Measurements of the spread in the initial electron energy in a gyrotron

An analyzer intended for measuring electron energy in gyrotrons is described. Electron energy spectra are measured in various operating modes of an experimental gyrotron. It is shown that, even when no microwave field is generated, the spread in electron energy due to the space-charge effect can be as high as several percent.     

Effect of electron velocity spread on gyro-peniotron efficiency

The paper describes the influence of the velocity spread of the electrons on the interaction efficiency and on other operational characteristics in the gyro-peniotron oscillator. Numerical simulation shows a drastic efficiency reduction from more than 45% to less than 30% with an electron velocity spread of 10% for a 35GHz, TE₀₃ mode gyro-peniotron operating at the third harmonic. The operation ranges of the device parameters at a defined efficiency level are also decreased when the velocity spread of the electrons increases.     

Eccentricity of the electron beam in a gyrotron cavity

Dependence of the coupling of the electron beam and RF field in a gyrotron cavity on eccentricity of the electron beam is studied.     

Influence of the axial misalignment of the electron beam and the cavity on the gyrotron parameters

We consider some effects connected with the axial misalignment of the electron beam and the cavity in gyrotrons. These effects are studied by the example of three gyrotrons with different parameters of the electron beam and output characteristics. The experimental data and the results of computer calculations are compared on the basis of a multi-mode nonstationary model of the electron-wave interaction in the gyrotron cavity.     

Development of the method for determining the electron velocity in a gyrotron

We have analyzed the possibility of determining the electron velocity using the data on propagation of a microwave low-power signal in a slow-wave system located in the region of electron beam formation. The diagnostic section is designed for implementing this method in an experimental gyrotron with a working frequency of 74.2 GHz and an output power of 100 kW. Numerical simulation shows that information on the longitudinal velocity of electrons can be obtained from an analysis of the frequency dependence of the amplification factor for a signal propagating in the slow-wave system at frequencies in the gigahertz range.     

The energy spectrum of an electron beam after interaction with an RF field in a gyrotron

The electron distribution over residual energy after electron interaction with an rf field was determined using the method of decelerating electric field. It is demonstrated that, under the maximum efficiency conditions, electrons with an energy lower than 30% of their initial energy are absent in the spectrum. Hence, the efficiency of energy-recuperation gyrotrons can substantially be increased.     

Observation of the anomalous increase of the longitudinal energy spread in a space-charge-dominated electron beam

We report a new experimental study of the growth of longitudinal energy spread in a space-charge-dominated electron beam, with a beam energy of several keV and beam current of approximately 100 mA. At relatively low beam densities, we measure growing energy spreads with distance along the transport channel, which are in remarkably good agreement with the theory of energy relaxation via Coulomb collisions. At higher beam densities, however, anomalous energy spreads exceeding the predictions of the relaxation theory are observed, which, we believe, could be caused by collective longitudinal-transverse instabilities observed in computer simulation studies. The onset of these instabilities occurs after several plasma periods according to calculations.     

Experimental studies of gyrotron electron beam systems

In order to provide electron beams of powerful gyrotrons, magnetron-injection guns operating in the regime of the temperature limited current are used. The electron beam quality and gyrotron performances are defined both by the cathode emission processes and the processes occurring in the electron beam during its formation and transportation. The results of measurements of the energy spectrum and velocity spread of the gyrotron electron beam in different regimes are given. Experimental data on the parameter of efficient emission inhomogeneity for different regimes are presented, as well as the dependencies of electron beam parameters on efficient inhomogeneity of the cathode     

Space charge effects in a gyrotron employing a solid electron beam

The influence of space charge forces on the performance of a single cacity gyrotron oscillator which uses a solid electron beam was investigated. It was found that space charge effects cause a large efficiency degradation as the beam current is increased, if the other experimental parameters are unchanged. A small increase in the magnetic field, however, can restore the efficiency to higher values.This research was supported by the U.S. Office of Naval Research.     

Diocotron instability of the electron beam in the drift tube of a gyrotron

A cold fluid model is used to investigate instabilities associated with a velocity shear due to the self fields of a relativistic electron beam inside the beam tunnel of a gyrotron. General statements concerning the stability of an electron beam like the frequency and the growth rate are possible. The growth is expressed as a length that can be compared with the geometry in the gun-tube-resonator system.     

Angular velocity spread of relativistic electron beam generated by high current diode

Theoretical analysis and numerical simulations of the Relativistic Electron Beam (REB) generation in a high current diode immersed in an external magnetic field has been done. The calculations have confirmed that the generated beam is homogeneous and monoenergetic in a broad central region. In the case of cylindrical diode the mixing of electron trajectories has been observed only in a narrow periphery beam region. The angle between particle trajectories and external longitudinal magnetic field varies chaotically from 0° to –25°. This phenomenon suppresses the excitation of two stream instability excited by the REB in the plasma column.     

