Research on Advanced High Power Gyrotrons at FZK

B. Piosczyk^1a, A. Arnold², G. Dammertz^1a, O. Dumbrajs³, R. Heidinger^1b S. Illy^1a, J. Jin^1a,

K. Koppenburg^1a, G. Michel⁴, T. Rzesnicki^1a, M. Thumm^1a,2, X. Yang^1a

¹Forschungszentrum Karlsruhe, Association EURATOM-FZK,

(a) Institut fuer Hochleistungsimpuls- und Mikrowellentechnik (IHM), (b) Institut fuer Materialforschung I (IMF I), Postfach 3640, D-76021 Karlsruhe, Germany,

² Universitaet Karlsruhe, Institut fuer Hoechstfrequenztechnik und Elektronik (IHE),

D-76128 Karlsruhe, Germany

³Department of Engineering Physics and Mathematics, Helsinki University of Technology,

Association EURATOM-TEKES, FIN-02150 Espoo, Finland.

⁴Max-Planck-Institut fuer Plasmaphysik, Ass. EURATOM-IPP,D-17491 Greifswald, Germany,

The R&D activities at 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 fusion plasmas consist of: (1) the development of a coaxial cavity gyrotron capable of delivering 2 MW continuous wave (CW) at 170 GHz and (2) investigations on tunable multi-frequency gyrotrons.

In the case of the coaxial cavity gyrotron the feasibility of manufacturing multi-megawatt gyrotrons in continuous wave (CW) operation has been demonstrated in proof of principle experiments at pulses up to a few ms. Problems specific to the coaxial arrangement have been investigated and information relevant for an industrial realization of a high power coaxial gyrotron has been obtained. Based on these results the development of a coaxial cavity gyrotron with an RF output power of 2 MW, CW at 170 GHz started recently in cooperation between EURATOM Associations (CRPP Lausanne, FZK Karlsruhe and HUT Helsinki) together with European tube industry (Thales Electron Devices, Velizy, France). To verify experimentally the design of the main components (electron gun, cavity and RF output system) of the industrial prototype, the experimental coaxial gyrotron at FZK has been redesigned for operation at 170 GHz. The maximum magnetic field of the superconducting magnet at FZK of only 6.667 T requires a reduction of the operating voltage from 90 kV (as foreseen for the industrial prototype) to 80 kV. The cavity and the RF output coupler are identical as foreseen for the industrial tube. The new design of the electron gun avoids regions in which electrons can be trapped in order to avoid the built up of a Penning discharge. The manufacturing of all components is practically finished. The experiments which are expected to start in May 2004 will allow to investigate the RF generation and mode competition as well as the efficiency of the RF output system. Measurements of the properties of the RF output beam radiated out of the gyrotron and the amount of internal stray radiation will be performed and compared with the calculations.

In the case of step frequency tunable gyrotrons the possibility of multi-frequency operation of a gyrotron designed to oscillate in the TE_22,8 mode at 140 GHz, the TE_17,6 mode at 105 GHz and six other modes at frequencies in between is currently under investigation. Key issues are here the development of a broadband quasi-optical (QO) output system and the CVD-diamond Brewster window. The QO mode converter of the gyrotron consists of a dimpled-wall antenna (Denisov-type launcher) and a beam forming mirror system. 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. The QO system has been optimized for broadband operation in the various modes. A 140 mm diameter disk is being developed at Element Six (formerly DeBeers Industrial Diamonds) for a full-size diamond Brewster angle window.