APPARATUS FOR CHARACTERIZING HIGH TEMPERATURE SUPERCONDUCTING THIN FILM

Abstract: A dielectric resonator apparatus for measuring the parameters of high temperature superconducting thin film is disclosed having improved means for positioning the dielectric and substrates, holding the resonator components in place during use, suppressing undesirable modes, adjusting the magnetic dipole coupling, and coupling to an electrical circuit.

Description: APPARATUS FOR CHARACTERIZING HIGH TEMPERATURE SUPERCONDUCTING THIN FILM FIELD OF THE INVENTION This invention relates to an apparatus which is used to measure high temperature superconducting thin film parameters such as surface resistance, critical current density, and critical magnetic field. BACKGROUND OF THE INVENTION The quality of superconductor thin films is described by several parameters: critical temperature, T c , surface resistance, R s , critical current density, J c , critical magnetic field, H c , etc. For microwave and millimeter wave applications, the most important parameter of a superconductor film is the surface resistance, R s , at a given frequency, measured as a function of temperature, current density (or rf magnetic field) . Measurement of these parameters accurately is not only necessary for superconductor material research and applications, but is also important for controlling the quality of manufacturing superconducting film. One method for measuring surface resistance is called "TEon mode cavity end wall replacement", Muller et al., J. Superconductivity, Vol. 3, p. 235-242 (1990) . It utilizes a copper cylindrical cavity operating at TEon mode with one of its two end walls replaced by a superconductor film. The R s of the film can be determined by comparing the Q-values of the cavity with the sample to the same cavity with a calibration standard film (such as niobium or copper) having a known R s value. This method has the following shortcomings: 1) it requires calibration, so it is not an absolute measurement; 2) the accuracy is limited by the fact that the R s of the sample film under test only contributes a small portion of the loss in the cavity; 3) the measurable range of the R 3 is limited at the low end by the poor sensitivity of this method. Another method for measuring R s is called "parallel plate resonator" as disclosed in Taber, R., Rev. Sci. Instrum., Vol. 61, p. 2200-2206 (1990) . It is constructed by two pieces of superconducting film separated by a thin dielectric spacer. The R s of the superconductor film can be determined by measuring the Q-value of the resonator. This method has the following shortcomings: 1) because the spacer is very thin, it is very difficult to couple the rf power in and out of the resonator; 2) since the Q-value is relatively low, the measurable range of R s is limited at the high end by too weak of a coupling; 3) since the parallel plate resonator is an open structure, the rf magnetic field is not confined, which results in poor accuracy and case mode interference; 4) it is not an absolute method, calibration is required. Yet another method is called "dielectric resonator" as disclosed by Fiedziusko et al., IEEE-MTT-S Inter¬ national Microwave Symposium Digest, Vol. 2, p. 555-558 (1989) and by Llopis et al., J. Less-Common Metals, Vols. 164, 165, p. 1248-1251 (1990) . There are two different versions. One version involves putting a dielectric resonator on top of the sample super¬ conducting film under testing. Again the R 3 of the film can be determined by measuring the Q-value of the resonator. This method has the following shortcomings: 1) the open structure makes it difficult to confine the rf fields, which results in poor accuracy and moding problems; 2) means for holding the dielectric resonator in the right place is a problem. The second version is a superconductor-dielectric-superconductor sandwich. Adding the second superconductor film solved the problems encountered in use of the first version. However, since the dielectric material used has a poor loss tangent factor, the sensitivity of this method is limited. Currently available apparatus for measuring surface resistance are not suitable for use as a production tool because of their limitations. Films cannot be tested at high power. Measurement is not accurate, and its reproducibility is poor. The dynamic range is limited. The assembly is time consuming. Finally, the measurement is very sensitive to how the films are assembled in the apparatus . Thus there is a need for an apparatus suitable for use in quality control operations for monitoring superconducting film manufacturing processes. The present invention provides apparatus suitable for improved characterization of high temperature superconducting thin films. Major improvement in performance has been achieved as well as the ability to use the same concept design to make the resonators that can characterize different sizes of superconducting thin films . BRIEF DESCRIPTION OF THE DRAWING Figure 1(a) is a top view of resonator #1 with the top plate removed. Figure 1 (b) is a cross-sectional view of resonator #1. Figure 1 (c) is a cross-sectional film of a substrate coated with superconducting film. Figure 1 (d) illustrates the film-sapphire-film structure. Figure 1(e) is an explode cross-sectional view of the connector assembly. Figure 2 (a) is a top view of the design of drawer #1 of resonator #1. SUBSTITUTE SHEET ISA/EP Figure 2 (b) is a front view of drawer #1 of resonator #1. Figure 2(c) is a cross-sectional view of sapphire locater #1 of resonator #1. Figure 2(d) is a top view of sapphire locator #1 of resonator #1. Figure 2(e) is an enlarged view of arc 125 of sapphire locator #1 of resonator #1. Figure 2(f) illustrates the design of drawer #2 of resonator #1. Figure 2 (g) is a front view of drawer #2 of resonator #2. Figure 2 (h) is a