EXPERIMENTAL-STUDY OF A HIGH-EFFICIENCY QUASI-OPTICAL MODE CONVERTER FOR WHISPERING GALLERY MODE GYROTRONS

A mode converter for whispering gallery mode gyrotrons has been designed and experimentally demonstrated. Experiments were performed on a megawatt power level, 3-mu-s pulsed gyrotron operating in the TE16,2,1 whispering gallery mode at 146 GHz. The gyrotron cavity employs a non-linear uptaper to minimize radial mode conversion. About 99% of the output power is in the TE16,2 mode. The quasi-optical converter consists of a helically cut Vlasov-type waveguide launcher and a reflector. The doubly curved reflector, designed using geometric optics and vector diffraction theory, was built to focus the full radiation pattern to a small, gaussian-like focal spot. Of the power incident in the TE16.2 mode, 96% is directed by the launcher and reflector to a gaussian-like focal spot in the far field. Small fractions of other modes were found to form distinct focal spots in the far field. Analysis of the power in the other focal spots allows for a good quantitative measurement of gyrotron output mode content, potentially on a single shot basis.