High temperature superconductor support structures for dielectric resonator

The invention is directed to a superconducting microwave resonator, to holding devices for those resonators, and to their methods of manufacture. The superconducting microwave resonator employs at least two superconducting films on substrates positioned on a dielectric. The holding devices include a variety of configurations, such as, a spring loaded device. The superconducting microwave resonators have Q values of as high as microwave resonators formed of Nb, but operate at much higher temperature.

FIELD OF THE INVENTION 
This invention relates to microwave resonators formed of high temperature 
superconductor and dielectric materials as well as to electronic circuits 
that employ those microwave resonators. 
BACKGROUND OF THE INVENTION 
Microwave resonators are known for use in time and frequency standards, 
frequency stable elements, as well as building blocks for passive devices 
such as filters and the like. The performance of the microwave resonator 
is gauged by its Q-value, expressed as 
EQU Q=2.pi. f.sub.0 * (Storage energy/Loss power), (1) 
where f.sub.0 is the resonant frequency of the microwave resonator. (See 
Hayt, J. R., "Engineering Electromagnetics", 1981, p. 472). As shown in 
Equation (1), the Q-value of the microwave resonator can be increased by 
reducing the loss power associated with factors such as conductor loss, 
dielectric loss, and radiation loss. 
Low temperature (T.sub.c), such as 4K, superconducting microwave resonators 
which employ a superconducting cavity made of Nb are known to have 
Q-values from about 10.sup.6 to 10.sup.9. (See V. B. Bragrinskii, et al: 
"The Properties of Superconducting Resonators on Sapphire", IEEE Trans. on 
Magn. Vol. 17, No. 1, P955, 1981, as a reference.) Although low T.sub.c Nb 
microwave resonators have high Q-values, they must operate at very low 
temperatures (below 9K). These microwave resonators require use of curved 
cavity walls. Curved cavity walls of materials which have a high T.sub.c, 
of for example 77K, however, are difficult to produce. On the other hand, 
high Q-value microwave resonators formed merely from a dielectric without 
an associated conducting medium also have high Q-values (see D. G. Blair, 
et al: "High Q Microwave Properties of a Sapphire Ring Resonator", J. 
Phys. D: Appl. Phys., 15, P1651, 1982.) However, the problems associated 
with the far reaching evanescent fields make them very bulky and 
vulnerable to microphonic effect, which limits the applications. 
The need therefore exists for microwave resonator made of high T.sub.c, 
such as 77K, superconductor that have Q-values comparable to low T.sub.c 
superconducting microwave resonators made of Nb.

SUMMARY OF THE INVENTION 
The invention is directed to high temperature superconductor-dielectric 
microwave resonators, to holding devices for those resonators, coupling of 
those resonators to electronic circuits, and to their methods of 
manufacture. The superconducting microwave resonator of the invention 
employ a superconducting film on substrates positioned on a dielectric. 
The holding devices include a variety of configurations, such as, a spring 
loaded device. The microwave resonators can be readily coupled to 
electronic circuits. The superconducting microwave resonators have Q 
values that are as high as low temperature microwave resonators formed of 
Nb, but operate at much higher temperature. 
In accordance with the invention, a high temperature superconducting 
microwave resonator comprising a dielectric and a plurality of substrates 
bearing a coating of high temperature superconducting material is 
provided. The substrates are positioned relative to the dielectric to 
enable the coating to contact said dielectric. 
The invention also includes devices for retaining the configuration of the 
superconducting microwave resonator of the invention. These devices 
comprise means to retain the relative positions of the substrate and the 
dielectric during use of the microwave resonator in an electrical circuit. 
These devices further comprise means for coupling of the microwave 
resonator to electrical circuits. 
The invention is further directed to a method for coupling the 
superconducting microwave resonator of the invention to an electric 
circuit by employing means positioned on the substrate for transferring 
electromagnetic energy between the dielectric of the superconducting 
microwave resonator and an electrical circuit via openings on the 
superconducting films and coupling lines. 
The invention is still further directed to passive devices such as filters 
that are formed of a plurality of dielectrics positioned between a 
plurality of substrates bearing a coating of high temperature 
superconducting material, or wherein the dielectrics and substrates are in 
alternating positions relative to each other. 
