Development of a Compact sub-THz Gyrotron FU CW CI for Application to High Power THz Technologies

For application of high frequency gyrotron to high power THz technology, Gyrotron FU CW series is being developed in FIR FU. Gyrotron FU CW CI is developed as one of sub-THz gyrotrons included in the series. The advantage of the gyrotron is compactness using a compact superconducting magnet and compact power supply system, which makes the accesses of the gyrotron to applied large-scale devices easier and extends the applications of gyrotron to wider fields. The designed frequency and cavity mode are 394.5 GHz and TE₂₆ mode for application to the 600 MHz DNP-NMR spectroscopy. As the operation results, the frequency and the output power were 394.03 GHz and around 30 W, respectively, which are available for the application to the 600 MHz DNP-NMR measurement. In addition, this gyrotron can operate at many other frequencies and cavity modes for application to high power THz technologies in wide fields. In this paper, the design and the operation results including long pulse or CW mode are presented.     

一种用于谐波倍增回旋放大器的耦合腔互作用电路

该文提出了一种用于回旋管放大器的新型耦合腔互作用电路模型。基于Ansoft HFSS高频计算软件,分析计算了矩形波导TE□10模输入,圆波导TE○01模输出模式转换系数随几何参数和工作频率的变化关系,讨论了结构变化对模式选择和抑制的作用。HFSS的模拟计算结果表明,对TE□10到TE○01转换,TE○01模能够稳定工作,且能量转换效率最大可以大于97%,－3dB带宽最大可以大于327MHz,是谐波倍增回旋行波放大器的一种可选输入耦合结构。     

High Frequency and High Mode Purity Operations of Gyrotron FU IVA

The cavity of the newest gyrotron, Gyrotron FU IVA in the high frequency series of Fukui University gyrotrons (the Gyrotron FU series) is designed to minimize the mode conversion from the main cavity mode to the higher modes. In this paper, the experimental results are compared with simulations for the complete gyrotron oscillator. The gyrotron has an additional advantage to cover the wide frequency range in submillimeter wave region.     

Nondestructive Gyrotron Cold-Cavity Q Measurements

A novel method for cold testing a gyrotron resonator to determine its total Q is presented. Probing radiation is coupled into the resonator through its radiation pattern. A sensitive heterodyne receiver is used in the far field to detect the reradiated cavity resonances. Good agreement between measurement and calculated total Q is found for several 140-GHz gyrotron resonators in the TE/sub 031, TE/sub 032/, TE/sub 231/, and TE/sub 611/ modes.     

在迴旋管中采用中等偏心率椭圆横截面谐振腔的探讨

本文综述了几种在迴旋管中采用大横截面谐振腔的办法,计算了中等偏心率椭圆波导各TE模的归一化截止频率。归一化截止频率的数据表明,用椭圆横截面谐振腔的迴旋管具备以下特性:(1)谐振腔体积大;(2)可以振荡的工作模式少;(3)可以使用大直径电子束;(4)可以工作于低次角向腔模。     

Resonant frequencies of a rising-sun gyrotron cavity

New resonant frequency calculations for a tunable gyrotron cavity with a magnetron-like rising-sun cross-section give results that are measurably different from those found by the traditional method, especially as the wavelengths become short enough for the radial slots to support higher-order modes. Moreover, in many instances a large fraction of the microwave field energy resides in these slot modes and as a result any gyrotron interaction is expected to be small.     

A cavity with reduced mode conversion for gyrotrons

Mode conversion at discontinuities of an ordinary gyrotron cavity is examined. It is shown how the production of unwanted spurious modes can be reduced substantially by introducing smooth transition regions between the individual parts of the cavity. The cavity of the 140 GHz KfK gyrotron operating in the TE_10,4 mode is used as an example. Cavity and adjacent tapered output waveguide are optimized as a single unit.     

Nonstationary Behavior of a Gyrotron in the Presence of Reflections

Nonstationary behavior of a gyrotron in the presence of reflections is investigated by means of our time-dependent self-consistent code. It is shown that the traditionally accepted scenario of the transition from the single-frequency stationary operation regime to the chaotic nonstationary regime due to an increase in the magnitude of the reflection coefficient is not correct if a realistic non-fixed field profile is used in the simulations. In this case, the increase of the reflection coefficient above a certain threshold value (R ≥ 0.4) leads to an excitation of a number of modes of the mismatched gyrotron cavity. Then the gyrotron shows a very complicated periodic or quasiperiodic behavior. For higher reflection coefficients (R ≥ 0.85), the competition between the modes of the mismatched cavity leads to the suppression of the main mode by another one operating at higher frequency. The influence of the distance to the reflecting load on the gyrotron behavior is also demonstrated.     

