Tunable microwave resonator for static dynamic nuclear polarization (DNP)
A tunable millimeter-wave (mmw) DNP probe head is disclosed that is compatible with efficient H/X/Y/e − DNP in samples that may have volume large compared to λ 0 3 , where λ 0 is the free-space wavelength at the frequency f e of the electron paramagnetic resonance (EPR) when placed in an external polarizing field B 0 , where B 0 is typically in the range of 6.5 T to 35 T, corresponding to f e in the range of 180-1000 GHz, and corresponding to proton resonance frequency f H in the range of 280 MHz to 1500 MHz. The probe head comprises a tune cavity of adjustable volume, a sample cavity that is large compared to the sample wavelength, a tapered conical feed transition filled with a high dielectric material at the small end, and a selectively reflective wall that is substantially reflective of electromagnetic radiation at f e but substantially transparent to electromagnetic radiation at f H .
1. A tunable static Dynamic Nuclear Polarization probe for characterization of a sample by Dynamic Nuclear Polarization techniques suitable for use in an external polarizing field B 0 at Nuclear Magnetic Resonance frequency f N and electron paramagnetic resonance frequency f e of free-space wavelength λ 0 , said probe characterized as comprising
a sample cavity of transverse dimension greater than 2λ 0 /3 substantially bounded by a reflective side wall and reflective end surfaces,
a tapered waveguide transition_ 1 having a small-end conical-section solid dielectric core at its smaller end,
said sample cavity fed transversely through a sample iris near an axial center of its reflective cylindrical wall from a dielectric tip of said tapered transition_ 1 ,
a tune cavity having an adjustable effective volume,
a waveguide transition_ 2 connecting said tune cavity to the larger end of said tapered waveguide,
said reflective side wall further characterized as being substantially reflective to electromagnetic radiation at frequency f e but substantially transparent to electromagnetic radiation at frequency f N and further characterized as being mechanically supported on an inside of a ceramic coilform, and
an rf coil on the outside of said coilform.
2. The Dynamic Nuclear Polarization probe of claim 1 in which the effective volume of said tune cavity can be varied by a movable metallic rod extending into said tune cavity.
3. The Dynamic Nuclear Polarization probe of claim 1 further characterized as including two adjustment mechanisms so both the effective volume of said tune cavity and the effective centering of its fields with respect to said transition_ 2 can be adjusted.
4. The Dynamic Nuclear Polarization probe of claim 1 in which the effective volume of said tune cavity can be varied by a movable dielectric component extending into said tune cavity.
5. The Dynamic Nuclear Polarization probe of claim 1 in which said tune cavity is further characterized as being fed through a tune-cavity iris connected to the small end of a tapered waveguide transition_ 3 of minor radius greater than λ 0 /3.
6. The Dynamic Nuclear Polarization probe of claim 1 in which said reflective side wall is further characterized as comprised of double-clad laminate with overlapping foil patches.
7. The Dynamic Nuclear Polarization probe of claim 1 in which a portion of said reflective end surfaces are located on removable dielectric plugs in the ends of a sample tube that fits inside said sample cavity.
8. The Dynamic Nuclear Polarization probe of claim 1 in which said sample cavity has length greater than 2λ 0 .
9. The Dynamic Nuclear Polarization probe of claim 1 in which said small-end conical-section dielectric core is of alumina.
10. The Dynamic Nuclear Polarization probe of claim 1 in which said first tapered waveguide further comprises a larger conical-section dielectric core adjacent to said small-end core, said larger core being made of a material of lower permittivity than that of said small-end core.
11. The Dynamic Nuclear Polarization probe of claim 1 in which said rf coil is a solenoid of at least two turns.
12. The Dynamic Nuclear Polarization probe of claim 1 in which said sample cavity is oriented such that its axis will be inclined at an angle between 55° and 90° when inserted into the bore of a superconducting magnet.
13. The Dynamic Nuclear Polarization probe of claim 1 further characterized as comprising
an input microwave port connected to an overmoded circular waveguide of radius greater than 1.2λ 0 ,
a fundamental-mode circular waveguide of radius less than 0.8λ 0 , and
a third tapered waveguide between said overmoded waveguide and said fundamental-mode waveguide.
14. The Dynamic Nuclear Polarization probe of claim 1 further characterized as comprising a small tube connected to transition_ 2 for optical irradiation.
15. The Dynamic Nuclear Polarization probe of claim 1 further characterized as including additional coils and capacitors to tune said rf coil to three Nuclear Magnetic Resonance frequencies simultaneously.
16. The Dynamic Nuclear Polarization probe of claim 1 in which said reflective side wall is further characterized as a conductive film of as alloy including a noble metal and having resistivity greater than 4 μΩcm.
17. The Dynamic Nuclear Polarization probe of claim 1 in which said sample cavity iris has minimum diameter greater than λ d /2, where λ d equals λ 0 divided by the square root of the dielectric constant of said solid dielectric core.
18. A sample cell for Dynamic Nuclear Polarization comprising a dielectric tube of outer radius r, length L, and wall w, with metallization covering its end surfaces and extending toward the center on a cylindrical surface a distance greater than w and less than L/3, said metallization having thickness greater than 0.1 micron and less than 2 microns, said dielectric selected from the set of quartz, silicon nitride, mullite, and aluminum oxide, and further comprising dielectric plugs within the ends of the dielectric tube.
19. A method of performing a Dynamic Nuclear Polarization experiment that includes loading sample material onto Dynamic Nuclear Polarization sample trays, inserting a stack of the loaded trays into a Dynamic Nuclear Polarization sample tube, inserting a reflective dielectric plug into an end of the sample tube, inserting the sample tube into a cylindrical Dynamic Nuclear Polarization sample cavity in a Dynamic Nuclear Polarization probe head, inserting the Dynamic Nuclear Polarization probe head into a polarizing magnet, and performing a Dynamic Nuclear Polarization measurement,
said Dynamic Nuclear Polarization sample trays further characterized as being of a material of relative permittivity less than 3.5, having depth less than λ 0 /2 and each lateral dimension greater than λ 0 /2 but less than 20λ 0 ,
said reflective dielectric plug further characterized as having an end surface that is substantially reflective to electromagnetic radiation at frequency f e .