Design of an ultra-broadband single-disk output window for a frequency step-tunable 1 MW gyrotron

Output window design is one of the key issues to realize broadband output of a multi-frequency gyrotron. Corresponding to the recent development of a frequency step-tunable 1 MW gyrotron at Forschungszentrum Karlsruhe (FZK), this paper reports about a newly designed ultra-broadband CVD-diamond disk Brewster window. The thickness of the disk and the geometry of the window unit have been optimized to get low power reflection over a broadband incident angle range around the Brewster angle at different operating frequencies. Detailed calculations of mm-wave transmission characteristics for this Brewster window have been performed for all the considered operation modes from TE17,6 at 105 GHz up to TE23,8 at 143 GHz, and for thickness of the disk from 1.5 to 2.0 mm. Calculations show if one prefers to place the low reflection area in the middle of the discussed frequency range (120–130 GHz), the thickness of 1.6 mm is near optimum and its −20 dB bandwidth angle is more than 30°. For operation near 105 and 140 GHz, a 1.9 mm disk is preferable and its −20 dB bandwidth angle is around 30°.