microwave radiation in the frequency range of 5.2-5.7 GHz. Overmoded BWOs reproducibly generated /spl sim/200 MW of peak power with corresponding efficiencies of /spl sim/4%. Pulse shortening was not observed in any of the experiments. The radiation generated by the devices was highly coherent (typically, /spl Delta/f/f<0.5%) and corresponded to a fundamental TM/sub 01//sup /spl odot//-mode interaction. The experimental results were compared with calculations made with recently developed nonlinear models; the measured data are shown to agree favorably with theory. The results of the experiments and modeling demonstrate that overmoded electrodynamic structures can be used to decrease internal electric field stresses while avoiding multimode generation and maintaining good spectral purity.

Relativistic backward‐wave oscillators: Theory and experiment

Microwave sources based on backward‐wave oscillators driven by relativistic electron beams are capable of producing high‐power coherent radiation in cm and mm wavelength regime. Although there have been a number of experiments reported over the last decade on this topic, there are only a few publications providing a theoretical description of these devices. Thus, there is a need for theoretical models which can be compared in detail with the experimental data. This work is devoted to fill this need and applied to the University of Maryland backward‐wave oscillator experiment. It is shown that the theoretical predictions for the threshold current to start the oscillations, the frequency characteristics, and the efficiency of the device compared favorably with the experimental data.

Relativistic backward‐wave oscillators operating near cyclotron resonance

A linear and nonlinear, time dependent self-consistent theory for relativistic backward wave oscillator is developed. In this theory the transverse motion of the electrons is taken into account. Analytical and numerical analysis of the model equations near the cyclotron resonance gives a good estimate for the power drop due to the cyclotron absorption observed in many experiments and predicts an increase in the power under some conditions.

Theory of relativistic backward wave oscillators operating near cutoff

Backward-Wave Oscillators utilize a high-current electron beam to produce high-power, coherent radiation in the centimeter and millimeter wavelength regime. Under certain voltage and beam current operating conditions, a Backward-Wave Oscillator (BWO) can operate near the upper edge of the transmission band where the group velocity of the electromagnetic wave goes to zero. In this regime, the cold structure dispersion relation can be approximated as a quadratic function of the wavenumber. A theoretical model similar to those presented in [1-3] has been developed to describe the operation of the device in this regime. We solve a self-consistent set of equations to describe the slow evolution of the envelope of the radiation field and the relativistic motion of the particles along a strong magnetic field. Included in the theoretical model are the effects of D.C. and A.C. space charge, and velocity spread in the beam. Numerical calculations of the starting current are performed and compared with an analytic expression for the starting current derived by assuming a fixed field profile.

High-efficiency relativistic backward wave oscillator: theory and design

Backward wave oscillators (BWOs) driven by high-current relativistic electron beams are capable of producing high-power coherent radiation in the centimeter and millimeter wavelength regions. However, the efficiency of these devices is usually limited to 15-20% when a homogeneous slow-wave structure is used. Utilizing a two-section slow-wave structure, where the spatial period of the second section is larger than that of the first section, a BWO efficiency of greater than 50% was calculated. A conceptual design of a high-efficiency S-band BWO driven by a 500-kV 5-kA electron beam has been developed and analyzed.

Relativistic backward wave oscillators: Theory and experiment

The linear and nonlinear theory of backward-wave oscillators (BWOs) is developed taking into account reflection of the electromagnetic wave at the boundaries of the slow wave structure. The effects of finite duration and rise time of the electron beam pulse on device operation are discussed. A series of low-current experiments attempting to measure the start current has been conducted. The main challenge in the experiments was to achieve BWO operation over a wide range of electron beam energy and current. Since for a particular gun geometry the variation in the beam current is limited, the authors built a number of electron guns which made it possible to cover a broad range of beam parameters.<<ETX>>

Overmoded backward-wave oscillator: a comparison of experimental results with non-linear analysis

Summary form only given, as follows. Overmoded structures have the potential for increasing the power handling capability of linear beam, high power microwave (HPM) devices, reducing internal rf electric fields to below field-emission breakdown levels. However, potential detrimental effects include multi-mode generation and a reduction in efficiency and power. To study the feasibility of using overmoded structures in HPM generation, we conducted a series of experiments with an overmoded (D//spl lambda/-3), relativistic backward-wave oscillator (BWO) in the 5.4 to 5.6 GHz frequency range. The experiment, which was designed to operate near the edge of the passband of the TM/sub 01/ mode, produced highly coherent radiation (/spl Delta/f/f/spl les/0.5%) with a maximum power of 320 MW. Power, efficiency, and frequency were measured as a function of both electron beam parameters and electrodynamic structure length. In parallel, we developed theoretical models of BWOs operating near cutoff, which included end-reflections and finite magnetic field effects. For most of the experiment, the measured frequencies were within 5% of the model, which correctly tracked the trend of efficiency with structure length. We present a comparison of the theoretical and experimental results and an interpretation of the physical processes.