Uniform Field Re-entrant Cylindrical TE_{01text {U}} Cavity for Pulse Electron Paramagnetic Resonance Spectroscopy at Q-band

Abstract

Uniform field (UF) resonators create a region-of-interest, where the sample volume receives a homogeneous microwave magnetic field (B_1) excitation. However, as the region-of-interest is increased, resonator efficiency is reduced. In this work, a new class of uniform field resonators is introduced: the uniform field re-entrant cylindrical TE_{text {01U}} cavity. Here, a UF cylindrical TE_{text {01U}} cavity is designed with re-entrant fins to increase the overall resonator efficiency to match the resonator efficiency maximum of a typical cylindrical TE_{011} cavity. The new UF re-entrant cylindrical TE_{text {01U}} cavity is designed for Q-band (34 GHz) and is calculated to have the same electron paramagnetic resonance (EPR) signal intensity as a TE_{011} cavity, a 60% increase in average resonator efficiency Lambda _mathrm{ave} over the sample, and has a B_1 profile that is 79.8% uniform over the entire sample volume (98% uniform over the region-of-interest). A new H-type T-junction waveguide coupler with inductive obstacles is introduced that increases the dynamic range of a movable short coupler while reducing the frequency shift by 43% during over-coupling. The resonator assembly is fabricated and tested both on the bench and with EPR experiments. This resonator provides a template to improve EPR spectroscopy for pulse experiments at high frequencies.