This subproject is one of many research subprojects utilizing theresources provided by a Center grant funded by NIH/NCRR. The subproject andinvestigator (PI) may have received primary funding from another NIH source,and thus could be represented in other CRISP entries. The institution listed isfor the Center, which is not necessarily the institution for the investigator.Currently, there are two types of resonant structure in common use for DEER. They are a high-Q bimodal cavity resonator and smaller low-Q resonators that are typically loop-gap and dielectric types. At high working frequencies, the filling factor of smaller structures begins to degrade due to the necessity to provide practical gaps and because of the sample tube wall thickness. For this reason a cavity resonator becomes a viable alternative. However, bimodal resonators at mm-waves pose a challenge, therefore cavity resonators, such as dielectrically loaded cylindrical TE011 or Fabry-Perot may become the probes of choice. Sensitivity will increase with Q until it is limited by a trade-off between spectral excitation in the pumping mode. The optimum is expected at Qs that matches excitation and detection pulse bandwidths. For example, at 35 GHz a loaded Q of 2000 permits excitation of at least 6 G spectral extents. A single-mode high-Q resonator however requires rapid pulsing of magnetic field to pump spins at a different frequency, which seems to be an adequate alternative for use with 4-pulse DEER. A cylindrical dielectrically-loaded resonator has been demonstrated with its side wall made of thin metal disks, which therefore can admit short pulses of magnetic field. The resonator of such type is in use in the Dr. D. Lee group at Cornell, and their design can be scaled to a higher frequency in Ka band in straightforward fashion. We plan to simulate by CST Microwave Studio on how to transform its shape to be more spherical and how to shape the dielectric insert in order to optimize both field homogeneity and filling factor and still be able to admit the pulsed magnetic field with low effects caused by eddy currents. A suitable current pulser is part of another ongoing project.
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