Experimental Results of the Novel 1.5-MW-Class 140-GHz Continuous-Wave Gyrotron for the Wendelstein 7-X Stellarator

In this work, we present the achievements obtained during the commissioning phase of the newly developed 140-GHz continuous-wave tube TH1507U at the gyrotron test stand of the electron-cyclotron resonance heating facility of the Wendelstein 7-X stellarator. The gyrotron is based on the successful 1-MW class industrial TH1507 gyrotron, which operates in the TE $_{{28},{8}}$ mode, and has been optimized for operation in the higher-order TE $_{{28},{10}}$ mode. The 1-ms short-pulse tests confirmed the nominal output power of 1.5 MW. In a long-pulse operating regime, an output power of 1.3 MW with total efficiency 45.9% was demonstrated at pulse lengths of 3 minutes. Different regimes where the beam current is above 50 A demonstrated a saturation of output power at 1.3 MW, that can be explained by the presence of parasitic modes. A parasite-free operation with an output power of 1.2 MW was achieved with pulses up to 580 s in length. The pulse length was limited due to the existing capabilities of the cooling system at the test stand, and is foreseen to be extended in the future.