EU megawatt-class 140 GHZ CW gyrotron

Summary form only given. The development of gyrotrons with 1-MW output power at the Forschungszentrum Karlsruhe (FZK) in collaboration with EURATOM Associations and Thales Electron Devices for continuous wave (CW) operation at 140 GHz is linked with the construction of the new 10 MW ECRH system of the superconducting stellarator Wendelstein 7-X at the Institute of Plasma Physics (IPP) Greifswald, Germany. The first series tube tested at FZK yielded a total output power of 980 kW with an efficiency of 31% in short pulse operation and of 920 kW in pulses of 180 s (efficiency of 45% at a collector depression voltage of 29 kV). The Gaussian mode output power was 906 kW. The pulse length at full power is limited at FZK by the available power supply. At reduced electron beam current, it is possible to operate at longer pulse lengths. At an output power of 570 kW (beam current of 29 A), the pulse length was increased to 1893 s. There was no physical reason for a limitation of this pulse: the pressure increase during the pulse was less than a factor of two and ended up at a very low value in the 10^-9 mbar range. The tube was delivered to IPP for tests at full power and up to 30 minutes pulse length. The Gaussian mode RF output power measured in a calorimetric load was 870 kW at a total output power of 920 kW in 30 min. pulses. Again, no indications for a limitation in pulse length was found. The second series tube was delivered to FZK in Nov. 2005. Up to now, this tube could only be tested in short pulse operation. At the design value of 40 A for the electron beam current, an output power of 960 kW was found in very short pulse operation (1 ms)