Electron Beam Misalignment Research Articles

Effect of off-axis electron beam on the performance of a gyroklystron amplifier

A crucial part of a gyroklystron is an electron beam. For the efficient operation of the gyroklystron, effective interaction between the radio frequency wave and the electron beam is necessary. Although the manufacturing and assembly of the gyroklystron are complicated, during the assembly process of various components of the gyroklystron, there is some possibility that the components may be misaligned from their actual positions. The operation of a gyroklystron is very sensitive if its components are misaligned. Electron beam misalignment is one type of such misalignment. This paper studies the effect of an off-axis electron beam on gyroklystron performance. Due to misalignment, the electron beam radius will vary in its position as the electron beam gyrates. First, the nonlinear single-mode analysis describing the beam–wave interaction process within the gyroklystron amplifier is discussed. As reported in the literature, an experimental, two-cavity 35 GHz fundamental harmonic gyroklystron amplifier has been used to conduct the present study. The results obtained from the analytically developed approach are benchmarked by the results of the experimental device. Subsequently, the analysis developed has been extended to investigate the impact of the misaligned electron beam on the gyroklystron performance characteristics. The study shows that efficiency, output power and gain decrease with misalignment, whereas bandwidth does not change due to such misalignments. These findings demonstrate the critical role that the misaligned electron beam plays in the operation of a gyroklystron amplifier.

Linear and Saturated Behaviors of Gyrotron Oscillator With a Misaligned Electron Beam

Linear and Saturated Behaviors of Gyrotron Oscillator With a Misaligned Electron Beam

Mode Distortion and Performance Degradation Caused by Electron-Beam Misalignment in W-Band Gyro-TWTs

Mode distortion and performance degradation caused by an OFF-axis gyrating electron beam in our W-band gyrotron traveling wave tube (gyro-TWT) were studied by the particle-in-cell (PIC) simulation and verified with the hot test experiments. The simulated electron beam misalignment may lead to a mixed pattern, mainly composed of TE01, TE02, TE21, and TE22 modes. They will further deteriorate the performance by disturbing the bunched electron beam. When the misalignment is over 0.1 mm, the operating mode shows a significant distortion, and the output power is reduced to 66% of that without misalignment at 93 GHz. For the prototype gyro-TWT with an estimated beam misalignment of about 0.1 mm, the measured saturated output power drops by more than 57% below 93.5 GHz compared with the normal one, whose estimated misalignment is less than 0.05 mm. The downward trend proves that the beam misalignment is the main factor causing the performance degradation at the W-band and above.

Particle-in-Cell Algorithm-Based Computation of Time-Dependent-Multimode Behavior of 42-GHz Gyrotron

The analysis of the multimode behavior of the interaction cavity for 42-GHz gyrotron is presented here. The in-house developed codes single-mode cold cavity solver (SMCCS) and GYCAD and commercial particle-in-cell (PIC) simulator MAGIC are used in the design and analysis of 42-GHz gyrotron cavity. SMCCS is used for the calculation of cold cavity parameters such as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> factor, resonant frequency, and the axial electric field profile, which are further used in the beam–wave interaction computation. The PIC code is used here for the parametric optimization and time-dependent-multimode beam–wave interaction simulations of the interaction cavity. The mode purity in terms of the electromagnetic power in individual modes is also calculated using the data generated in the beam–wave interaction simulations. The single-mode analysis of the cavity is also performed using the analytical code GYCAD based on the generalized nonlinear theory. The results obtained by the PIC simulator and GYCAD are discussed and compared here. The analysis of the electron beam misalignment, which may occur due to the radial shifting or tilting of the different components during the fabrication of the device, is also performed and the effects on the efficiency, mode competition, and mode stability are discussed in detail.

Investigations on the Effect of the Electron Beam Misalignments on a Continuously Frequency-Tunable Gyrotron

The effect of the electron beam misalignments, including the parallel shifted electron beam and the tilted electron beam on the operating frequency, the beam-wave interaction efficiency, and the frequency-tunable range for a continuously frequency-tunable gyrotron has been studied based on a self-consistent nonlinear beam–wave interaction theory. And the effect of the parallel shifted electron beam on the starting current has also been investigated based on linear theory. It is found that the misaligned electron beam has a little effect on the operating frequency and the frequency-tunable range but a significant effect on the beam-wave interaction efficiency. The misaligned electron beam has a different effect on different axial modes, the influences on the operating frequency, the frequency-tunable range, and the beam-wave interaction efficiency of a higher order axial mode are more obvious. Meanwhile, the influence of the parallel shifted electron beam on the starting current is significant; not only the order of magnitude of starting current increases, but the operating magnetic field is also changed. These results present that the effect of the misaligned electron beam is significant; the electron beam misalignments should be taken into account and avoided in the design and fabrication of a continuously frequency-tunable gyrotron.