Coaxial cavities with corrugated inner conductor for gyrotrons

This paper investigates coaxial gyrotron cavities with longitudinal slots on the inner conductor as a means to reduce the number of possible competing modes. In the analytic theory the corrugated surface is treated as a homogeneous impedance surface (“impedance corrugation”) to obtain simple formulas for the characteristic equation of the eigenmodes, for the electromagnetic fields and the wall losses. The developed model applies if the number of slots is sufficiently high (cutoff wavelength much larger than the corruption period). The characteristic equation in terms of the ratio C of the outer wall radius to the inner conductor radius is solved numerically to determine a range of eigenvalues and C where the eigenvalue curves are monotonically decreasing. In such a region a cavity having its inner conductor downtapered (radius decreasing toward the cavity output) can be used to reduce the diffractive quality factors of several modes, leaving the working mode undisturbed and without favoring other modes. In addition the electromagnetic field profiles are investigated, and in particular it is shown that for certain cavity parameters a mode could have its energy concentrated close to the inner conductor. As a check on the validity of the theoretical approximations, simulations with the MAFIA code are carried out. These give good agreement with the results of the analytic equations     

一种超低电压宽带调谐太赫兹回旋管互作用结构

低电压频率可调太赫兹回旋管在生物医学和波谱学等领域具有重要应用。文章分析了超低电压(<1 kV)下采用传统开放腔互作用电路的330 GHz 回旋管输出功率和频率调谐特性,探讨了超低电压下由于电子相对论效应减弱而导致的回旋管中电子注-波互作用耦合强度降低的问题。在此基础上,针对330 GHz 超低电压回旋管提出了一种改进的互作用电路结构,其下倾式尾端结构有助于增大反向波幅度,提高弱相对论电子注与电磁波之间的耦合强度,从而提高回旋管的输出效率及频率调谐带宽。非线性模拟结果表明,在低至0.3 kV 的超低电压下,采用此种互作用电路结构仍可获得大于1 W 的连续波输出功率及22 GHz 的连续调谐带宽,峰值输出效率大于7%。     

A W-band Third Harmonic Gyrotron with an Iris Cavity

The design and experimental results of a W-band gyrotron operating at the third cyclotron harmonic are presented. The gyrotron is designed to operate at the TE₆₁ mode, which is significantly distinct from competing modes. An iris cavity is employed for the purpose of trapping the third harmonic mode more effectively and lowering its start current. In the experiment, the gyrotron is drived by a triode magnetron injection gun (MIG) which can produce a 45 kV, 3 A electron beam. When maximum axial magnetic field is 1.22 T, a single mode third harmonic gyrotron radiation is observed with the frequency of 94.86 GHz. The maximum output power is 5.5 kW, corresponding to an efficiency of 4%. Another third harmonic mode TE₀₂ is also detected at 88.8 GHz, with maximum output power of 1.5 kW.     

带状电子注驱动的准光谐振腔回旋管

回旋管作为一种重要的真空电子源,能够在毫米波和太赫兹频段生成高峰值以及高平均功率的电磁辐射,在波谱学、雷达、通信、生物医学等领域具有广泛的应用前景。传统的回旋管将不可避免地面对激烈的模式竞争问题,引入准光谐振腔将有望大大减小模式竞争的激烈程度。本文基于电子回旋脉塞理论,将准光谐振腔及带状电子注进行结合,以期实现更高的输出功率及效率。仿真结果表明,在电子电压为40 kV,背景磁场为8.4 T时,设计的准光回旋管能在220 GHz频点处产生6.1 kW的输出,电子效率达到6.1%且在一段时间内能稳定运行。本文结构将可能为高频段乃至高次谐波工作的回旋管设计提供全新方案,从而进一步用于通信、雷达等领域。     

Ohmic effects in quasioptical resonators

Several properties of the Fabry-Perot-type open resonator used in the quasioptical gyrotron (QOG) and the quasioptical induced resonance electron cyclotron (IREC) maser are derived. The electric fields of the normal modes are given for the general case of the resonator axis tilted with respect to the direction perpendicular to the electron beam axis. The ohmic quality factor and the power dissipated in the mirrors are derived, as is the energy stored in the resonator. The time dependence of the mirror heating, relevant for pulsed experiments, is also derived. The formulae are applied to an example of current relevance, the quasioptical IREC maser resonator.     

