EPR Uniform Field Signal Enhancement by Dielectric Tubes in Cavities

Abstract

The dielectric tube resonator (DTR) for EPR spectroscopy is introduced. It is defined as a metallic cylindrical TE011 microwave cavity that contains a dielectric tube centered on the axis of the cylinder. Contour plots of dimensions of the metallic cylinder to achieve resonance at 9.5 GHz are shown for quartz, sapphire, and rutile tubes as a function of wall thickness and average radius. These contour plots were developed using analytical equations and confirmed by finite element modeling. They can be used in two ways: design of the metallic cylinder for use at 9.5 GHz that incorporates a readily available tube such as a sapphire tube intended for NMR, or design of a custom procured tube for optimized performance for specific sample-size constraints. The charts extend to the limiting condition where the dielectric fills the tube. However, the structure at this limit is not a dielectric resonator due to the metal wall and does not radiate. In addition, the uniform field (UF) DTR is introduced. Development of the UF resonator starting with a dielectric tube resonator is shown. The diameter of the tube remains constant along the cavity axis, and the diameter of the cylindrical metallic enclosure increases at the ends of the cavity to satisfy the uniform field condition. This structure has advantages over the previously developed UF TE011 resonators: higher resonator efficiency parameter Λ, convenient overall size when using sapphire tubes, and higher quality data for small samples. The DTR and UF DTR structures fill the gap between free space and dielectric resonator limits in a continuous manner.

Uniform Field Resonators for EPR Spectroscopy: A Review

“…The DTR was introduced by Hyde and Mett in 2017 (7). Their work was motivated by the early patent by Rempel et al, which discloses the use of a quartz Dewar inserted into an Xband cavity for temperature control (11).…”

“…An effort was made in Ref. (7) to cite every paper on dielectric resonators. This was done in part as a convenience to the reader, and in part to define the difference between the dielectric resonator and the DTR.…”

“…Methods developed to achieve uniformity in cavity resonators have been extended to LGRs. Other advances that are reviewed here include (a) structures based on circular waveguide that contains dielectric tubes (7), and (b) extension of UF technology to ridge circular waveguide or perhaps other re-entrant metallic geometries (8).…”

“…And at X-band, both designs lead to miniaturization for enhanced sensitivity with samples of limited volume. Hyde et al and Sidabras et al provide new insight into resonator design for EPR beyond the stated goal of achieving UF over the sample (7,8).…”

Розробка Та Характеризація Керамічних Вставок В Металеві Резонатори ЕПР Спектрометрів Для Підвищення Їх Чутливості

“…Another effective approach to increase the sensitivity of EPR spectrometers is the application of dielectric resonators (DRs) or dielectric inserts (DIs) in standard metallic resonators. Really, since DIs are made of materials with a high value of dielectric constant , the microwave field becomes concentrated in the sample region, which increases the resonator fill factor [9][10][11][12][13]. The advantages of DRs were considered in work [14] in detail.…”