Design status of the ITER ECH Upper Launcher Steering Mirror Mechanism

Abstract

The ITER ECRH upper port antenna (or launcher) will be used to drive current locally for stabilising the neoclassical tearing mode (NTM) by depositing ram-wave power inside of the island which forms on the q=3/2 or 2 rational magnetic flux surfaces and control the sawtooth instability by driving current near the q=l surface. This requires the launcher to be capable of steering the focused beam deposition location across the resonant flux surface over the range where the q=l, 3/2 and 2 surfaces are expected to be found (roughly the outer half of the plasma). ITER'S present reference design uses a front steering (FS) concept, which uses a moveable mirror close to the plasma. Two separate mirrors are used to decouple the focusing and steering aspects resulting in an optimized optical configuration providing a well focused beam over a large steering range. The steering mechanism providing the mirror rotation uses a frictionless and backlash free mechanical system based on the elastically compliant deformation of structural components to avoid the in vessel tribological difficulties. Traditional designs are based on push-pull rods acting on a mirror which rotates with ball bearings, they present the risk of gripping or result in stick-slip movements. The ball bearings are replaced with a set of flexure pivots while the classic actuation through a push-pull rod scheme is replaced by a directly acting pneumatic system consisting on a fast feed line, bellows and springs, in which the pressure acting on the bellows pushes the mirror against the compressive springs. The rotation of the mirror is thus produced by the counteraction between the forces exerced by the springs and the bellows, themselves piloted by the pressure of the system. A servovalve placed outside of the port plug and connected to the bellows by a small tube will control this pressure. The system also includes flexible water cooling pipes which allow the removal of heat generated by the ohmic surface losses of the reflected mm-wave beams and the nuclear and radiation volumic heating of the rotating mirror components. This paper will give an overview of the engineering and design issues and their solutions, and provide the development status of the different components of the mechanism. Special attention will be given to the engineering analysis performed to ensure compliance of the steering mechanism with the various ITER requirements.