回旋管同轴谐振腔模式的选择

 为了研究回旋管工作模式的选择机理,分析了圆柱波导开放式谐振腔、光滑同轴谐振腔和纵向内开槽同轴谐振腔相应波导的色散特性,进而研究了这些谐振腔内模式竞争的问题。对同轴结构波导色散方程作了相应的数值计算,结果表明,同轴谐振腔利用自身色散曲线与谐振腔内外半径比值的数值关系,能够使在开放式谐振腔内无法工作的高阶模式在其内部稳定工作。最后,以德国卡尔斯鲁厄理工学院(KIT)研制的同轴回旋管为例,进行了相应的分析计算,结果表明纵向内开槽同轴谐振腔相对于光滑同轴谐振腔更适合高阶模式的稳定工作。     

毫米波回旋速调管放大器的自洽非线性数值模拟

为了实现回旋速调管放大器的快速设计,基于经典的回旋管的稳态单模非线性理论方法,开展了回旋速调管放大器的束波作用效率的理论模拟研究。由于单模理论无法匹配回旋速调管放大器的输入腔、中间腔两端的突变边界条件,所以输入腔与中间腔都只能采用给定场法进行求解。回旋速调管的输出腔的功率输出端通常采用缓变结构,这种腔体可以采用单模自洽理论进行求解。对两腔毫米波回旋速调管放大器进行了理论模拟,并与商业粒子模拟软件的结果进行对比,验证了该数值理论模拟方法的有效性。     

Improved gyrotron cavity with high quality factor

For gyrotrons operating at the second harmonic of the electron cyclotron frequency cavities with high quality factors are needed. Up to now steep output tapers or abrupt output irises are used in order to provide a high reflection coefficient. Such circuits suffer from the problem of strong conversion to unwanted output modes. A promising possibility to achieve a high reflection at the cavity output (and thus a high quality factor) combined with high mode purity is the use of a rounded iris and an optimized nonlinear taper with very low unwanted mode conversion at the output of the resonator.     

Enhancement of cavity selectivity in relativistic gyrotrons operated at axisymmetric modes

Mode selection is a key problem from the viewpoint of maintaining the single-mode generation regime in high-power gyrotrons operated at axisymmetric modes. In this paper, we propose several electrodynamic methods of mode selection, which allow separating the electrically strong axisymmetric higher-order mode from spurious modes. The possibility of suppressing the nearest spurious modes efficiently, by using either wide slits in the cavity, or azimuthal corrugations on the walls of the tapered cavity, is shown. A method of mode selection at cyclotron frequency harmonics is proposed. The results of studying two types of gyrotron cavities experimentally at a low power level confirmed their high selective properties. The cavities were calculated aiming at using them as the basis for creation of a gyrotron operated at the TE0.3 mode and producing a power of 5–10 MW in 1 μs long pulses at a wavelength of 1 cm. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 10, pp. 837–849, October 2008.     

A rigorous time domain analysis of gyrotrons

A self-consistent time domain analysis of gyrotrons is presented which allows studying multi-mode, multi-frequency operation. The electromagnetic field in the gyrotron cavity is expanded with respect to complete sets of eigenfunctions so that space charge effects are included in the analysis. Two improvements of the modal expansion inside gyrotron cavities are suggested which significantly increase the accuracy and the numerical efficiency of this method, namely, the removal of the non-uniform convergence of some field series at the coupling apertures and the estimation of the asymptotic values of some slowly converging series related to the modal analysis by a moderate number of cavity eigenfunctions. Discrete Fourier transform is used to obtain the time dependence of the electromagnetic field. The electron beam is described by a set of relativistic single particles. It is demonstrated that the strong numerical requirements of the suggested method can be overcome by using a vector computer. Two gyrotrons are investigated, namely, a low Q 35 GHz TE₀₁- and a 150 GHz TE₀₃-gyrotron. Both oscillation build-up and steady state operation are investigated including mode competition and window reflections. The simulations show that the assumption of a monofrequent steady state operation of gyrotrons, which is made by the commonly used frequency domain methods, is not always justified.     

