This is an excellent review about uniform field resonators for EPR. These resonators typically have larger filling factors and therefore increased sensitivity. Not directly related to DNP-NMR spectroscopy, but definitely an inspiring article to think about high (and low) field resonators for DNP-NMR.
Hyde, James S., Jason W. Sidabras, and Richard R. Mett. “Uniform Field Resonators for EPR Spectroscopy: A Review.” Cell Biochemistry and Biophysics 77, no. 1 (March 2019): 3–14.
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.