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

For the future stellarator W7-X in Greifswald, a powerful electron cyclotron resonance heating (ECRH) system at 140 GHz with a total RF power of 10 MW CW is under construction. The transmission of the radiation from the gyrotron hall to the machine will be realized via a fully optical system consisting of more than 160 water cooled mirrors. This paper describes the design of the mirrors and presents results of calculations of the thermo–mechanical properties. Experimental verification of the calculations showed that mirror deformation under application of a heat load equivalent to the absorbed power from a 1 MW mm-wave beam is in the tolerable range. Water cooled mirrors of the single-beam type have been used successfully during conditioning of a prototype gyrotron with a mm-wave power of 850 kW and a pulse length of up to 180 s. To test and optimize in-situ measurements of beam parameters and power of the CW ECRH system on W7-X, these mirrors are equipped with different diagnostics.