David K. Abe

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. >

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.

Novel method for determining the electromagnetic dispersion relation of periodic slow wave structures

A novel method for calculating the dispersion relation of electromagnetic modes in an arbitrary periodic slow wave structure is reported. In this method it is sufficient to know the frequencies corresponding to three special wave number values, with other points calculated using an approximate analytical expression. This technique was successfully applied to determine the dispersion relation of the TM01 mode in a sinusoidally corrugated slow wave structure. This structure is commonly used in relativistic high‐power backward wave oscillators and traveling‐wave tubes, and is expected to have many additional applications.

Design Study of a Permanent-Magnet Quadrupole Focusing Lattice for a mm-wave Traveling Wave Tube

Linear beam tubes have traditionally relied on axisymmetric solenoidal or PPM focusing to transversely confine the electron beam. In the particle accelerator community, however, it is well-known that alternating-gradient focusing using sequences of quadrupole magnets, known as strong focusing, is capable of transporting more current for comparable field strengths (Reiser, 1994). At mm-wavelengths, a key limitation to the peak and average power of vacuum electronics devices stems from the difficulty of control and confinement of high-current density beams. In this paper we summarize an ongoing design study of a strong-focusing lattice employing permanent magnet quadrupoles (PMQs) capable of transporting a sub-mm radius electron beam for vacuum electronics applications. The study has the aim of producing a practical design for an experimental study at NRL for comparing quadrupole lattices with standard ones based on solenoidal periodic permanent magnets (PPMs). In addition, a design code and methodology for designing these PMQ channels is under development

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>>

Theoretical and experimental investigations of the cyclotron interaction effect on relativistic backward-wave oscillator operation

Microwave sources based on backward-wave oscillators (BWOs) driven by relativistic electron beams are capable of producing high power coherent radiation in the cm and mm wavelength region. When the axial magnetic field is used in these devices to confine the electron beam satisfies the condition of cyclotron resonance there is a significant modification in the behavior of BWO due to beam coupling to cyclotron modes. A time-dependent, self-consistent theory of BWOs is developed taking into account a possible cyclotron interaction. The analysis of the system near the cyclotron resonance yields a number of physical effects including the power drop due to the cyclotron absorption observed in many BWO experiments. A series of experiments has been conducted to compare the measured BWO characteristics with the theoretically predicted ones.

Studies of Relativistic Backward Wave Oscillators: Comparison between Theory and Experiment

Microwave sources based on backward-wave oscillators driven by relativistic electron beams are capable of producing high power coherent radiation in the 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 filling 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.

High Power Microwave Generation From Plasma-Filled Backward-Wave Oscillators

Experimental and theoretical studies of high power microwave generation from three different plasma-filled backward-wave oscillator (BWO) configurations are reported. In the first, a neutral fill gas inside the BWO slow wave structure is ionized by the electron beam and a resonant enhancement of microwave output is observed at optimum values of fill gas pressure and applied axial magnetic field. In the second, a low energy electron beam is used to preionize the fill gas in advance of electron beam injection. In the third configuration, plasma is injected into the slow wave structure in advance of beam injection using a Marshall plasma gun. A fivefold increase in microwave output power and efficiency is observed in this case. Possible theoretical explanations for the observed interaction enhancement are also discussed.