S.M. Miller

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.

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.

Cyclotron resonances in relativistic BWO's operating near cutoff

A numerical model is presented for analyzing backward-wave oscillator (BWO) operation in the presence of a constant finite axial magnetic field. The model is appropriate for devices that operate near the upper edge of the transmission band. An analytic calculation of starting currents is presented. The operating frequency and efficiency versus magnetic field predicted by the nonlinear numerical simulations are presented.

Ponderomotive effects in plasma-filled backward-wave oscillators

A numerical model is presented for analyzing plasma-filled backward-wave oscillators (BWOs) operating near cutoff. The model allows for the investigation of the effects of the ponderomotive potential of the high-frequency electromagnetic waves on the motion of plasma electrons. As a result of their motion the electron plasma density is modified, and this affects the high-frequency radiation by modifying the dispersion characteristics of the slow wave structure. Two approaches for modeling the plasma are considered, a fluid model and a particle-in-cell model. Nonlinear simulations are performed to investigate the possible excitation of plasma waves over a range of background plasma densities. Results from nonlinear simulations show that for low plasma densities, electrons clump in regions of low high-frequency electric field. At somewhat higher densities nonlinear instabilities of the Raman type are excited. The model does not indicate the cause of the observed efficiency enhancement in plasma filled backward wave oscillators.

Experimental studies of overmoded high power microwave generators

Summary form only given. Experiments with overmoded (Dspl lambda/ /spl sim/3) high-power microwave generators have been performed. Two classes of devices were investigated: (1) single-stage backward wave oscillators (BWOs) with varying electrodynamic structure lengths; and (2) multiwave Cerenkov generators (MWCGs), consisting of two stages of seven sinusoidal periods each, connected by a variable length drift section. Both types of devices were driven by large-diameter, magnetized, annular, intense relativistic electron beams (250-900 keV, 2.5-9 kA). High-power microwave radiation in the 5.5 to 6 GHz frequency range was generated with both types of structures. A maximum radiated power of 320 MW was obtained with an overmoded BWO structure, and a maximum power of 102 MW was obtained with an MWCG. Start oscillation currents in the range of 204 kA were observed for both classes of devices.

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.

Cyclotron effects in relativistic backward wave oscillators

Summary form only given, as follows. The operation of high power Cherenkov devices such as backward wave oscillators is based on the interaction of high current, relativistic electron beams with synchronous electromagnetic fields of periodically corrugated waveguides. The high current electron beam is guided by a strong axial magnetic field. Therefore, in addition to the Cherenkov synchronism, the cyclotron resonance conditions can be realized for different spatial harmonics of both forward and backward waves. The cyclotron interaction manifests itself in the output power dependence on the axial focusing magnetic field. We developed linear and nonlinear theories that predict this dependence and the results from the numerical modeling compared favourably with the existing experimental data. We also found that the combined Cherenkov and cyclotron interaction may result in a higher efficiency conversion of the electron beam energy into radiation.

Theory and modeling of high power backward wave oscillators

Summary form only given, as follows. Microwave sources driven by high current relativistic electron beams are capable of producing high power coherent radiation in the centimeter and millimeter wavelength regions. In particular, Backward Wave Oscillators (BWOs) are very promising sources of high power microwaves. Over recent years we have employed a general model to analyze these devices. This model describes the interaction of the radiation and beam electrons self consistently in time and space. In addition, the model has been extended to include the effects of non-synchronous spatial harmonics and the effects of the AC space charge field. The theory is used to describe the operation of BWOs in a number of regimes: with current close to and much above the starting current, with beam voltage corresponding to operation far and close to the /spl pi/ point, with a guided magnetic field far and close to the cyclotron resonance value. The results of the modeling compared both with PIC simulations using MAGIC and with the data obtained from BWO experiments.