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.     

8mm TE13 Mode Gyrotron

The design of a 8mm TE ₁₃ mode gyrotron is given in this paper. Discussions about the selection of the dimensions of the RF structure and electron gun are presented. Calculated results of the operating parameters of the gyrotron are also given. At 37.5GHz, pulse power 56kW is obtained for the gyrotron manufactured according to our design. Mode pattern obtained by scorching method shows that the mode of the output millimeter wave is TE ₁₃.     

Investigation of a Broadband Quasi-Optical Mode Converter for a Multi-Frequency 1 MW Gyrotron

A broadband quasi-optical (QO) mode converter for a multi-frequency gyrotron has been designed and tested at Forschungszentrum Karlsruhe (FZK). The launcher is optimized for the TE_22,8 mode at 140 GHz, but the radiated beams present an almost identically focused pattern for all 9 considered modes between 105 GHz (TE_17,6) and 143 GHz (TE_23,8). Combining with a beam-forming mirror system, which consists of a quasi-elliptical mirror and two phase-correcting mirrors with non-quadratic surface contour, further calculations show that efficiencies of more than 94% have been achieved for converting the rotating high-order cylindrical cavity modes into the usable fundamental Gaussian mode. Low power (cold) measurements show a good agreement with theoretical predictions. This QO mode converter can be used for the broadband operation of a multi-frequency 1 MW gyrotron.     

Mode Selection for a Terahertz Gyrotron Based on a Pulse Magnet System

The TE_6,11 mode has been selected as a candidate for the second harmonic operation of a terahertz gyrotron at 1007.68 GHz. The predicted efficiency is 8.6 percent for the output power 0.38 kW. Time-dependent, multi-mode calculations have been carried out to investigate stability of a single-mode operation at second harmonic. It has been found that with the beam current 0.111 A and the magnetic field 19.282 T the second harmonic operation in the TE_6,11 mode is possible.     

A Quasi-Optical System for Converting TE0n Mode Outputs of a Gyrotron into Gaussian Beams

In this paper we present a novel design of a quasi-optical system for conversion of gyrotron outputs into Gaussian-like beams. It consists of a quasi-optical antenna, two focusing mirrors and a filter which removes the side lobes of the beam. The system is appropriate as a transmission line for frequency tunable gyrotrons operating at TE_0n mode. As an illustration of our approach, we present results which demonstrate the applicability of the developed system for conversion of the radiation generated by the Gyrotron FU IVA. The examples include conversion of three TE_0n modes (TE₀₂, 223 GHz; TE₀₃, 323 GHz; TE₀₄, 423 GHz) into Gaussian- like beams.     

Determination of the Mode Content in Spurious Microwave Radiation of the Gyrotron with a Straight Axisymmetric Output

The method for determination of the mode content of a spurious microwave radiation of the gyrotron with a straight circular axisymmetrical output is discussed. As demonstrated, in the test task accuracy of this method is several percents. The experimental results on the determination of the mode content at the output of the coaxial gyrotron, operating on the mode TE _14.8 are in a good agreement with the theoretical calculations.     

Rippled wall mode converters for circular waveguide

We present the general theory of rippled wall mode converters. Coupling coefficients for TE and TM waves, of both fixed and rotating polarizations, are calculated. The waveguide ripples considered may be axisymmetric, fluted or helical. We describe in detail a 97% efficient TE₀₄/TE₀₁ converter designed for use with a 35 GHz gyrotron. Cold test results confirm its performance.     

Numerical Study of Processes in the Cavity of the 170 GHz Gyrotron for ITER Operating at the TE25.10 Mode

The opportunity of use of a TE_25.10 operating mode for CW 170 GHz/1 MW gyrotron for ITER is estimated. Parameters are optimized to achieve maximum efficiency of the gyrotron with an acceptable Ohmic load on the cavity. The influence of unwanted mode conversion at the output of the resonator, mode competition, electron beam potential depression, ion compensation of the space charge and beam energy recovery is investigated.     

