EPR microwave cavity for small magnet airgaps

1. A microwave resonator, the resonator comprising:

a metal cavity body structured for supporting an electromagnetic microwave resonance mode, said microwave resonance mode having an even number of local maxima of microwave electric field energy, said metal cavity body having at least one first opening for inserting a sample tube to a center position of the resonator, wherein a center of said first opening and a center position of the resonator define an x-axis, said metal cavity body also having at least one second opening for transmitting microwave radiation into the resonator; and

at least two substantially identical dielectric elements disposed symmetrically with respect to an E-field nodal plane, said E-field nodal plane containing said x-axis and a z-axis which is perpendicular to said x-axis, wherein each dielectric element is geometrically formed and positioned to provide an equal overlap with a local maximum of said microwave electric field energy, wherein the resonator has a flat structure having a smallest internal extension along said z-axis substantially equal to a thickness of said first opening or of said dielectric elements, whichever is greater.

2. The resonator of claim 1 , wherein each of said dielectric elements is elongated along an axis parallel to said x-axis.

3. The resonator of claim 1 , wherein a ratio of a thickness of said dielectric elements to a dimension of said first opening, both in a direction of said z-axis of the resonator, is in a range of 0.5 to 1.5.

4. The resonator of claim 1 , wherein said equal overlap is such that at least 50% of said microwave electric field energy is within said dielectric elements.

5. The resonator of claim 1 , wherein said dielectric elements change a resonance frequency of said cavity body.

6. The resonator of claim 1 , wherein the resonator is of cylindrical shape.

7. The resonator of claim 1 , wherein the resonator is box-shaped.

8. A microwave resonator, the resonator comprising:

a metal cavity body structured for supporting an electromagnetic microwave resonance mode, said microwave resonance mode having an even number of local maxima of microwave electric field energy, said metal cavity body having at least one first opening for inserting a sample tube to a center position of the resonator, wherein a center of said first opening and a center position of the resonator define an x-axis, said metal cavity body also having at least one second opening for transmitting microwave radiation into the resonator; and

at least two substantially identical dielectric elements disposed symmetrically with respect to an E-field nodal plane, said E-field nodal plane containing said x-axis and a z-axis which is perpendicular to said x-axis, wherein each dielectric element is geometrically formed and positioned to provide an equal overlap with a local maximum of said microwave electric field energy, and further comprising at least one set of coils for creating a low frequency magnetic field traversing said cavity body and the sample tube, said coils being located at least partly inside the resonator, said coils having a connection to an outside of the cavity body via openings in the side walls of said cavity body which are perpendicular to said z-axis.

9. The resonator of claim 8 , wherein windings of said coils are completely outside said cavity body.

10. The resonator of claim 8 , wherein a section of windings of said coils inside said cavity body has a general orientation parallel to said x-axis.

11. The resonator of claim 8 , wherein the resonator comprises metallized side plates having openings for providing access to said coils inside the resonator.

12. The resonator of claim 1 , wherein said cavity body of the resonator operates in dielectric loaded rectangular TE102 or cylindrical TM110 resonance modes, and said dielectric elements are placed parallel to said x-axis centered to points of microwave E-field maxima.

13. An EPR (electron paramagnetic resonance) probe head, the probe head comprising:

a microwave resonator, wherein said resonator comprises a metal cavity body structured for supporting an electromagnetic microwave resonance mode, said microwave resonance mode having an even number of local maxima of microwave electric field energy, said metal cavity body having at least one first opening for inserting a sample tube to a center position of the resonator, wherein a center of said first opening and a center position of the resonator define an x-axis, said metal cavity body also having at least one second opening for transmitting microwave radiation into the resonator, said resonator also having at least two substantially identical dielectric elements disposed symmetrically with respect to an E-field nodal plane, said E-field nodal plane containing said x-axis and a z-axis which is perpendicular to said x-axis, wherein each dielectric element is geometrically formed and positioned to provide an equal overlap with a local maximum of said microwave electric field energy;

a housing for holding said microwave resonator in a static magnetic field along a z-axis; and

a cryostat within which said probe head is placed, wherein said cavity body of the resonator is spaced from innermost walls of said cryostat, and a space between said cryostat and the resonator is equipped with modules containing stacks of low frequency planar coils for creating main magnetic field modulation, gradient fields, and/or for fields for ENDOR or NMR excitation and detection.