Multipurpose EPR loop-gap resonator and cylindrical TE011 cavity for aqueous samples at 94GHz

Abstract

A loop-gap resonator (LGR) and a cylindrical TE(011) cavity resonator for use at W band, 94 GHz, have been designed and characterized using the Ansoft (Pittsburgh, PA) high frequency structure simulator (HFSS; Version 10.0). Field modulation penetration was analyzed using Ansoft MAXWELL 3D (Version 11.0). Optimizing both resonators to the same sample sizes shows that EPR signal intensities of the LGR and TE(011) are similar. The 3 dB bandwidth of the LGR, on the order of 1 GHz, is a new advantage for high frequency experiments. Ultraprecision electric discharge machining (EDM) was used to fabricate the resonators from silver. The TE(011) cavity has slots that are cut into the body to allow penetration of 100 kHz field modulation. The resonator body is embedded in graphite, also cut by EDM techniques, for a combination of reasons that include (i) reduced microwave leakage and improved TE(011) mode purity, (ii) field modulation penetration, (iii) structural support for the cavity body, and (iv) machinability by EDM. Both resonators use a slotted iris. Variable coupling is provided by a three-stub tuning element. A collet system designed to hold sample tubes has been implemented, increasing repeatability of sample placement and reducing sample vibration noise. Initial results include multiquantum experiments up to 9Q using the LGR to examine 1 mM 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in aqueous solution at room temperature and field modulation experiments using the TE(011) cavity to obtain an EPR spectrum of 1 microM TEMPO.

Saturation recovery EPR and ELDOR at W-band for spin labels

“…The W-band LGR [16] is the enabling technology for the experiments described here. The active region of the resonator is about 1 wavelength.…”

“…The resonator is a five-loop-four-gap LGR [16]. The LGR is positioned into a Magnex Split Pair EPR Superconducting Magnet System (Oxford, UK) via a side access port, which is advantageous for room temperature sample handling and to minimize waveguide path lengths.…”

“…16. Small water-cooled plates were attached to the assembly, and since the silver LGR is in good thermal contact, all parts of the assembly are clamped to the temperature of the water bath.…”

“…The combination of the LGR [16] and high nominal frequency (W-band) present significant advantages for SR EPR and SR ELDOR. As with CW EPR, the low Q-factor inherently suppresses source phase noise and is less sensitive to frequency drift between the incident frequency and the resonator frequency.…”

“…The issue of bandwidth can be overcome using bimodal resonators with one mode tuned to a first frequency and the other to a second frequency, but resonators of this type are complex to build and use. In the new instrument, we use a W-band loop-gap resonator (LGR) that has a bandwidth of about 1 GHz [16]. One thesis of this paper is that W-band SR ELDOR is greatly facilitated by the high bandwidth of this resonator.…”