140-GHz gyrotron experiments based on a confocal cavity

We have designed and experimentally demonstrated the operation of a novel quasioptical gyrotron oscillator based on an overmoded confocal waveguide cavity. This cavity effectively suppresses undesired modes, and therefore has extremely low mode density. Stable single-mode, single-frequency operation was achieved in the TE₀₆ mode at 136 GHz. A peak RF output power of 66 kW, corresponding to an efficiency of 18%, was measured. By varying the cavity magnetic field, high-power generation was observed at 136 GHz in the TE₀₆ mode and at 114 GHz in the TE₀₅ mode. These frequencies correspond to the high Q modes of the confocal resonator. The low Q modes were either weak or not observed. In this paper, we will review the design procedure for this cavity and present experimental data verifying its effectiveness in reducing the number of modes that can be excited. The confocal waveguide could also be used in high-power, gyro-TWT amplifiers to provide greater operating stability and bandwidth, especially in an overmoded waveguide structure     

Experimental Study of a Ka-Band Cylindrical Open Resonator

A study of the electrodynamical properties of a Ka-band gyrotron open resonator was experimentally conducted. Experiments were accomplished to measure resonant frequencies and their respective loaded quality factors for TE modes in the frequency range from 26 to 40 GHz. In particular, a perturbation technique was used to determine the axial, radial and azimuthal electric field profiles, as an identification method of the TE₀₂₁ mode operating around 35 GHz. In any experimental event, good agreement with the values predicted by theory was found.     

RF behavior of triple-frequency high power fusion gyrotron

The RF behavior of high power, triple frequency (170-, 127.5-, and 85 GHz) gyrotron for fusion application is presented in this paper. The operating mode selection is discussed in detail for each corresponding frequencies and TE_34,10, TE_25,8 and TE_17,5 modes are selected as the operating mode for 170 GHz, 127.5 GHz and 85 GHz operation of the device, respectively. The interaction cavity geometry and beam parameters are finalized by the cold cavity analysis and beam-wave interaction simulations. Considering the beam parameters and the beam launching positions in cavity (beam radius), the design of Magnetically Tunable MIG (MT-MIG) is also presented. Results of MT-MIG confirm the beam launching with desired beam parameters at the beam radius corresponding to the selected operating modes for all three frequencies. The CVD diamond window is also designed for RF power transmission. The beam-wave interaction simulations confirm more than 1 MW power at all three frequencies (170-, 127.5-, and 85 GHz).     

5.8GHz磁控管的数值模拟和实验研究

C波段高稳定度磁控管是目前磁控管的研究重点。对5.8GHz磁控管进行模拟研究,冷腔计算磁控管π模频率为5.863GHz,阳极用双端双隔模带结构磁控管的工作频率与相邻模式频率分隔度为44%。模拟磁控管输出频率为5.856GHz,输出微波功率约1.2kW。对研制的磁控管进行注入锁定实验研究,输出微波功率1.047kW,效率约为58%。磁控管锁频锁相后输出的频率和相位稳定。     

A Ku-band magnetically insulated transmission line oscillator with overmoded slow-wave-structure

In order to enhance the power capacity, an improved Ku-band magnetically insulated transmission line oscillator(MILO) with overmoded slow-wave-structure(SWS) is proposed and investigated numerically and experimentally. The analysis of the dispersion relationship and the resonant curve of the cold test indicate that the device can operate at the near π mode of the TM01 mode, which is useful for mode selection and control. In the particle simulation, the improved Ku-band MILO generates a microwave with a power of 1.5 GW and a frequency of 12.3 GHz under an input voltage of480 k V and input current of 42 k A. Finally, experimental investigation of the improved Ku-band MILO is carried out. A high-power microwave(HPM) with an average power of 800 MW, a frequency of 12.35 GHz, and pulse width of 35 ns is generated under a diode voltage of 500 k V and beam current of 43 k A. The consistency between the experimental and simulated far-field radiation pattern confirms that the operating mode of the improved Ku-band MILO is well controlled inπ mode of the TM01 mode.     

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.     

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.     

Theory of step-tunable gyrotrons operating at two cyclotronharmonics

The scaling of operating parameters and the tendencies in the efficiency of the step-tunable gyrotron operating in a set of modes in a cavity of a given length are considered. Results of calculations are reported for the design of a ~1-kW, step-tunable gyrotron oscillator operating from 150 to 600 GHz in steps of about 15 GHz. Detailed results are presented for the efficiency, power losses in the cavity, and the starting conditions. Designs of the electron gun electrodes and of an output mode transducer are also presented. Full modulation of the radiation via the modulating anode of a magnetron injection gun is considered. The gyrotron would be of use for plasma diagnostics and spectroscopy, including electron paramagnetic resonance spectroscopy (EPR)     

