AbstractThe project W7-X is a new stellarator facility built as a branch of IPP Garching at Greifswald, Germany. The main heating system is electron cyclotron resonance heating (ECRH) with a total power of 10 MW in continuous wave (CW) operation at a frequency of 140 GHz and it consists of 10 gyrotrons with an output power of 1 MW each. The power of 1.2 MW (1.6 MW for modulated operation) dissipated in each gyrotron has to be cooled away by powerful cooling modules which are part of the gyrotron system. Because of the very special operating conditions for these components, e.g. high power density, high voltage potential, high pressure drop and the extreme corrosion sensitivity of diamond windows, the cooling systems have to operate very reliably. The ERCH with gyrotrons and cooling modules will be built up by the Forschungszentrum in collaboration with IPP Garching, IPF Stuttgart and industry.