DETAILED DESCRIPTION OF THE INVENTION 
Having briefly summarized the invention, the invention will now be 
described in detail by reference to the following specification and 
non-limiting examples. Unless otherwise specified, all percentages are by 
weight and all temperatures are in degrees Kelvin. 
FIG. 1 shows superconducting microwave resonator and a holding device for 
that resonator. As shown in FIGS. 1(a) and 1(4), a superconducting 
microwave resonator 100 with cavity 90 is provided in the form of 
substrates 20 bearing superconducting film 10 positioned on dielectric 30. 
Substrate 20 is a single crystal that has a lattice matched to 
superconductor film 10. Preferably, substrates 20 are formed of 
LaAlO.sub.3, NdGaO.sub.3, MgO and the like. 
Generally, superconductor film 10 may be formed from any high T.sub.c 
superconducting material that has a surface resistance (R.sub.s) that is 
at least ten times less than that of copper at any specific operating 
temperature. T.sub.c can be determined by the "eddy current method" using 
a LakeShore Superconductor Screening System, Model No. 7500. Surface 
resistance of superconducting film 10 can be measured by the method 
described in Wilker et al., "5-GHz High-Temperature-Superconductor 
Resonators with High Q and Low Power Dependence up to 90K" IEEE, Trans. on 
Microwave Theory and Techniques, Vol. 39, No. September 1991, pp. 
1462-1467. Generally, superconductor film 10 is formed from materials such 
as YBaCuO (123), TlBaCaCuO (2212 or 2223), TlPbSrCaCuO (1212 or 1223), or 
the like. 
Superconducting film 10 can be deposited onto substrate 20 by methods known 
in the art. See, for example, Holstein et al., "Preparation and 
Characterization of Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub.8 Films on 100 
LaAlO.sub.3 ", IEEE, Trans. Magn., Vol. 27, pp. 1568-1572, 1991 and 
Laubacher et al., "Processing and Yield of YBa.sub.2 Cu.sub.3 O.sub.7-x 
Thin Films and Devices Produced with a BaF.sub.2 Process", IEEE, Trans. 
Magn., Vol. 27, pp. 1418-1421, 1991. Generally, the thickness of film 10 
is in the range of 0.2 to 1.0 micron, preferably 0.5 to 0.8 micron. 
Microwave resonator 100 is formed by positioning substrates 20 bearing 
superconducting film 10 on dielectric 30. Substrates 20 can be placed on 
the surface of dielectric 30, or, alternatively, low loss adhesive 
materials may be employed. Polymethyl methacrylate optionally may be 
deposited onto the surface of superconducting film 10 to more firmly bond 
dielectric 30, as well as to protect superconducting film 10. 
Dielectric 30 may be provided in a variety of shapes. Preferably, 
dielectric 30 is in the form of circular cylinders or polygons. Dielectric 
30 may be formed of any dielectric material with a dielectric constant 
.epsilon..sub.r &gt;1. Such dielectric materials include, for example, 
sapphire, fused quartz, and the like. Generally, these dielectric 
materials have a loss factor (tan .delta.) of from 10.sup.-6 to 10.sup.-9 
at cryogenic temperatures. The .epsilon..sub.r and tan .epsilon. of the 
dielectric material can be measured by methods known in the art. See, for 
example, Sucher et al., "Handbook of Microwave Measurements", Polytechnic 
Press, Third Edition, 1963, Vol. III, Chapter 9, pp. 496-546. 
The configuration of the microwave resonator 100, when in use, is 
maintained by holding device 25 e.g., see FIG. 1(a). The holding device 
can be any embodiment that maintains the relative positions of the 
components of the resonator during thermal cycling associated with use of 
the resonator. FIG. 1(a) shows a first embodiment of a holding device that 
employs spring loading. As shown in FIG. 1(a), the configuration of 
microwave resonator 100 is maintained by holding device 25. Holding device 
25 includes sidewalls 45, bottom plate 50, top lid 60, pressure plate 70, 
and load springs 80. Load springs 80 are sufficiently strong to retain the 
configuration of the microwave resonator during thermal cycling. Load 
springs 80 preferably are formed of nonmagnetic material in order to 
prevent disturbing the radio frequency fields in the resonator to achieve 
the highest possible Q-values. Load springs 80 preferably are formed of 
Be-Cu alloys. 
