Design and Test of the Remote-Steering ITER ECRH Upper-Port Launcher

The design of the mm-wave system for the ITER upper ports is being carried out with the aim to inject Electron Cyclotron Waves (ECW) in the plasma in order to stabilize neoclassical tearing modes (NTM). Four ports are reserved for the upper launchers which are equipped with six to eight mm-wave lines, each capable of transmitting high power up to 2 MW at 170 GHz. In order to exploit the capability of ECW for localized heating and current drive over a large range of plasma radii in ITER, the ECH(and)CD upper port launcher needs a beam steering capability. Two alternative approaches are being studied in Europe under EFDA tasks. The '' classic '' layout, Front Steering (FS), has rotating mirrors near the plasma. To avoid moving water-cooled components at the plasma-facing end of the launcher, e.g., mirrors and cooling lines, an alternative solution, Remote Steering (RS) is studied by a number of European institutes, as well. In the case of RS, the mm-wave beam is brought into the ITER vacuum vessel by a long square corrugated waveguide. The mm-wave beam will leave the waveguide at the plasma side under the same (but opposite) variable angle as the input angle and reflect on a set of 2 mirrors towards the ITER plasma, a so-called '' dog-leg '' set-up. The design work so far has demonstrated the feasibility of the remote-steering approach in the ITER environment. To validate (with respect to power-loss, antenna patterns and polarisation) the design work a full-scale mock-up line at 170 GHz has been built and successfully tested both at low power at Rijnhuizen and at high power on the coaxial, short pulse gyrotron at FZK. New design efforts are ongoing in order to improve the limited current-drive capability of the RS set-up at the relevant NTM positions. An increase of the square waveguide length to 8 m is foreseen to enable the efficiency to increase the current-drive capability to the minimum levels that are needed to stabilize the 3/2 and 2/1 NTM modes in nearly all ITER scenarios. (author)