- experimental and technical conditions -

B. Piosczyk, A. Arnold1, H. Budig, G. Dammertz, O. Dumbrajs2, O. Drumm1,

M.V. Kartikeyan, M. Kuntze, M. Thumm1 and X. Yang

Forschungszentrum Karlsruhe, Association EURATOM-FZK, Institut für Hochleistungsimpuls- und Mikrowellentechnik, D-76021 Karlsruhe, Germany

1 also Universität Karlsruhe, Institut für Höchstfrequenztechnik und Elektronik

2Department of Engineering Physics and Mathematics, Helsinki University of Technology, Association EURATOM TEKES, FIN-02150 Espoo, Finland.

e-mail: bernhard.piosczyk@ihm.fzk.de

The development work on coaxial cavity gyrotrons at the Forschungszentrum (FZK) Karlsruhe demonstrated the feasibility of manufacturing a 2 MW, CW coaxial gyrotron operated at 170 GHz and provided all information necessary for a technical design and industrial manufacturing. All problems specific of the coaxial arrangement have been investigated.

- The stability and the losses of the coaxial insert have been measured and the influence of a misalignment of the insert on the losses has been studied.

- The amount of microwave radiation captured inside the tube has been measured to be (9 ±  1) % of the RF output power. The captured stray radiation has been found to be distributed more or less uniformly inside the mirror box.

- In single-pulse operation the pulse length has been extended up to 17 ms. A sudden increase of the current Iins to the coaxial insert has been observed which limited the achievable pulse length. The increase of Iins is caused by a kind of Penning discharge in the technical part of the electron gun. The occurrence of such a discharge can be avoided by a proper design of the gun geometry.

- Parasitic low frequency oscillations have been successfully suppressed and stable operation has been achieved over a wide range of parameters.

By applying a rapid variable bias voltage at the coaxial insert fast (~ 0.1 ms) frequency tuning has been demonstrated. In particular, step frequency tuning by ±  2.2 GHz due to switching from the nominal mode at 165 GHz to its azimuthal neighbors has been done and continuous tuning by up to 70 MHz within the bandwidth of the TE31,17 mode has been performed.

Based on the experimental results a draft design of a 170 GHz coaxial gyrotron for 2 MW, CW output power has been performed. The usability of the components for CW operation and their compatibility with technical restrictions has been proven. A suitable cavity mode has been selected and a collector has been designed. In addition, an integrated design of the tube has been performed.