Experimental study of a plasma-filled backward wave oscillator

Experimental studies of a plasma-filled X-band backward-wave oscillator (BWO) are presented. Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement by a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from ⩽0.5 GHz to 2 GHz when the BWO was working at the RF power enhancement pressure region. The RF power enhancement appeared over a much wider pressure range in a high beam current case (10-100 mT for 3 kA) than in a lower beam case (80-115 mT for 1.6 kA). The plasma-filled BWO has higher power output than the vacuum BWO over a broader region of magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) are observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the T-G modes and the X-band Tm₀₁ mode prevailed when the background plasma density was below 6×10¹¹ cm^-3 . At a critical background plasma density of ≃8×10¹¹ cm^-3 power enhancement appeared in both X-band and the T-G modes. Power enhancement of the S-band in this mode collaboration region reached up to 8 dB. Electric fields measured by the Stark-effect method were as high as 34 kV/cm while the BWO power level was 80 MW. These electric fields lasted throughout the high-power microwave pulse     

A dual-mode microwave resonator for double electron-electron spin resonance spectroscopy at W-band microwave frequencies

We present a dual-mode resonator operating at/near 94 GHz (W-band) microwave frequencies and supporting two microwave modes with the same field polarization at the sample position. Numerical analysis shows that the frequencies of both modes as well as their frequency separation can be tuned in a broad range up to GHz. The resonator was constructed to perform pulsed ELDOR experiments with a variable separation of "pump" and "detection" frequencies up to Δν=350 MHz. To examine its performance, test ESE/PELDOR experiments were performed on a representative biradical system.     

Investigation about the modes in the cylindrical cavity of an ECR ion source

Both theoretical and numerical electromagnetic analyses about the modes inside the cylindrical cavity of superconducting electron cyclotron resonance ion source (SERSE) have been performed. Modes close to 14, 18 and 28 GHz frequencies, usually employed in SERSE operating conditions, have been calculated in vacuum and when the chamber is filled with a uniform non-collisional plasma at different electron densities. To consider the holes present in the chamber flanges, a numerical approach has been used, by means of the HFSS^TM electromagnetic simulator, for the first mode in the cavity. Modes in a plasma-filled cylindrical cavity with holed bases have full width half maximum bandwidths larger with regard to the closed cavity in vacuum, and it leads to an increased mode frequency overlap. A monochromatic electromagnetic wave feeding this cavity can, in principle, excite different modes. Further investigations about the coupling between the SERSE cavity and its feeding waveguides have to be performed.     

Phase constant peculiarities of cylindrical zero-index anisotropic metamaterial waveguide

Here, we present the phase constant dependencies of propagating eigenmodes of an open cylindrical anisotropic metamaterial waveguide when the metamaterial permittivity and permeability tensor components may accept values close or equal to zero. Dispersion characteristics of rod and hollow-core waveguides with the radii 0.5, 2.5, and 5 mm at the left-handed polarization of the microwave are shown here. There are unusual shapes of eigenmode dispersion characteristics and anomalous sectors of the characteristics at certain frequencies. The first eigenmode of the rod waveguide with the lowest cutoff frequency is a particularly important mode because it is a single one in the frequency range 1.0?C1.9 GHz, and some small variations on the frequency produce large changes in the phase constant. We can observe packages of dispersion characteristic branches when their cutoff frequencies closed to the metamaterial electric and magnetic plasma frequencies between 1.9 and 3.5 GHz. There are only three modes in the hollow core anisotropic metamaterial waveguide at the frequency range 1.4?C2.8 GHz.     

Measured mixer noise temperature and conversion loss of a cryogenic Schottky diode mixer near 800 GHz

We accurately measured the noise temperature and conversion loss of a cryogenically cooled Schottky diode operating near 800 GHz, using the UCB/MPE Submillimeter Receiver at the James Clerk Maxwell Telescope. The receiver temperature was in the range of the best we now routinely measure, 3150 K (DSB). Without correcting for optical loss or IF mismatch, the raw measurements set upper limits ofT _M=2850 K andL _M=9.1 dB (DSB), constant over at least a 1 GHz IF band centered at 6.4 GHz with an LO frequency of 803 GHz. Correction for estimated optical coupling and mismatch effects yieldsT _M=1600 K andL _M=5.5 dB (DSB) for the mixer diode itself. These values indicate that our receiver noise temperature is dominated by the corner cube antenna's optical efficiency and by mixer noise, but not by conversion loss or IF mismatch. The small fractional IF bandwidth, measured mixer IF band flatness from 2 to 8 GHz, and similarly good receiver temperatures at other IF frequencies imply that these values are representative over a range of frequencies near 800 GHz.     