cross-sectional view of sapphire locator #2 of resonator #1. Figure 2(i) illustrates the design of sapphire locater #2 of resonator #1. Figure 3(a) illustrates the design of the body of resonator #1. Figure 3(b) is a cross-sectional view of the body of resonator #1. Figure 3 (c) is a bottom view of the body of resonator #1. Figure 3 (d) is the design of the assemblage for fastening connectors (95) onto the body of resonator #1. Figure 3(e) illustrates the design of the top plate of resonator #1. Figure 3(f) illustrates a cross-sectional view of the top plate of resonator #1. Figure 3 (g) illustrates the design of the bottom plate of resonator #1. Figure 3 (h) is a cross-section view of the bottom plate of resonator #1. Figures 3(i) and 3(j) illustrate the design of the transition of resonator #1. SUBSTITUTE SHEET ISA/EP 4A Figures 3 (k) and 3 (m) illustrate the design of the insulator of resonator #1. Figure 4 (a) illustrates the design of the piston of resonator #1. Figure 4 (b) is a front view of the piston of resonator #1. Figure 4 (c) is an enlarged view of a portion of the piston of resonator #1. Figure 4 (d) is a cross-sectional view of the piston of resonator #1. Figure 4 (e) illustrates the design of the piston locater of resonator #1. Figure 5(a) is a top view of sapphire locating device #3. Figure 5(b) illustrates the design of sapphire locating device #4. Figure 5(c) is a bottom view of platform (223) of sapphire locating device #4. Figure 5(d) is a side view of platform (223) of sapphire locating device #4. Figure 6(a) is a cross-sectional view of holding device #1. Figure 6(b) is a cross-sectional view of holding device #2. Figure 6(c) is a cross-sectional view of holding device #3. Figure 7 (a) is a top view of resonator #2 with the top plate removed. Figure 7 (b) is a cross-sectional view of resonator #2. SUBSTITUTE SHEET ISA/EP Figure 7 (c) is an explode cross-sectional view of the connector assembly. Figure 8(a) illustrates the design of the body of resonator #2. Figure 8(b) illustrates tapped holes (315) for screw subassemblies (260) . Figure 8(c) illustrates holes used for attachment of connectors (95) to the body of resonator #2. Figure 8(d) illustrates the design of the top plate of resonator #2. Figure 8(e) is a front view of the top plate of resonator #2. Figure 8(f) illustrates the design of the bottom plate of resonator #2. Figure 8 (g) is a cross-sectional view of the bottom plate of resonator #2. Figure 9(a) illustrates the design of the piston of resonator #2. Figure 9(b) is a cross-sectional view of the piston of resonator #2. Figure 9(c) is a bottom view of the piston of resonator #2. Figure 9(d) illustrates the design of the piston locater of resonator #2. Figures 9(e) and 9(f) illustrate the design of the screw subassembly of resonator #2. Figure 10(a) is a cross-sectional view of a modified design of resonator #1. Figure 10 (b) is an explode cross-sectional view of a sapphire rod holding device used in modified resonator #1. Figure 10 (c) is a top view of modified resonator #1. Figure 11 is a graph of surface resistance of (TlPb) SrCaCuO vs. temperature. 2 33TITUTE SHEET ISA/EP 5A Figure 12 is a graph of surface resistance of YBaCuO vs. maximum magnetic field. SUMMARY OF THE INVENTION The present invention comprises a dielectric resonator apparatus having a dielectric element of sapphire positioned between and in contact with two discrete films of at least one superconducting material, all encased in an outer enclosure having means for magnetic dipole coupling, wherein the improvement comprises: (a) means for positioning the sapphire dielectric; SUBSTITUTE SHEET ISA/EP 6 (b) means for positioning, loading and unloading the superconducting films; (c) means for holding the dielectric and superconducting films in place during use; (d) means for suppressing undesirable modes; and (e) means for adjustment of the magnetic dipole coupling. The resonator comprises a sapphire and two substrates bearing a coating of high temperature superconducting (HTS) material, hereinafter referred to as films or HTS films. The films are positioned relative to the sapphire to enable the coating to contact said sapphire. The structure of HTS film- sapphire-HTS film in the resonator can be modified, so the microwave will either be exposed to both the high temperature superconducting material and the outer enclosure material, or it will only be exposed to the high temperature superconducting material at the end planes. The present invention also comprises a dielectric resonator apparatus having a dielectric element of sapphire positioned between and in contact with two discrete films of at least one superconducting material and a means for magnetic dipole coupling, wherein the improvement comprises a means for moving the resonator to test the superconducting films at multiple discrete areas . The advantages of the present invention are achieved by devices to obtain exact locations of resonator components, to retain the locations of resonator components during the usage of the resonator in an electrical circuit, to position, to load and to unload the superconducting films, to suppress undesirable modes, to adjust the magnetic dipole 7 coupling, and to couple to an electrical circuit. These devices permit very accurate and repeatable measurements. They also allow for easy assembly, and consequently, reduce the assembly time. DETAILED DESCRIPTION OF THE INVENTION Throughout the following description, similar reference numerals refer to similar elements in all figures of the drawings. The present invention comprises three variations of a sapphire resonator for measuring high temperature superconducting thin film parameters. The first resonator design has the body design shown in Figure 3(a) and can accommodate two different drawer designs as shown in Figures 2 (a) and 2 (c) . The second resonator design is shown in Figure 7 (a) . The third design is shown in Figure 10(a) . In the first resonator as shown in Figures 1 (b) and 1(d), a sapphire dielectric, conveniently a rod (5) is sandwiched between two films (17) . Figure 1(c) shows the film (17) as a substrate (10) which is coated on one or both sides with superconducting materials (15) . Substrate (10) must be a single crystal that has a lattice matched to superconducting material. As shown in Figures 1(a) or 1(b) the resonator body (25) (outer enclosure) sits on top of the bottom plate (20) . The bottom film (17) is positioned on a drawer (40) which can be withdrawn out of body (25) for film loading and unloading. In operation, drawer (40) is attached to body (25) by two screws (75) . The sapphire rod (5) is centered in the resonator by a locating device to be described later. Piston (35) sits on top of sapphire rod (5) and holds the top film (17) . A piston locater (52) is used to guide the piston (35) to its exact location relative to the body (25) . 8 A holding device, described later, together with the top plate (30) will exert a pressure on the film- sapphire-film structure, and hold the structure in place during the characterization process. Two connectors (95), two transitions (60), two cables (55), and two insulators (65) will couple the resonator to an electrical circuit. The sapphire rod (5) used in this embodiment had a dielectric constant 6 r =9.2, and a loss factor (tan δ) of from 10~ 6 to 10" 9 at cryogenic temperatures. Its C-axis must also be perpendicular to its end surfaces within ±1° Depending upon the method of locating the sapphire rod (5), there are two different drawer (40) designs. Figure 2(a) shows the design of drawer #1. It can be made of any non-ferrous metal which has appropriate electrical conductivity and thermal conductivity. The material used in this embodiment is oxygen free copper. This drawer consists of a recess (100) to hold film (17), a recess (105) to hold a microwave frequency absorber, two clearance holes (102) for two screws (75), and two clearance holes (80) for two dowel pins (115) . The depth of recess (100) is such that the surface of the film is slightly above the surface of the drawer, preferably 0.025mm. The bottom plate, the top of the recess, and the bottom of the drawer must be parallel to each other to within 0.07mm, preferably 0.025mm. The above surfaces must also be perpendicular to axis "A" within ±1°. The drawer #1 has chamfers (135) to ease its insertion into the body (25) . The sapphire locater to be used with drawer #1 is shown in Figure 2 (b) . This locater is made of two non- metallic sheets (120) each with a maximum dielectric constant of about 4, and a low dielectric loss such as Kapton®, or any polymer film. Its thickness should be chosen to compromise for the mechanical strength and the 9 dielectric loss. The material used for this embodiment is a polyimide Kapton 300HN® film, .076mm thick. Sheet (120) has a clearance hole (130) for dowel pin (115), an arc (125) with a radius that matches the radius of sapphire rod (5) . The width of the sheet (120) is made to fit the slot in body (25) which will be described later. The locater has chamfers (135) to ease its insertion into the body (25) . Figure 2(c) shows the design of drawer #2. It is made of a non-ferrous metal which has appropriate electrical conductivity and a good thermal conductivity. The material used in this embodiment is oxygen free copper. Similar to drawer #1, it consists of a recess (100) to hold film (17), a recess (105) to hold a microwave frequency absorber, two clearance holes (102) for two screws (75) . The sapphire locater is fixed in place by two pins (110), or by other known methods. The depth of recess (100) is such that the surface of the film is slightly above the surface of the drawer, preferably 0.025mm. The bottom, the top of the recess, and the bottom of the drawer must be parallel to each other within 0.076mm, preferably 0.025mm. The above surfaces must also be perpendicular to axis "B" within ±1°. The drawer #2 has chamfers (135) to ease its insertion in body (25) . The sapphire locater to be used with drawer #2 is shown in Figure 2 (d) . This locater is made of a non metallic sheet (145) with a maximum dielectric constant of about 4, and a low dielectric loss such as Kapton®, or any polymer film. Its thickness should be chosen to compromise for the mechanical strength and the dielectric loss. The material used for this embodiment is a polyimide Kapton 300HN® film, .076mm thick. Sheet (145) has a hole (140) with a radius through which sapphire rod (5) will pass with minimal clearance. 10 Sheet (145) width is made to fit the slot in body (25) which will be described later. The locater has chamfers (135) to ease its insertion in body (25) . Figure 3(a) shows the design of the body (25) of the resonator. At the center of the body is a hole (165) . Its diameter is determined such that, including all tolerances built into all components, only film is visible when viewed from the top. No film edges are visible when viewed from the top. Its diameter is also calculated proportionally to the sapphire rod diameter to maximize performance. The ratio between the diameter of hole (165) and the diameter of the sapphire rod must be greater than 2.0. The ratio used in this embodiment is 2.23. The surface created by hole (165) must be polished to reduce the energy loss caused by microwave absorption. On top of hole (165) is a recess (155) . The size of this recess is-larger than the size of piston (35) to create an even rectangular gap for helium gas to reach the center of the resonator. At the top surface of body (25) are two dowel pins (70) which are used to guide the top plate (30) and the piston locater (52) . There are also four tapped holes (85) for