Design methods for mirrors in a quasi-optical mode converter for a frequency step-tunable gyrotron

For controlling instabilities in magnetically confined plasmas, in large thermonuclear fusion reactors by localized current drive, frequency step-tunable gyrotrons are of great interest. In order to achieve a Gaussian distribution of the output beam of a gyrotron for all frequencies, the mirrors of the quasi-optical mode converter should be optimized for all exited modes. Two methods for generating an optimized contour of the surface of a nonquadratic phase-forming mirror are introduced: the well-known Katsenelenbaum-Semenov algorithm and the gradient method. A mirror system, using an advanced launcher as source, optimized for 9 frequencies is shown in order to demonstrate the presented methods.