Gordon Coupler with Inductive or Capacitive Iris for Small EPR Resonators for Aqueous Samples

Abstract

The Gordon coupler was introduced for use in EPR experiments at liquid helium temperatures. It provides an evanescent wave incident on the iris of a microwave resonator. Match of power incident on the coupler to the resonator is obtained by variation of the amplitude of an evanescent wave that arises from displacement of a dielectric wedge in a tapered waveguide. Reduced microphonics from helium bubbling was reported. The Gordon coupler was subsequently extended from cavity resonators to loop-gap resonators, initially at helium temperatures but later for aqueous samples. Plastics with low dielectric constants, usually Teflon, were used. Here, we extend the Gordon coupler for application in X-band five-loop-four-gap resonators using fused quartz, sapphire, or rutile dielectrics, noting that the size of the coupler can then be commensurate with dimensions of dielectric loop-gap resonators as well as dielectric tube resonators. Finite element modeling of electromagnetic fields has been carried out, and use of a capacitive iris that interfaces with the Gordon coupler reduces pulling of the resonant frequency when matching the resonator.

Keywords: Gordon coupler; capacitive and resonant iris; dielectric resonator; electron paramagnetic resonance; inductive; loop-gap resonator; sapphire; single crystal rutile.

Figure 1:

Cross sectional view, left to right, of dLGR (purple), capacitive iris, reduced-size rectangular waveguide, sapphire wedge (pink), waveguide taper, and standard WR90 waveguide. The taper length of both the wedge and waveguide is 40 mm. Other dimensions are shown in Table 2.

Figure 2:

Zoomed cross sectional view from top of dLGR showing one large LGR inner loop and four small outer loops with conductor shown purple, rutile dielectric resonator blue, Teflon sample holder white, aqueous sample red, reduced-size rectangular waveguide brown and sapphire wedge pink. Dimensions are shown in Table 2.

Figure 3:

Cross sectional view of Fig. 1 except showing the magnitude of the peak rf magnetic field, red maximum to blue minimum, in a logarithmic scale. The dLGR is critically coupled with 1 W input power. The peak field in the dLGR is 86.3 G and the peak field in the WR90 is 72.3 mG. This corresponds to a resonator efficiency of 43 G/W^1/2. The PC rutile DR contains a AWG32sw sample tube with 135 nL of liquid water (see Ref. [12]).