FOR A 1 MW, 140 GHZ, CW 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
Current high power gyrotrons
employ an internal quasi-optical mode converter to extract the rotating
high-order cavity mode 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 inner
tube structure. For a 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 very low diffraction losses, a highly efficient
quasi-optical mode converter system with several novel features has been designed and tested at Forschungszentrum Karlsruhe (FZK).
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 with R(z) = 21.9 mm +0.004 z. 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 helical Dm1 = 1 and Dm2 = 3 wall perturbations for the longitudinal and azimuthal field bunching, respectively, use the same amplitude of 0.041 mm and perturbation length of 70 mm (with 10 mm tapering), but they start at different positions along the z-axis of the launcher. The Dm1 = 1 perturbation is between z = 0 mm and z = 70 mm, the Dm2 = 3 perturbation between z = 17 mm and z = 87 mm. The helical cut begins at z = 160 mm. The coupled mode theory has been used to optimize 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 shows that
the dimple-wall launcher can generate a well-focused Gaussian radiation
pattern with low diffraction losses.
One quasi-elliptical mirror
and two toroidal mirrors can be used as the beam-forming mirror system to
obtain a desired beam pattern at the gyrotron output window. 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 agrees well with
the desired fundamental Gaussian distribution. Further calculations of power
conservation show that an efficiency of more than 98% has been achieved.
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, a low power TE28,8
mode generator, the gyrotron mode converter-system as device under test, and a
pick-up antenna to measure the E-field distribution in a defined linear
polarization (horizontal and vertical). The E-field distribution is
investigated at the output flange of the mode generator; and 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. Low power
measurement results of the launcher and the beam-forming mirror system show
that the beam patterns agree well with the theoretical predictions.