EP1955404A1

EP1955404A1 - Method for production of hollow bodies for resonators

Info

Publication number: EP1955404A1
Application number: EP06818910A
Authority: EP
Inventors: Xenia Singer; Waldemar Singer; Johannes Schwellenbach; Michael Pekeler
Original assignee: Deutsches Elektronen Synchrotron DESY
Current assignee: Deutsches Elektronen Synchrotron DESY
Priority date: 2005-12-02
Filing date: 2006-11-29
Publication date: 2008-08-13
Anticipated expiration: 2026-11-29
Also published as: US8088714B2; DE502006003219D1; ATE426255T1; US20090215631A1; JP5320068B2; DE102006021111B3; JP2009517817A; EP1955404B1; WO2007062829A1

Abstract

A method for production of hollow bodies (8), in particular for radio-frequency resonators, is illustrated and described. The object of producing hollow bodies or a resonator which have or has improved characteristics is achieved by a method having the following steps: provision of a substrate having a monocrystalline area, definition of a slice surface through the substrate, fitting of markings (3, 3') on both sides of the slice surface, production of two wafers by cutting along the slice surface, with the wafers being completely removed from the monocrystalline area, forming of the wafers into half-cells (5, 5'), with the half-cells having a joint surface (6, 6'), and joining of the half-cells together to form a hollow body (8), with the joint surfaces resting on one another and with the markings on the half-cells being oriented with respect to one another on both sides of the joint surface as on both sides of the slice surfaces.

Description

Process for producing hollow bodies for resonators

The present invention relates to a process for producing hollow bodies, in particular for high-frequency resonators.

High frequency resonators comprising a plurality of hollow bodies are particularly used in particle accelerators which use electric fields to accelerate charged particles to high energies.

In such high-frequency resonators, also called cavity resonators, an electromagnetic wave is excited, which accelerates charged particles along the resonator axis. The thus accelerated particle experiences a maximum possible energy gain when it passes through the resonator with respect to the phase and the high-frequency field so that it is located in the middle of a cavity cell just when the electric field strength reaches its maximum there. Here, the cavity cell length and the frequency are adjusted so that the particles in each cell experience the same energy gain. In this case, superconducting resonators for the provision of large field strengths have the advantage that far less energy has to be expended due to the very low high-frequency resistance.

For a long time one method for resonator production was to connect the so-called semi-hollow bodies, which were produced from a polycrystalline niobium sheet by deep-drawing, to one another by electron beam welding. From DE 37 22 745 Al also a method in which half-cells are made of coated sheets known. Further, this document discloses a resonator fabricated thereafter, and more particularly, a superconducting high frequency resonator made of Ni coated with copper. Furthermore, US Pat. No. 5,500,995 discloses producing multicellular cavity resonators without welding seams by applying and deforming the desired material by means of a spinning technique on a forming, removable substance which serves as a substrate, and then subsequently removing the forming substance.

The sheets used in the two methods known in the art are coated with or consist entirely of a suitable superconducting material. A preferred material in this case is the superconducting niobium, since it can be processed very well on the one hand and on the other a high critical temperature T _c = 9.2 K and a high critical magnetic field H _c ≡ 200 mT (temperature or magnetic field, above that the superconductivity breaks down).

After forming, the material is further treated in a conventional manner in order to obtain a surface with the lowest possible roughness, since roughening of the surface generally occurs when forming a polycrystalline material. In addition, the inner surface should be free of impurities and foreign particles. Because surface defects are u.a. responsible for causing the superconductivity to collapse, because the currents circulating in the surface layer of the superconductor, which prevent an external magnetic field from penetrating into the interior (Meissner-Ochsenfeld effect), are interrupted. Finally, a rough surface causes locally very high field strengths occur, which is also undesirable.

A common method of surface treatment is a chemical (pickling) process with an acid mixture called BCP (Buffered Chemical Polishing), wherein HF (48%), HNO ₃ (65%) and H ₃ PO ₄ (85%) in a ratio of 1: 1: 2. There However, the grain boundaries of polycrystalline material are attacked more than the material of the grains themselves, is still a relatively rough surface after this treatment. In addition, this method is relatively time consuming. One method that gives better results is electro polishing ("EP") using HF and H ₂ SO ₄ in the ratio 1: 9 and applying an electric field. By electropolishing a very smooth surface is achieved even with polycrystalline material, so that in the case of hollow bodies made of polycrystalline niobium by means of electropolishing a roughness of 250 nm can be achieved.

