Aftercavity Interaction Research Articles

Influence of the Output Window Reflection on the Performance of W-Band Gyrotron Traveling Wave Tubes

Effects of the output window reflection instability on the performance of our designed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${W}$ </tex-math></inline-formula> -band broadband gyrotron traveling wave tube (gyro-TWT) were studied by the simulation and hot test experiments. Strong oscillations and frequency self-modulation (FSM) in the after-cavity interaction (ACI) region were observed in the zero-driven state with a two-layer window and are verified with the beam-wave particle-in-cell simulation. Oscillations and FSM of 80–81.8 GHz and 91.03–94.68 GHz are, respectively, generated from the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> modes in the ACI region due to the high window reflection level. By using the “sandwich” and meta-surface windows with low reflection and broad bandwidth, the parasitic ACI oscillations can be effectively suppressed. The maximum saturated output power is improved to 156.8 kW at 94 GHz and is zero-driven stable in our recent hot test experiments.

Multifaceted Simulations Reproducing Experimental Results From the 1.5-MW 140-GHz Preprototype Gyrotron for W7-X

A multifaceted simulation procedure, addressing the electron beam properties, the beam-wave interaction, and the internal losses, has been used for the simulation of the experimental operation of a 1.5-MW 140-GHz short-pulse preprototype gyrotron. The preprototype is related to the development of 1.5-MW gyrotrons for the upgrade of the electron cyclotron resonance heating system at the stellarator W7-X. A very good reproduction of experimental results has been achieved by simulation, without resorting to arbitrary speculations. This validated the numerical tools as well as the design and fabrication of the short-pulse preprototype, which fully reached the target of efficient 1.5-MW operation in millisecond pulses. Special attention has been given to simulating the possibility of parasitic after-cavity interaction in the gyrotron launcher. Also, parasitic backward-wave excitation in the gyrotron cavity has been demonstrated by simulation, at a frequency and voltage range in agreement with experimentally observed parasitic oscillations. This offers an additional possibility with respect to the origin of deleterious parasitic oscillations in high-power gyrotrons, which are usually attributed mainly to the gyrotron beam tunnel.

Study on After Cavity Interaction in a 140-GHz Model TE0,3Gyrotron Using 3-D CFDTD PIC Simulation

The possibility of dynamic after cavity interaction (ACI) in a down-scaled gyrotron model has been studied using a 3-D conformal finite-difference time-domain particle-in-cell simulation. The purpose has been to benchmark this type of simulation against previous numerical results obtained by other methods and tools and to obtain additional physical insight on the effect of dynamic ACI. The benchmarking has been successful, as a good agreement with the older results has been achieved. Thus, the algorithms developed and validated can be used for studying the issue in a larger scale or more realistic model with affordable computational resources in the near future.

Self-consistent non-stationary theory of the gyrotron

For a long time, the gyrotron theory was developed assuming that the transit time of electrons through the interaction space is much shorter than the cavity fill time. Correspondingly, it was assumed that during this transit time, the amplitude of microwave oscillations remains constant. A recent interest to such additional effects as the after-cavity interaction between electrons and the outgoing wave in the output waveguide had stimulated some studies of the beam-wave interaction processes over much longer distances than a regular part of the waveguide which serves as a cavity in gyrotrons. Correspondingly, it turned out that the gyrotron theory free from the assumption about constant amplitude of microwave oscillations during the electron transit time should be developed. The present paper contains some results obtained in the framework of such theory. The main attention is paid to modification of the boundary between the regions of oscillations with constant amplitude and automodulation in the plane of normalized parameters characterizing the external magnetic field and the beam current. It is shown that the theory free from the assumption about the frozen wave amplitude during the electron transit time predicts some widening of the region of automodulation.

