Search

The complex dielectric permittivity of electrically lossy AlN-SiC composites has been measured over the 26.5-40 GHz frequency range (Ka band), at temperatures from -180 to 200 °C. The dielectric permittivity is found to be strongly sensitive to temperature. Additional broadband, room-temperature dielectric measurements indicate that the Ka-band response is situated on the high-frequency tail of a broad Maxwell-Wagner relaxation. Analysis of the temperature-dependent losses at 30 GHz in a composite containing 20% SiC by weight, using an effective cluster model, indicates the existence of two conduction processes within the SiC, with activation energies of 0.0064 and 0.095 eV. The low-energy process is consistent with a hopping conductivity and the higher-energy process is attributed to a nitrogen donor ionization. [ABSTRACT FROM AUTHOR]

Presents an analysis of the radiation from betatron oscillations for the stellatron, a strong-focused betatron, to diagnose the energy of an electron beam propagating in a focusing channel. Particle orbits in the stellatron; Standard expression for radiated intensity for prescribed particle motion; Effect of geometric line broadening, an inhomogeneous broadening mechanism due to orbit curvature.

Details a calculation on spontaneous radiation emitted by an electron beam passing through a continuously rotating quadrupole magnetic undulator. Information on the use of free-electron laser; Equations of motion; Characteristics of the radiation spectrum.

Provides information on the components of simple-particle trajectories of relativistic electrons in a magnetic field configuration. Analysis of gain mechanism in free-electron laser devices; Particle orbit equations; Stability analyses.

Presents a study which reduced the free-electron-laser (FEL) equations to a set of one-dimensional, normalized equations that allow a universal dimensional analysis. Performance characteristics of specific high-gain FEL; Estimates for the efficiency enhancement achieved with a tapered section; Design of high-gain millimeter-wave FEL amplifier using a superconducting short-period wiggler and sheet electron beam.

Presents procedures and quantitative analyses for obtaining and evaluating experimental short-period-wiggler free-electron lasers (FEL) oscillators that have been optimized with respect to efficiency. Applications for candidate sources of high-power, tunable, coherent radiation; Equilibrium operating point for the FEL oscillator; Effects of beam axial energy spread on the FEL interaction efficiency.

Analyzes the effects of anomalous permittivity on the microwave heating of zinc oxide. Experimental procedure used; Results of the study.

Presents a study that investigated the effect of porosity on the complex dielectric permittivity of microwave sintered zinc oxide. Description of microwave sintering of ceramics as a complex process; Sample preparation and dielectric properties measurement; Analytical dielectric mixing theories and result of the study.

Discusses a study which considered processes leading to atmospheric cleansing of hazardous species responsible for the destruction of ozone layer. Analysis of requirements and restrictions on operating parameters of the purification process; Investigation on the removal of halocarbons from air by using microwave sources; Factors that causes the destruction of halocarbons.

We introduce a new, alternative form of the 3-D alternating direction implicit finite-difference time-domain (ADI-FDTD) algorithm that has a number of attractive properties for electromagnetic simulation. We obtain a leapfrog form of the time-advance equations, where the E and H fields are staggered at half-integer and integer time steps, respectively, that preserves the unconditional stability of the ADI-FDTD method. The resulting equations resemble the explicit leapfrog-FDTD method, but the field update equations are modified to include the solution of sets of tri-diagonal equations at each step, similar to the original ADI-FDTD scheme, so that the scheme is not constrained by the Courant–Friedrichs–Lewy limit. The algorithm is simpler than the ADI-FDTD method but algebraically equivalent, allowing a reduction in computation to achieve the same numerical solution. We discuss the advantages of the formulation over the original FDTD and ADI-FDTD methods, and confirm our results numerically. Published in 2008 by John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Accurate evaluation of the ac space-charge electric field is required in order to obtain accurate predictions for linear-beam-tube performance using steady-state simulation codes. The space-charge field is commonly calculated under the assumption that the beam current density and space-charge fields are all periodic in time and locally periodic in axial distance. In this brief, it is shown by example that there are cases in which it is important, in both the small and large signal regimes, to include departures from this "local spatial periodicity" assumption. We find in the cases that we have studied that it is sufficient to keep a single additional term, proportional to the spatial derivative of the bunched beam current density, in the expression for the space-charge electric field. The resulting expression is much faster to evaluate numerically than a full convolution integral over all axial wavelengths. The improved representation is suitable for use in both small- and large-signal linear-beam simulation codes. It has been implemented and tested in the CHRISTINE large-signal TWT simulation code. [ABSTRACT FROM AUTHOR]

Presents a study which described a general procedure to efficiently stimulate the spatial distribution of electron impacts in misaligned gyro-amplifier beam collectors, using adiabatic theory and analytic geometry. Information on the simulation procedures and the aligned collector example; Description of the angular misalignments; Discussion and conclusion.

This article explores the history and diversity of this remarkable technology, with emphasis on advances in vacuum-electronic amplifiers, including the microwave power module (MPM), that have been enabled by the ongoing development of modeling and simulation tools. These physics-based codes, enabled by rapid advances in computational hardware, allow simulation-based design and optimization of complex vacuum-electronic circuits. The growth of satellite-based digital communications technology over the past decade has opened lucrative commercial opportunities for vacuum-electronic amplifiers. This exciting new area requires the efficient production of high-frequency power and the ability to handle spectrally efficient modulations within stringent packaging constraints at affordable cost [ABSTRACT FROM PUBLISHER]

