Design of a system converting an output radiation of frequency tunable gyrotron into a gaussian beam

A novel design of a versatile quasi-optical system for conversion of gyrotron radiation into collimated gaussian beams is presented and discussed. The proposed system consists of a quasi-optical antenna, two ellipsoidal mirrors and a spatial filter which truncates the sidelobe radiation. The system is appropriate as a transmission line for frequency tunable gyrotrons operating at TE_0n and TE_1n modes. 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 four TE_1n mode outputs (TE₁₂, 170 GHz; TE₁₃, 271 GHz; TE₁₄, 372 GHz; TE₁₅, 472 GHz) into gaussian-like beams and three TE_0n modes (TE₀₂, 223 GHz; TE₀₃, 323GHz; TE₀₄, 423 GHz) into bigaussian-like beams.     

Design of a quasi-optical mode conversion system with variable output beam size

The quasi-optical system consisting of a quasi-optical antenna and two parabolic mirrors can convert the TE₀₃ mode output (f = 301 GHz) of the Gyrotron FU IV into a well collimated, linearly polarized beam. This system offers the possibility of tuning the size of the beam produced from 5 mm up to 24 mm by rotating and moving the two parabolic mirrors.     

Experimental study of a single-mode quasi-optical gyrotron

The first experimental results of a high-power quasi-optical gyrotron are presented. The cavity was designed to eliminate all mode competition, longitudinal as well as radial. Powers to 80 kW and efficiencies to 11% were measured with indications that the actual electronic efficiency was at least a factor of 1·5 greater.     

Cold tests of quasi-optical gyrotron resonators

Cold tests which have been performed on quasi-optical gyrotron resonators at frequencies near 94 and 120 GHz to measure cavity Q are discussed. The separation between the resonator mirrors was varied between 0.15 and 0.35 m. Quality factors ranging from 10000 to 100000 are shown. Good agreement is shown between the measured data and values calculated from scalar diffraction theory. The experimental effect of misaligning the mirrors is examined     

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.     

Design of a low-power tunable quasioptical gyrotron with a slotted output mirror

A low-power tunable quasi-optical gyrotron was designed for driving free electron laser amplifiers in the frequency ránge of 220–270 GHz. The gyrotron cavity consisted of a spherical mirror and a plane slotted mirror to produce a Gaussian radiation output beam. Output powers of 1–10 kW were obtained in the 4 cm length cavity by a few amperes of electron beams with a voltage of 30 kV.     

Sub-nanosecond pulselength millimetre wave generation with a phase modulated quasi-optical gyrotron

It is shown that a quasi-optical gyrotron operating with a current modulated second beam can produce a train of phase-locked millimetre wave pulses.     

Operation of the quasi-optical gyrotron at harmonics of the gyrofrequency

This paper investigates the performance of a quasioptical gyrotron, when the electron beam interacts with the radiation fields at harmonics of the gyrofrequency. The nonlinear equations of motion are obtained in the slow-timescale. The expression for the linear gain is derived and the conditions for excitation are given (frequency threshold, optimal operating point, bean current and resonator quality thresholds). In the nonlinear regime, it is shown that maximum efficiencies comparable to those at the fundemental (～50%) are possible, albeit at a prohitively high radiation field amplitude, while realistically feasible field amplitudes can give somewhat smaller, but nevertheless still high efficiencies (～15%). Finally, the results are suplemented by empirical scaling laws, useful for experimental designs.     

Frequency measurement of a submillimeter wave gyrotron output

Measurement of frequency spectra of the output of a gyrotron in the millimeter to submillimeter wavelength range using a Michelson interferometer have been made. The results are compared with frequency measurements using a Fabry-Perot interferometer. A correction for mesh transmittance is used to give the true frequency spectrum. The method is used to search for the presence of higher cyclotron harmonics (n=f /f _c=3,4,…) which may be generated along with the more intense fundamentals (n=1) or second harmonics (n=2), and to study mode competition between the fundamental and the second harmonic modes.     

First experiment of gyrotron tube with quasi-optical cavity of special configuration

A new kind of quasi-optical cavity—Axisymmetrical Quasi-Optical Cavity of Oblique Rotation (AQCOR) and the new scheme for quasi-optical gyrotron with AQCOR had been proposed in [1], and the experimental tube was built in our institute. Now the initial test results of the tube are reported in this paper.     

A high-efficiency quasi-optical mode converter for a 140-GHz 1-MW CW gyrotron

A highly efficient quasi-optical mode converter system with several novel features has been designed and tested at Forschungszentrum Karlsruhe (FZK). The converter consists of a dimpled-wall waveguide launcher, one quasi-elliptical mirror and two toroidal mirrors. The coupled-mode theory has been used to analyze the operation of the prebunching waveguide launcher; the radiated fields from the cut of the launcher have been calculated by the scalar diffraction integral. Simulation results show that the advanced dimpled-wall launcher generates a well-focused Gaussian radiation pattern with low diffraction losses. In this case, toroidal mirrors are sufficient to obtain a desired output beam pattern. An efficiency of more than 98% has been achieved to convert the rotating TE/sub 28,8/ cavity mode at 140 GHz into a fundamental Gaussian beam. Experimental measurements show close agreement with theoretical predictions.     

