A Coaxial Cavity Gyrotron

- experimental results and technical conditions -

B. Piosczyk, A. Arnold1, H. Budig, G. Dammertz, O. Drumm1, O. Dumbrajs2, M. Kuntze, and M. Thumm1

Forschungszentrum Karlsruhe, Association EURATOM-FZK,

Institut für Hochleistungsimpuls- und Mikrowellentechnik (IHM), D-76021 Karlsruhe

1 also Universität Karlsruhe, Institut für Höchstfrequenztechnik und Elektronik (IHE)

2Department of Engineering Physics and Mathematics, Helsinki University of Technology,

Association EURATOM TEKES, FIN-02150 Espoo, Finland.

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



In accordance with the goal of the ITER task on development of coaxial cavity gyrotrons which ended in 2001 the potential of coaxial gyrotrons has been investigated and as a result data necessary for an industrial realization of a 2 MW, CW 170 GHz tube have been obtained. In addition, first work on tube integration has been done.

The measurements have been performed with an experimental tube constructed in a modular way and designed to operate in the TE31,17 mode at 165 GHz. The cavity has been optimized for the nominal parameters: cathode voltage Uc = 90 kV, beam current Ib = 50 A, microwave output power Pout = 1.5 MW. In general, the measurements have been performed at short pulses (typically 1 ms) with a repetition rate of 1 Hz. In order to investigate the behavior at longer pulses, single pulse operation with extended length has been examined.

The achievements can be summarized as follows:

¨ The mechanism of parasitic low frequency oscillations has been understood and the

occurrence of such oscillations has been suppressed successfully.

¨ Efficient microwave output power generation has been achieved in single mode operation.

maximum RF-output power: Pout @ 2.2 MW

maximum output efficiency at Pout @ 1.5 MW: h out @ 30 %

with depressed collector h out @ 48 %

¨ The electron beam remained stable up to 84 A at Uc @  90 kV and velocity ratio a  @  1.4.

¨ The microwave pulse length has been extended up to 17 ms. The limitation has been found

to be due to a Penning discharge in the gun region. Possibilities to avoid this limitation are


¨ The current to the insert saturates at nominal parameters after a few ms at a value < 30 mA.

¨ The amplitude of the mechanical vibrations of the insert has been measured to be within

±  0.03 mm caused mainly due to the flow of the cooling water.

¨ The losses at the insert have been found to be about 0.1% of the RF output power.

¨ The amount of the microwave power captured inside the gyrotron tube has been found to

be fairly large, namely about 11% of the RF output power. The captured radiation is

distributed approximately uniformly inside the mirror box.

¨ A fast (within »  0.1 ms) frequency tuning has been demonstrated by biasing the coaxial

insert. In particular, a fast step frequency tuning between the 165 GHz nominal mode and

the azimuthal neighbors at 162.75 GHz and 167.2 GHz have been performed. In addition,

at the nominal mode a continuous frequency variation within the bandwidth of up to

70 MHz have been done.