Mirror development for the 140 GHz ECRH transmission system on the stellarator W7-X

For the future stellarator W7-X of IPP at Greifswald, a powerful electron cyclotron resonance heating (ECRH) facility is under design, which will include ten gyrotrons at 140 GHz with a CW power of 1 MW each plus two optional gyrotrons at 70 GHz. The millimetre wave power will be transmitted via two multi-beam waveguides (MBWG) with a length of about 50 m. This paper shows results for the optimum shape of the mirrors and the mode conversion of the MBWG transmission system. The design which will be used for water-cooled mirrors and cooling structures is discussed. Concepts for the back reflectors, which will be mounted at the inner side of the vacuum vessel of W7-X to redirect the non-absorbed part of the beam back into the plasma are presented. A test facility to test the transmission properties of quasi-optical lines, including a measurement system for ohmic loss of mirrors, a reflectometer to detect possible surface deformations and an alignment control system is described