Today's performance requirements for future DEMO gyrotrons are an operating frequency > 200 GHz and an output power of 2 MW. A total gyrotron efficiency of better than 60 % must be achieved. Multi-purpose/multi-frequency operation and frequency step-tunability are required also. It has been shown earlier, that the coaxial-cavity technology is a promising candidate [1] for future fusion power plants. In [1] a world record RF output power at 170 GHz of 2.2 MW has been presented in short-pulse

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... ation (~ few milliseconds). However, it has to be proven that the coaxial-cavity technology can be used in steady-state operation. That shall be achieved by upgrading the existing 2 MW 170 GHz short-pulse prototype with a cooling system and the implementation of advanced Magnetron Injection Guns (MIGs) [3] . Both, the advanced MIGs as well as the longpulse gyrotron will show the way towards a possible solution for DEMO with a significantly higher output power at higher operating frequencies.