Electron Beam Misalignment Study of MIG for 42 GHz, 200 kW Gyrotron

AbstractThis paper presents the electron beam misalignment study with respect to cathode position and cathode magnetic field of 42 GHz, 200 kW gyrotron. The performance of gyrotron is affected with the misalignment of cathode position. The simulation results confirm the tolerance of cathode misalignment with respect to the design parameters such as the transverse-to-axial velocity ratio, the maximum transverse velocity spread, etc.

Method to eliminate the impact of magnetic fields on the position of the electron beam during EBW

The paper presents the approximate formulas for calculating the deflection angle and the misalignment of the electron beam from the optical axis of the electron gun caused by the action of magnetic fields during the electron beam welding. Mathematical model of the effect of magnetic field induced by thermoelectric currents on the electron beam position in the process of electron beam welding of dissimilar materials is presented. The method of monitoring of the misalignment of the scanning electron beam and its mathematical model are proposed. Monitoring of the misalignment of the scanning electron beam is based on the processing of the signal of the collimated X-ray sensor directed to the optical axis of the electron gun by synchronous detection method. The method of compensation of the effect of magnetic fields by passing through the welded seam the currents which compensate thermoelectric currents is considered.

Nonlinear Analysis of a Gyroklystron Amplifier with Misaligned Electron Beam

In this paper, a nonlinear single mode analysis has been developed for studying the beam-wave interaction behavior of gyroklystron amplifiers. The normalized parameters: field amplitude, length and detuning parameter, are optimized for the efficient device operation through the contour plots, and have been extended here in terms of a mathematical model. Further, in the present work, the momentum and phase for each cavity of the gyroklystron amplifier have been obtained by considering the actual device parameters instead of arbitrarily chosen parameters to estimate the efficiency in individual cavities to get the overall efficiency of the device. The bandwidth of the device is thus obtained by estimating the variations of RF power output with frequency. Moreover, the effects of practical problems of the electron beam misalignment and axial velocity spread on the performance of gyroklystron amplifier have also been analyzed in the presented model. It has been found that the RF output power is more sensitive in comparison to the device bandwidth to the electron beam misalignment. To validate the developed mathematical model, an experimental 35 GHz four-cavity gyroklystron amplifier, reported in the literature, has been selected. The results from the nonlinear formulation predicted a stable RF output power of 180 kW at 34.9 GHz, with ~56 dB gain, 26 % efficiency and a bandwidth of 0.54 % for a 72 kV, 9.6 A gyrating electron beam. The analytically obtained values of RF output power and gain are found to be in agreement with the experimental results within ~5 %. The analytical result has also been validated through the 3D PIC simulation using a commercially available code within ~5 %.

Effects of misaligned electron beam on inverse free electron laser acceleration

Abstract In this paper, we discuss the effects of misaligned electron beam on an inverse free electron laser with both an electromagnetic wave wiggler and magnetostatic wiggler acceleration scheme. It is shown analytically that electromagnetic wiggler IFEL energy gain distance is substantially smaller when compared to the standard IFEL i.e. with a magnetostatic wiggler. The analysis further explains a better tolerance of the electromagnetic wiggler IFEL with respect to the misaligned electron beam in comparison to a magnetostatic wiggler IFEL scheme.

Time domain analysis of a gyrotron traveling wave amplifier with misaligned electron beam

This article develops a time-domain theory to study the beam-wave interaction in gyrotron traveling wave amplifier (gyro-TWA) with a misaligned electron beam. The effects of beam misalignment on the TE01 mode gyro-TWA operating at the fundamental are discussed. Numerical results show that the effect of misalignment is less obvious when the input power is larger, and the influences of misalignment on the stable gain and the stable time are basically opposite.

Theoretical Study of the Effect of Electron Beam Misalignment on Operation of the Gyrotron FU IV A

In this paper, the theory describing the processes in gyrotrons with misaligned electron beams is used for interpreting some experimental results obtained in the FU IV A, 335-GHz gyrotron operating at the fundamental cyclotron resonance. The theory describes the effect of the beam misalignment on the single-mode operation in the small-signal (start currents) and large-signal (efficiency) regimes. It also describes the effect of the beam misalignment on the interaction between two oppositely rotating modes. Comparison of theoretical and experimental results demonstrates good qualitative agreement; in particular, both of them show that 0.3-mm misalignment of the beam axis causes efficiency degradation by a factor of two.

On the dependence of the efficiency of a 240 GHz high-power gyrotron on the displacement of the electron beam and on the azimuthal index

Two issues in the cavity design for a Megawatt-class, 240 GHz gyrotron are addressed. Those are first, the effect of a misaligned electron beam on the gyrotron efficiency and second, a possible azimuthal instability of the gyrotron. The aforementioned effects are important for any gyrotron operation, but could be more critical in the operation of Megawatt-class gyrotrons at frequencies above 200 GHz, which will be the anticipated requirement of DEMO. The target is to provide some basic trends to be considered during the refinement and optimization of the design. Self-consistent calculations are the base for simulations wherever possible. However, in cases for which self-consistent models were not available, fixed-field results are presented. In those cases, the conservative nature of the results should be kept in mind.