Spectral Domain Analysis of Coaxial Cavities

Coaxial cavities are used in high power gyrotrons as the beam-wave interaction structure. Much research has been devoted to their mode selective properties. A coaxial cavity lacks a sharp boundary at its open end, so it has some physical features that can only be observed using a spectral model, such as frequency-dependent field profiles and mode overlapping effects. These properties are important since cold tests are usually conducted in the frequency domain. This study applies the incident/reflected wave boundary condition to the wave equation of a weakly irregular coaxial waveguide. The resistivity of the wall is considered in the analysis. Calculations reveal that the fixed-position spectrum yields an uncertain resonant frequency and quality factor. Although the maximum-field spectrum can uniquely determine the properties of the coaxial cavity, the resonant frequency obtained using the maximum-field spectral model is inconsistent with that obtained using the temporal model. The field-energy spectrum explains the low Q nature of the coaxial cavity. Moreover, resonant frequencies evaluated using the field-energy spectrum agree precisely with those evaluated using the temporal model.     

Selective excitation of high-order modes in circular waveguides

The excitation of very high-order modes in circular waveguides has been performed in a cavity with a connected up-taper with a geometry similar to those used in gyrotrons. A Gaussian beam was coupled to the cavity which was made translucent by an array of holes. With the help of a special optics, the amplitude as well as the phase distribution of the beam was matched to the mode to be excited in the resonant cavity. By simple rotation of one mirror to adjust the phase distribution together with the change of frequency to match the resonance condition, a large number of modes could be produced with one experimental set-up. Field measurements in the output waveguide show a high mode purity of the radiation and confirm the calculations. The method can be used for cold tests of electrodynamic systems operating with these modes, e.g. quasi-optical converters for gyrotrons.     

Start-up scenario in gyrotrons with a nonstationary microwave-field structure

Megawatt class gyrotrons operate in very high-order modes. Therefore, control of a gyrotron oscillator's start-up is important for excitation of the desired mode in the presence of the many undesired modes. Analysis of such scenario using the self-consistent code MAGY [M. Botton, IEEE Trans. Plasma Sci. 26, 882 (1998)10.1109/27.700860] reveals that during start-up not only mode amplitudes vary in time, but also their axial structure can be time dependent. Simulations done for a 1.5 MW gyrotron show that the excitation of a single operating mode can exhibit a sort of intermittency when, first, it is excited as a mode whose axial structure extends outside the interaction cavity, then it ceases and then reappears as a mode mostly localized in the cavity. This phenomenon makes it necessary to analyze start-up scenarios in such gyrotrons with the use of codes that account for the possible evolution of field profiles.     

Numerical simulation of mode interaction in 170 GHz/1 MW gyrotrons for iter

The paper presents an advanced method for and results of calculating main parameters of CW 170 GHz/1 MW gyrotrons operating at the TE_28.7 and TE_31.8 modes for ITER. Parameters are optimized to achieve maximum efficiency of the gyrotron with an acceptable Ohmic load on the cavity. Numerical modeling of starting up a gyrotron with an optimized cavity and processes of mode interaction are discussed.     

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.     

Cold-cavity and self-consistent approaches in the theory of modecompetition in gyrotrons

On the basis of a generalized theory, the cold-cavity and the self-consistent methods of calculation of mode competition in gyrotrons are compared. This comparison enables the conditions when the cold cavity approximation is justified to be determined. Specific examples are presented and numerical problems are discussed. It is concluded that mode competition can be studied by neglecting the interaction of the electrons and the RF field in the output taper in those cases when the parameter of irregularity of the output taper is >15. Here, the results obtained in the cold-cavity approximation overestimate the starting current of the parasitic mode by ~25% in comparison with the self-consistent approach. For values of the parameter of irregularity <15, the mode competition in gyrotrons must be studied only self-consistently, taking into account the interaction of the electrons and the RF field in the output taper as well as the factors which influence the interaction along the axis     