Since the superconductivity at the grain boundaries of a polycrystalline material is disturbed, more recently, experiments have been conducted on the usability of niobium ingots (Residual Resistance Ratio RRR> 250) for the production of half-cells with a positive result (P. Kneisel, GR Myeni, G. Ciovati, J. Sekutowicz and T. Carneiro, Preliminary Results From Single Crystals and Very Large Crystal Niobium Cavities, Proceedings of the 2005 Particle Accelerator Conference, Knoxville, Tennessee, USA). In this case, two slices were cut by means of a wire erosion machine from a coarse-crystalline niobium ingot to produce a small cavity resonator and then brought by deep drawing in the desired shape, without the crystalline properties have been changed; Here, too, however, it came at the points where the transformed crystalline discs were assembled into a hollow body, to defect sites.

In addition to the defect-free crystal structure in the cavity resonators, it is very important for the quality of superconducting cavity resonators that no superconducting losses occur at the joints. Another factor that interferes with superconductivity is hydrogen, which is incorporated in the superconducting material. This problem is conventionally solved by performing a heat treatment.

Starting from the prior art, it is therefore an object of the present invention to provide a method in which the hollow body produced or the entire resonator have improved electrical properties.

This object is achieved by a method with the following steps:

Providing a substrate having a monocrystalline region,

Defining a cut surface through the substrate,

- applying markings on both sides of the cut surface,

Producing two slices by cutting along the cut surface, wherein the slices are completely removed from the monocrystalline region,

Forming the slices into half-cells, wherein the half-cells have a joining surface,

Joining the half-cells to a hollow body, wherein the joining surfaces abut each other and wherein the markings on the half-cells are oriented on both sides of the joining surface to each other, as on both sides of the cut surfaces.

In the method according to the invention, in a first step, a substrate with a monocrystalline region is provided. represents, which is in a preferred embodiment of superconducting material. A preferred material here is superconducting niobium, since it is very readily moldable and also has a high critical temperature T _c = 9.2 K and a high critical magnetic field H _c = 200 mT. In this context, "superconducting" material is understood as meaning a material which has superconducting properties under suitable ambient conditions and below a critical temperature, thus abruptly losing its electrical resistance and displacing subcritical magnetic fields from its interior he is easily accessible.

In a second step, at least one cut surface is defined by the substrate, and in a subsequent third step markings are applied to both sides of the cut surface. Preferably, these markers are stamped or embossed because superconducting materials are metals that have a hard surface. The markings are designed such that adjacent areas in the substrate can be identified again after a separation and their original orientation can be restored to one another. The markings are preferably mounted on the outer surface or on the peripheral surface of the discs.

After the markings have been applied, two slices are made by cutting along the cut surface, and the slices are further cut out of the substrate so as to have only single crystalline material. In a preferred embodiment, the discs are about 5 mm thick and have a diameter or extension in the plane of the cut surface of 200 mm. In a subsequent step, the disks are transformed into half-cells, wherein the half-cells have a joining surface. These joining surfaces serve to be able to join two half cells together. In a preferred embodiment, the half-cells furthermore have a termination surface running parallel to the joining surface, which makes it possible for the half-cell also to be connected to a further half-cell on the side opposite the joining surface.

The forming is preferably carried out by pressing, deep drawing and optionally rolling, which are known metal processing techniques. The area of the disc may have previously been enlarged in this regard, which is also possible with the aid of the already mentioned techniques.

In forming, a preferred embodiment involves creating a hollow truncated cone having two parallel open end surfaces. Furthermore, the half-cells are preferably shaped rotationally symmetrical, so that half-cells can be connected as easily as possible.

Alternatively, the forming can also take place in such a way that the production of a hollow cone by deep drawing or pressing against a mold is included, wherein in a further preferred Äusführungsform the largest diameter of the hollow cone is greater than or equal to the outer diameter of the half cell. This makes it possible to bring the cone later with the least possible number of processing steps to the desired shape and size of the half-cell, without the single-crystal structure is lost.

