Design concept and thermal-structural analysis of a high power reflective mm-wave optical mirror (M2) for the ITER ECH-UL

Each of the 4 ITER Electron Cyclotron Heating Upper Launcher (ECHUL) features 8 transmission lines used to inject microwave power of up to 1.31 MW per line (at the FCS diamond window), into the plasma at a frequency of 170 GHz. The millimetre waves are guided through a quasi-optical section consisting of three fixed mirror sets (M1, M2 and M3) and the front steering mirror set (M4). The M2 mirror set is composed of an upper and lower part, each reflecting 4 Gaussian beams coming from the M1 passing to the M3 mirror, which ultimately will reach the M4 steering mirror that will aim at the correct location in the plasma for suppression of the Neoclassical Tearing Modes (NTMs) that occur at the q=3/2 and q=2/1 rational surfaces. Reflecting mm-wave power generates heat by ohmic dissipation, reaching a peak power density of approximately 4 MW/m(2) on each of the 4 beam centre spots of the M2 mirror, and totaling 19.4 kW of absorbed power. EPFL-SPC has developed a novel water cooled mirror design concept which is able to dissipate such high heat loads (with up to 60,000 thermal cycles) and also to resist the applied external loads and dynamic displacements arising from plasma disruptions and seismic events, while complying with material and space restrictions. This study describes the main design features of the upper M2 mirror, its design conformity in accordance to thermal and structural integrity, as well as its compliance with the Essential Safety Requirements for nuclear in-vessel components.