High-Power Experiments with 140 GHz Series Gyrotrons for W7-X

Electron-cyclotron-resonance heating (ECRH) is an important heating system for the stellarator W7-X. For this system a total power of 10 MW at 140 GHz is needed in 30 min operation. In a collaboration with European research laboratories and European tube industry (Thales Electron Devices (TED) in France), cylindrical cavity gyrotrons designed for an output power of 1 MW, CW at 140 GHz have been developed. They are equipped with a single-stage depressed collector for increasing the efficiency and reducing the power loading at the collector wall, an advanced quasi-optical (q.o.) mode converter system with minimized stray radiation, and a single-disk diamond window made by chemical vapor deposition (CVD diamond). Details of the design can be found in Ref. (Dammertz et al, 2002). The first series gyrotron (SN1) has been tested successfully in long pulse operation with full power (30 min, 920 kW, efficiency close to 45 %) and meets the specifications (Dammertz et al., 2006). In this contribution we discuss experimental results of the second and third series gyrotron (SN2, SN3).