Loaded Q's and field profiles of tapered axisymmetric gyrotron cavities

A theoretical investigation of the vacuum electromagnetic properties of a class of tapered cylindrical resonant cavities employed in gyrotron design is reported. The cavity properties which determine gyrotron interaction efficiency, field profiles and loaded Q's, are predicted as a function of the geometric parameters. It is shown that this resonator geometry is superior to other designs for several reasons: (1) its properties are less sensitive to machining tolerances; (2) it produces negligible mode conversion at the output; and (3) in contrast with the limitations of iris-coupled designs, is capable of providing loaded Q's as low as about 0.3 Q(diff) where Q(diff) equals 4Pi(L/lambda)-squared is the 'diffraction limit'. Cold tests in the millimeter bands are reported which verify these conclusions.