X.Yang 1, O.Drumm 3, A.Arnold 1,3, E.Borie 1, G.Dammertz 1,
R.Heidinger
2, K.Koppenburg 1, B.Piosczyk 1, D.Wagner 4, M.Thumm 1,3
Forschungszentrum
Karlsruhe, Association EURATOM-FZK,
1Institut fuer Hochleistungsimpuls- und Mikrowellentechnik,
2Institut
fuer Materialforschung I,
D-76021 Karlsruhe, Germany
3 also
Universitaet Karlsruhe, Institut fuer Hoechstfrequenztechnik und Elektronik,
In order to achieve a
low diffraction loss Gaussian distribution of the output beam of a gyrotron for
all designed frequencies, both the launcher and the mirrors of the quasi
optical mode converter have to be optimized for all excited modes. The
quasi-optical mode converter for a frequency step-tunable gyrotron consists of
a dimpled-wall antenna (Denisov-type launcher) and a beam-forming mirror system
optimized for nine modes from TE17,6 to TE23,8. The first
mirror is a large quasi elliptical one; the second and third are phase
correcting mirrors with a non-quadratic shape of the surface. Calculation
results show that for these modes the dimpled-wall antenna has a well-focused
beam with low diffraction losses. A multi-mode optimization of a mirror with
two different methods has been tested. The simulations have shown that the
phase correcting mirrors can be used in a rather broad frequency range from 105
GHz to 143 GHz of a step-tunable gyrotron. Brewster window and Brewster-angle window
have been considered as gyrotron window to realize ultra-broadband frequency
output. This quasi optical mode converter can achieve
efficiencies of 94%--98% for converting the rotating high-order cylindrical
cavity modes into the usable fundamental Gaussian mode.