Crystalline alumina loaded cavity resonator.

FIELD OF THE INVENTION

0001This invention relates to a crystalline alumina loaded cavity resonator and to a method of making such a resonator.

0002The resonators with which the invention is particularly concerned are those which have low losses and high frequency stability such that their frequency may be well-defined and only weakly perturbed by temperature, pressure and mechanical changes in the environment of the resonator, especially when brought to crogenic temperature below 20° KELVIN. The resonators also have capability of high power or high electric field operation.

DISCUSSION OF THE PRIOR ART

0003It is known that dielectric resonators exhibit radiation losses, see for example Richtmeyer R D (1939) J Appl Phys 10, 391-8.

0004It has already been shown by Braginsky et al, (1981) IEEE Trans Magn 17, pp 955-957 that the very low loss tangent of sapphire, the low coefficient of thermal expansion and the high Young's modulus, makes a sapphire dielectric resonator (SDR) suitable as an extremely stable frequency standard.

0005For cylindrical resonators the high dielectric constant of sapphire (E ≃ 10) enables "whispering gallery" modes to exist, with low radiation losses and consequently high Q-factors as long as the wavelength is sufficiently small compared with the radius of curvature.

0006During the past decade several superconducting cavity stabilised oscillator (SCSO) systems have been developed (Stain S R and Turneare J P (1973) Proc 27th Annual Symposium on Frequency Control Washington, DC: Elec Industries Assoc pp 414-20; Jiminez J J and Septier A (1973) Proc 27th Annual Symposium Frequency Control Washington DC: Elec Industries Assoc pp 406-13; Mann A G and Blair D G (1983) J phys D: Appl Phys 16, 105) that have exceptional frequency stability (Stein S R and Turneare J P (1973) Proc 27th Annual $ymposium on Frequency Control Washington DC: Elec Industries Assoc pp 414-20) and exceptionally low phase noise (Mann A G and Blair D G (1983) J Phys D: Appl Phys 16, 105). Low phase noise is achieved by locking an external oscillator to the cavity, and then using the cavity as a high Q transmission filter, and for this application it is important to have the highest possible Q-factor. Long, term frequency stability depends primarily on environmental control, since temperature variations and mechanical movement are transformed into frequency variations of the resonant cavity, Chief limitations are the coefficient of thermal expansion of the cavity, temperature dependence of the surface reactance of the superconductor and mechanical deformations due to vibration and due to tilt variations in the presence of the earth's gravitational field.

0007The intrinsic radiation loss from dielectric resonators (Richtmeyer R D (1939) J Appl Phys 10, 391-8) has led to the idea of coating a sapphire resonator with superconductor to obtain a high Q-factor (Strayer D M, Dick G J, Tward E (1983) IEEE Trans Magn 19, 512). Although this is an elegant solution, it does not avoid problems arising from the temperature dependence and the microwave power dependence of the surface reactance (Braginsky V B and Panov V I (1979) IEEE Trans Magn 15, pp 30-32) of the superconductor. The superconductor experiences the full electromagnetic field of the SDR, and power dependent Q-degradation has been observed (Braginsky V G and Panov V 8 (1979) Private Communication). These problems can be avoided by using an uncoated SDR. Braginsky has suggested the use of a large sapphire torus to prevent radiation losses (Braginsky V G, Panov V I, Timashov A V (1982) Sov Phys Doklady 267, 74). However study of a 50mm diameter torus at 10-20 GHz has shown that radiation losses are still a problem with this geometry (Blair D G and Vyatchanin S P (1978) Sov Phys JEIP 47, 433), while the torus is difficult to mount rigidly without introducing field perturbations and losses.

SUMMARY OF THE INVENTION

0008It is an object of this invention to provide a resonator with frequency stability superior to existing resonators through a system in which the magnitude of all known environmental perturbation are reduced compared with known systems.

0009Accordingly, this invention provides a crystalline alumina loaded cavity resonator comprising a crystalline alumina dielectric resonator having at least one protrusion whereby it can be rigidly mounted inside a metallic housing such that the main body of the said crystalline alumina dielectric resonator is separated a significant distance from the inside walls of the said housing constituting an electromagnetic cavity.

0010A preferred resonator comprises a spindle shaped sapphire dielectric resonator mounted inside and occupying some 25% of the volume of a superconducting niobium cavity.

DESCRIPTION OF THE DRAWING

0011A clearer understanding of this invention will be gained by a consideration of the drawing of a preferred embodiment and the further description of preferred embodiments.

0012In the drawing there is shown a vertical section of the resonator of the invention.

Turning to the drawing -

0013Numeral 1 designates a single crystal of alumina of generally cylindrical shape being a sapphire having protrusions 2 and 2' which fit closely in recesses in the base 3 and lid 3' of a niobium housing. The lid 3¹ can be clamped by means of the groove 4 (using clamping means not shown) to hold the sapphire rigidly between the lid and the base. To prevent radiation losses, an indium seal of suitable dimensions is provided whereby on clamping, the indium seals the lid 3' to the side walls 5 at groove 6 in lid 3'. One or more microwave probes 7 (schematically shown) are used to couple microwave power into the cavity through one or more holes 8 . The hole dimension and the probe position are designed to optimise the coupling to the resonator without degrading its performance.

0014The body of the sapphire dielectric resonator is a cylinder 30mm diameter and 30mm long. The protrusions 2 and 2' are about 7mm in diameter and 12mm long, and fit into recesses at the ends of the housing which is a 50mm diameter x 50mm long cylindrical niobium cavity. The system is designed to have a fundamental TE₀₁₁ mode at about 1 GHz, and for the SDR to be spaced about 5 scale lengths of the evanescent field from the cavity walls. This greatly reduces any perturbing effects of the cavity.

0015The cylindrical symmetry is also chosen so that transverse and longitudinal vibrations or fluctuations in the position of the SDR relative to the niobium cavity will, to first order, have a null contribution to the frequency of the resonator. This property will occur so long as the particular modes of the SDR have sufficient symmetry. This requires, firstly, that the symmetry axis of the sapphire be chosen to be parallel to the resonator axis, otherwise the anisotropy of the dielectric constant will cause angular distortion of the resonator field leading to incomplete nulling of frequency fluctuations. Secondly, nulling requires that modes with sufficient symmetry are selected.

0016In a further preferred embodiment the high Q-factor and the decoupling of the microwave energy from the walls allows much higher electric fields to be generated in a sapphire loaded conducting cavity than in other configurations. By using an appropriate mode in the sapphire dielectric resonator, and by placing appropriate beam entry holes in the housing in line with a small hole in the dielectric resonator (to allow penetration of a charged particle beam), it is possible to use this resonator as a high efficiency particle accelerator element.

0017The preferred substance from which the crystalline alumina dielectric resonator is constructed is a single crystal of sapphire but ruby or emerald may also be used. The metallic housing is preferably constructed from niobium although other high conductivity metals such as copper, silver, lead, tin and alloys and mixtures (including intermetallic compounds) may be used.

0018It is to be noted that this invention is to be given a broad connotation and is not to be limited to the invention specifically described.