Filling Factor and Isolator Performance of the Traveling-Wave Maser*

Abstract

In designing a large gain and simultaneously large instantaneous bandwidth traveling-wave maser (TWM), the filling factor and the isolator performance should be optimized. The filling factor is a measure of the efficiency of interaction between the spin system of the maser material and the RF magnetic fields of the slow-wave structure. For the TWM using the 90° operation of ruby and the comb as the slow-wave structure, the c axis of the ruby should be parallel to the z axis of the structure (the direction of the signal wave propagation) for the largest filling factor. The improvement of the filling factor by the proper orientation of the c axis of the ruby is larger at the lower signal frequencies because the transition probability of ruby is more nearly linear at those frequencies. The isolator should provide sufficient reverse absorption to make the TWM short-circuit stable and yet add the minimum forward absorption to the TWM. Both the reverse and the forward absorption of the isolator depend critically on the size of the ferrite disks and the position in which they are imbedded in the comb structure. An analysis of the filling factor and isolator performance and its comparison with measurements was made. Together with Refs. 1 and 2, this paper is intended to reduce the amount of experimental work involved in developing traveling-wave masers. Although the discussion is centered on the comb-type ruby TWM, the data provided also apply to other tape slow-wave structures using different active crystals.