In the forming step, it is possible for a disk, before, for example, a hollow cone or a truncated cone to be formed, to be converted into a disk by means of rolling or pressing, which faces the original disk has enlarged diameter. This makes it possible to form monocrystalline half-cells of the desired size even from slices derived from a small-diameter ingot.

In a further step of the method, the half-cells are joined together to form hollow bodies, wherein the joining surfaces lie against one another and the markings on both sides of the joining surface are oriented relative to each other, as on both sides of the cut surfaces. This means that half cells produced from the slices abut each other along the joining surfaces, as was the case in the substrate prior to the cutting of the cut surfaces. As a result, the monocrystalline orientation is maintained in both disks converted into hollow bodies.

Because of the sensitivity of high-purity niobium to impurities of any kind, the surfaces to be joined can be cleaned shortly before joining, which is preferably done with a chemical pickling treatment (with BCP).

The joining is (mbar <10 ^{~ 4)} by electron beam welding in a high vacuum and preferably carried out optionally at a defined residual gas composition. This technique has a high power density so that components can be welded with a smooth seam that is 5 to 7 mm wide, as it results in a localized energy input.

In a preferred embodiment, the joining and / or closing surfaces are chemically treated. This is preferably carried out by a pickling treatment, in particular with BCP (1: 1: 2). This avoids that foreign material is introduced into the material in the region of the weld. The hollow body is subsequently heat treated. As a result, remaining defects and the joints are annealed, the hydrogen contained in the material is expelled and the RRR value, which describes the purity of the niobium preferably used, is thus increased.

A preferred embodiment of the heat treatment μmfasst in the case of an existing niobium hollow body comprises a first heating step of 400 ⁰ C to 500 ⁰ C for 2 to 6 hours and a second heating step of 75O ⁰ C and 85O ⁰ C, preferably from 750 ⁰ C to 800 ⁰ C. The aim of the first heating step is to reduce the stresses created by the transformations and to eliminate newly formed nuclei. The second heating step serves to remove existing hydrogen from the material and to relax the entire hollow body. Hereby, the single crystal is retained since nucleation nuclei have been previously eliminated, so that grain growth by the heat treatment can not occur.

The heat treatment is dependent on the degree of deformation ε of the material, which in the preferred embodiment with niobium is about 40%. The degree of deformation ε of a material is understood in this context to mean the percentage of the deformation. The degree of deformation ε is calculated to

where to is the thickness of the undeformed disk and t is the thickness of the deformed disk.

With the method according to the invention, it is possible to produce a monocrystalline resonator comprising monocrystalline hollow bodies or half-cells. Such single-crystal resonators have excellent electrical properties. par- In addition, there are also circulating currents in the monocrystalline surface layer of the superconductor (niobium) which prevent an external magnetic field from penetrating inside, whereby superconductivity is not disturbed. In addition, in the case of monocrystalline material, significantly lower roughnesses, in particular of the inner surface, can be achieved, which are 25 nm in the case of a final BCP treatment. This means an improvement by a factor of 10 compared to comparable polycrystalline material after a more expensive after-treatment.

The above object is further achieved by a method comprising the following steps:

Manufacture of a plurality of hollow bodies according to one of claims 2 to 18 and

Joining the hollow bodies along the end surfaces, wherein half cells of originally adjacent slices are joined in the substrate and wherein the adjacent to the end surfaces markings are associated with each other, as on both sides of the cut surface between the discs.

In the method according to the invention, first a multiplicity of hollow bodies are produced and these are then joined together along the end surfaces. In this case, the hollow bodies are always connected to hollow bodies produced from adjacent slices of the raw material, wherein the markings adjacent to the end surfaces are associated with one another as on both sides of the cut surface. This ensures that the monocrystalline structure is maintained even between adjacent hollow bodies. In a preferred embodiment, the surface of the resonator is treated. This is preferred by a chemical Procedure made with BCP (1: 1: 2). In principle, the chemical process can be carried out before or after the joining. It is very important to prepare an inner surface of the resonator hollow body so that it is free of impurities and foreign particles to produce high electric fields without losses. This occurs subsequent to or even without a prior heat treatment with a standard chemical or electrical process.