Parts 45, 50, 60 and 70 of holding device 25 are made of thermally and 
electrically conductive materials in order to reduce radio frequency loss 
as well as to enable efficient cooling of resonator 100. Parts 45, 50, 60 
and 70 therefore may be formed of, for example, oxygen fired copper, 
aluminum, silver, preferably oxygen fired copper or aluminum. 
The high T.sub.c superconductor-dielectric microwave resonators of the 
invention are capable of attaining extremely high Q-values, due in part, 
to the ability of substrate 20 bearing film 10 to prevent axial radio 
frequency fields from extending beyond the London penetration depth of the 
superconducting film 10. This is accomplished where substrates 20 are 
substantially greater than the diameter of dielectric 30 so that radio 
frequency fields are confined within the cavity region between substrates 
20. 
The high Q-value superconducting microwave resonators provided by the 
invention have a variety of potential applications. Typically, these 
resonators may be employed in applications such as filters, oscillators, 
as well as radio frequency energy storage devices. 
The microwave resonators of the invention also may be employed as frequency 
stable elements to reduce the phase noise for oscillators. As shown in 
FIG. 2, circuit 51 employs a microwave resonator 100 of the invention that 
is inserted into a closed feedback loop of, preferably, a low noise 
amplifier 15. Where the product of the gain of amplifier 15 and the 
insertion loss of resonator 100 is greater than one, and where the total 
phase of the closed loop, as adjusted by phase shifter 17, is a multiple 
of 2.pi., then, due to the extremely high Q-values of the superconducting 
microwave resonators of the invention, the oscillator can be made to 
oscillate at the microwave resonator's resonant frequency to yield-lower 
phase noise in the oscillator. The term "out" indicates a line out of the 
loop. 
The superconducting microwave resonators of the invention also may be 
employed to provide highly stable frequencies suitable for secondary 
standards for frequency or time. Since the microwave resonator has an 
extremely high Q-value and operates at a constant cryogenic temperature, 
the microwave resonator has a very stable resonate frequency that makes 
the resonator useful for serving as a secondary standard. 
The superconducting microwave resonators of the invention further may be 
employed as building blocks in passive devices such as filters. Examples 
of such filters are shown in FIGS. 3(a) and 3(b). As illustrated in FIG. 
3(a), filter 110 shown in the form of a series of dielectrics 30 
sandwiched between substrates 20 bearing superconducting films 10. 
Coupling between dielectrics 30 is achieved by the evanescent fields of 
dielectrics 30. Coupling of filter 10 to electronic circuits (not shown) 
can be achieved by coaxial cable 18 bearing coupling loop 21. 
FIG. 3(b) shows an alternative embodiment of a filter. As shown in FIG. 
3(b), filter 120 employs a series of dielectrics 30. Coupling between 
dielectrics 30 is achieved by the evanescent fields of dielectrics 30 via 
openings (not shown) on substrates 20. Coupling of filter 120 to an 
electronic circuit (not shown) can be achieved by couplings 13. Couplings 
13 can be coaxial lines, waveguides, or other transmission lines. In 
either of the embodiments of FIGS. 3(a) or 3(b), the high Q-values of the 
superconducting microwave resonators reduces the in-band insertion loss of 
the filter so as to make the skirt of the frequency response curve of the 
filter steeper. 
An additional application of the superconducting, microwave resonators of 
the invention is to measure the surface impedance (Z.sub.s) of 
superconductor materials and the complex dielectric constant 
.epsilon..sub.r =.epsilon..sub.r '-j.epsilon..sub.r " of dielectric 
materials, where Z.sub.s and .epsilon..sub.r have been determined by 
measurement of f.sub.0 and Q at two differing modes in accordance with 
methods known in the art. The surface impedance (Z.sub.s) is defined as 
the ratio of the voltage to the current. The complex dielectric constant 
.epsilon..sub.r =.epsilon..sub.r '-j.epsilon..sub.r " is defined as the 
ratio of the electrical displacement vector (D) and the electric field 
strength vector (E). E.sub.r ' is a measure of the capability of 
electrical energy storage of the dielectric material. E.sub.r " is a 
measure of the electrical loss in the dielectric material. The symbol j is 
the unit of an imaginary number. 
Generally, high Q-values for the superconducting microwave resonators of 
the invention may be obtained by selecting the proper electromagnetic 
modes to prevent flow of radio frequency current across the edges of 
superconducting films 10. These proper modes are TE.sub.oin modes where 
the radial mode index has a value of i=1,2,3, . . . and the axial mode 
index has a value of n=1,2,3, . . . All TE.sub.oin modes have only 
circular radio frequency currents that do not cross the edge of films 10. 
