7th Biennial ECH Transmission Line Workshop, Virginia Beach, Virginia, USA, September 14 – 16, 2005

 

 

 

 

QUASI-OPTICAL MODE CONVERTERS IN ADVANCED HIGH-POWER GYROTRONS FOR FUSION PLASMA APPLICATIONS

 

 

M. Thumm1,2, A. Arnold2, G. Dammertz1, J. Jin1, K. Koppenburg1, G. Michel3, B. Piosczyk1,
O. Prinz1, T. Rzesnicki1, D. Wagner4, X. Yang1

 

 

1Karlsruhe Research Center, Association EURATOM-FZK, Institute for Pulsed Power and Microwave Technology,

 D-76021 Karlsruhe, Germany

2University of Karlsruhe, Institute of High-Frequency Techniques and Electronics, D-76128 Karlsruhe, Germany

3Max-Planck-Institute for Plasma Physics, Association EURATOM-IPP, D-17491 Greifswald, Germany

4Max-Planck-Institute for Plasma Physics, Association EURATOM-IPP, D-85748 Garching, Germany

 

 

The R&D activities at the Karlsruhe Research Center (Forschungszentrum Karlsruhe: FZK) on advanced high-power millimeter (mm)-wave gyrotrons for future use in electron cyclotron heating and current drive (EC H&CD) in magnetically confined thermonuclear fusion plasmas consist of:

 

(1)      Development of a tunable multi-frequency gyrotron (105 GHz to 140 GHz) for ASDEX Upgrade in collaboration with IAP Nizhny Novgorod/GYCOM,

(2)      Development of a coaxial cavity gyrotron capable of delivering 2 MW CW at 170 GHz for ITER in collaboration with EFDA and TED, and

(3)      Investigations on a 4 MW 170 GHz coaxial cavity gyrotron with a two-beam output (2x2 MW) for a future DEMO fusion reactor.

 

The present paper discusses the different quasi-optical (q.o.) mode converter schemes employed in these various types of advanced mm-wave gyrotrons to convert the very high order rotating cavity modes into a linearly polarized fundamental Gaussian beam (TEM00 mode) for low-loss transmission and optimum radiation into the plasma.

All the q.o. mode converters employ dimpled-wall launchers in order to reduce diffraction losses and thus to avoid parasitic stray radiation inside the tubes as much as possible. A weak diameter cone is used to prevent self excitation in the launcher waveguide by the not completely worked out electron beam.