The ECH front steering launcher for the ITER upper port

The purpose of the ITER electron cyclotron resonance heating (ECRH) upper port antenna will be to drive current locally inside the island which forms on the q = 3/2 or 2 rational magnetic flux surfaces in order to stabilize neoclassical tearing modes (NTMs). The launcher should be capable of steering the focused beam deposition location across the resonant flux surface over the range in which the q = 3/2 and 2 surfaces are found, for the various plasma equilibria susceptible to the onset of NTMs. ITER's present reference design uses a front steering (FS) concept, with the moveable mirror close to the plasma. Two separate mirrors are used to decouple the focussing and steering aspects resulting in an optimized folded beam path.The FS launcher is capable of steering eight 2 MW beams via two sets of steering mirrors out of each of the four allocated upper port plugs. Among the relevant mechanical design aspects are the systematic reliance on frictionless and backlash free mechanical movements based on the compliant deformation of structural components to avoid the invessel tribological difficulties [J.-D. Landis, et al., Design of the critical components in the ITER ECH upper launcher steering mechanism, this conference]. An inert gas pressure controlled bellows system provides accurate angular positioning of the steering mirror. The antenna is designed fail-safe to provide full bandwidth beam steering capabilities during the twenty years lifetime of ITER. Details of the FS launcher design relating to mm-wave, thermohydraulic, electromagnetic, neutron shielding and structural aspects and possible options regarding the integration of launcher components into the port plug are discussed.