Quasi-optical Mode Research Articles

Design, Analysis, and Simulation Studies of TE10,4Mode, 100-kWW-Band Gyrotron Oscillator

The design studies of a relatively higher-order mode, TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10,4</sub> , gyrotron have been presented for reduced beam interception, ohmic loss, and dimensional tolerance issues. The tapered RF interaction cavity and the nonlinear taper (NLT) designed using axial mode profile calculations were optimized with time-dependent, multimode theory with tapered magnetic field profile. 3-D particle-in-cell (PIC) simulation predicted the desired device temporal growth with ~40% efficiency and the percentage of agreement with the analytical values is presented. Device behavior under practical tolerances has also been studied confirming the rated device performance. Further, its interaction structure design has been checked from thermal considerations without any additional cooling fins. Furthermore, Denisov type quasi optical mode (QOM) launcher along with a sapphire disk window has been designed using in-house developed algorithm and coupled-mode theory-based code. The designed profile provides >98% Gaussian content on the launcher cut.

Quasi-Optical Mode Converter for a 0.42-THz TE26Mode Pulsed Gyrotron Oscillator

A quasi-optical (QO) mode converter for a 0.42-THz, TE26 mode, pulsed gyrotron is designed, fabricated, and tested. Based on the geometric optics principle, vector diffraction theory, and the iterative Stratton–Chu method, a computer code is developed to design and optimize the QO mode converter. The scalar Gaussian mode content is about 93.7%, which agrees well with that obtained by the electromagnetic simulation software. The preliminary experiment of the 0.42-THz, TE26 mode QO mode converter shows that a well-focused Gaussian-like beam with 90.4% scalar Gaussian mode content has been observed on the output window by using an infrared video camera. The experimental results agree well with the theoretical prediction.

High Efficiency Mode Converter for Low-Frequency Gyrotron

A high efficiency quasi-optical (QO) mode converter for high-power, low-frequency gyrotron have been designed and tested. For low-frequency gyrotrons, the scales of the mode converter are comparatively small on the wavelength scale, thus causing significant diffraction losses. Over-1 MW power gyrotron with TE8,3 cavity at 28 GHz have been developed, which has a high efficiency mode converter designed by the use of numerical methods for launcher optimization. This calculation is sufficiently optimized to maximize the fractional Gaussian content of the far field. The total transmission efficiency from the mode converter to output window is 94.7%. For the experimental result of first tube, the output power of more than 1 MW has been obtained with about 40% efficiency and output burn pattern agrees fairly with the calculated profiles, which imply the design appropriateness. Besides, the frequency dependence for diffraction loss is discussed, and these results give the guiding design principle of the mode converter for high-power, low-frequency and long-pulse gyrotrons.

Synthesis of quasioptical waveguide mode converters on the basis of the Fox–Li method

We consider a generalization of the Fox-Li method to the problems of analysis and synthesis of weakly irregular quasioptical waveguides. It is shown that the field representation in the form of integral expansions in terms of convergent and divergent cylindrical waves makes it possible to efficiently solve such problems with allowance for the effects of nonparaxiality of wave beams and the vector character of electromagnetic fields. An example of synthesizing a converter of the output radiation of a gyroklystron with an operating frequency of 30 GHz at the TE 5,3 mode to the Gaussian wave beam is considered.

Sub-THz Beam-Shaping Mirror System Designs for Quasi-optical Mode Converters in High-power Gyrotrons

Methods, algorithms and theories required in sub-THz beam-shaping mirror designs for Quasi-Optical (QO) mode converters (launchers) in high-power gyrotrons are presented in this article. The state-of-the-art design procedure highlights many novel ideas like the Iterative Phase Retrieval (IPR) technique, the Taylor-Interpolation FFT (TI-FFT) algorithm and the iterative Geometrical Optical Phase Correction (GOPC) method, which ensure fast, reliable, flexible and accurate sub-THz beam-shaping mirror system designs in the high-power gyrotron applications. The efficiency of the sub-THz beam-shaping mirror system procedure has been successfully tested in a sub-THz beam-shaping 4 -mirror system design for a single-mode (TE22,6/110GHz), 1.25 MW CPI gyrotron to shape the input beam from a QO launcher into the target Fundamental Gaussian Beam (FGB) to be injected into the corrugated waveguide through the diamond window, with up to 99.85% coupling between the designed output beam and the target FGB.

