Matching the output beam of a multi-frequency gyrotron to a Brewster window with small aperture

Multi-frequency megawatt gyrotrons are of great interest for controlling instabilities in magnetically confined fusion plasmas. For multmode operation of a gyrotron a broadband output window such as a Brewster window is necessary. But the aperture of such a window is limited because of the available CVD-disks. Thus the quasi-optical mode converter in the gyrotron has to be optimized for a given window position and aperture size.