Mirror Design for Use in Gyrotron Quasi-Optical Mode Converters

An iterative planar- and cylindrical-based mirror design procedure is discussed for use in a gyrotron quasi-optical mode converter. The planar-based mirrors were designed using the iterative Katsenelenbaum-Semenov phase corrector procedure with an input beam obtained from a planar phase reconstruction. Many improvements were made to the design procedure including the use of an advanced phase unwrapping technique. The more robust design procedure resulted in smooth mirror surfaces. It was found that the fraction of power coupled between the target beam and the output of the planar-based mirrors measured in a cold test setup was 0.982. A similar design process was used for a cylindrical-based mirror pair. This design was performed using a simulated output of the launcher. It was found that the fraction of power coupled between the target beam and the theoretical output of the system was 0.990. The fraction of power coupled between the target beam and the phase-reconstructed beam at the output of the second mirror was 0.961. By using a cylindrical phase reconstruction, it was found that the reason for the slightly degraded performance of the cylindrical-based mirrors tested in the low power test setup was due to inaccuracies in modeling the field radiated from the launcher. These inaccuracies are presumed to come from impurities in the mode generator. The techniques discussed in this article can be used to improve the efficiency of quasi-optical mode converters and thus improve the operation of gyrotrons.