A 2 MW, 170 GHz Coaxial Cavity Gyrotron

- experimental verification of the design of main components-

B. Piosczyk¹, T. Rzesnicki¹, G. Dammertz¹, O. Dumbrajs³, S. Illy¹, J. Jin¹,

W. Leonhardt¹, G. Michel⁴, M. Schmid¹, M. Thumm^1,2 and X. Yang¹

¹Forschungszentrum Karlsruhe, Association EURATOM-FZK, Institut für Hochleistungsimpuls- und Mikrowellentechnik (IHM),Postfach 3640,

D-76021 Karlsruhe, Germany

²⁾ Universität Karlsruhe, Institut für Höchstfrequenztechnik und Elektronik,

D-76128 Karlsruhe, Germany

³Department of Engineering Physics and Mathematics, Helsinki University of Technology, Association EURATOM-TEKES, FIN-02150 Espoo, Finland,

⁴Max-Planck-Institut für Plasmaphysik, Ass. EURATOM-IPP, D-17491 Greifswald, Germany; e-mail: bernhard.piosczyk@ihm.fzk.de

A 2 MW, CW, 170 GHz coaxial cavity gyrotron is under development in cooperation between European Research Institutions (FZK Karlsruhe, CRPP Lausanne, HUT Helsinki) together with European tube industry (TED, Velizy, France). The design of critical components has been verified experimentally at FZK Karlsruhe. For that a short pulse (~ few ms) coaxial cavity gyrotron has been brought into operation which uses the same cavity and same quasi-optical (q.o.) RF-output system as designed for the industrial prototype and a very similar electron gun. In recently performed experiments at FZK the following results have been obtained:

Electron gun and electron beam:

The performance of the electron gun is according to the design. No limitations in high voltage operation due to built-up of a Penning discharge have been observed. In addition, a stable electron beam up to a current of 80 A without any instabilities has been obtained.

Cavity and RF-interaction:

The TE_34,19 mode, which is the nominal mode of the 2 MW, CW prototype, has been excited at 170 GHz over a reasonably wide parameter range. At a magnetic field of 6.68 T, limited by the available SC-magnet, a maximum RF output power of about 1.1 MW has been measured. The oscillation range of the nominal mode, however, is somewhat reduced due to more severe mode competition compared to theoretical predictions. As a consequence, a more dense mode spectrum has been excited in the experiment than expected from simulations. The reason for that is under investigation.

Quasi-optical (q.o.) RF-output system:

The designed q.o. system consists of a dimpled-wall launcher with a straight and helical cut and of three mirrors - one quasi-elliptical mirror followed by a toroidal mirror and a phase correcting mirror with a non-quadratic surface contour. The q.o. system has been tested both at low power ("cold") and in the gyrotron ("hot"). Both, the results from the "cold" and "hot" measurements show significant disagreement in comparison with the design calculations. In very recent simulations it has been found that there was an error in the field pattern from the launcher taken for designing the mirrors. With the correct RF output pattern of the launcher a good agreement is obtained between measurements and simulation. This confirms the confidence into the numerical design tools. A redesign of the q.o. system is in progress.