Experimental study of a quasi-optical cavity with slotted output mirror

This paper describes an experimental study of a quasi-optical cavity to be used in qyrotrons. The cavity configuration was designed so that its output wave could pass into a quasi-optical transmission line. The test cavity was coaxially composed of a pair of annular mirrors, one of which was a curved conical mirror and the other was a plane mirror having a radial slot array for wave output. A mode excited in the cavity at 56.4 GHz was observed by detecting the radiation wave that leaked out through the slotted mirror. It was shown that the present slotted mirror served as a partially and uniformly transparent mirror to radiate the output wave with the same mode as in the cavity.     

Design of a 1-MW,CW coaxial gyrotron with two gaussian beam outputs

The design of a 170 GHz, 1 MW-CW gyrotron for electron cyclotron heating of nuclear fusion plasmas is presented. The designed gyrotron incorporates a coaxial cavity to reduce mode competition, and a coaxial electron gun to support the cavity inner conductor. A new mode converter splits the generated wave into two beams and radiates them in different directions. The radiated beams are transmitted to two output windows through two mirror systems, being transformed into Gaussian-like beams. A single-stage depressed collector improves the overall efficiency of the gyrotron and reduces the heat flux to the collector surface.     

A depressed collector system for a quasi-optical gyrotron with precisely controlled magnetic flux lines

Design of a depressed collector system for a quasi-optical gyrotron, which had a severe constraint on the maximum allowable radius of the collector region is outlined. The needs for unwinding of spent beam and for energy sorting could be accommodated by precise control of the magnetic field profile, especially in the collector region. Techniques used for defining and obtaining such profiles; and for dovetailing the profile with the collector geometry are discussed. Results on profiles and electron trajectories are presented, which demonstrate the feasibility of the design. From primary electron trajectories a collector efficiency of up to 68% has been calculated for a three collector design.     

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.     

Eccentricity of the electron beam in a gyrotron cavity

Dependence of the coupling of the electron beam and RF field in a gyrotron cavity on eccentricity of the electron beam is studied.     

Generation of the Gaussian-like HE11 mode from gyrotron TEOn mode mixtures at 70 GHz

This work reports calculations and measurements on mode converters for the transformation of TEOn gyrotron mode mixtures (primarily TEO2) into the linearly polarized HE11 hybrid mode at 70 GHz. This mode is ideal for quasi-optical launching systems for ECRH of plasmas. Mode transducers with axisymmetric radius perturbations convert the gyrotron TEOn mode composition into the TEO1 mode. Proper matching of the phase differences between the various modes and of the perturbation amplitudes of the several converter sections is required. The TEO1 wave is used for long-distance transmission through smooth-walled overmoded waveguides. A mode converter with constant diameter and periodically perturbed curvature transfers the unpolarized TEO1 mode into the polarized TE11 wave. The experimentally determined ΣTEOn-to-TEO1 conversion efficiency is (98.5±1)% (99% predicted) while the TEO1-to-TE11 transformer has a (94±2)% efficiency (93% theoretically); ohmic losses are included. The “Gaussian-like” HE11 mode with axisymmetric power distribution and almost no cross polarization is produced in a circumferentially corrugated TE11-to-HE11 mode transducer with a measured conversion efficiency of (98.3±1.5)% (98.5% predicted). The overall efficiency of the complete mode converter system in the desired mode was determined to be (91±2.5)%. High-power operation (200kW, 100ms) has been successfully demonstrated.     

Amplitude modulation of submillimeter wave gyrotron output

Amplitude modulation of gyrotron by a small modulation of the anode voltage is calculated using an energy transfer formula. Experimental measurements using a submillimeter wave gyrotron are in good agreement. One hundred percent modulation of the output at frequencies up to several hundred kilohertz has been achieved with anode modulation levels of only a few percent. Numerical calculations lend further support to the experimental results.     

Design of a 35 GHz gyrotron

The conceptual design of a 35 GHz gyrotron has been developed consistently with the complex formulation of the electric field longitudinal distribution in the resonant cavity. Some models of magnetron injection guns able to produce laminar beams have been investigated leading to the design of an electron gun capable of generating a current of 5 A with a perpendicular velocity dispersion of 0.5%. The device includes three magnetic systems producting flat axial magnetic induction profiles of 1.05 kG, 13.2 kG and 0.65 kG in the cathode, cavity and collector regions, respectively. The gyrotron has been designed for pulsed operation in the TE₀₂₁ mode. Under the soft self-excitation condition, the maximum attainable efficiency is 40% with an output power of 100 kW. An analysis of the collector thermal behaviour has been carried out as well as a study of the thermophysical properties of the alumina window to be used.     

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.