Mode Selection for a 2 MW, CW 170 GHz Coaxial Cavity Gyrotron

 

O. Dumbrajs+ , B. Piosczyk, C.T. Iatrou++

 

Forschungszentrum Karlsruhe, Association EURATOM-FZK, Institut für Hochleistungsimpuls- und

Mikrowellentechnik (IHM), D-76021 Karlsruhe

+Department of Engineering Physics and Mathematics, Helsinki University of Technology,

FIN-02150 Espoo, Finland.

++National Technical University of Athens, Athens, Greece

e-mail: dumbrajs@csc.fi

Based on results achieved during the development work on coaxial cavity gyrotrons at FZK a 2 MW, CW 170 GHz gyrotron is under design. A selection of suitable operating modes is underway. The boundary conditions are:

generated RF-power: Prf = 2.2 MW

electronic efficiency: h el ³  35 %

peak surface losses: ppeak £  1 kW/cm², ideal copper

electron beam radius: 9.5 mm £ Rb £ 11 mm.

The presently used TE31,17 cavity at 165 GHz with a radius of 27.38 mm could be used for generation of the TE33,17 mode at 170 GHz. However, at Prf = 2.2 MW corresponding to an RF-output power of 2 MW, the surface losses are 1.1 kW/cm², slightly above the accepted technical limit. Thus, several different modes with an eigenvalue around 100 has been extensively examined. To perform a selection of a suitable mode, in particular the sensitivity of the RF-generation to a variation of operating parameters has been investigated numerically with a self consistent, time dependent multimode code available at FZK. To cross-check the results corresponding calculations have been performed with a new extended code developed by one of the authors. The results will be presented and discussed.