In the following, the present invention will be explained with reference to a drawing showing only a preferred embodiment. In the drawing show

1 shows a cross-sectional view of a substrate with a monocrystalline region and fixed cut surfaces,

FIG. 2 is a cross-sectional view of slices made by cutting along the cut surface; FIG.

FIG. 3 is a cross-sectional view of a half-cell made of a disc by forming; FIG.

FIG. 4A is a cross-sectional view of slices made by cutting along the cut surface; FIG.

4B is a cross-sectional view of a disc which has been made to a suitable size by reshaping;

4C is a cross-sectional view of a cone made from a disc by forming, Fig. 5 is a cross-sectional view of a hollow body of two assembled half-cells, and

Fig. 3 is a cross-sectional view of a resonator composed of a plurality of hollow bodies.

The figures show the steps of a preferred embodiment of the method according to the invention.

In Fig. 1, a substrate 1 with a monocrystalline region (hatched) is shown, which is provided for the production of hollow bodies for resonators. The single-crystalline region is preferably in a cylindrical shape, and the material of the substrate is preferably niobium because it can be processed well and has a high critical temperature T _c = 9.2 K and a high critical magnetic field H _c ≦ 200 mT. Subsequently, three adjacent cut surfaces 2, 2 \ 2 ^{> λ} , which run through the substrate 1, set. On both sides of the cut surface 2 ^y markings 3 and 3 ^{λ are} mounted on the surface of the substrate 1, which is preferably realized by punching or embossing. The markings 3, 3 ^λ are designed so that they are still visible after forming. One of the cut surfaces 2, 2 ^λ , 2 ^λλ can also form an end of the substrate 1, so that only two of the cut surfaces must be defined.

Then 2 ^λ 2 ^yx and discs 4 and 4 are formed by cutting along the predetermined cut surfaces 2, manufactured ^λ (see Fig. 2), the discs 4, 4 ^λ completely from the monocrystalline region have been removed. The latter means that the disks 4, 4 ^λ comprise only monocrystalline material and possibly existing polycrystalline or amorphous regions are separated. Preferably, the markings 3, 3 ^{λ are} punched or embossed, since the material is preferably a metal having a hard surface. The Markings 3, 3 ^Λ are designed such that in the substrate 1 adjacent areas can be identified again after a separation and their original orientation can be restored to each other.

Both disks 4 and 4 * are approximately 5 mm thick in this preferred embodiment and, since they preferably come from a cylindrical single crystal, have a diameter of 200 mm. In the case of a non-cylindrical single-crystal region, the disks 4 and 4 ^{Λ have} an extension in the plane of the cutting surfaces 2, 2 \ 2 ^λX of 200 mm.

In Fig. 3, a first possibility for the following step of -Vmformens the disc 4 is shown to a half-cell 5. The forming of the disc 4 is preferably carried out by pressing, deep drawing and optionally rolling, wherein the half-cell 5 shown in cross-section in Fig. 3 and a half-cell 5 shown in Fig. 5 in cross-section 5 ^Λ are formed accordingly. Also, a forming intermediate step, in which the surface of the disc is first increased and / or the creation of a hollow truncated cone with two parallel open end faces, is possible. Preferably, the half-cells 5, 5 'are rotationally symmetric. The half-cell 5 also has a joining surface 6 and a closing surface 7. In this case, the joining surface 6 and the end surface 7 preferably run parallel to one another. The marker 3 is mounted on the disc 4 so that it is still visible after forming a disc 4 to a half-cell 5.

4, a second possibility for the forming of the discs 4, 4 'is shown. Here, the forming includes the creation of a hollow cone by deep drawing or pressing, wherein the pressing takes place against a negative mold. It is possible that the discs 4, 4 ', which initially have a diameter a, before forming, for example, a cone or a truncated cone are first converted by means of rolling or pressing to discs 4, which have a diameter b which is greater than a. This makes it possible, even from discs 4, 4 ^Λ derived from an ingot having a small diameter, half-cells 5, 5 ^λ of the desired size to form. The largest diameter c of the hollow cone after forming is greater than or equal to the outer diameter of the half-cell 5. This makes it possible to bring the hollow cone with the smallest possible number of processing steps to the desired shape and size of the later half-cell 5, without the monocrystalline properties the material is lost.

