Corrugated Horn Antenna for Low-Power Testing of the Quasioptical Transmission Lines at TJ-II Stellerator

This paper presents the design and characterisation of a millimetre-wave corrugated horn antenna to perform low power measurements of the main parameters of the quasioptical transmission lines of the Electron Cyclotron Resonance Heating (ECRH) at TJ-II stellerator experiment working in Madrid (E). The antenna generates a practically pure Gaussian Beam at 53.2 GHz that simulates the stellerator gyrotron output beam. Simulations of the designed corrugated horn, by using a specially developed computer code, are presented. The far field radiation pattern of the constructed prototype was measured at the ESTEC Compact Antenna Test Range (CATR). The measurement results are in excellent agreement with those obtained from simulations.     

Quasioptical Transmission Lines for ECRH at TJ-II Stellarator

Two mirror lines have been used to transmit the microwave power from the powerful microwave generators to the TJ-II plasmas. Both lines have been tested at nominal power level and they are in operation now. This paper deals with the final design of the transmission lines and their testing. Before starting operation at high power level, measurements of the wave beam parameters at low power level were performed. Two horn antennas were designed to simulate the gyrotron output. The results are presented in this paper. A computer code based on Huygens diffraction theory was developed to simulate the propagation of the beam along the mirror lines. A comparison of the theoretical and the experimental results is also shown here.     

Initiation of dusty structures in chain reactions under the action of gyrotron radiation on a mixture of metal and dielectric powders with an open boundary

A dusty plasma formed in chain exothermal reactions initiated by radiation of a high-power gyrotron in mixtures of metal and dielectric powders has been described. An oscillatory character of such chain reactions, as well as the appearance of dust particles at the first (explosive) stage, has been detected. The tracks, velocities, and sizes of dust particles have been measured. It has been revealed that ensembles of dust particles appear in a reactor after switching-off of the gyrotron against the background of development of chemical reactions. The time of existence of these ensembles is three or four orders of magnitude larger than the duration of a microwave radiation pulse. The quasistationary state of the low-temperature plasma with charged macroparticles appears because of both the chemical heating of the mixture in the reactor and thermophoresis. It has been shown that dust particles are necessary as crystallization nuclei for the creation (or deposition) of complex composites of nano- and micromaterials produced in secondary plasma chemical synthesis.     

Moderately relativistic high-harmonic gyrotrons formillimeter/submillimeter wavelength band

A large orbit gyrotron with a 300 keV, 30 A, 20 ns electron beam has provided selective generation with radiation power from 1.5 MW at fundamental (wavelength of 14 mm) to 100 kW at the fifth cyclotron harmonic (wavelength of 4 mm). The next experiment with a unique electron beam (250 keV, 18 A, 10 μs, 0.6 mm in diameter) is in progress     

Using a standard sample to estimate the X‐ray wavelength of the 1W2A SAXS beamline at BSRF

This contribution describes a method for measuring diffraction peaks of a standard sample to estimate the incident X‐ray wavelength at the 1W2A SAXS beamline at BSRF. A simple simulation has been performed to establish the factors influencing the accuracy of the wavelength measurement. Appropriate measurement conditions and error control measures are presented. An actual experimental example further verifies the effectiveness of the simulation. This method is particularly suitable for synchrotron radiation beamlines using bent triangular crystal monochromators.     

Generation of 1.5-kW, 1-THz coherent radiation from a gyrotron with a pulsed magnetic field

To cover a so-called terahertz gap in available sources of coherent electromagnetic radiation, the gyrotron with a pulsed solenoid producing up to a 40 T magnetic field has been designed, manufactured, and tested. At a 38.5 T magnetic field, the gyrotron generated coherent radiation at 1.022 THz frequency in 50 musec pulses. The microwave power and energy per pulse were about 1.5 kW and 75 mJ, respectively. Details of the gyrotron design, manufacturing, operation and measurements of output radiation are given.     

Experimental study of a continuous-wave high-stability second-harmonic gyrotron for spectroscopy of dynamically polarized nuclei

We present the results of the experiments on a continuous-wave gyrotron with an output frequency of about 260 GHz, which has been developed for dynamic polarization of nuclei and is operated at the second cyclotron-frequency harmonic. For an accelerating voltage of 16 kV and a beam current of 0.3 A, a maximum continuous-wave radiation power of about 200 W has been obtained. Measurements of the thermal load on the resonator cavity allow evaluating its actual ohmic Q-factor. Gyrotron radiation is also observed at other frequencies and modes. Their generation zones and the structure of the output radiation have been studied. Long-term stability of gyrotron parameters has been determined.     

