The ITER ECH FS Upper Launcher mm-wave Design based on a Synergy Study with the Equatorial Launcher

The ITER ECH heating and current drive system consists of 24 MW (20 MW delivered) at 170GHz, which can be directed to either the equatorial (EL) or upper (UL) port launching antennas (launchers) depending on the desired physics application. The UL reference design uses a front steering (FS) mirror that sweeps the beam in a poloidal plane providing co-ECCD over the outer third of the plasma cross section. A novel frictionless, backlash-free steering mechanism has been developed for an increased reliability. The design avoids components such as bearings and push-pull rods, which tend to grip in conventional launchers in use on present day ECH systems. Flexure pivots replace bearings and a pneumatic seal-less actuator using pressurised helium integrated into the rotating mirror assembly offers a fast and precise response avoiding push-pull rods, linkages representing sliding bearings and remote actuators. The result is a complete self-contained frictionless kinematic assembly rotating the steering mirror up to ±7o (±14º for RF beam). The launcher has a single dedicated purpose of stabilising the neoclassical tearing modes (NTM), with the launcher steering range accessing the region in which the q=3/2 or 2 flux surfaces are expected for scenarios susceptible to NTMs. The performance of the FS launcher far exceeds (by a factor of 1.5 to 3) that required by the physics to stabilize the NTM. The two mirror (focusing and steering) system of the FS launcher essentially decouples the steering and focusing functions of the launcher, offering the flexibility to increase the access range beyond that required by the NTM stabilisation such that the launcher can access further inward for sawteeth control. Extending the range of the UL can relax the EL steering range, which can then be optimised for enhanced performance for improved central deposition and potentially for counter ECCD.