Quasi-Optical Mode Converter/Mirror System for a High-Power Coaxial-Cavity Gyrotron

This paper presents the investigation of a mirror system of a quasi-optical (QO) mode converter for a high-power coaxial-cavity gyrotron. The mirror system consists of three mirrors. The first mirror is a quasi-elliptical one. Based on the Katsenelenbaum-Semenov Algorithm (KSA), the second and the third mirrors are iteratively optimized as adapted phase-correcting mirrors to transform the outgoing wave beam into a fundamental Gaussian structure. The investigation shows that the focal length of the quasi-elliptical mirror has a great influence on the optimized conversion efficiency, and, hence, it should be chosen to match the asymptotic beam growth (ABG) angle well in order to obtain high conversion efficiency. The design of a mirror system has been performed for a 2 MW, continuous wave (CW), 170 GHz, and TE34,19 -mode coaxial-cavity gyrotron, which is under development at Forschungszentrum Karlsruhe, Germany. Taking into account the size of the mirrors and the conversion efficiency, a mirror system has been designed with a conversion efficiency of 98.3%

Efficient Broad Band HE11 Mode Exciter

A new scheme of quasi-optical HE11 mode exciter is presented. It consists of a smooth rectangular horn, dielectric focusing lens and output waveguide section. The main advantages of the converter are frequency broad band operation and simple design. Results of measurements of converter parameters performed at millimeter waves agree well with the design parameters: total conversion efficiency exceeds 96% in the frequency band of 20%.

TE01-TEM00 Quasi-Optical Mode Converter

A new quasi-optical TE01-TEM00 mode converter is presented. It consists of four mirrors sequentially correcting diffraction divergence of TE01 mode, its polarization, then amplitude and phase patterns. Results of the measurement performed at millimeter waves agree well with the design parameters: total conversion efficiency exceeds 90% in the frequency band of 20%.

A 1.5-MW, 140-GHz, TE/sub 28,16/-coaxial cavity gyrotron

The design of a 1.5-MW, 140-GHz, TE-/sub 28,16/-coaxial cavity gyrotron is presented and results of experimental operation are given. A cavity with a cylindrical outer wall and a radially tapered inner rod with longitudinal corrugations was used. A maximum output power of 1.17 MW has been measured in the design mode with an efficiency of 27.2%. Single-mode operation has been found over a wide range of operating parameters. The experimental values agree well with the results of multimode calculations. Frequency-step tuning has been performed between 115.6 and 164.2 GHz. In particular, an output power of 0.9 MW has ben measured in the TE/sub 25,14/ mode at 123.0 GHz and 1.16 MW in the TE/sub 32,18/ mode at 158.9 GHz. At frequencies its with strong window reflections the parameter range for which stable operation is possible is reduced significantly. In order to obtain results relevant for a technical realization of a continuously operated gyrotron, a tube with a radial radio frequency (RF)-beam output through two output windows and a single-stage depressed collector has been designed and is under fabrication. A two-step mode conversion scheme-TE-/sub 28,16/ to Te/sub +76.2/ to TEM/sub 00/-which generates two narrowly directed (60/spl deg/ at the launcher) output wavebeams has been chosen for a quasioptical (q,o) mode converter system. A conversion efficiency of 94% is expected.

TE03 to TE01 mode converters for use with a 150 GHz gyrotron

Two-step high-power TE03-to-TE01 mode transducers (TE02 is the intermediate mode) were designed to transform the TE03 output mode of the 150 GHz KfK gyrotron into the fundamental circular symmetric TE01 mode for low-loss propagation and further conversion into the linearly polarized quasioptical HE11 mode. The advanced perturbation structures of the ruppled-wall mode converters (with input- and output-diameter D=27.8 mm) were optimized by numerically solving the proper coupled-wave differential equations. The overall TE03-to-TE01 conversion efficiency at 150 GHz was calculated to be η0=99.1% for a long converter (total length L=2.67 m) and η0=97.8% for a short converter (L=1.07 m); ohmic attenuation is included. The corresponding theoretical bandwidth (for η≧95%) is ±1.4 GHz and ±2 GHz, respectively. Low-power experimental data are in excellent agreement with these computed values.