Summary Fabrication of a resonator that optimizes liquid-phase concentration sensitivity at Q-band (35 GHz) was completed. The resonator has a coaxial TM020 mode, which is uniform in all but the radial direction. The mode is fed by an azimuthally symmetric capacitive iris and a cylindrical TM010 mode near cutoff. The cavity body is divided in two parts at an rf current null and can be disassembled for sample access. The 50 l sample volume in the form of a thin (0.09 mm) cylindrical shell of an 8 mm radius is predicted to give a saturable EPR signal 11 times the standard 0.5 l sample in the cylindrical TE011. With sample, the Q-value was measured as 1600, close to the predicted value. Since the original design,1 the Rexolite sample holder thickness was increased to approximately one-sixth of a wavelength to improve sample handling, saturable EPR signal, and manufacturability. The thick sample holder also permitted a novel modulation scheme to be implemented. A 10-turn solenoidal coil mounted inside the cavity on the sample holder provides a modulation amplitude of 4.5 G/A with a uniformity of 85%. EPR signal measurements from the resonator using a Varian E-12 spectrometer with an E-110 bridge are reported.

Design The cylindrical TM020 mode is the smallest cavity mode with a cylindrical electric field node between the cavity walls. The electric fields of this mode are parallel to the cylinder axis, and the magnetic fields are perpendicular. The rf fields are uniform in the two dimensions of the node, making the mode a natural choice for EPR. The mode is coupled from the inside with a cylindrically symmetric long slot capacitive iris. A central conducting wall houses a cylindrical TM010 mode inside the center conductor. This center cavity facilitates coupling between the rectangular waveguide and the iris, Fig. 1. The iris symmetry and placement suppress undesired modes in both the coaxial and cylindrical cavities. Dimensions and theoretical EPR signal are shown in the table. The mode is relatively insensitive to sample non-uniformities.

Coupling adjustments of the cavity can be accomplished either by adjusting the iris thickness via four set screws (Fig. 3) or by moving a 1-mm-wide, 2-mm-diameter metallic disc in the waveguide taper.

Fabrication of the two-part resonator assembly in Aluminum 6062-T6 was done first by CNC lathe (IDC Precision, Mukwonago, WI), Fig. 3. Then the rectangular waveguide and taper were cut by plunge EDM, Fig. 4. A 54 µm deep rectangular groove milled into each end of the cavity is designed to hold the Rexolite sample holder sleeves at a precise radius. The Rexolite sample holder was made on a precision lathe, Figs. 3-5. The two parts snap together and maintain a seal indefinitely. The sample size was made to optimize saturable signal. The 25 AWG (0.45 mm od) modulation coils are

Resonator for Optimization of Liquid-Phase EPR Concentration-Sensitivity for Spin Labels at Q-Band: Preliminary Results

Richard R. Mett,1,2 James R. Anderson,1 Jason W. Sidabras,1 and James S. Hyde1 1Department of Biophysics, Medical College of Wisconsin, Milwaukee, WI, USA

2Department of Physics and Chemistry, Milwaukee School of Engineering, Milwaukee, WI, USA

sample holder thickness was adjusted to tune the resonator frequency and the sample radius was adjusted for maximum Q value. With sample, the measured Q-value with the modulation coil in place is nearly the same as the Q-value without.

Testing of the resonator indicates that the mode is robust. The mode frequency is stable. The Q value is maximum when the sample holder is concentric with the resonator and declines rapidly when the sample is off-center. The 0.56 H modulation coil was put in series with a standard set of modulation coils. An attempt to obtain an EPR spectrum from 1M 2,2,6,6,-tetramethylpiperidine 1-oxyl (TEMPO) was made.

Reference 1Mett, Anderson, Sidabras, and Hyde, EPR Symposium Poster, 52nd RMCAC, Snowmass, CO, 3 August 2010.

Acknowledgements This work was supported by grants P41 EB001980 and R01 EB001417 from the National Institutes of Health.

Fig. 1 – Driven-mode finite-element simulation of the TM020 uniform-field cavity and coupling system. Electric field magnitude at 35.006 GHz is shown. The modulation coil is mounted on the sample holder.

Fig. 2 – Half section of cavity and coupling system (gray) with: the aqueous sample (blue, left), the Rexolite sample holder (pink, right), and 5 out of 10 modulation coils (purple).

Fig. 3 – Coaxial TM020 resonator components: cavity top with WR28 waveguide (left), bottom (upper right), and assembled sample holder with modulation coil (bottom right).

WR-28 flange

taper

TE10 - TM01 transition

TM010 coupling cavity

long slot iris

TM020 cavity

aqueous sample

Fig. 5 – Resonator with waveguide (center and right) and Rexolite sample holder assembly with modulation coil (left). The Delrin ends permit attachment of the modulation coil ends to the driver unit.

cavity mode TM020 TE011

dimensions (mm)

coupling cavity OD 6.56 -

cavity ID 7.57 -

cavity OD 23.46 11.3

cavity length 12 11.3

sample radius (position) 7.75 0

sample holder wall thickness 0.92 0.13

optimum EPR saturable signal parameters

sample volume (L) 45.9 0.78

sample thickness (mm) 0.08 0.30

long slot iris thickness (mm) 0.19 0.07

loaded Q-value 1672 3542

Λ (G/W1/2) 0.65 7.2

signal (W1/2/G) 24.0 2.2

optimum EPR unsaturable signal parameters

sample volume (L) 34.4 0.13

sample thickness (mm) 0.06 0.24

loaded Q-value 2513 7518

Λ (G/W1/2) 0.79 9.6

signal 17.0 9.5

modulation coilsFig. 4 – Partially disassembled two-piece Rexolite sample holder and modulation coil ready for sample loading. Left: on top of sample loading fixture. Right: sample loading position. The sample is fed into the groove by syringe as the inner section is slid into the outer section.

glued into half-round threads milled into the sample holder and fed out of holes on each end of the cavity. The modulation coil was found to perturb the mode. Using Ansoft HFSS (ver. 13, Pittsburgh, PA), the