A High-Efficiency Second-Harmonic Gyrotron with a Depressed Collector

The gyrotron with an electrically insulated collector has been manufactured and tested. The 60% output efficiency is obtained for the Collector Potential Depression (CPD) operation regime. The results of the numerical simulation of the electron energy spectrum in the collector region of the second-harmonic technological 24 GHz gyrotron are presented. It is shown, that the minimal electron beam energy is about 20% of the initial value and that the gyrotron cavity should be expanded to achieve the highest output efficiency of the gyrotron with energy recovery. The highest experimental decelerating voltage is in good agreement with the calculation of the electron energy spectrum.     

Electromagnetic mode analysis on photonic-band-gap resonant cavity in a gyrotron

The traditional cylinder resonant cavity in a gyrotron is replaced with a photonic-band-gap resonant cavity (PBGC) with a structure of triangular arrays of metal posts, and the cold character of the TE operating modes in the PBGC are analysed and calculated. Under the consideration of the cavity structure features, a research method for the mode theory on PBGC is provided, single-mode operation in the cavity is found, and the distribution characteristics of the azimuthally inhomogeneous electromagnetic modes are reported. The study provides a physical theory fundament which reveals possibilities for changing the essential operating status of a gyrotron.     

The research of resonant cavity with elliptical cross-section at moderate eccentricity in gyrotron

In this paper, the method of calculating cut-off frequencies of TE modes in elliptical waveguide is given. From the calculations, it is demonstrated that the gyrotron utilizing a cavity with elliptical cross-section of moderate eccentricity has the following characteristics: (a) a large volume cavity can be used; (b) the density of modes which can oscillate is low; (c) it is allowable to use an electron beam with a large diameter; (d) it is possible to use a cavity mode with low azimuthal number.     

Numerical solution of fields in lossy structures using MAGY

Lossy structures are used in vacuum electronic devices to control and suppress modes. Numerical simulation of the effect of these lossy structures is critical to the design and optimization of devices. The gyrotron simulation code MAGY makes use of the generalized telegraphist's equations in which the transverse structure of fields is represented as a sum of local modes of a metallic waveguide. If the wall is not a perfect conductor then sum over modes is not uniformly convergent. We have developed an algorithm to deal with this problem and allow for the simulation of structure with highly lossy walls. The theory and implementation of this algorithm are presented     

Cylindrical cavity-backed suspended stripline antenna-theory andexperiment

The problem of a cylindrical cavity-backed suspended stripline (SSL) antenna is viewed as a transition of the SSL to a circular cylindrical waveguide opening into an infinite ground plane. The fields in the waveguide are expanded in terms of TE and TM modes. The effect of the radiating aperture on the modal expansion of the fields is taken into account by introducing reflection coefficients for each mode. The current on the SSL probe is assumed to have sinusoidal distribution. These simplifications reduce the original problem to that of a known radially oriented current residing on a dielectric sheet inside a circular-cylindrical cavity whose top wall has known impedances corresponding to different modes. The Green's function for this modified structure is found and is used to obtain a general expression for the input impedance. This expression is specialized to the case where the SSL probe and the radiating aperture are coupled through the dominant TE ₁₁ mode only. This input impedance is translated to the measurement plane of the antenna. The computed and measured results are found to be in good agreement     

Conceptual Design Study of a Novel Gyrotron for NMR/DNP Spectroscopy

In this paper we present some initial results from a conceptual design study focused on the development of a novel frequency tunable gyrotron for nuclear magnetic resonance (NMR) spectroscopy with signal enhancement based on the utilization of high field radiation and dynamic nuclear polarization (DNP) technique. The first variants of both the electron optical system and the resonant cavity which have been designed aiming continuous frequency tunability in a broad frequency band are presented and discussed. The selected method for frequency tunability is based on the excitation of higher order axial modes and smooth transition between them. It was selected after a critical examination of the known theoretical and practical results related to the frequency control in gyrotrons. It is believed that the current conceptual design is an appropriate basis for development of the next (optimized) design which will include also a detailed design of other components (mode converter, output window etc.) and magnetic circuit (superconducting magnet and supplementary solenoids) as well as for the overall mechanical design and fabrication of the prospective gyrotron.     

Properties of a Second Harmonic Gyrotron Cavity with Internal Blazed Grating

Spectral domain analysis, scattering matrix analysis and PIC simulations are used here to design a second harmonic gyrotron cavity with an internal blazed grating which favors operation at the second harmonic of the electron cyclotron frequency rather than operation at the electron cyclotron frequency itself. Based on these simulations, the cavity design is optimized. The results show that a new gyrotron with higher power and higher frequency can be achieved by incorporating such a blazed grating.