Current design studies on electron cyclotron heating and current drive (ECH&CD) systems for a DEMO fusion reactor demand gyrotron frequencies of above 200 GHz for efficient CD and a total gyrotron efficiency above 60% to achieve an efficient fusion power plant operation. Considering the mandatory total heating power for DEMO and the space and maintenance requirements per tube, gyrotrons with a unit power of above 1 MW will be needed. Furthermore, for plasma stability control using a simple fixed ECCD launcher, fast frequency tunability (in a few seconds) in steps of about 2-3 GHz is necessary. Such tubes require broadband or tunable output windows and their quasi-optical mode converter must support the conversion of the various modes to a fundamental Gaussian wave beam. Slow tunability of gyrotrons (within a few minutes) in leaps of about 30–40 GHz is considered advantageous (multi-frequency gyrotrons), eg for DEMO plasma start-up, for DEMO variants with a relatively low magnetic field or for multi-purpose use in other fusion devices such as ITER. A multi-frequency gyrotron can operate with a simple, single-disk synthetic-diamond output window which is transparent at the relevant frequencies.