Recent Developments of the Upper Port ECH&CD Launcher Systems for ITER Based on the Remote Steering Concept

ITER employs Electron Cyclotron Waves (ECW) for heating of the plasma and to stabilize MHD modes including the neoclassical tearing modes (NTM). To cover all the significant ITER plasma scenarios, the upper port ECW launcher needs a beam steering mechanism to reach the magnetic q-surfaces where NTM instabilities can grow. Two alternative approaches are being studied in Europe under EFDA tasks: Front Steering (FS), which has rotating mirrors situated near the plasma, and Remote Steering (RS), which has the mirror steering mechanism situated in the secondary vacuum. The steering mirrors in the RS system project the mm-wave beam onto a Square Corrugated Waveguide (SCW) of a length such that the input beam profile is imaged at the output, producing a steered output beam. The advantage of RS is that the steering mechanism is shielded from direct plasma exposure and neutron bombardment and it also allows simpler maintenance. One of the drawbacks of RS is that the output steering range is inversely coupled to the focusing. The optimization of the quasi-optical system is done for a short (4.3 m) and a long (9.1 m) SCW. The results are evaluated for their effectiveness in NTM stabilization by calculating the parameter ? which represents the ratio between the EC wave driven current and the bootstrap current, preferably ? should exceed 1.2. The performance is also evaluated in terms of beam focusing properties and power loading on the mirrors. The ? achieved so far meets the requirements in the ITER reference scenarios for q=2/1 and partly for q=3/2. Additional tools under investigation for optimization are: non spherical mirror curvatures, tapered SCWs and the use of general astigmatic beams. This work has been carried out as part of the EFDA work program Task TW6-TPHEECHULB for the development of the ITER ECRH Upper Port Launcher system, in collaboration with the Euratom Associations IFP Stuttgart, IPP Garching, FZK, IFP/CNR and CRPP.