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 c yclotron resonance heating (ECRH) facility is under design, which will incl ude 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 th e MBWG transmission system. The design which will be used for water-cooled mirrors and cooling structures is discussed. Concepts for the back reflecto rs, 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 presen ted. A test facility to test the transmission properties of quasi-optical l ines, including a measurement system for ohmic loss of mirrors, a reflectom eter to detect possible surface deformations and an alignment control syste m is described. (C) 2001 Elsevier Science B.V. All rights reserved.