Detailed Consideration of Experimental Results of Gyrotron FU CW II Developed as a Radiation Source for DNP-NMR Spectroscopy

A CW gyrotron for the sensitivity enhancement of NMR spectroscopy through dynamic nuclear polarization has been designed. The gyrotron operates at the second harmonic and frequency of 394.6 GHz with the main operating mode TE_0,6. Operating conditions of other neighboring cavity modes such as TE_2,6 at frequency of 392.6 GHz and TE_2,3 at frequency of 200.7 GHz were also considered. The experimental conditions of the gyrotron at low and high voltages are simulated. The output power of 56 watts corresponds to the efficiency of 2 percent at low voltage operation and frequency of 394.6 GHz is expected.     

Numerical simulation of a processes in the cavity of 170 GHz/CW/1 MW gyrotron for iter

We study two versions of a gyrotron operating in the TE_28.7 and TE_31.8 modes for electron-cyclotron heating of plasma in the international thermonuclear reactor ITER. The gyrotron cavity parameters are optimized allowing for ohmic losses, dips in electron beam potential, and velocity spread. The influence of the ion compensation for the space charge, the setting of oscillations at when the gyrotron is switched on, and the competition between the operating and parasitic modes are discussed. The possibility of attaining an efficiency of 32 to 36% for a specific power of ohmic losses in the cavity of less than 2.5 kW/cm² in the TE_28.7 mode and 2 kW/cm² in the TE_31.8 mode is demonstrated. Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 6, pp. 788–797, June, 1997.     

非均匀扰动结构TE0n模式变换器研究

本文提出一种半径和周期双重扰动的非均匀圆波导TE_0n模式变换器. 通过耦合波理论(CWT)和数值优化方法对该模式变换器进行研究, 计算结果与电磁仿真软件基本一致. 与传统的均匀结构半径微扰模式变换器比较, 非均匀扰动结构TE_0n模式变换器可以在更少的波纹周期内实现高于均匀结构的模式转换效率, 95%功率转换绝对带宽增加150%, 器件长度减小接近一半. 本文的研究工作为设计轴向尺寸短、工作带宽大、 转换效率高的高功率回旋管外接模式变换器提供了重要参考.     

RADIATION PATTERN MEASUREMENTS ON A MILLIMETER-WAVE OPEN RESONATOR

Far-field radiation patterns can be used for identifying different kinds of resonant modes in a gyrotron cylindrical open resonator. The operating TE₀₂₁ mode is identified among its closest competitors TE₂₂₁ and TE₆₁₁ by measuring radiation patterns obtained experimentally based on millimeter wave source for exciting the open resonator. A good agreement between experimental and, theoretically predicted values was found.     

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.     

Submillimeter-wave large-orbit gyrotron

We develop a thermionic-emission electron-optical system forming a dense beam of electrons moving along helical trajectories round the axis of a gyrotron cavity. The maximum beam current is 4 A and the pitch-factor of electrons is 1.0 for a particle energy of 250 keV and a pulse duration of 10 µs. Using such a beam in a gyrotron operated at the third cyclotron harmonic, we obtain single-mode oscillation with a power of 10 µs in the TE_3,8 and TE_3,9 modes with frequencies 371 and 414 GHz, respectively.     

An improved design for quasi-optical mode conversion of whispering gallery mode gyrotron radiation

Results are reported of a theoretical and experimental investigation of a quasi-optical mode converter for the transformation of whispering gallery mode gyrotron output into a linearly polarized Gaussian like beam. The mode converter consists of a helically cut waveguide launcher, similar to that originally proposed by Vlasovet al, followed by a focusing mirror. Theoretical results using aperture field methods indicate that the length of the waveguide launcher is of critical importance in providing a confined radiation pattern. Experimental results on the radiation pattern were obtained for several launcher lengths using a 0.6 MW, 149 GHz pulsed gyrotron operating in the TE_16,2 mode. Radiation pattern results for the optimum launcher length agree well with theoretical calculations using the Stratton-Chu aperture radiation theory for unperturbed waveguide modes. A mirror focusing in the azimuthal direction was designed by a geometrical optics approach to focus the radiation coming from the launcher. Good focusing with 91.4% efficiency (power in the focused beam divided by gyrotron power) was found experimentally using the combined launcher and mirror with the pulsed gyrotron. These results indicate that quasi-optical antennas are useful for transforming high order, high frequency gyrotron modes into directed beams in free space.     

