Abstract :
Electron Cyclotron Resonance Heating (ECRH) is a technique for plasma breakdown and heating in nuclear fusion experiments. Hundreds of KW of microwave power are generated by gyrotron oscillators that operate with a strong magnetic field inside. Power is then transferred by using two different approaches: overmoded waveguides, used when the available space is limited or electromagnetic compatibility (EMC) and interferences are relevant factors. Otherwise, quasioptical (QO) waveguides where power is guided using metallic mirrors are preferred, since power coupling is much simpler and they are less sensitive to arcing. On the other hand, QO solutions need extra room to accommodate big metallic mirrors to avoid spillover effects, and are susceptible to the excitation of higher order modes and aberrations in the beam due to focusing with off-axis metallic mirrors. Lenses are interesting candidates to overcome all these problems. However, conventional lenses are made of dielectric materials which are easily degraded under high power exposition.
