B. Levush

MAGY: a time-dependent code for simulation of slow and fast microwave sources

We present the newly developed Maryland Gyrotron (MAGY) code for modeling of slow and fast microwave sources. The code includes a time-dependent description of the electromagnetic fields and a self-consistent analysis of the electrons. The calculations of the electromagnetic fields are based on the waveguide modal representation, which allows the solution of a relatively small number of coupled one-dimensional partial differential equations for the amplitudes of the modes, instead of the full solution of Maxwells equations. Moreover, the basic time scale for updating the electromagnetic fields is the cavity fill time and not the high frequency of the fields. The equations of motion of the electrons are formulated within the framework of the guiding-center approximation and solved with the electromagnetic fields as the driving forces. Therefore, at each time step, a set of trajectories are calculated and used as current sources for the fields. We present two examples for the operation of the code, namely the two-cavity gyroklystron and the backward-wave oscillator (BWO). These examples demonstrate the possible usage of the code for a wide variety of electron-beam systems.

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. >

Traveling-wave tube devices with nonlinear dielectric elements

The performance of traveling-wave tube (TWT) amplifiers incorporating nonlinear dielectric elements is studied via computer simulation. Two different situations are investigated: (1) the use of nonlinear dielectric elements to reduce intermodulation distortion and (2) the use of voltage-controlled dielectrics to provide rapid modification of the dispersion characteristics of the slow-wave structure. In the first case, the use of dielectrics with negative second-order susceptibilities is studied as a means of reducing phase and intermodulation distortion. Use of these dielectrics along with dynamic velocity taper to reduce amplitude modulation distortion results in marked reduction of predicted intermodulation distortion. In the second case, the goal is to design an amplifying structure whose gain as a function of frequency can be varied electrically. Preliminary design studies show that relatively large changes in the center frequency of the amplification band can be achieved with relatively modest changes in the dielectric constant of helix support structure.

Mode competition and control in higher-power gyrotron oscillators

One of the most important problems in the design of high-power millimeter-wave sources such as gyrotron oscillators is insuring that the device operates in the desired mode. For high-power and short-wavelength devices the effective mode density is high, in that the current is above threshold for many modes. One then is led to ask whether operation in a single mode is possible and what steps must be taken to maximize the electronic efficiency of the device while ensuring single-mode operation. The answer to the first question has been determined to be yes. Provided that certain conditions are met, single-mode operation is stable. The present results emphasize time-dependent multimode simulations showing how these stable states can be accessed. In particular, the accessibility to the stable single mode with maximum efficiency is studied. Regions of parameter space for which stable single-mode operation is possible are plotted for an annular beam for a closed-cavity gyrotron operating at a high-order whispering-gallery mode (TE/sub 80.4/). These results also apply to the quasioptical gyrotron with a pencil electron beam. >

Multifrequency theory of high power gyrotron oscillators

Abstract A multifrequency time dependent model for high power gyrotron oscillator operation is developed and presented. A computer code based on this model is then used to simulate recent experiments carried out at the Massachusetts Institute of Technology. The simulation code indicates that the probable cause for the degradation of efficiency observed in the experiment is mode competition.

Relativistic plasma microwave electronics : studies of high-power plasma-filled backward-wave oscillators

The area of relativistic plasma microwave electronics has only recently generated renewed interest. New experimental data are presented demonstrating that the presence of a low‐density background plasma in a relativistic backward‐wave oscillator leads to several beneficial effects, including (a) enhanced interaction efficiency (40%), (b) operation at very low and possibly zero guiding magnetic field, (c) tunability by controlling the plasma density, (d) high degree of spectral coherency, and (e) operation well above the vacuum limiting current.

A dielectric mixing law for porous ceramics based on fractal boundaries

The effect of porosity on the complex dielectric permittivity of microwave sintered zinc oxide at room temperature and 2.45 GHz is reported. The predictions of conventional Maxwell–Garnet theory and the effective medium approximation are in poor agreement with the experimental results. Various methods are employed to investigate the system in an effort to come up with new mixing laws, including combinations of these two analytic theories and finite difference electromagnetic simulations of representative microstructures. A model that assumes the existence of dielectrically inactive, fractal‐geometry boundaries between ceramic grains provides an excellent description of the results with no free parameters. It gives physical insight into the experimentally observed mixing law.

Spontaneous radiation of an electron beam in a free‐electron laser with a quadrupole wiggler

A calculation is presented of spontaneous radiation emitted by an electron beam passing through a continuously rotating quadrupole magnetic undulator. It is shown that radiation spectrum emitted in forward direction of beam propagation has four peaks, corresponding to four betatron frequencies. Utilizing the Madey theorem, a stimulated emission is calculated and presented as gain versus frequency curves, for different values of the quadrupole magnetic field. A free‐electron laser operating at two or three radiation frequencies with a quadrupole magnetic wiggler is suggested.

Mode competition and suppression in free electron laser oscillators

The stability of single‐mode operation of free electron laser (FEL) oscillators is investigated. Two models of an untapered FEL oscillator are considered. The first model is called the klystron model. In the klystron model the FEL interaction occurs at two points: a prebunching point and an energy extraction point. In this model the nonlinear electron dynamics are solvable exactly, leading to a complex delay equation for the wave fields. The stability of single‐mode operation can then be determined easily as a function of a single‐pass gain, energy mismatch–frequency, and the difference between the group velocity of the radiation and the beam velocity. The second, more realistic model has a distributed interaction region of finite length: Stability of single‐mode operation in this device must be determined numerically. The results of the models in the low gain regime are compared and the parameter regimes where stable single‐mode operation is possible are determined. It is found that the time to reach a single‐mode state can be extremely long.

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.