Optimization of axial RF field distribution in low-frequency EPR loop-gap resonators.

A novel coupling method that optimizes the axial RF distribution of low-frequency EPR loop-gap resonators is presented. It consists of a resonant coupling loop positioned at the centre of a two-section loop-gap resonator. This arrangement ensures a symmetrical distribution of the radio frequency field along the axis of the resonator. The design of a central coupling system suitable for EPR resonators operating at about 220 MHz is described. Experimental results show that with the central coupling system the RF field is symmetrical and has a very good axial homogeneity (100% of the resonator length).