Design considerations for future DEMO gyrotrons: A review on related gyrotron activities within EUROfusion

Long-term options for a steady-state DEMOnstration power plant may require the availability of gyrotrons with an operating frequency significantly above 200 GHz together with an RF output power of more than 1 MW and a total gyrotron efficiency of better than 60%. Frequency tuning in steps of around 2-3 GHz might be needed for control of plasma stability. Multi-purpose operation at frequencies with leaps of about 30 GHz might be considered for plasma start-up, heating and current drive at different operation scenarios. The combination of those requirements clearly challenges present-day technological limits. The R&D work within the EUROfusion WP HCD EC Gyrotron R&D and Advanced Developments is focusing on named targets. In particular, a centre frequency of around 240 GHz is under investigation. The coaxial-cavity gyrotron technology, and, as a possible fallback solution, the conventional hollow-cavity are under investigation. Both options are studied with regards to maximum achievable output power versus efficiency, operation stability and tolerances. Concerning the coaxial-cavity technology, an additional experimental investigation shall verify the predicted operation capabilities. Various promising concepts for multi-stage depressed collectors (MSDC) are under investigation. The research and development are completed by advancing the simulation and test tools capabilities significantly.