Uniform Field Resonators for EPR Spectroscopy: A Review
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Cavity resonators are often used for electron paramagnetic resonance (EPR). Rectangular TE102 and cylindrical TE011 are common modes at X-band even though the field varies cosinusoidally along the Z-axis. The authors found a way to create a uniform field (UF) in these modes. A length of waveguide at cut-off was introduced for the sample region, and tailored end sections were developed that supported the microwave resonant mode. This work is reviewed here. The radio frequency (RF) magnetic field in loop-gap resonators (LGR) at X-band is uniform along the Z-axis of the sample, which is a benefit of LGR technology. The LGR is a preferred structure for EPR of small samples. At Q-band and W-band, the LGR often exhibits nonuniformity along the Z-axis. Methods to trim out this nonuniformity, which are closely related to the methods used for UF cavity resonators, are reviewed. In addition, two transmission lines that are new to EPR, dielectric tube waveguide and circular ridge waveguide, were recently used in UF cavity designs that are reviewed. A further benefit of UF resonators is that cuvettes for aqueous samples can be optimum in cross section along the full sample axis, which improves quantification in EPR spectroscopy of biological samples.
KeywordsUniform field EPR Ridge waveguide Dielectric tube resonator Q-band W-band
This work was supported by grant P41 EB001980 from the National Institutes of Health. Jason W. Sidabras is currently funded by the European Union Horizon 2020 Marie Skłodowska-Curie Fellowship (Act-EPR; http://act-epr.org) and the Max Planck Society.
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Conflict of Interest
The authors declare that they have no conflict of interest.
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