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

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 TE_31,17 mode at 165 GHz. The cavity has been optimized for the nominal parameters: cathode voltage U_c = 90 kV, beam current I_b = 50 A, microwave output power P_out = 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: P_out @ 2.2 MW

maximum output efficiency at P_out @ 1.5 MW: h _out @ 30 %

with depressed collector h _out @ 48 %

¨ The electron beam remained stable up to 84 A at U_c @  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

suggested.

¨ 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.