High Power, Frequency Tunable, Submillimeter Wave ESR Device Using a Gyrotron as a Radiation Source

ESR device using a submillimeter wave gyrotron as a radiation source and a pulse magnet for high field up to 30 T has been constructed. Our gyrotrons (Gyrotron FU series) were developed as millimeter and submillimeter wave radiation sources and have attractive advantages for ESR spectroscopy, for example, high power and frequency tunability over broad range. The ESR device has been successfully applied to three cases of ESR measurements. In the first case, the temperature dependence of ESR was measured for a typical antiferromagnetic material MnO at the frequency of 301 GHz. In the second case, the dependence of the fine structure constant of the ruby on the magnetic field intensity was measured in the millimeter to submillimeter wave region. In these two cases, the gyrotron was operated by complete cw mode. In the final case, a pulse technique was applied to the ESR, the gyrotron was operated in pulse mode and the pulsed magnetic field was generated in the synchronized phase with the gyrotron operation.     

Optical spectroscopy of plasma in high power microwave pulseshortening experiments driven by a μs e-beam

Microwave pulse shortening experiments have been performed on a rectangular-cross-section (RCS) gyrotron driven by the Michigan Electron Long Beam Accelerator (MELBA) at parameters V=-800 kV, I_tube=0.3 kA and pulselengths of 0.5-1 μs. Pulse shortening typically limits the highest (10 MW level) microwave power pulselength to 100-200 ns. Potential explanations of pulse shortening are being investigated, particularly plasma production inside the cavity and at the e-beam-collector. We report the first optical spectroscopy diagnostic measurements inside an operating gyrotron as a means of exploring plasma effects on pulse shortening. Plasma hydrogen H-α line radiation has been characterized in both time-integrated and temporally-resolved measurements and correlated with microwave power/cutoff. Hydrogen is believed to originate from water absorbed on internal tube surfaces in the gyrotron     

Ten-Megawatt Pulsed Gyrotron with a 1-cm Wavelength and a 50% Efficiency

Development of a gyrotron driven by a relativistic electron beam and generating radiation at the wavelength 1 cm with an output power of 10 MW, an efficiency of 50%, and a pulse duration of a few hundreds of nanoseconds is reported.     

A submillimeter gyrotron with a pulsed magnetic field

Pulse solenoids seem promising for sumbillimeter wave generation in gyrotrons. In this paper a single-pulse submillimeter gyrotron with a pulsed magnetic field is described. At the wavelength 0.8 mm the output power 120 KW with the efficiency 15% has been obtained.     

Design of a CW 1 THz Gyrotron (Gyrotron Fu Cw III) Using a 20 T Superconducting Magnet

Design of a CW 1 THz gyrotron at second harmonic operation using a 20 T superconducting magnet has been described. The mode competition analysis is employed to investigate operation conditions of second harmonic mode, which is being excited at the frequency ranging from 920 GHz to 1014 GHz. The output power up to 250 watt corresponding to the efficiency of 4.16 percent could be achieved by using an electron beam with accelerating voltage 30 kV and current 200 mA. The important advantage of this gyrotron is that the single mode excitation at second harmonic, and extremely high frequency of the radiation, could be maintained even at high currents. It opens possibility to realize a high power radiation source at 1 THz. Such gyrotron is under construction at FIR Center, University of Fukui.     

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.     

Whispering-Gallery-Mode Gyrotron Operation with a Quasi-Optical Antenna

High-frequency gyrotron operation in the TE611 whispering gallery mode is reported. Powers as high as 112 kW and an efficiency as high as 25.4 percent have been obtained. In addition to the TE611 mode, ten other fundamental modes with frequencies between 133.9 and 216.4 GHz were observed. A quasi-optical antenna for whispering gallery modes has been tested for the TE611 mode. The combination of this antenna and a reflector produces a well-collimated linearly polarized RF beam suitable for electron cyclotron resonance heating (ECRH) or plasma diagnostics. The experimental conversion efficiency was determined to be 89 percent, and the cross polarization was down by 25 dB. A new quasi-optical transmission line employing this antenna has also been tested with the gyrotron. The absence of mode competition for the TE611 mode, as well as the efficient conversion of the output radiation into a linearly polarized Gaussian-like beam, substantiates the arguments for operation in surface modes in high-power gyrotrons. The implications of quasi-optical antennas for megawatt gyrotron window design are discussed.     

Determination of gyrotron wave beam parameters

The output radiation of a gyrotron has the form close to the Gaussian beam. Knowing parameters of the wave beam along its way from a gyrotron to an object permits one to optimize the transmission losses. By this, the structure of the wave beam can be recorded with an infrared camera in several cross-sections of the beam. Real gyrotron wave beam is approximated with high precision by the Gaussian beam.     

Axisymmetric optical system for quasi-optical gyrotron

An analysis is carried out with a profile of hollow Gaussian radiation beam propagating in an optical system composed of coaxial annular curved mirrors. The result will be a useful tool in designing an axisymmetric quasi-optical gyrotron oscillator (ASQUOTRON).     

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.     

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.     

Highly Efficient Gyrotron with Multi-Stage Recuperation of Residual Electron Energy

Technical Physics - The results of complex modeling of physical processes in a gyrotron of moderate power in the 4-mm wavelength range have been presented. Methods for improving the quality of the...