Design Strategies for Second Harmonic Gyrotrons in Nuclear Fusion Applications

Gyrotrons are high-power microwave sources that play an important role in the heating of plasmas for magnetically confined thermonuclear fusion applications. This paper presents a comprehensive study of two potential strategies for operating high-power megawatt-class fusion gyrotrons at the second harmonic of the electron cyclotron frequency which requires only half of the gyrotron cavity magnetic field. The first approach focuses on a coaxial cavity design that effectively suppresses fundamental competing modes, making it a robust solution for second harmonic operation. The second strategy discusses the injection of an external locking signal. Therefore, a quasi-optical mode converter was designed and tested capable of handling both, co- and counter-rotating modes.