ECRH Beam Optics Optimization for ITER Upper Port Launcher

The electron cyclotron resonance heating (ECRH) launchers are going to be installed in four ITER upper ports for stabilizing the neoclassical tearing mode (NTM) by driving currents (co-ECCD) locally inside either the q = 3/2 or 2 island. The efficiency in stabilizing the NTM depends on the peak current density (jCD), relative to the local bootstrap current. The mm-wave optical design has been optimized to provide the largest jCD over the region, where NTMs are expected to occur in the plasma cross section. The optimization has been carried out for two designs: one applied only to NTM stabilization and another for Extended Physics launcher for an enhanced ITER ECRH physics programme. The main limitation to the optical system is the spatial restrictions of the port plug and blanket shield module. The ECRH launcher has 8 beams (4 beams × 2 vertical rows) per port, with each beam incident on a focusing and steering mirror. The beam optics has been optimized by overlapping 4 beams on each mirror, this overlap maximizes the beam spot size (~64mm) on the focusing mirror within the available space, which can focus a narrow beam waist (~21 mm) far into the plasma (~2 m). The output beam characters can be controlled by modifying the focusing mirror curvature.