screws (87) . Hole (170) and tapped holes (175) are used to fasten the connectors (95) onto body (25) . In the front of the body (25) are two tapped holes (160) for screws (75) . There is a slot (150) at the bottom. For drawer #1, this slot extends through the entire length of body (25) . For drawer #2, this slot is only machined partially through. The width of this slot is precisely made to match that of the drawer to ensure the film and the sapphire are exactly located. The top plate (30) is shown in Figure 3 (b) . Its slot (74) depth is calculated to ensure proper compression of the holding device. The top plate has two locating holes (79) for dowel pins (70) , four 11 countersink holes (72) that match tapped holes (85), and a through hole (77) for helium gas to enter the resonator. The bottom plate (20) is shown in Figure 3(c) . It has four tapped holes (84) that are used to mount the resonator onto a supporting device which is inserted into a dewar. Four countersink holes (89) are used to bolt it to the body (25) . There are two slots (82) for mounting several heaters to control and stabilize the resonator temperature. The cable size to be used in this resonator is one of the factors that affects the sensitivity and the accuracy of the measurements. The performance of the resonator improves when small cable is used, because less case mode is developed. The cable used in this embodiment has an 0.94mm outer diameter. To use this cable with a commercial connector such as Omni Spectra Flange Mount Cable Jack #1006-7985, having an inner diameter of 2.2mm and an outer diameter of 3.07mm, available from Omni Spectra, 740 4th Avenue, Waltham, MA 02254, a transition (60) and an insulator (65) are employed to achieve mating. They are shown in Figure 3(d) and 3(e). The transition is an electrical conductor tube. The outside diameter of the transition fits the inside diameter of the commercial connector, (for this embodiment 2.2mm). The inside diameter of the transition fits the outside diameter of the cable (0.94mm in this embodiment). The insulator is a dielectric ring. Its outside diameter is exactly the same as that of the transition. Its inside diameter is smaller than the diameter of the center conductor of the connector to avoid a short circuit between the transition and the center conductor of the connector. The transition (60) is soldered to the connector. The materials used for the transition and the insulator in 12 this embodiment are oxygen free copper and Teflon®, respectively. The penetration of the cables into the resonator is adjusted by inserting/removing shims (32) at the connector flanges as shown in Figure 1 (b) . Figure 4 (a) shows the design of the piston (35) . It is made of a non-ferrous metal which having appropriate electrical conductivity and thermal conductivity. The material used in this embodiment is oxygen free copper. This piston consists of a recess (185) to hold film (17), a recess (190) to hold a microwave frequency absorber, a through hole (180) for vacuum to reach film (17), and a counterbore (195) for placement of a holding device. The depth of recess (185) is such that the surface of the film is slightly above the surface of the drawer. The bottom plate, the top of the recess, and the bottom of the piston must be parallel to each other to within 0.076mm, preferably 0.025. The above surfaces must also be perpendicular to axis "C" within ±1°. Figure 4 (b) shows the design of piston locater (52) . It has two holes (200) that are used with dowel pins (70) to position the piston locater relative to the body (25) . The size of slot (205) is large enough, so piston (35) can drop through. The clearance between the piston (35) and the piston locater can be in the range from .025mm to .15mm. In this embodiment, all the dimensions are controlled to create at least .025mm clearance between them. The performance of the resonator greatly depends on the placement of the sapphire rod (5) and the films (17) . There are several locating devices for the sapphire rod. The first design is shown in Figure 2(b) and Figure 2(d), e.g. sapphire locaters #1 and #2. A film is either positioned in the resonator by pin-and- hole fashion, or by confining it into place by any of 13 the known prior art methods such as by use of pins, recess, walls, and the like. The film has a hole at the exact location where the sapphire rod has to be placed. The film can be designed to remain in or to be removed from the resonator before measurements are made. Sapphire locating devices #3 and #4, described hereinafter for resonator #2, can be employed in the resonator #1 if the body is modified in design to accommodate these locating devices. Several holding devices can be used to keep all the resonator components from moving during the operation. Figure 6(a) shows a holding device which is a system of helical compression springs (240) that are distributed evenly on top of the piston (35) . These springs are attached to either piston (35) or top plate (30) by soldering or by other known methods. Once the top plate is screwed on to body (25) , the springs will be compressed to produce a desired holding force. The holding force can be adjusted by adding or removing springs, by selecting springs with higher tension rate, or by changing the distance the springs are compressed. Another holding device is shown in Figure 6(b) . In this design, plate (245) is inserted between top plate (30) and piston (35) . Plate (245) material is selected such that it will compensate for all the different thermal contractions of material involved in the resonator at cryogenic temperature, such as Teflon®. Thus, the final compression force on film (17) and rod (5) are very close to the force applied on them before the resonator is cooled down. The holding device used in this embodiment is shown in Figure 6(c) . A