Having selected the specific electromagnetic mode of the microwave 
resonator, the Q and the resonant frequency f.sub.0 for the microwave 
resonator can be calculated by solving Maxwell's Equations for the 
boundary conditions of the resonator, as is known in the art. 
The loss power associated with parasitic coupling to low Q-value modes such 
as non-TE.sub.Oin modes or "case modes" may be minimized in the microwave 
resonators of the invention by assuring that substrates 20 are flat and 
parallel to within a tolerance of less than 1.degree.. Loss power also may 
be minimized by ensuring that the C-axis of anisotropic materials such as 
sapphire, when employed as dielectric 30, is perpendicular to substrate 20 
to within .+-.5.degree. preferably 1.degree.. 
As also is shown in FIG. 1(a), microwave resonator 100 can be coupled to an 
electric circuit (not shown) by coaxial cable 18 that includes coupling 
loop 21 protruding into cavity 90 of microwave resonator 100. The 
orientation of coupling loop 21 and the depth of insertion of coaxial 
cable 18 into cavity 90 readily can be adjusted to ensure coupling to the 
electronic circuit. 
In a preferred aspect of the invention, superconducting film is formed by 
epitaxially depositing 0.5 micron superconducting films of Tl.sub.2 
Ba.sub.2 Ca.sub.1 Cu.sub.2 O or YBa.sub.2 Cu.sub.3 O on 2 inch diameter 
substrates of LaAlO.sub.3 positioned on cylindrical dielectrics of 
sapphire. The superconducting film is deposited so that the C-axis of the 
film is perpendicular to the surface of the substrate. The dielectrics of 
sapphire typically measure 0.625 inch diameter by 0.276 inch tall, 0.625 
inch diameter by 0.552 inch tall, or 1.00 inch diameter by 0.472 inch 
tall. The substrates and dielectric are retained in position by a holding 
device formed of oxygen free copper. Coupling of the microwave resonator 
to an electrical circuit can be achieved by inserting two 0.087 inch 
diameter copper or stainless steel, 50 ohm coaxial cables with coupling 
loops made of extended inner conductor into the cavity of the resonator. 
The surface of the coupling loops is perpendicular to the vertical axis of 
the sapphire dielectric to enable selective coupling to the TE.sub.011 
(i=1, n=1) mode of the dielectric. 
The Q values of the above described microwave resonators, when employing 
YBa.sub.2 Cu.sub.3 O as the superconducting film, are shown in FIG. 4. As 
shown in FIG. 4, Q values of 5 million, 1.5 million, and 0.25 million are 
found at temperatures of 4.2K, 20K and 50K, respectively. The Q values of 
the above described microwave resonators, when employing Tl.sub.2 Ba.sub.2 
Ca.sub.1 Cu.sub.2 O as the superconducting film, are shown in FIG. 5. As 
shown in FIG. 5, Q values of 6 million, 3 million, and 1.3 million are 
found at temperatures of 20K, 50K, and 77K, respectively. 
The dependence of Q values of the above described microwave resonators that 
employ Tl.sub.2 Ba.sub.2 Ca.sub.1 Cu.sub.2 O as the superconducting film 
on the size of the sapphire dielectric is shown in FIG. 6. As shown in 
FIG. 6, the Q values increase from 3 million to 6 million with increasing 
size of the sapphire dielectric. 
Device 25 shown in FIG. 1(a) that employs spring loading is only 
illustrative. Other means for holding microwave resonator 100 are shown 
below. 
FIGS. 7(a) and 7(b) show an alternative embodiment for holding the 
microwave resonators of the invention. As shown in FIG. 7, the microwave 
resonator is held by holding device 27. Device 27 is indentical to device 
25 except that, as shown in FIG. 7(a), spring loaded holding device 27 
employs three dielectric rods 35 positioned 120.degree. relative to each 
other to further support dielectric 30. Dielectric rods 35 are inserted 
through side walls 47 of holding device 27 into cavity 95. Dielectric rods 
35 have a low loss and a dielectric constant less than that of dielectric 
30. The tips of rods 35 are pointed to minimize contact area with 
dielectric 30 to minimize loss power. Superconducting film 10 is in 
contact with dielectric element 30. Coupling of the resonator to 
electronic circuits (not shown) is achieved by coaxial cable 18 bearing 
coupling loop 21. 