THE DESIGNS OF HIGH EFFICIENCY LAUNCHER OF QUASI-OPTICAL MODE CONVERTER FOR HIGH POWER GYROTRONS

A high efficiency launcher of quasi-optical (QO) mode converters for high power gyrotrons have been designed and tested. A helical cut launcher radiates the RF power via its straight cut onto the first phase correcting mirror. The launchers have been optimized for the TE_31.8 mode at 170 GHz and TE_22.6 mode at 110 GHz by numerically optimizing a launcher surface. The helical cut of the launcher has been optimized by taking the taper angle into account. Further more, the amplitude of the surface perturbation have been optimized for improved focusing in order to reduce the diffraction losses at the helical cut. Low power measurement shows a good agreement with the design. High efficiency characteristics of the design have also been calculated on the assumption of frequency downshift due to the thermal expansion of the cavity and stepwise frequency tuning by changing the operating mode. Besides, the possibility of high efficiency launcher for higher mode is discussed, and these results give the prospect to high efficiency long pulse gyrotrons.     

THE FIRST EXPERIMENT OF A THz GYROTRON WITH A PULSE MAGNET

A THz gyrotron with a pulse magnet has been designed, constructed and operated in FIR FU. It is developed as one of high frequency gyrotrons included in Gyrotron FU Series. The gyrotron has already achieved the first experimental result for high frequency operations whose radiation frequency exceeds 1 THz. In this paper, the design detail and the operation test results for sub-terahertz to terahertz range are described. The second harmonic operation is confirmed experimentally at the expected frequency of 1.005 THz due to TE_6,11 cavity mode at the magnetic field intensity of 19.0 T.     

Effect of beam potential dip in gyrotrons

The effect of the electron beam potential dip in high-power gyrotrons with overdimensional open cavities is studied theoretically. The gyrotron power and efficiency limitations due to the beam potential dip in the cavity are determined. Variations in the self-excitation conditions, efficiency, and optimal parameters are discussed for the case of ion compensation of the space charge of an electron beam in the short-pulse and long-pulse generation regimes. Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 8, pp. 1042–1049, August, 1997.     

双频回旋管内置准光模式变换器设计

回旋管一般使用准光模式变换器实现高阶腔体模式到高斯波束的转换。结合标量衍射理论、KS迭代算法、几何光学、最小均方法等方法设计了工作频率为140 GHz(TE24,9)和105 GHz(TE18,7)的双频准光模式变换器。仿真结果显示所设计的准光模式变换器工作频率为140 GHz(TE24,9)时能量传输效率99.0%、高斯含量99.7%,工作频率为105 GHz(TE18,7)时能量传输效率97.3%、高斯含量98.0%。能够满足MW级双频回旋管的应用需求。     

原子-腔-光纤复合系统中的纠缠特性

采用Negativity熵来描述两子系统间的纠缠,研究了由两个二能级原子与光纤联接的单模腔构成的系统的纠缠特性.利用数值计算方法讨论了原子与腔场的耦合强度和原子选择性测量对纠缠特性的影响.研究结果表明:对一个原子的选择性测量,可增强原子间与腔场间和腔场与腔场间的纠缠.     

Possibilities for Optimizing the Cavity of a High-Power Continuous-Wave Gyrotron

We calculated the optimal parameters of a low-Q cavity of a millimeter-wavelength continuous-wave gyrotron which ensure that the maximum efficiency is reached for a limited heat load on the cavity wall. The influence of the cavity optimization on the efficiency of energy recovery of a collector electron beam is considered. Stability of the operating mode to self-excitation of other modes is studied. Gyrotrons with radiation power 1 MW, frequency range 140–170 GHz, and operating modes TE_22.6 and TE_25.10 are studied as the example. The obtained results are generalized to gyrotrons with other operating modes and frequencies.     

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.     

原子-腔-光纤复合系统中的纠缠特性

采用Negativity熵来描述两子系统间的纠缠,研究了由两个二能级原子与光纤联接的单模腔构成的系统的纠缠特性.利用数值计算方法讨论了原子与腔场的耦合强度和原子选择性测量对纠缠特性的影响.研究结果表明:对一个原子的选择性测量,可增强原子间与腔场间和腔场与腔场间的纠缠.