The loop-gap resonator. II. Controlled return flux three-loop, two-gap microwave resonators for ENDOR and ESR spectroscopy

In a previous paper ( I, W. Froncisz and J. S. Hyde, J. Magn. Reson.47, 515 (1982) ), one-loop, multigap microwave resonators were described with emphasis on applications to ESR spectroscopy. In the present paper, n-loop, m-gap resonators are introduced, where there are n parallel loops with a shared gap connecting adjacent loops, there being ( n - 1) = m gaps in all, and an analysis of three-loop, two-gap resonators is presented. Calculations of resonant frequencies are compared with experiment. The structure exhibits two resonant modes, a fundamental mode where magnetic flux is excluded from the central loop, and a higher frequency mode in which flux links all three loops. This latter mode is used for ESR and ENDOR spectroscopy. The sample fills the central loop with a filling factor of about 0.4, and one side loop is used for inductive coupling. A 25% tuning range can be achieved by introduction of a metallic conductor into the intense magnetic field of the other side loop. Lines of magnetic flux are well controlled in the three-loop, two-gap resonator, permitting juxtaposition of radiofrequency coils suitable for ENDOR without interference between the coils and the microwave fields.