Development of a Quasi-optical Transmission System for Gyrotron Application as a Radiation Source
I.Ogawa, T.Idehara, M.Myodo, H.Ando, D.Wagner1 and M.Thumm2
Research Center for Development of Far-Infrared Region, University of Fukui
3-9-1 Bunkyo, Fukui 910-8507, Japan
1Max-Planck-Institut für Plasmaphysik, Garching, D-85748, Germany
2Institut für Hochleistungsimpuls- und Mikrowellentechnik, Forschungszentrum Karlsruhe
P.O. Box 3640, D-76021 Karlsruhe, Germany
Abstract. High frequency gyrotrons are characterized by their capacity to generate powerful radiation in millimeter to
submillimeter wavelength range. Functional quasi-optical systems to treat a Gaussian beam converted from gyrotron
output are necessary to the application of gyrotron as a radiation source. They are versatile systems to transmit and to
tune the waist size of output beam.
Keywords: Gyrotron, Submillimeter wave, Gaussian beam, Quasi-optical system
Among radiation sources in millimeter to submillimeter wavelength range, high frequency gyrotrons are
characterized by their capacity to generate powerful radiation. In addition, the gyrotrons have the advantage of
tuning the output frequency by selecting the operating mode.
Some applications like plasma diagnostics and material processing need high power radiation. However, a
gyrotron produces a spreading radiation with a spatial structure of the TEmn circular waveguide mode. Because the
polarization, the phase and the intensity of the radiation have complicated profiles, such radiation is not suitable for
the applications. In this respect, a Gaussian beam (TEM00 mode) with a linear-polarization, simple profiles of the
intensity and the phase is more suitable for effective irradiation of the sample and transmission using mirror system
and waveguide system (dielectric waveguide and corrugated waveguide).
In order to widen the use of the high frequency gyrotron, we need functional quasi-optical systems to transmit
and to tune the waist size of output beam. The function to tune the waist size of Gaussian beam provides the
effective coupling of the beam to the sample and the waveguide.
HIGH PURITY MODE OPERATION OF A GYROTRON
In order to convert effectively the gyrotron output radiation into a Gaussian beam, we have developed a new
gyrotron, Gyrotron FU VA (Fig.1) which produces high purity mode outputs. The Gyrotron FU VA has a carefully
designed cavity with nonlinear up-tapers and a rounded iris at the output (Fig.2). The cavity was designed by means
of a scattering matrix formalism (SM-code) taking into account the complete gyrotron geometry including the
pumping sections (slots) and the window1). The design and the fabrication of the cavity were carried out in
collaboration with the Max-Planck Institute for Plasma Physics and the Institute for High Power Pulse and
Microwave Technology at the Research Center (FZK) Karlsruhe (Germany).
Gyrotron FU VA consists of a gyrotron tube and a helium-free superconducting magnet. This magnet can
produce a magnetic field up to 8T without using liquid helium. The tube is demountable, because we will try to
optimize all components, the cavity, the transmission waveguide and the output window.
The radiation patterns are measured by two-dimensionally (x-y plane) moving pyro-electric detector array over
the gyrotron window. The measurement demonstrates that Gyrotron FU V can produce outputs of high purity mode
for several operating modes. The patterns for these modes agree well with the intensity profiles calculated. This
demonstrates that Gyrotron FU VA can produce outputs.
When the gyrotron delivers the TE13 output radiation consisting of rotating clockwise and counterclockwise
modes, the high quality beam is obtained by matching appropriately the nodes of the radiation pattern with the stepcut
of the quasi-optical antenna. This was confirmed by an experiment. The measurements of the final beam
structure have demonstrated that the system developed produces bi-Gaussian beams from the outputs of TE03 and
TE13 operating modes (Fig.4).
S.C. magnet Cavity
Turbo molecular pump Collector
The measurements of the final beam structure have demonstrated that the system developed produces high
quality beams from the TE03 and TE13 operating modes.
If the Gaussian beam is obtained from gyrotron output, the versatile transmission system for a Gaussian beam
will be realized. In this system, con-focal mirror systems are installed in a special shaft which goes through the
second floor, the third floor and the forth floor of the building of the Research Center for Development of Far-
Infrared Region (Fig.5). The Gaussian beam can be transmitted to those three floors keeping the waist size of the
output beam equal to that of the input beam by rotating and moving the flat mirror located at the third floor (Fig.6).
The radiation patterns of transmitted beam are shown in Fig.7.
The measurement of radiation patterns demonstrates high purity non-rotating mode operations. The resonance
calculation for a complete gyrotron geometry using scattering matrix formalism (SM-code) was carried out and the
results were compared with the measurement. The quasi-optical system consisting of a Vlasov antenna and a
focusing mirror can convert the high purity mode outputs of TE03 and TE13 modes into Gaussian-like beams. The
Gaussian-like beams are effectively transmitted by the quasi-optical transmission system.
1. D.Wagner et al., Int. J. of Infrared and Millimeter Waves, 19, 185 (1998)
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