series of Belleville springs (50) (or disc springs) are stacked on top of piston (35) or (290) . These springs compress the film-sapphire-film structure against the bottom plate when the top plate is 14 screwed on to body (25) . The spring force is adjusted by adding or removing springs, by selecting springs with higher tension rate, by stacking the springs in different configurations (parallel or series forms), or by changing the distance the springs are compressed. This holding device is compact because of the Belleville spring design. It is also more stable than the first holding device because the spring force is concentrated directly on top of the rod. The piston (35) and the slot (155) of the body (25) can be made of any shape. The rectangular shape is chosen in this embodiment because it permits compactness. The microwave frequency absorber eliminates all modes but TE01N (N=l,2,3) modes because TE01N modes carry only circular currents. The absorber can be any ferrite material. The absorber used in this embodiment is ECCOSORB® (available from Emerson & Cuming Inc., Woburn, M.A. 01888) . The absorber is located in recess 105 as shown in Figure 2 (a) . There are several steps required to assemble the resonator. If drawer #1 and locater #1 as shown in Figures 2(a) and 2(b) respectively are used, the first step is to position the bottom film (17) , and sapphire rod (5) in the resonator. This step is carried out by placing the film in the recess (100) of drawer #1. Two sapphire locaters #1 are then placed opposite to each other on drawer #1. At this time, drawer #1 is slid in and bolted on body (25) . Next, dowel pins (115) are inserted through holes (130) of sheet (120) and holes (80) of drawer #1 to locate exactly the circle formed by arcs (125) relative to body (25) . The sapphire rod (5) is positioned into the circle formed by arcs (125) . The second step is to position the top film (17) in the resonator. To accomplish this step, the piston locater 15 (52) is set on top of body (25) , with the help of dowel pins (70) . The film (17) is then dropped into recess (185) of piston (35) . After covering the hole (180) with a suction device, piston (35) is turned upside down and guided into the resonator. Then, the suction device and the piston locater are removed. The last step is to place a holding device in recess (195) of the piston, put the top plate (30) on top of body (25), and tighten screws (87) . When the resonator is ready for testing, dowel pins (115) and sapphire locaters #1 are removed from body (25) . If drawer #2 and locater #2 as shown in Figures 2(c) and 2(d) respectively are used, the first step is to position the bottom film (17), and sapphire rod (5) in the resonator. This step is carried out by placing the film in the recess (100) of drawer #2. Sapphire locaters #2 is then placed on drawer #2. Sapphire locater #2 is confined by the walls of body (25) and pins (110) . The sapphire rod (5) is positioned into circle (140) . The second step is to position the top film (17) in the resonator. To accomplish this step, the piston locater (52) is set on top of body (25) , with the help of dowel pins (70) . The film (17) is then dropped into recess (185) of piston (35) . After covering the hole (180) with a suction device, piston (35) is turned upside down and guided into the resonator. Finally, the suction device and the piston locater are removed. The last step is to place a holding device in recess (195) of the piston, put the top plate (30) on top of body (25), and tighten screws (87) . This invention further comprises a sapphire resonator of a second design, denoted sapphire resonator #2. The design of this resonator is shown in Figures 7 (a) and 7 (b) . The major components of resonator #2 are 16 bottom plate (305), piston (290), piston locater (285), top plate (280), Belleville spring (50), shims (292), and body (295) . The film (17) construction and the film-sapphire- film construction is the same as that of resonator #1, as previously described and as shown in Figures 1(c) and 1(d) . Figure 8(a) shows the design of body (295) (outer enclosure) of resonator #2. At the center is a hole (320) . Its diameter is calculated proportionally to the sapphire dielectric diameter, conveniently a rod, to maximize the performance. The ratio between the hole (320) diameter and the diameter of the sapphire rod must be greater than 2.0. The ratio used in this embodiment is 3.3. The surface created by hole (320) is polished to reduce the energy loss caused by microwave absorption. There are two dowel pins (250) for guiding the top plate (280) and the piston locater (285) . Three tapped holes (315) are for screw subassemblies (260) . Three tapped holes (310) are used to attach the body to the bottom plate (305) . Tapped holes (330) and hole (335) are used to attach connectors (95) to the body. There is an engraved line (325) to indicate the orientation of the bottom film. Figure 8(b) shows the design of top plate (280) of resonator #2. It has a counterbored hole (340) to clear the piston diameter, two clearance holes (345) for pins (250), and three countersank holes (342) for screws (265) . Figure 8(c) shows the design of the bottom plate. Similar to resonator #1, it has four tapped holes (350) to mount the resonator onto a supporting device, which is inserted into a dewar flask. There are three countersink holes (380) for screws (265), one recess (355) to locate film (17), another recess (370) for 17 microwave absorber. The depth of recess (355) is such that the surface of the film is slightly above surface "A", preferably 0.025mm. The bottom of the recess, surface "B", and surface "A" must be parallel to each other within 0.076mm, preferably 0.025mm. The above surfaces must also be perpendicular to axis "D" within ±1°. To center the film in the resonator, diameter (360) is made to slip fit to the diameter of the hole (320) . Diameter (365) is smaller than diameter (360) to create a circular gap; so microwaves can reach the microwave absorber painted in the recess (370) . This gap could raϊige from 0.254mm-1.016mm (total), and preferably is 0.508mm. Two slots (375) are used to mount several heaters to control and stabilize the resonator temperature. Figure 9(a) shows the design of the piston (290). It has a recess (390) for the film (17) , a recess (395) for microwave absorber, a through hole for vacuum to reach the film, and an engraved line (405) to indicate the orientation of the film. The depth of recess (390) is such that the surface of the film is slightly above the surface of the piston, preferably 0.025mm. Diameter (385) is made smaller than the hole (320) to create an even circular gap for cooling gas such as helium to reach the center of the resonator. The bottom of the recess, the top of the recess, and the bottom of the piston must be parallel to each other within 0.076mm, preferably 0.025mm. The above surfaces must also be perpendicular to axis "E" within ±1°. Figure 9(b) shows the design of the piston locater (285) . It is a plate of either plastic or metal which has a hole (415) through which the piston will pass with minimal clearance to guide the piston, and two holes to slip fit on pins (250) . 18 Figure 5(a) shows a sapphire locating device #3. In this design, sapphire rod (5) is dropped roughly at the center of the resonator. There are a plurality of modified screws (210) with three non-metallic tips (215) which are glued, soldered, or fastened by any suitable means to the screws . The modified screws turn, and move the rod to the center. The rod will be centered as the screws are stopped by the bottoms of threaded holes (220) . Screws (210) will be removed or left in place after rod (5) is secured in place by a holding device. The non-metallic material used for the tips (215) in this embodiment is sapphire. Another sapphire locating device (#4) is shown in Figure 5(b) . In this design, platform (223) has a perpendicular extension having a recess (225) . Sapphire rod (5) is fit into recess (225) , and is held in place by a vacuum through hole (235) . The device is positioned on top of the body of the resonator by employing two dowel-pin-guiding-holes (230) . Once it is set, the vacuum is turned off to release the sapphire rod (5), and the device is removed. The rod is then secured by a holding device. The sapphire locator devices #1 and #2 as shown in Figures 2(b) and 2(d) may also be employed in resonator #2 if their shape and size is modified to fit the body of this resonator. Figure 9(c) shows the design of the screw subassembly (260) which is a modified shoulder screw (210) with a sapphire rod (215) at the end. The rod is inserted into the screw and held in place by adhesive or by other known method. Any of the holding devices shown in Figures 6 (a) , 6(b), or 6(c) can be employed with resonator #2. Resonator #2 uses identical transition (60) and insulator (65) as in resonator #1. The penetration of 19 the cables into the resonator is adjusted by adding or removing the shims (292) . There are two major differences between resonator #1 and resonator #2. Unlike resonator #1, which does not allow the microwave to be exposed to the material of the drawer, film (17) of resonator #2 is smaller than surface "A", Figures 8(c); so microwave will be exposed to both the superconducting material as well as the platform material. The other difference is the height of the sapphire rod whereas the height is 2.5mm in resonator #1, it is 2mm in #2 resonator. There are several steps required to assemble resonator #2. The first step is to place film (17) in the recess (355) of bottom plate (305) . Next, screw subassemblies (260) are partially turned into threaded holes (315) of body (295). At this time, the sapphire rod (5) is placed roughly at the center of body (295) . Then, the screw subassenblies are turned until they are stopped. During that time, the sapphire rod is moved to the exact center of the body. The next step is to position the top film (17) in the resonator. To accomplish this step, first, the piston locater (285) is set on top of body (295) , with the help of dowel pins (250) . The film (17) is then dropped in recess (390) of piston (290) . After covering the hole (400) with a suction device, the piston is turned upside down and guided into the resonator. Then, the suction device and the piston locater are removed. The last step is to place a holding device in recess (402) of the piston, put the top plate (280) on top of the body, and tighten screws (265) . When the resonator is ready for testing, the srew subassemblies are taken out. The design of resonator #1 and #2 can be modified to test any film sizes, provided related dimensions such as film recesses, overall resonator dimensions, sapphire 20 rod dimensions are changed to accommodate for the new films and to assure the performance of the resonator is not affected. The present invention further comprises a third variation of a sapphire resonator which is a modification of resonator #1. The apparatus is modified to determine the local surface resistance and the uniformity of a superconducting thin film on a large substrate as shown in Figures 10(a), 10(b), and 10(c). A large high temperature superconducting thin film can be tested at various discrete areas. This design has two subassemblies. The first assembly, shown in Figure 10(a), has a cylindrical plate (460) that holds a wafer with superconducting film (455) on its underside. A vacuum hole (490) keeps film (455) from falling during the operation. Plate (460) is rotated around its center Z axis manually or mechanically, or by any other rotating device. The second subassembly is called a testing head. This