EPR resonator with extended transparency and homogeneity in RF range
An EPR resonator for a cylindrical TE01n microwave mode, where n=1, 2, 3, or 4, has: a cylindrical body ( 10 ) which has an RF absorption of less than 5% at RFs below 1 kHz, a first plunger ( 11 ) delimiting the resonating volume within the body in an axial direction at a first end and a second plunger ( 12 ) delimiting the resonating volume within the body at a second end, the second plunger having an opening ( 13 ) for inserting an EPR sample. The first and second plunger each has a spiral winding of an electrically conductive filament wherein neither the ends nor neighboring turns of the spiral windings have electrically conductive connections prone to forming electrically closed loops. Using spiral winding plungers for cylindrical TE01n microwave modes provides equivalent functionality compared to conventional plungers, but without creating Eddy currents at frequencies lower than the frequency of the TE01n microwave mode.
1. An electron paramagnetic resonance (EPR) resonator for a cylindrical TE01n microwave mode where n is an integer, comprising:
a cylindrical body which has a radio frequency (RF) absorption of less than 5% at RFs below 1 kHz,
a first plunger delimiting a resonating volume within the cylindrical body in an axial direction at a first end and
a second plunger delimiting the resonating volume within the cylindrical body in the axial direction at a second end,
wherein the second plunger has an opening configured to receive an EPR sample inserted into the EPR resonator,
wherein the first and the second plunger each comprise at least one spiral winding of an electrically conductive filament, wherein neither ends nor neighboring turns of any of the spiral windings connect electrically conductively to form any electrically closed loop.
2. The EPR resonator according to claim 1 , wherein the integer n is 1, 2, 3 or 4.
3. The EPR resonator according to claim 1 , wherein a ratio D/δ of a minimum extension D of the filament to a distance δ between neighboring turns of the spiral winding is bigger than 1:1.
4. The EPR resonator according to claim 3 , wherein the ratio D/δ is bigger than 10:1.
5. The EPR resonator according to claim 1 , wherein a ratio of an axial length of at least one of the plungers to an inner bore diameter of the cylindrical body is at least equal to 1.
6. The EPR resonator according to claim 1 , wherein the first plunger and/or the second plunger includes, in at least two layers, at least two spiral windings of the electrically conductive filament arranged in an axial direction of the cylindrical body without electrically conductive connection between neighboring ones of the layers.
7. The EPR resonator according to claim 6 , wherein the first plunger and/or the second plunger include, in at least 30 layers, at least 30 spiral windings of the electrically conductive filament arranged in the axial direction without electrically conductive connection between the neighboring layers.
8. The EPR resonator according to claim 1 , wherein the opening of the second plunger has a size smaller than a cut-off diameter of the cylindrical TE01n microwave mode.
9. The EPR resonator according to claim 1 , wherein the first plunger has an opening for a support of a sample positioned in a working position in the EPR resonator.
10. The EPR resonator according to claim 1 , wherein the first plunger or the second plunger has a second opening configured to transmit microwave radiation with an antenna.
11. The EPR resonator according to claim 3 , wherein space between neighboring turns of the filament is filled with an insulator.
12. The EPR resonator according to claim 1 , wherein the filament is coated with an insulator.
13. The EPR resonator according to claim 3 , wherein the extension D of the filament is in a range of 5 μm to 5 mm.
14. The EPR resonator according to claim 13 , wherein the extension D of the filament is in a range of 30 μm to 1 mm.
15. The EPR resonator according to claim 1 , wherein the filament comprises a wire made entirely of metal.
16. The EPR resonator according to claim 1 , wherein the metal wire is Ag or Au coated copper, brass or tungsten, or Ag or Au wire.
17. The EPR resonator according to claim 1 , wherein an RF field is generated by RF coils arranged outside the EPR resonator and a lateral dimension of the RF coils relative to an axial length of the resonating volume within the cylindrical body is in a range of 50% to 200%.
18. The EPR resonator according to claim 17 , wherein a homogeneity of the RF field at the sample volume is in a range of 0.01% to 10%.
19. The EPR resonator according to claim 18 , wherein the homogeneity of the RF field at the sample volume is in a range of 0.1% to 1%.