A further embodiment of a device for holding the microwave resonators of 
the invention is shown in FIG. 8. As set forth in FIG. 8, the microwave 
resonator is retained in position by holding device 28. Holding device 28 
includes sidewalls 45, bottom plate 50, top lid 60, pressure plate 70, and 
load springs 80, and is identical to holding device 25 except for the 
additional use of retainer 77. As shown in FIG. 8, substrate 20 bearing 
superconducting film 10 is positioned on bottom-plate 50. Dielectric 30 is 
positioned on substrate 20. Retainer 77 is positioned about dielectric 30. 
Retainer 77 contacts sidewalls 45 and superconducting film 10 on substrate 
20. Retainer 77 and side walls 45 have openings for receiving coaxial 
cables 18. Cables 18 have loops 21 for coupling of the resonator to an 
electric circuit(not shown). Retainer 77 is formed of materials that have 
low dielectric constant of nearly 1 and low tan .delta. of &lt;10.sup.-4. As 
shown in FIG. 8, retainer 77 is hollow, and is solid neat sidewalls 45 
where the electrical fields are minimum. The wall thickness of retainer 77 
is minimized to reduce the contact area between retainer 77 and dielectric 
30 to minimize loss power. 
Still yet another embodiment of a holder device for the microwave 
resonators of the invention is shown in FIG. 9. Holding device 29 shown in 
FIG. 9 is identical to holding device 25 except for the use of additional 
dielectric 65. As shown in FIG. 9, cavity 91 between dielectric 30 and the 
interior surface of sidewall 45 of device 29 is filled with dielectric 
material 65. Dielectric material 65 has a tan .delta. of less than 
10.sup.-5. Examples of dielectric material 65 include styrofoam, porotic 
TEFLON.RTM., and the like. 
FIG. 10 shows a further embodiment of a holding device suitable for use 
with the superconducting microwave resonators of the invention. Holding 
device 24 shown in FIG. 10 is identical to holding device 25 except for 
additional use of holding pins 71. As shown in FIG. 10, pins 71, formed of 
low tan .delta. dielectric materials such as sapphire, quartz, polymers, 
polytetrafluoroethylene TEFLON.RTM., DELRIN.RTM., registered trademarks of 
E. I. du Pont de Nemours and Company, and the like are inserted into 
substrate 20 bearing superconducting film 10 and into dielectric 30. 
FIGS. 11(a) to 11(d) show alternative embodiments for coupling of the 
microwave resonators of the invention to an electronic circuit (not 
shown). Generally, the embodiments shown in FIGS. 11(a)-11(c) entail use 
of substrates that bear superconducting films on the surfaces of the 
substrate that directly contacts dielectric 30. Openings are provided on 
the superconducting film on the side which directly contacts dielectric 
30. A coupling device is located over the opening on surface of the 
substrate that does not contact dielectric 30. 
FIG. 11(a) shows a microstrip line coupling mechanism for coupling of the 
microwave resonators of the invention to an electronic circuit (not 
shown). In FIG. 11(a), microstrip line 15 is formed by depositing 
superconducting film material on that surface of substrate 20 that is 
remote to dielectric 30. Microstrip line 15 serves as the lead to an 
electronic circuit (not shown). Opening 12 is provided in film 10 on the 
surface of substrate 20 that contacts dielectric 30. Opening 12 extends 
through film 10 but not through substrate 20. Opening 12 does not contact 
dielectric 30 in order to minimize the effects of magnetic fields on 
dielectric 30. Opening 12 is parallel to the local magnetic field. 
Coupling is achieved by magnetic field leakage through opening 12 to line 
15. Microstrip line 15 extends over opening 12 by a distance of 
.lambda./4, where .lambda. is the wavelength of the radio frequency field 
at the operating frequency of the resonator. 
FIG. 11(b) shows a coplanar line coupling mechanism for coupling the 
microwave resonators of the invention to an electronic circuit (not 
shown). The coplanar line coupling is formed by depositing superconducting 
film material on that surface of substrate 20 that is remote to dielectric 
material 30 to form center line 19 and ground plane 21. The coplanar line 
coupling serves as the lead to an electronic circuit (not shown). The 
coplanar line coupling extends over opening 12. Opening 12 is provided by 
film 10 on the surface of substrate 20 that contacts dielectric material 
30. Opening 12 extends through film 10 but not through substrate 20. 
Opening 12 does not contact dielectric material 30. 
