ÌÇÐÄTV

 'Uniform field resonators for EPR and DNP in aqueous samples at high frequencies'

Two themes are combined in this presentation: design of uniform field (UF) resonators and aqueous sample-cells to achieve increased concentration sensitivity and uniform excitation at high microwave frequencies. Several UF resonators will be considered including (i) cavities with fields that are uniform in the dimension corresponding to transverse electric (TE) field propagation, (ii) transverse magnetic (TM) field propagation, or (iii) both (which for a rectangular cavity results in fields that are uniform over a plane); (iv) cylindrical cavities with uniform fields over a cylindrical surface, including both TM and TE modes; and (v) loop-gap resonators that are uniform in the axial dimension. Two principles of aqueous sample-cell design have been developed: placement of the aqueous sample in regions of zero electric field or in cells that are oriented such that the incident electric field is perpendicular to the cell surface. In both of these methods, either separately or in combination, the purposes of the sample-cell design are to minimize dielectric loss and maximize sample volume. UF designs result in uniform fields over two arbitrarily large dimensions, although practical problems arise if they are too large. Both planar and cylindrical geometries permit introduction of increased amounts of water. However, aqueous sample cells become very thin at high frequencies, which is a practical limitation. Another problem occurs as the extent of the null surface increases: troublesome higher order modes are found. Methods of suppression of these higher order modes have been studied. Microwave coupling to the UF mode at multiple points that are strictly in phase is one such approach. Numerous resonators have been designed and some have been constructed as we strive for improved concentration sensitivity for aqueous samples at high microwave frequencies.