Megawatt-Class Sub-THz Gyrotrons for ECRH in Future High-Field Fusion Devices

The demand for megawatt-class gyrotrons for Electron Cyclotron Resonance Heating (ECRH) of nuclear fusion devices is growing significantly. It goes hand in hand with growing requirements for output power, efficiency, operating flexibility, and reliability of gyrotrons used in nuclear fusion, as well as higher demanding of the manufacturability for serial production. In Europe research and development is ongoing to fulfil that future needs, particularly, with regards to output power and efficiency. It is complemented by the work on gyrotrons for the future EU DEMOnstration power plant which, according to the latest re-design, most probably, will be a device working at a low aspect ratio, hence, with a moderate toroidal magnetic field (~ 4.39 T) requesting for gyrotrons operating significantly below 200 GHz and at a power level of 2 MW CW. That is in line with the fact, that, today, ECRH systems operate at frequencies below 200 GHz, e.g., at 140 GHz for Wendelstein 7-X and 170 GHz for ITER, respectively. In parallel to this, several new start-ups are proposing concepts for compact nuclear fusion devices that shall operate at much higher toroidal magnetic fields. Hence, it will require gyrotrons operating significantly above 200 GHz, even above 300 GHz. That raises the question about the suitable concepts of megawatt-class sub-THz gyrotrons for ECRH in future low-field and high-field fusion devices. Design considerations ongoing in Europe addressing those challenges will be discussed.