A High Efficiency Quasi-Optical Mode Converter

for a 140 GHz 1 MW Gyrotron


M. Thumm 1, 2, A. Arnold 1, 2, G.Dammertz1, G. Michel 3,

J. Pretterebner 4, D.Wagner 5, X.Yang 1


1Forschungszentrum Karlsruhe, Association EURATOM-FZK,
Institut fuer Hochleistungsimpuls- und Mikrowellentechnik, 76021 Karlsruhe, Germany

2 Universitaet Karlsruhe, Institut fuer Hoechstfrequenztechnik und Elektronik,

Kaiserstr. 12, 76128 Karlsruhe, Germany

3Max-Planck-Institut fuer Plasmaphysik,Teilinstitut Greifswald, Association EURATOM-IPP,

Wendelsteinstr. 1, 17491 Greifswald, Germany

4DaimlerChrysler AG, 70546 Stuttgart, Germany

5 Max-Planck-Institut fuer Plasmaphysik, Association EURATOM-IPP,

Boltzmannstr.2, 85748 Garching, Germany

e-mail: xiaokang.yang@ihm.fzk.de

Tel: +49-7247-822455, Fax: +49-7247-4874



Current high power gyrotrons employ an internal quasi-optical mode converter to extract the rotating high-order cavity modes into a linearly polarized fundamental Gaussian beam. Conversion efficiencies close to unity are required since trapped RF energy jeopardizes the stable operation regime and may lead to heating of the tube structure. For the 1 MW continuous wave (CW) gyrotron operated in the TE28, 8 mode at 140 GHz, in order to achieve a usable fundamental Gaussian distribution of the output beam with low diffraction losses, a high efficiency quasi-optical mode converter has been optimized and designed at Forschungszentrum Karlsruhe (FZK). It consists of an advanced dimple-wall antenna as a launcher, one quasi-elliptical mirror and two toroidal mirrors as the beam-forming system.

The launcher employs an irregular cylindrical waveguide section (perturbations of the waveguide surface) followed by a helical-cut launching aperture. The purpose of the pre-bunching section is to obtain a mode mixture in the launching waveguide such that the field intensity on the wall has a Gaussian profile. In addition, the mean radius of the launcher section is slightly up-tapered. This configuration reduces the Q factor of the section between the cavity and the helical cut, and suppresses spurious oscillations generated by the spent electron beam in the launcher section. The coupled mode theory is used to analyze the operation of the pre-bunching waveguide. The fields radiated from the cut of the launcher are calculated by the scalar diffraction integral. The calculated radiation pattern on the aperture of the launcher is shown in Fig.1 (the contour map of all the figures mentioned in this paper are plotted at 3 dB increments from 30 dB to 0). It is obvious that

the dimple-wall launcher can generate a well-focused Gaussian radiation pattern with low diffraction losses. The calculated distribution of the electrical field radiated from the launcher on a plane at the position of x = 100 mm away from the gyrotron axis has been calculated as shown in Fig. 2. This figure is used to compare with the result of low power measurement of the launcher.

One quasi-elliptical mirror and two toroidal mirrors can be used as the beam-forming mirror system to obtain a desired beam pattern on the gyrotron output window, since calculations also show that the toroidal mirrors are sufficient in this case; at the same time, they are inexpensive, easy to manufacture, and relatively easy to align. The positions and the orientations of the mirrors are found by evaluating the moments of the first and second order of the power distribution. This design technique requires knowledge of both the amplitude and the phase distribution of the input beam and the desired output beam. The design procedure incorporates a fast scalar diffraction code for a nonparallel aperture, which allows for a rapid synthesis of the mirror profiles. Including this beam-forming system, the calculated power density distribution on the gyrotron window is shown in Fig.3. Obviously, the calculation result agrees well with the desired fundamental Gaussian distribution. Further calculation of power conservation shows that an efficiency of more than 98% has been achieved to convert the TE28,8 mode at 140 GHz into fundamental Gaussian beam.

A low power test facility has been built to check the performance of the quasi-optical mode converter system. The transmission measurement device consists of a network analyser, the low power TE28,8 mode generator, mode converter-system as device under test, and the pick-up antenna to measure the E-field distribution in a defined linear polarization (horizontal or vertical). The E-field distribution is investigated at the output flange of the mode generator; or for the quasi-optical mode converter, at the position where the gyrotron output window is located. The pick-up antenna is fixed on a programmed 3-dimentional movable table in order to scan the distribution at an arbitrary position.

After exact adjustment of the beam-shaping mirror system, low power measurements of the gyrotron output beam at different positions have been done perfectly. Fig.4 shows one example at the position of gyrotron window. To compare with the theoretical predictions, it is obvious that cold measurements and calculations give the same output beam patterns in both near field and far field, they agree well with each other at different positions outside the gyrotron output window.



Fig.1 Calculated radiation pattern Fig. 2 Calculated distribution of electrical field on a

on the aperture of the launcher. plane at x = 100 mm away from gyrotron axis.


Fig. 3 Calculated result of the output beam Fig. 4 Low power measurement result of the output

on the gyrotron window. beam on the gyrotron window