In the ECRH experiments at the frequency of 140 GHz on the Frascati Tokamak Upgrade (FTU), four transmission lines will deliver the plasma microwave pulses at a total power of 1.6 MW for 0.5 s. Each line transfers 450 kW of millimeterwave power generated by a Gycom gyrotron with an efficiency higher than 90%. The gyrotron's output is in the form of a linearly polarized gaussian beam, matched by a set of mirrors to a polarized hybrid HE1,1 waveguide mode. The two corrugated mirrors, which provide the polarization control, are remotely positioned, thus allowing the polarization adjustment of the beam launched into the plasma. Because of the high dimensional accuracy required for the profile of the corrugated surface, the mirrors were machined with special tools. The linear-to-elliptical and the linear-to-linear polarization rotation capability of the two mirrors have been tested at low and high power as well.

All the mirrors in the quasi-optical gyrotron-to-waveguide matching section are finely adjustable for optimum coupling. 90 degree bents are quasi-optical, with a flat mirror between the input and output arms, bent at 90 degrees. In some of the bends an array of holes is drilled in the reflecting mirror, in order to allow directive measurements of the transmitted and reflected RF power. The directional coupler is tested at low power and calibrated at high power.

The transmission lines are made of oversized circular corrugated waveguides. Each line is composed by a set of contiguous straight sections, connected to each other by joints laying on adjustable stands placed on a rigid supporting beam, and by 90 degree bends. Each stand is regulated to achieve ideal alignment, which is controlled by optical techniques.

Radiation leakage from the joints is prevented by damping the leaking RF power by resin absorbers, and by sealing all apertures by metallic gaskets. The joint high power capability and the effectiveness of its RF sealing have been tested also in the case of non-ideal HE1,1 propagation by inserting smooth sections instead of the standard corrugated waveguides. Each gyrotron's output beam can be switched from the line to a matched load during tune-up and periodical conditioning. A second matched load can be installed along each line to allow high power testing of the transmission lines an calibration of the couplers. The paper will illustrate the construction procedure, machining, testing and assembly of the ECRH microwave system for FTU.