A comparative study on the modeling of dynamic after-cavity interaction in gyrotrons

There are cases where gyrotron interaction simulations predict dynamic After-Cavity Interaction (ACI). In dynamic ACI, a mode is excited by the electron beam at a dominant frequency in the gyrotron cavity and, at the same time, this mode is also interacting with the beam at a different frequency in the non-linear uptaper after the cavity. In favor of dynamic ACI being a real physical effect, there are some experimental findings that could be attributed to it, as well as some physical rationale indicating the possibility of a mode being resonant with the beam at different frequencies in different regions. However, the interaction codes used in dynamic ACI prediction up to now are based on simplifications that put questions on their capability of correctly simulating this effect. In this work, the shortcomings of the usual simplifications with respect to dynamic ACI modeling, namely, the trajectory approach and the single-frequency boundary condition, are identified. Extensive simulations of dynamic ACI cases are presented, using several “in-house” as well as commercial codes. We report on the comparison and the assessment of different modeling approaches and their results and we discuss whether, in some cases, dynamic ACI can be a numerical artifact or not. Although the possibility of existence of dynamic ACI in gyrotrons is not disputed, it is concluded that the widely used trajectory approach for gyrotron interaction modeling is questionable for simulating dynamic ACI and can lead to misleading results.

Study of Dynamic After Cavity Interaction in Gyrotrons—Part II: Influence of a Nonuniform Magnetic Field

Spurious oscillations, particularly dynamic after cavity interaction (ACI) in high-power gyrotrons gained attention as an influencing factor on the overall gyrotron performance. Numerical investigations are a main pillar for a deeper understanding of this phenomenon. The Karlsruhe Institute of Technology in-house self-consistent, time-dependent self-consistent multimode code was modified to accommodate the influence of an axial variation of the external magnetic field on the after cavity dynamics. Inclusion of the influence of the nonuniform distribution of the magnetic field in the time-dependent gyrotron beam-wave interaction differential equations is necessary to make proper numerical investigations in terms of the appearance of the dynamic ACI particularly in the output waveguide section (uptaper), where the nonuniform distribution of the magnetic field is more significant. We evaluate the importance of this for modeling of three different high-power gyrotron cavities, where partially undesired weak interactions in the up-taper/launcher region have been observed.

Study of Dynamic After Cavity Interaction in Gyrotrons—Part I: Adiabatic Approximation

In recent years, the after cavity interaction (ACI) in high-power gyrotrons operating in the 100-200 GHz range gained special attention as an influence factor on overall efficiency. While investigations concentrated on ACI as a stationary effect until now, recent simulations show that undesired interactions in the output waveguide region (uptaper) can also result in additional parasitic oscillations. In this paper, such nonstationary, dynamic processes are investigated in first simulations and results are described using the self-consistent time-dependent Karlsruhe Institute of Technology multimode code. In this paper, results are generated along with the assumption of constant B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> (z) = B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (maximum magnetic field) as well as with the implementation of the adiabatic approximation in the case of varying magnetic field for determination of the axial velocity of each particle. The simulations show weak parasitic/spurious oscillations in the uptaper of the gyrotron cavity that may be attributed to the occurrence of dynamic ACI.

Analysis of Aftercavity Interaction in European ITER Gyrotrons and in the Compact Sub-THz Gyrotron FU CW-CI

Possibilities of arising of aftercavity interaction are analyzed in the ITER 170 GHz 2 MW coaxial cavity gyrotron and the 170 GHz 1 MW cylindrical cavity gyrotron, as well as in the compact 394.5 GHz low power gyrotron FU CW-CI. Also, the simulations for the gyrotron efficiency in the presence of aftercavity interaction are performed in the cold cavity approximation. Results of the analysis illustrate the subtle interplay between the geometry of the output taper and the profile of the magnetic field.

Numerical study of efficiency for a 670 GHz gyrotron

In this paper, the results of the efficiency study of a 670 GHz gyrotron operating at TE31,8-mode are presented. Calculations are performed by using the self-consistent nonstationary code MAGY. Three cavity configurations were examined. The effects of ohmic losses and electron velocity spread were included in the simulation. The results show that the output efficiency can reach 35% and the velocity spread in the electron beam does not degrade the operation significantly. Furthermore, we verified that the smoothing of the sharp corners for a small tapering angle would reduce mode conversion; the parasitic excitation of neighboring radial modes is less than 1% of the amplitude of the operating mode and the effect on efficiency is small. Lastly, the simulation results show that the after-cavity interaction causes only slight variations in the efficiency.

