Analysis of two stacked cylindrical dielectric resonators in a TE102 microwave cavity for magnetic resonance spectroscopy

The frequency, field distributions and filling factors of a DR/TE102 probe, consisting of two cylindrical dielectric resonators (DR1 and DR2) in a rectangular TE102 cavity, are simulated and analyzed by finite element methods. The TE+++ mode formed by the in-phase coupling of the TE01 delta(DR1), TE01 delta(DR2) and TE102 basic modes, is the most appropriate mode for X-band EPR experiments. The corresponding simulated B+++ fields of the TE+++ mode have significant amplitudes at DR1. DR2 and the cavity's iris resulting in efficient coupling between the DR/TE102 probe and the microwave bridge. At the experimental configuration, B+++ in the vicinity of DR2 is much larger than that around DR1 indicating that DR1 mainly acts as a frequency tuner. In contrast to a simple microwave shield, the resonant cavity is an essential component of the probe that affects its frequency. The two dielectric resonators are always coupled and this is enhanced by the cavity. When DR1 and DR2 are close to the cavity walls, the TE+++ frequency and B+++ distribution are very similar to that of the empty TE102 cavity. When all the experimental details are taken into account, the agreement between the experimental and simulated TE+++ frequencies is excellent. This confirms that the resonating mode of the spectrometer's DR/TE102 probe is the TE+++ mode. Additional proof is obtained from B-1x, which is the calculated maximum x component of B+++. It is predominantly due to DR2 and is approximately 4.4G. The B-1x maximum value of the DR/TE102 probe is found to be slightly larger than that for a single resonator in a cavity because DR1 further concentrates the cavity's magnetic field along its x axis. Even though DR1 slightly enhances the performance of the DR/TE102 probe its main benefit is to act as a frequency tuner. A waveguide iris can be used to over-couple the DR/TE102 probe and lower its Q to approximate to 150. Under these conditions, the probe has a short dead time and a large bandwidth. The DR/TE102 probe's calculated conversion factor is approximately three times that of a regular cavity making it a good candidate for pulsed EPR experiments. (c) 2011 Elsevier Inc. All rights reserved.