Two of the most significant issues of our time reside in the continuous increase in the energy demand and, on the other hand, the urgent need to reduce the carbon footprint of traditional energy sources. Nuclear fusion energy is expected to provide a solution for both instances: it should be a clean, safe and practically unlimited source of energy, at least in human time frameworks. This new technology relies on fusing light particles in an extremely hot and ionized gas, the plasma. For the external heating of the latter, different techniques are currently used and envisaged, and among them the gyrotron, a high-power (∼MW) linear-beam vacuum tube which employs the cyclotron resonance of electrons to generate high-frequency (∼ 100 GHz) EM waves to irradiate the plasma throughout its operation.