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. We show 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 ∼0.3 Q
diff
where Q
diff
= 4π(L/λ)
²
is the "diffraction limit." Cold tests in the millimeter bands are reported which verify these conclusions.