Possibilities for reducing the aftercavity interaction effect in gyrotrons

This paper addresses the problem of aftercavity interaction in gyrotrons. The term “aftercavity interaction” is used for the cyclotron resonance interaction between electrons in a spent beam exiting the microwave cavity and traveling electromagnetic waves propagating from the cavity exit into the output part of a gyrotron. Aftercavity interaction may reduce the interaction efficiency by several percent and spoil the energy distribution of spent electrons, thus reducing the possibility of efficient utilization of depressed collectors. We have used a simple theory to find conditions resulting in the reduction of this effect. In particular, we have shown that both the axial waveguide wall profile and magnetic field distribution should be modified in order to weaken the effect of aftercavity interaction. We verified results of our simple analysis with advanced simulations and demonstrated that they are in good agreement. Result of these studies can be beneficial for the gyrotron efficiency enhancement.

Analysis of aftercavity interaction in gyrotrons

This paper is devoted to the analysis of a so-called “aftercavity interaction” in gyrotrons. The term “aftercavity interaction” is used for defining the cyclotron resonance interaction between electrons in a spent beam exiting the cavity and traveling electromagnetic waves propagating from the cavity exit into the output part of a tube. A simple theory allowing one to analyze the role of aftercavity interaction in gyrotron operation has been developed. With the use of only two parameters characterizing the profile of the waveguide wall and the profile of the magnetic field, one can find conditions resulting in weakening this parasitic effect. Also, the simulations for the gyrotron efficiency in the presence of aftercavity interaction are performed for an existing device. Results of these simulations agree with analytical predictions. They also show how the axial distribution of the magnetic field in an existing cryomagnet should be modified in order, first, to use the aftercavity interaction for improving the interaction efficiency and, second, for weakening the role of this interaction in degrading the wall-plug efficiency in gyrotrons with depressed collectors. Results of such analysis can be beneficial for the gyrotron efficiency enhancement.

Experimental observation of the effect of aftercavity interaction in a depressed collector gyrotron oscillator

This paper presents the experimental observation of the effect of an aftercavity interaction (ACI) in a depressed collector gyrotron oscillator. The gyrotron generates an output power of 1.5MW at 110GHz in 3μs pulses with a 96kV and 40A electron beam and has a single-stage depressed collector. The ACI arises from an unintended cyclotron resonant interaction between the microwave beam traveling out from the cavity and the gyrating electron beam. The interaction occurs in the uptaper of the launcher, immediately downstream from the cavity, where the magnetic field is slightly lower than its value in the cavity region. The ACI results in a reduction in efficiency since the electron beam tends to extract power from the wave. There is also a broadening of the spent beam energy profile, which reduces the effectiveness of the depressed collector and in turn limits the overall efficiency of a gyrotron. Measurements of the maximum depression voltage of the collector vs beam current at 96kV are compared with simulations from the MAGY code [M. Botton et al., IEEE Trans. Plasma Sci. 26, 882 (1998)]. Excellent agreement is obtained between theory and experiment but only if the ACI is included. In the present experiment, it is estimated that the observed efficiency of 50% would have been about 60% in the absence of the ACI. These results verify the role of the ACI in reducing the efficiency of the gyrotron interaction.

Influence of the energy and velocity spread in the electron beam on the starting conditions and efficiency of a gyrotron

We study theoretically the influence of the spread of initial energies and velocities in the electron beam on the starting conditions and efficiency of a gyrotron. We compare various analytical and numerical models and the results of experimental studies of gyrotrons in which the interaction takes place at the first and second harmonics of the cyclotron frequency. The aftercavity interaction of the electron beam with the high-frequency field in the output waveguide transition is taken into account. The influence of the energy spread on the recuperation efficiency is estimated. Permissible spreads of the initial energies and electron velocities are determined.

Influence of Aftercavity Interaction on Gyrotron Efficiency

We consider theoretically the influence of interaction of a used electron beam with a concurrent wave in the output waveguide transition outside the cavity on the efficiency and energy recovery in a powerful millimeter-wave gyrotron. Without energy recovery, parasitic interaction in the transition reduces the efficiency and output power of a gyrotron by 5–10%. In a gyrotron with energy recovery, losses due to interaction in the transition can become most significant compared with other losses, and the efficiency is reduced by 20–30%. The influence of the transition decreases with decreasing transition length and increasing ratio of the maximum radius to the minimum radius of the transition.