In gyro-devices, a nonlinear output taper is often employed as the transition from the near cutoff radius of the interaction circuit to a much larger output waveguide. The tapers are usually designed to avoid passive mode conversion, and thus do not consider the effect of a bunched beam. However, recent simulations with the self-consistent MAGY code [Botton et al., IEEE Trans. Plasma Sci. 26, 882 (1998)] indicate that higher order mode interactions with the bunched electron beam can substantially compromise the mode purity of the rf output. The interaction in the taper region is traveling wave in nature, and is strongly dependent on the residual beam bunching characteristics resulting from the upstream operating mode interaction. An experiment has been performed to quantify the rf output mode content from a Ka-band gyroklystron. The agreement between salient theoretical and measured rf output characteristics confirms the existence of higher order mode excitation in output tapers as predicted by theory. [ABSTRACT FROM AUTHOR]

Since there is always some ambient gas in electron beam devices, background ionization is ubiquitous. For long pulse times, the electrostatic potentials associated with this ionization can reach significant levels and give rise to such observed phenomena as phase noise in microwave tubes. This noise is usually associated with the motion of ions in the device; therefore, it is called ion noise. It often manifests itself as a slow phase fluctuation on the output signal. Observations of noise in microwave tubes such as coupled-cavity traveling wave tubes (CC-TWTs) and klystrons have been discussed in the literature. In this paper, a hybrid model is discussed in which the electron beam is described by the beam envelope equation, and the ions generated by beam ionization are treated as discrete particles using the one-dimensional equations of motion. The theoretical model provides good qualitative as well as reasonable quantitative insight into the origin of ion noise phenomena. The numerical results indicate that the model reproduces the salient features of the phase oscillations observed experimentally. That is, the scaling of the frequency of the phase oscillations with gas pressure in the device and the sensitive dependence of the phase oscillations on the focusing magnetic field. Two distinct time scales are observed in simulation. The fastest time scale oscillation is related to the bounce motion of ions in the axial potential wells formed by the scalloping of the electron beam. Slower sawtooth oscillations are observed to correlate with the well-to-well interactions induced by the ion coupling to the electron equilibrium. These oscillations are also correlated with ion dumping to the cathode or collector. As a practical matter, simulations indicate that the low frequency oscillations can be reduced significantly by using a well-matched electron beam propagating from the electron gun into the interaction circuit. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents direct measurements of electronic noise in gyroklystrons. Basis of carrier-free noise temperature measurements; Calculations of growth rates expected for electrostatic cyclotron instabilities; Suppression of noise growth by perpendicular velocity.

Highlights the results of a series of high power, high efficiency K[suba]-band and W-band gyroklystron experiments. Stagger tuning of the cavities for bandwidth enhancement commonly used in the conventional multicavity klystrons; Desired stagger tuning; Effects of the dielectric insert.

Reports on the experimental demonstration of a high average power W-band gyroklystron amplifier. Achievement of larger bandwidths at the expense of peak and average output power at higher magnetic fields and lower beam voltages; Gyrotron amplifier's advantage over conventional millimeter wave sources; Absence of heating and breakdown problems associated with small interaction circuits.

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. [ABSTRACT FROM AUTHOR]

The spectral characteristics of a continuous beam free electron laser are studied both numerically and analytically. The average spectral width and the probability distribution of mode amplitudes determined from nonlinear numerical simulations are found to agree with a simple theory based on the linear growth of each mode in the presence of a white noise source. Included in the numerical simulation model is the time dependence of the beam energy and its effect on the spontaneous emission rate. Based on this model predictions of the frequency of observing a single dominant mode in the recent University of California at Santa Barbara experiments [FEL ’89, 11th International Conference on Free Electron Lasers, Conference Digest (IEEE), New York, 1989), Paper p. 2.19] are made. The predicted frequency is somewhat lower than observed. However, other features of the simulations do agree with experimental results. [ABSTRACT FROM AUTHOR]

The regions in parameter space of stable single mode operation of a quasioptical gyrotron are determined theoretically for a number of configurations. A pencil beam is found to give rise to large operating regimes where single mode operation is stable. These regions shrink dramatically when an annular beam is considered. However, stable operation can be restored by slightly tilting the axis of the cavity. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Using a normalized set of nonlinear equations, which describe a free-electron laser (FEL) oscillator, the efficiency of energy extraction from the electron beam to the radiation can be optimized. The optimum values of this efficiency are presented for (a) an untapered and (b) a tapered wiggler FEL oscillator. [ABSTRACT FROM AUTHOR]

A new rotating quadrupole wiggler configuration for free-electron lasers is proposed. The stability analyses of the particle trajectories in a continuously rotating quadrupole magnetic field and self-fields of a solid beam are performed. The resulting orbit equations are solved exactly and it is shown that high-current beams can be confined. To analyze the potential of this wiggler for free-electron lasers, a set of nonlinear orbit equations is derived which averages over the fast time variation in both the wiggler and radiation fields. By integrating these equations, the linearized single particle gain of the free-electron laser is calculated. It is shown that, at comparable wiggler strengths, the rotating quadrupole wiggler and conventional wiggler give similar gains. [ABSTRACT FROM AUTHOR]

The effect of thermal spread in pitch angle on the limiting current of an electron beam is calculated. The downstream beam parameters are also evaluated. At the limiting current the downstream value of the pitch angle and the downstream thermal spread are found to be sensitive to the initial thermal spread. [ABSTRACT FROM AUTHOR]

Focuses on the design of high power, continuous wave gyrotron cavities. Anticipated degradation of efficiency with beam velocity spread; Sensitivity of cavities to velocity spread, their mode competition and their maximum Ohmic power dissipation.

Reports on the weak coupling of electrons in low voltage gyrodevices operating at high cyclotron harmonics, to the synchronous harmonic of an electromagnetic field. Increase in the resonator Q-factor to compensate for the weak coupling; Modification of the optimization of gyrotron parameters.