FeCoBSiO2磁性纳米颗粒膜的微波电磁特性

采用交替沉积磁控溅射工艺制备了超薄多层的FeCoBSiO2磁性纳米颗粒膜.利用x射线衍射仪、扫描探针显微镜、透射电子显微镜分析了薄膜的微结构和形貌特征.采用振动样品磁强计、四探针法、微波矢量分析仪及谐振腔法测量薄膜试样的磁电性能和微波复磁导率.重点对SiO2介质相含量、薄膜微结构对电磁性能产生重要影响的机理做了分析和探讨.结果 表明：这类FeCoBSiO2磁性纳米颗粒膜具有良好的软磁性能和高频电磁性能，2GHz时的 磁导率μ′高于70，可以应用于高频微磁器件或微波吸收材料的设计. 关键词： 磁性纳米颗粒膜 高频特性 复磁导率 磁控溅射     

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.     

Design and experimental operation of a 165-GHz, 1.5-MW,coaxial-cavity gyrotron with axial RF output

The development of a coaxial-cavity gyrotron operating in TE_31,17 mode at 165 GHz is presented. The selection of the operating frequency and mode are based on the limitations imposed by the maximum held of the superconducting (sc) magnet at Forschungzentrum Karlsruhe, Institut fur Technische Physik (FZK), the use of the inverse-magnetron injection gun (IMIG) of the 140-GHz, TE_28,16 coaxial gyrotron and the possibility of transforming the cavity mode to a whispering gallery mode (WGM) appropriate for the dual-beam quasioptical (q.o.) output coupler and the two output windows, which are foreseen for the next lateral output version of the tube. The tube with axial output has been tested at FZK to deliver maximum output power of 1.17 MW in the designed TE_31,17 mode with 26.7% efficiency at 164.98 GHz. Maximum efficiency of 28.2% was achieved at 0.9-MW output power. The design operating point with output power 1.36 MW and 36.7% efficiency was net accessible because of beam instabilities at high electron-velocity ratio α, presumably caused due to high electron-velocity spread. Power at higher frequencies was also detected: 1.02 MW at 167.16 GHz in TE_32,17 mode with 26.88 efficiency, 0.63 MW at 169.46 GHz in TE_33,17 mode with 18% efficiency, and 0.35 MW at 171.80 GHz in TE_31,17 mode with 13.3% efficiency     

INFLUENCE OF MAGNETIC FIELD INHOMOGENEITY ON OPERATION OF THE THz GYROTRON WITH A PULSE MAGNET

Influence of magnetic field tapering on operation of a gyrotron working in the TE _4,12 mode at the second harmonic frequency 1013.67 GHz is investigated. It is found that the existing inhomogeneity of the magnetic field of the order of 0.25%–0.50% in the cavity allows one to achieve higher efficiencies. It improves also mode competition scenario by suppressing oscillations of the two parasitic TE _3,6⁺ and TE _5,5⁻ modes at the fundamental frequencies 513.35 GHz and 503.64 GHz, respectively.     

Towards a 1 MW, 170 GHz gyrotron design for fusion application

The electrical design of different components of 1 MW, 170 GHz gyrotron such as, magnetron injection gun, cylindrical interaction cavity and collector and RF window is presented in this article. Recently, a new project related to the development of 170 GHz, 1 MW gyrotron has been started for the Indian Tokamak. TE_34,10 mode is selected as the operating mode after studied the problem of mode competition. The triode type geometry is selected for the design of magnetron injection gun (MIG) to achieve the required beam parameters. The maximum transverse velocity spread of 3.28% at the velocity ratio of 1.34 is obtained in simulations for a 40 A, 80 kV electron beam. The RF output power of more than 1 MW with 36.5% interaction efficiency without depressed collector is predicted by simulation in single-mode operation at 170 GHz frequency. The simulated single-stage depressed collector of the gyrotron predicted the overall device efficiencies >55%. Due to the very good thermal conductivity and very weak dependency of the dielectric parameters on temperature, PACVD diamond is selected for window design for the transmission of RF power. The in-house developed code MIGSYN and GCOMS are used for initial geometry design of MIG and mode selection respectively. Commercially available simulation tools MAGIC and ANSYS are used for beam–wave interaction and mechanical analysis respectively.     

Operation of a 32 GHz gyrotron

The design and operation of a 32 GHz pulsed gyrotron are reported. The device is step-tuned between the TE_1,2 (24.16 GHz) and TE_0,2 (31.78 GHz) modes with cathode voltages ranging from 30 to 40kV and beam current up to 5.0A. Experimental frequencies are in close agreement with the self-consistent calculated values and in the TE_2,2 resonator mode an output peak power of 6kW corresponding to an 18% efficiency was measured by using a fast response calorimeter with a thermal sensitivity of 0.1°C/Wmin.     

High-Voltage Ka-Band Gyrotron Experiment

A new high-voltage, intense-beam, Ka-band gyrotron experiment is underway using a 350-400-keV 0.5-1-kA beam from a short-pulse Febetron pulser. High-power operation at 35 GHz has been observed, with 20 MW (±3 dB) produced at ~8.5-percent efficiency in a "whispering-gallery" TE62 mode. Operation in other modes and at other frequencies has also been studied.