A versatile loop-gap resonator probe for low-temperature electron spin-echo studies

Abstract

We have recently constructed a new high-power pulsed EPR spectrometer for use in biological electron spin-echo (ESE) studies. This spectrometer produces 25 ns microwave pulses with power levels up to one kilowatt. The bandwidth requirement for admitting such narrow pulses limits the quality factor, Q, for the EPR sample cavity or resonator to a value of a few hundred. A conventional microwave cavity, such as the TElo2 cavity often used in EPR, has a critically coupled Q of several thousand. Thus the Q must be greatly reduced to meet the bandwidth requirements for pulsed EPR. Without the asset of very high Q, the large volume and resultant poor filling factor of such a cavity limit sensitivity and increase the microwave power necessary to create a given microwave magnetic field amplitude. The stripline transmission cavity of Mims (I) provides a well-tested alternative for pulsed EPR. This device has a high filling factor and appropriately low Q, but is inconvenient to use because samples are placed directly into the cavity rather than into conventional EPR tubes, precluding measurements of pulsed and conventional EPR with identical samples. In recent years, other reduced volume resonators have been introduced to EPR (2-4). We have constructed a low-temperature pulsed EPR probe built around the loopgap resonator of Froncisz and Hyde (2), taking advantage of its moderate Q and high filling factor. This loop-gap probe mounts in a liquid He immersion dewar. The immersion dewar provides economical operation and excellent stability at temperatures as low as 1.5 K. The first loop-gap probe we constructed used a movable inductive loop to variably couple microwave power to the resonator. This system suffered from excessive microphonics, as the coupling loop vibrated in the bubbling liquid He. This led us to consider a system in which microwave power is coupled to the loop-gap resonator directly through a waveguide or cavity. In designing such a system we were constrained by the relatively small i.d. of the liquid helium dewar (3.2 cm). Many of our samples are poised in an EPR-active state at cryogenic temperatures and are unstable at room temperature. Thus an additional constraint was provided by the necessity of introducing these samples directly into the resonator while the probe system was immersed in liquid He. This capability would additionally allow us to change samples without removing the entire probe structure from the liquid He, conserving both liquid He and experimental time. We also planned to use the design for conventional EPR and for ENDOR experiments. In this note we present details of