subassembly includes the rest of components of the resonator as shown in Figure 10 (a) . A sapphire dielectric (5) , conveniently a rod, and a small high temperature superconducting film (17) are sandwiched between plate (460) and a piston (475) . Body (445) is used to hold cables (450). Body (445), piston (475), and bottom plate (440) are held and guided by shoulder screws (480) . Rod (430) is welded, or screwed, or fastened by any equivalent method, to bottom plate (440) of the resonator. The other end of rod (430) is connected to an actuator (420) . The actuator can be an air cylinder, a linear actuator, or any device that reciprocates rod (430) up and down in the Z direction perpendicular to plate (460) . A non-metallic sheet (485) is used to center and to keep sapphire rod (5) from moving during the operation as shown in Figure 10(b) . This sheet must have a low dielectric constant, 21 and a low dielectric loss such as Kapton® or any polymer film. The testing head is connected to another actuator. The actuator can be an air cylinder, a liner actuator, or any device that moves the testing head in the X direction. The design of piston (475) is identical to piston (35) described before except for additional tapped holes for shoulder screws (480) . The design of bottom plate (440) is identical to plate (30) described before except for additional holes for shoulder screws (480) . Sheet (485) is identical to sapphire locator #2 (Figure 2(d)) except its size is made to fit the recess of body (445) . In operation, the testing head is moved to a desired location on the film (455) . This is accomplished by the combination of the movement of the testing head in the X direction and the rotation of the plate (460) around its Z axis. Then, the actuator (420) moves the testing head in the +Z direction, compressing a holding device (435), previously described. At this time, film (17) , sapphire rod (5) , and film (455) forms a resonator similar to resonators #1 and #2. Film parameters at this location are recorded. To test another location on film (455) , actuator (420) lowers the testing head to keep the sapphire rod from touching film (455) . The testing head and plate (460) are again moved to position the testing head immediately below the desired testing location. The next steps described in the previous paragraph are then repeated. The invented apparatus are used for measuring the parameters such as surface resistance, R s , critical current density, J c , and critical rf magnetic field, H c , for high temperature supercoductors (HTS) . 22 For a given TEon mode HTS-sapphire-HTS resonator, the surface resistance, R s , is inversely proportional to the unloaded Q-value, Qo, of the resonator: R s =G/Qo (1) Here G is the geometry factor, which can be either calculated by using the formulas given in Shen et al., 1992 IEEE-MTT-S International Microwave Symposium Digest, pp. 193-196 or calibrated by a conductor such as copper with known R s . Therefore, the measurement of R s is simply to measure the Qo of the resonator, which is known in the art. In a TEon mode resonator, the maximum rf magnetic field, H ma , and the corresponding maximum current density, J max , in the HTS films are related to the R s and the dissipated power, PQ, in the resonator as follows: Here Ji (x) is the 1st order Bessel function of the 1st kind; Jι,m a x=0.582 is the maximum value of the Ji (x) ; ε r =9.32 and tanδ are the relative dielectric constant and the loss tangent of sapphire, respectively; R is the ratio of the electrical energy stored inside to that stored outside of the sapphire rod; a and L are the radius and the length of the sapphire rod, respectively; ξ l is the transverse wave number in the sapphire rod; λ is the wave length; λ < ι is the penetration depth. For a given resonator, all these quantities are constants. Therefore, the H max and J ma χ can be determined by measuring the R s and PQ (PQ can be measured by using a 23 power meter) . The resonators of the present invention provide measurements of improved accuracy and are reliably reproducible. For example, resonator #1 with drawer design #2 provided reproducibility at a level of 2%. The resonators of the present invention are easy to assemble while retaining superior sensitivity. Thus the apparatus are useful as a quality control tool for monitoring superconducting thin film manufacturing processes. The critical magnetic field, H c , can be determined by the Hmax value at which the R 3 value exceeds certain selected criteria. The J c is simply equal to H c /λ d . The surface resistance, R s , of the superconductor TlPbSrCaCuO (1212) and (1223) phases was measured using resonator #1 with drawer #2 assembled as previously described. The data measurements were performed in a liquid helium storage dewar with a glass epoxy insert. The resonator assembly was mounted on a copper plate at the end of a stainless steel probe. The assembly was then evacuated and lowered into the insert. Temperature was controlled with a pair of 100 W heaters. A HP-8510 vector network analyzer with 1 Hz frequency resolution was used for the measurements. A HP-8449A preamplifier and a Hughes 8030H02F TWT power amplifier with output power up to 30 W were inserted at the input of the resonator for the high powedered measureements. The R s was measured at 27 GHz. The resulting data are shown in Figure 11 plotted at 10 GHz, according to f 2 low as a function of temperature. The copper (Cu) data are for reference. EXAMPLE 2 The surface resistance, R s , of the superconductor YBa 2 Cuθ7- a was measured using resonator #1 with drawer #2 assembled as previously described using the procedure of 24 Example 1 except that the R s was measured at 5.55 GHz. The resulting data for various temperatures are shown in Figure 12 graphed as a function of maximum rf magnetic field.