Linear theory of large-orbit gyrotron traveling wave amplifiers with misaligned electron beam

A linear theory of large-orbit gyrotron traveling wave amplifiers (gyro-TWAs), which can be applied to analyze the effect of electron beam misalignment, is developed by specializing the corresponding theory of small-orbit gyro-TWAs. The linear theory is validated by comparing with a nonlinear theory. Numerical results show that beam misalignment can reduce linear gain and amplification bandwidth of large-orbit gyro-TWAs and increase the starting length of large-orbit gyro-BWOs for modes in accordance with the mode-selective condition. In addition, beam misalignment can also break the limitation of mode-selective condition and make the instability problem more complex.

Linear and nonlinear analysis of a gyrotron traveling wave amplifier with misaligned electron beam

This paper presents linear and self-consistent nonlinear theories to describe the beam-wave interaction in a gyrotron traveling wave amplifier (gyro-TWA) with a misaligned electron beam. A linear theory based on the linearized Vlasov equations is developed by using Laplace transforms. By setting appropriate initial values of the electron guiding center, the nonlinear theory without misalignment can be used to study the misalignment effect. The two theories are in good agreement in the small-signal region for the gyro-TWA. The effects of beam misalignment on the TE01 mode gyro-TWA operating at the fundamental are discussed and the corresponding physical mechanism is analyzed.

Gyrokinetic formula and experimental examination of the electron-beam misalignment effect on the efficiency of a cylindrical-cavity gyrotron oscillator.

By making use of the gyrokinetics of free-electron masers, the efficiency formula of a cylindrical-cavity gyrotron oscillator is presented, where the misalignment of the electron-beam axis to the cavity axis has been taken into account. Comparison with a recent experimental report [Int. J. Infrared and Millimeter Waves 19, 1303 (1998)] is made, which confirms the creditability of the gyrokinetic theory.

Gyrokinetic Analysis of Electron-Beam-Misalignment Effect on a Far-Infrared Gyrotron

Gyrokinetic study is presented to the effect of the electron-beam misalignment on a far-infrared cylindrical-cavity gyrotron oscillator constructed by a Japanese research group. Agreement is found between the theory and the experimental observation. The influences of the operating magnetic field and the electron-beam energy are discussed with the electron-beam misalignment taken into account. It is found that the starting current notably increased because of the existence of the electron-beam misalignment.

Comparison of Electron-Beam-Misalignment Effect on Starting Current and Output Power in Coaxial-Cavity and Cylindrical-Cavity Gyrotron Oscillators

Comparative study is presented to the effect of the electron-beam misalignment on the starting current and output power of the coaxial-cavity and cylindrical-cavity gyrotron oscillators operating in the millimeter wave ranges. The numerical analysis is based on the gyrokinetic formulas for a TE28,16,1 mode at a frequency of 140 GHz. Results show that the coaxial-cavity gyrotron oscillator has lower starting current and less power loss than the cylindrical-cavity gyrotron oscillator when the electron-beam axis has a misalignment to the cavity axis.

Study of Electron Beam Misalignment in a Submillimeter Wave Gyrotron

Electron beam misalignment in a submillimeter-wave gyrotron strongly affects its total efficiency as well as excitation of rotating and counterrotating modes. Some of misalignment phenomena were studied theoretically and experimentally.

Kinetic description of the influence of electron-beam misalignment on the performance of a coaxial-cavity gyrotron

Based on the gyrokinetics of free-electron masers, the influence of the electron-beam misalignment on the energy-transfer rate and starting current of a coaxial cavity gyrotron oscillator is studied. As a practical example, a numerical analysis is given to the 1.5 MW/140 GHz coaxial-cavity gyrotron that is being constructed at the Forschungszentrum Karlsruhe (FZK) [M. Thumm, Fusion Eng. Design 30, 139 (1995)]. Results show that the electron-beam misalignment will decrease the energy-transfer rate and increase the starting current, and the dependence of the output power on the magnetic field may become more sensitive due to a poor alignment of the electron-beam.

Design and experiment of a far-infrared FEL

To design a FEL system, we have developed a three-dimensional simulation code including nonlinear effects such as interaction of electrons with laser fields. The code can calculate the motion of electrons and the variation of laser fields in a wiggler. It can simulate processes of bunching of electrons, and amplification and saturation of laser fields. This code, however, cannot include the effects of inhomogeneity of the wiggler field and beam misalignment. To estimate the gain reduction due to the above effects, we have also developed a linear simulation code based on Madey's theorem including the following effects: actual wiggler field, beam envelope, energy spread, and electron beam misalignment to the wiggler axis. The FEL system we have designed consists of a 20 MeV, 100 mA rf linac and a wiggler with 6 cm pitch, 28 periods and 2.5 kG peak field. The simulation results show that the FEL system has about 30% gain and several MW output power at 37 μm laser wavelength. Our FEL system has been completed and experiments have been started.