Recent developments in the methodology and instrumentation of electron paramagnetic resonance (EPR) hold promise of revolutionary enhancement of applications of EPR to problems in materials science, chemistry, physics, biology, and medicine. One key missing development is an improvement in the coupling of the sample- containing EPR resonator to the microwave source and detector systems. In preliminary work basic engineering design equations were developed for three types of coupling of an EPR resonator to a transmission line: mutual inductive coupling, series capacitive coupling, and parallel capacitive coupling. These equations form an accurate basis for developing new, more optimum resonator coupling techniques. Focusing on the versatile loop-gap type~of EPR resonator (LGR), a test fixture was built in which were tested prototype LGR coupling networks. The tests verified predictions of the equations and demonstrated that a varactor (a voltage-variable capacitor) diode is a useful tuning element in LGR coupling networks. A prototype, novel, general-purpose resonator has been constructed. It yields useful low-temperature spectra of metalloenzymes. The design equations and the results of tests conducted on prototype resonators and coupling devices point directly to the specific aims of the proposed grant period. It is now proposed to build a resonator at S-band (2-4 GHz) implementing the predicted best low-noise coupling scheme, and incorporating design features that facilitate ease of sample change and flexibility for both continuous wave and pulsed EPR. The design will be consistent with convenient variable temperature (including cryogenic) operation. GaAs varactor diodes and field-effect transistor amplifiers will be used, since they have good performance at cryogenic temperatures. Several of the critical microwave components can be located in close proximity to the sample on a small circuit board, which itself can be cooled alo ng with the sample and resonator, with the goal of the lowest thermal noise in the electronic components. Careful attention to materials properties will result in a robust and usable assembly. The second phase of the work will be the development of a totally new approach to integrating the resonator, coupling assembly, means for tuning, and the microwave bridge element all on or close to a small hybrid microwave subs~te. Such a hybrid microwave bridge circuit will have lower losses resulting in improved EPR signal-to-noise ratio at room temperature. By cooling the circuitry to cryogenic temperatures there is potential for a large increase in signal-to-noise. Extension to the X-band (9 GHz) region, which requires making everything smaller and building it more precisely, will begin during the proposed grant period. The use of varactor tuning will be demonstrated at X-band, and a test fixture will be built to obtain practical operating experience with the sensitivity of the resonator and coupling system performance to various construction features.