In the coplanar line coupling of FIG. 11(b), center line 19 is short 
circuited to ground plane 21. Center line 19 extends across opening 12. 
Opening 12 is parallel to the local magnetic field. Coupling is achieved 
by magnetic field leakage through slot 12 to center line 19. 
FIG. 11(c) shows a parallel line coupling mechanism for coupling dielectric 
material 30 to an electronic circuit(not shown). The parallel line 
coupling includes parallel lines 31 and loop 32. The parallel line 
coupling is formed by depositing superconducting film material on that 
surface of substrate 20 that is remote to dielectric material 30. The 
parallel line coupling mechanism serves as the lead to an electronic 
circuit (not shown). Parallel lines 31 and loop 32 extend over opening 12. 
Opening 12 is provided in film 10 on the surface of substrate 20 that 
contacts dielectric material 30. Opening 12 extends through film 10 but 
not through substract 20. Opening 12 does not contact dielectric material 
30. Coupling is achieved by leakage of magnetic field through opening 12 
which is captured by loop 32. 
FIG. 11(d) shows a coupling mechanism useful for microwave resonators such 
as those used for a filter as shown in FIG. 3(b). As shown in FIG. 11(d), 
the coupling mechanism employs identical, congruent slot 12 through film 
10 of both surfaces of substrate 20. Slots 12 extend through films 10 but 
terminate at the surfaces of substrate 20. Slots 12 on each surface of 
substrate 20 may be the same or different in size. Coupling is achieved by 
leakage of evanescent magnetic field through slots 12. 
Coupling of the microwave resonator also may be achieved through dual 
couplings. FIG. 12 shows a dual coupling mechanism that utilizes dual 
identical coupling microstrip lines 44(a) and 44(b) that cross slots 12(a) 
and 12(b) on film 10 (not shown). Slots 12(a) and 12 (b) are provided in 
film 10 on that surface of the substrate 20 that contacts dielectric 30. 
Slots 12(a) and 12(b) terminate at the surface of substrate 20. Couplings 
44(a) and 44(b) are connected by lead line 41 that is divided into equal 
length branches 42(a) and 42(b). Lines 44(a) and 44(b) and lead line 41 
are formed by depositing superconductive material onto substrate 20. 
Coupling is achieved by leakage of evanescent magnetic field through slots 
12(a) and 12(b). The dual coupling mechanism shown in FIG. 12 enables 
selective coupling to the TE.sub.011 mode and suppresses competing 
electromagnetic field modes that have antisymmetrical magnetic field 
distribution. "To circuit" indicates that lead line 41 leads to an 
electrical circuit. 
The coupling mechanisms of the invention also provide for ease of 
connection to circuits integrated onto substrate 20. As shown in FIG. 13, 
a circuit is integrated onto the side of substrate 20 that bears coupling 
mechanisms 55(a) and 55(b). Couplings 55(a) and 55(b) may be formed by 
depositing superconductive film material onto substrate 20 over slots 
12(a) and 12(b). Slots 12(a) and 12(b) are provided in the superconducting 
film (not shown) on that side of substrate 20 that contacts dielectric 30. 
Slots 12(a) and 12(b) extend through the superconductor film but terminate 
at the surface of substrate 20. Coupling is achieved by leakage of 
magnetic field through slots 12(a) and 12(b). 
Integration of circuits onto substrate 20 as shown in FIG. 13 may be 
achieved by well known thin film printed circuit technology. If the 
circuit is a hybrid circuit that employs, for example, transistors, then 
the transistors can be integrated into the circuit by conventional wire 
bonding. The term "OUT" indicates a line out of the loop. 
FIG. 14 shows an alternative embodiment of the superconducting microwave 
resonator of the invention that is retained by holding device 25. As shown 
in FIG. 14, rings 61 with a dielectric constant much less than that of 
dielectric 30 are inserted between dielectric 30 and superconducting film 
10. Rings 61, by placing dielectric 30 further from superconducting film 
10, enable the microwave resonator to handle greater power levels. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention, and without departing 
from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various uses and conditions.