Design of a new ECRH launcher for FTU tokamak

The present ECRH launcher installed on a single equatorial port of FTU tokamak was designed to inject four beams independently steered continuously in poloidal direction and in a set of predetermined toroidal angles. The launching mirrors can be moved only shot by shot. New experimental programmes for control of MHD instabilities with ECH/ECCD and heating of over-dense plasmas with ECBW require new launcher respectively with fast-steerable mirrors and increased toroidal capabilities. The required scanning speed for tracking the rational surfaces in the FTU plasma is 1 cm in 10 ms in poloidal direction, while the maximum toroidal angle needed for O–X–B heating scheme is around ±40°. Two ECRH lines, feeding the old launcher, will be switched to the new launcher, located in a different equatorial position, capable of launching two independent beams from small movable mirrors in the plasma proximity. A control on the power deposition width will be achieved by changing the beam radius in the plasma using an optical system composed by two mirrors (zooming range 17–28 mm). Place has been reserved for future arrangements of additional components, e.g. a remote steering waveguide. A dedicated feedback control for the poloidal motion of the launching mirrors is being developed, in order to adapt the tracking of the power deposition location to the dynamic changes of magnetic surfaces in real-time. The maximum toroidal angle impacts strongly on the movable mirror design; dimensions (height around 90 mm, width around 54 mm) are limited by the port width (=80 mm) and the need to preserve the maximum steering angle. Since the mirrors will not be actively cooled, temperature control will be achieved by covering the backside with a high emissivity coating, to obtain an efficient radiative cooling. A detailed description of the launcher is presented in the paper.