Long Pulse Operation of the THz Gyrotron with a Pulse Magnet

Long pulse operation up to 1 msec of a high frequency gyrotron with a pulse magnet has been successfully carried out in a frequency range including 1 THz. In the experiments, the timing of an electron beam pulse injection is adjusted at the top of the magnetic field pulse, where the variation of field intensity is negligible. The operation cavity modes seem to be TE_{1, 12} and TE_4,12 at the second harmonics. The corresponding frequencies are 903 GHz and 1,013 GHz, respectively. Additionally several features of radiation measurement results of the gyrotron are described and brief considerations are presented.     

Development of 394.6 GHz CW Gyrotron (Gyrotron FU CW II) for DNP/Proton-NMR at 600 MHz

Gyrotron FU CW II with an 8 T liquid He free superconducting magnet, the second gyrotron of the THz Gyrotron FU CW Series, has been constructed and the operation test was successfully carried out. It will be used for enhancing the sensitivity of 600 MHz proton-NMR by use of Dynamic Nuclear Polarization (DNP). The designed operation mode of the gyrotron is TE_2,6 at the second harmonic. The corresponding frequency is 394.6 GHz. The real operation frequency is 394.3 GHz at TE₀₆ mode, because of fabrication error of the diameter of the cavity. The operation is in complete CW at the output power of around 30 W or higher at the TE₀₆ cavity mode. There are many other operation modes at the fundamental and the second harmonic. Typical output power of the fundamental and the second harmonic are higher than 100 W and 20 W, respectively. The highest frequency observed up to the present is 443.5 GHz at the second harmonic operation of TE_6,5 mode. The measured results are compared with the theoretical consideration.     

MAGICTRAC, a novel method for conversion of whispering-gallery modes into a free-space Gaussian-like beam

A novel device, MAGICTRAC, is described for efficient conversion at millimeter wavelengths of the TE _m,n whispering-gallery mode into a linearly polarized, free-space Gaussian-like beam. MAGICTRAC uses a mode-converting waveguide taper and three mirror optics, one of which incorporates a twist reflector to linearly polarize the output beam. An example design is presented for the TE_15,2 mode at 140 GHz with a calculated efficiency of 96%. Related possible applications include (1) installation of the MAGICTRAC within the vacuum envelope of a gyrotron to separarate the spent e-beam from the generated rf, (2) generation of a whispering-gallery mode by injection of a Gaussian-like beam into the output end, and (3) conversion of TE _m,n modes into TE_0n modes for low-loss transmission in smooth-wall waveguide.Work performed by LLNL for USDOE under contract W-7405 ENG-48.     

Design of a Quasi-Optical Mode Converter for a Frequency Step-Tunable Gyrotron

The quasi-optical mode converter for a frequency step-tunable gyrotron which consists of a dimpled-wall antenna (Denisov-type launcher) and a beam-forming mirror system has been optimized for 9 modes from TE_17,6 at 105 GHz to TE_23,8 at 143 GHz. The first mirror is a large quasi-elliptical focusing one; the second and third are phase-correcting mirrors with a non-quadratic shape of the surface. The results of calculations show that for these modes the Denisov-type launcher has a well-focused beam with low diffraction losses, and the radiation pattern presents an almost identical field shape for all modes considered. A multi-mode optimization of the phase-correcting mirrors with two different methods has been tested. The simulations show that the phase-correcting mirrors can be used for broadband operation in the frequency range from 105 GHz up to 143 GHz in the various design modes. This quasi-optical mode converter can achieve efficiencies of 94%-98% for converting the rotating high-order cylindrical cavity modes into the usable fundamental Gaussian mode.