Claims: 25 What is claimed is: 1. A dielectric resonator apparatus having a dielectric element of sapphire positioned between and in contact with two discrete films of at least one super¬ conducting material, all encased in an outer enclosure having means for magnetic dipole coupling, wherein the improvement comprises : (a) means for positioning the sapphire dielectric; (b) means for positioning superconducting films; (c) means for holding the sapphire dielectric and superconductor films in place during operation; (d) means for suppressing undesirable modes; and (e) means for adjustment of the magnetic dipole coupling. 2. The resonator of Claim 1 wherein the means for positioning the sapphire dielectric comprises two non- metallic sheets each having (1) a notch centered on one edge in size and shape suitable for being juxtaposed against the sapphire dielectric, (2) a hole to permit fastening in place with a dowel pin, and (3) chamfers at the corners of the edge containing the notch. 3. The resonator of Claim 1 wherein the means for positioning the sapphire dielectric comprises a non- metallic sheet having (1) an opening in size and shape through which the sapphire dielectric will pass with minimal clearance and (2) chamfers at two corners. 4. The resonator of Claim 2 or 3 wherein the sheet comprises a polymeric film having a dielectric constant of no greater than . 5. The resonator of Claim 2 or 3 wherein the sheet is polyimide Kapton 300HN®. 26 6. The resonator of Claim 1 wherein the means for positioning the sapphire dielectric comprises a plurality of screws inserted through openings in the outer enclosure, said screws having non-metallic tips. 7. The resonator of Claim 6 wherein the screws have sapphire tips. 8. The resonator of Claim 1 wherein the means for positioning the sapphire dielectric comprises (1) a device comprising a platform to which is attached perpendicularly an extension having a recess into which one end of the dielectric fits, (2) a hole running through said extension and platform through which a vacuum can be created to hold the dielectric in place in the recess, and (3) at least two additional holes for dowel pins for fastening the device to the outer enclosure . 9. The resonator of Claim 1 wherein the means for positioning the superconducting film comprises a drawer of non-ferrous metal having (1) a recess for holding the superconducting film above the drawer surface, (2) a grooved recess for holding a microwave frequency absorber; (3) at least two holes for screws; and (4) at least two holes for dowel pins. 10. The resonator of Claim 1 wherein the means for positioning the superconducting film comprises a plate to which is attached perpendicularly an extension having (1) a recess for holding the superconducting film above the plate surface, (2) a recess for holding a microwave frequency absorber, and said plate having (1) at least two holes for screws, (2) at least two tapped holes for securing the resonator to a support device, and (3) at least one slot on the side of the plate opposite the extension. 11. The resonator of Claim 1 wherein the means for positioning the superconducting film comprises (1) a 27 piston having (a) a recess to hold the superconducting film above the piston surface, (b) a recess to hold a microwave frequency absorber, c) a hole running through said piston through which a vacuum can be created to hold the sample in place, and (d) a counterbore, and (2) a piston locator in the form of a flat sheet containing an opening through which the piston will pass with minimal clearance and at least two holes for dowel pins for fastening said piston locator to the outer enclosure. 12. The resonator of Claim 11 wherein the piston diameter is such that the piston will fit into the outer enclosure with sufficient clearance to permit the passage of a cooling gas. 13. The resonator of Claim 1 wherein the means for holding the sapphire dielectric and superconducting film in place during operation comprises at least one helical compression spring secured to either (1) a top plate of the outer enclosure or (2) the top of a piston holding the superconducting film, said spring being compressed when the top plate is in place. 14. The resonator of Claim 1 wherein the means for holding the sapphire dielectric and superconducting film in place during operation comprises a plate of material which compensates for thermal contractions at cryogenic temperature. 15. The resonator of Claim 14 wherein the plate is Teflon®. 16. The resonator of Claim 1 wherein the means for holding the sapphire dielectric and superconducting film in place during operation comprises a plurality of disc springs stacked between the means for holding the superconducting film and the top of the outer enclosure. 17. The resonator of Claim 1 wherein the means for suppressing undesirable modes comprises microwave 28 absorber contained in a groove or recess surrounding the superconducting film. 18. The resonator of Claim 1 wherein the means for adjusting the magnetic dipole comprises adjusting the extent of the penetration of cables into the resonator enclosure by inserting or removing shims at flanges on a connector employed for the magnetic dipole coupling. 19. The resonator of Claim 1 further comprising a means for coupling to an electrical circuit which is a connector having (1) a transition element comprising an electrical conductor tube having an outer diameter which can be mated with a commercial connector and an inner diameter which can be mated with a cable having an outside diameter of a maximum of 0.94mm, and (2) an insulator comprising a dielectric ring having an outer diameter equal to the outer diameter of the transition element and an inner diameter less than the diameter of the center conductor of the connector. 20. The resonator of Claim 19 wherein the transition element is oxygen free copper and the insulator is Teflon®. 21. A dielectric resonator apparatus having a dielectric element of sapphire positioned between two discrete films of at least one superconducting material and a means for magnetic dipole coupling, wherein the improvement comprises a means for moving the resonator to test the superconducting films at multiple discrete areas. 22. A resonator of Claim 21 wherein the means for moving the resonator comprises (a) a plate capable of rotation which holds a discrete piece of the superconducting film on its underside, (b) a rod fastened to the bottom of the resonator at one end and to an actuator at the other end that reciprocates the 29 resonator in a direction perpendicular to the film on the plate. 23. The resonator of Claim 22 wherein the sample is held to the plate by means of a vacuum applied through a hole in the plate.

Inventors: DOROTHY ROBERT GLENN, NGUYEN VIET XUAN, SHEN ZHI-YUAN, DOROTHY, ROBERT, GLENN, NGUYEN, VIET, XUAN, SHEN, ZHI-YUAN
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1993
Main classification: G01R27
IPC codes: G01R27, H01P7