Studying the start-up scenario for a pulsed gyrotron with a relativistic electron beam

We present the results of studying the start-up scenario for a high-power pulsed gyrotron operated at the TE₅₃ mode for two variants of the current–voltage characteristic of a three-electrode magnetron-injection electron gun. In the standard variant of feeding the voltage to the first anode from the resistive divider, sequential generation of the TE₇₃ and TE₆₃ modes, and, finally, the operating TE₅₃ mode was observed at the edge of the pulse. The current–voltage characteristic for which only the operating mode is excited is obtained by decreasing the growth rate of the voltage applied to the first anode.     

The nonlinear theory of slow-wave electron cyclotron masers with inclusion of the beam velocity spread

The nonlinear theory of slow-wave electron cyclotron masers (ECM) with an initially straight electron beam is developed. The evolution equation of the nonlinear beam electron energy is derived. The numerical studies of the slow-wave ECM efficiency with inclusion of Gaussian beam velocity spread are presented. It is shown that the velocity spread reduces the interaction efficiency.     

Improvement of the helical electron beam quality and the gyrotron efficiency by controlling the electric field distribution near a magnetron injection gun

A technique for controlling the electric field distribution near the cathode of a magnetron injection gun is developed. The feasibility of improving the quality of a helical electron beam by optimizing the electric field distribution in a pulsed 4-mm-wave gyrotron is studied theoretically and experimentally. Field distributions are obtained that minimize the electron velocity spread in the beam, coefficient of electron reflection from a magnetic mirror, and intensity of parasitic low-frequency oscillations. It is demonstrated that the gyrotron efficiency can be increased through a rise in the beam quality at the optimized electric field distribution.     

Mode competition of the gyrotron under high efficiency operating conditions

Conclusion Mode competition — interaction between a mode and another mode — is due to the non-linearity of the relativisitc factor . At the beginning of a multimode oscillating, the behavior of the multimode oscillating is only dependent on the ratio of mode frequency and relativistic cyclotron frequency and is in agreement with that described by single mode theory. In non-linear domain mode interaction depends on both amplitudes of A₁ and A₂ and phase difference _i of the two modes. The main effect of mode interaction is that excitation of the parasitic mode suppresses growth of the operating mode.Under the operating conditions with high efficiencies, a mode interaction between the dominant mode and the lower lateral mode is almost unavoidable, especially in the gyrotron operating with a cavity at higher order modes to increase the capability of output power.A research fellow of the Alexander von Humboldt Foundation coming from Beijing Vacuum Electron Devices Research Institute, Beijing, China     

Influence of the beam-forming conditions on the development of space-charge oscillations in a long-pulse relativistic electron beam

The characteristics of the space-charge oscillations of a long-pulse relativistic electron beam in magnetically insulated diodes are determined for different geometries of the electron acceleration section and for explosive-emission cathodes of different materials. The important role of the stream of electrons having high transverse velocities in the evolution of the oscillations is demonstrated, and the laws governing the generation of this stream are determined. Possible mechanisms of the space-charge oscillations are described, taking into account the interaction of the electron stream in the beam halo with the main electron stream, the development of diocotron instability in the stream of electrons emitted by the outer lateral surface of the plasma emitter, and the instability of the space charge of “long-lived” electrons in the beam transport channel. Zh. Tekh. Fiz. 68, 102–106 (April 1998)     

Momentum spread in a relativistic electron beam in an undulator

The motion of the relativistic electron beam in the spatially periodic magnetic field of an undulator has been considered taking into account the effect of the incoherent field of the spontaneous undulator radiation on the motion of the electrons. An expression for the rms momentum of the electrons has been obtained. It has been shown that the momentum spread in the ultrarelativistic electron beam increases in the spontaneous incoherent emission mode. Conditions for the self-amplification of the spontaneous undulator radiation in ultrashort-wavelength free-electron lasers have been discussed.     

Results from gyrotron backward wave oscillator experimentsutilizing a high-current high-voltage annular electron beam

We report the first gyrotron-backward-wave oscillator experiments which utilize a high-current (1-4 kA) high-voltage (300-500 keV) annular electron beam. The experiment was designed to operate in the TE₀₁ backward wave mode. Radio frequency extracted power was 0.1-4 MW, with pulselengths of 80-500 ns. Experimental results suggest the possibility of competition with the TE₂₁ backward wave mode. Frequency spectrum measurements have shown a wide content of frequencies during the voltage pulse, an undesirable result for use in narrow-band devices