FIG. 5 shows a cross-sectional view of a hollow body 8 which has been assembled from two half cells 5 and 5 ^λ with markings 3 and 3 ^λ along the two joining surfaces 6 and 6 ^x , preferably by electron beam welding under high vacuum (<ICT ⁴ mbar). and further preferably occurs at a defined residual gas composition. With this technique, the half-cells 5 and 5 ^Λ can be welded with a smooth seam, which is 5 to 7 mm wide, with only a localized energy input. In addition, this technique ensures that the weld is absolutely tight.

Here, the joining surfaces 6 and β ^λ of two half-cells 5 and 5 ^Λ have been joined together such that the half cells are arranged 5 and 5 ^Λ from originally adjacent in the substrate 1 disks 4 and 4 ^Λ next to each other, wherein the ^λ to the Fügefϊächen 6 and 6 adjacent Markers 3 and 3 ^λ are arranged to each other, as was the case on both sides of the cut surface 2 between the discs 4 and 4 ^λ . The ^λ of the assembled half-cells 5 and 5 existing hollow body 8 has two mutually substantially parallel standing CAULK terminal areas 7 and 7 on ^λ. The ^λ from the half-cells 5, 5 hollow body 8 is produced over the entire volume, including in the region of the former the joining surfaces 6, 6 single crystals \ from linem material so that it has good electrical properties and flows in the surface layer of the superconductor (niobium) circulating currents, which prevent an external magnetic field from penetrating into the interior, whereby the superconductivity is disturbed.

Preferably, the joining surfaces 6 and 6 ^λ and / or end surfaces 7 and 7 ^λ are cleaned before joining. These surfaces are first rinsed and treated in an ultrasonic bath, then preferably by a chemical process with BCP (1: 1: 2) pickled to remove contaminants in this area, rinsed again with ultrapure water and finally dried in a clean room.

Subsequently, in a preferred embodiment of the method, a special heat treatment of the hollow body 8 take place, which is a heating for a period of two to six hours at 400 ⁰ C to 500 ° C and then heating for a period of one to three hours at 750 ⁰ C to 850 ⁰ C, preferably 750 ° to 800 ° C comprises. As a result, remaining defects are healed. The goal of the first heating step is to break down the stresses created by the transformations and eliminate newly formed nuclei ¹ . The second heating step serves to remove existing hydrogen from the material and to relax the entire hollow body.

The monocrystalline hollow bodies 8 thus produced have excellent electrical properties, in which circulating currents are present in the monocrystalline surface layer of the superconductor (niobium), which prevent an external magnetic field from penetrating into the interior, whereby a superconductivity is not disturbed. In addition, by the monocrystalline material significantly lower roughness in particular the inner Surface at 25 nm in the case of a final BCP treatment.

Fig. 6 shows a plurality of hollow bodies 8, 8 ^λ , S ^x \ which have been prepared according to the method described above and analogous to the addition of two half-cells 5 and 5 ^λ to a hollow body 8 at their end ^surfaces 7 \ 7 ^λ> , 7 ^{> Λλ} , 7 ^{λΛ >>} have been joined together, preferably also by electron beam welding. This means that the markings 3, 3 \ 3 ^λ \ 3 ^Λλ \ 3 ^ΛλΛ \ 3 ^λλλλλ adjacent to the end ^faces 7, 1 \ 7 ^λ \ 7 ^λΛ \ 7 ^λλλ \ 7 ^{λλλ> x} are arranged relative to one another, as on Both sides of the cut surfaces 2 and 2 ^λ between the discs 4, 4 \ from which the corresponding half-cells were produced. The resonator 9 produced by assembling a plurality of hollow bodies 8, 8 ^λ , 8 ^λλ can be polished, preferably by a chemical process with BCP (1: 1: 2).

It should be mentioned at this point the sake of completeness that it is also possible, of course, two half-cells 5 ^Λ and 5 ^{X λ} in such a way at their end surfaces 7 ^X and 7 ^λλ assemble (s. Fig. 6) that the adjacent marks 3 ^Λ and 3 ^{λλ of} the half cells 5 'and 5 ^{λ λ} have such an orientation, as was the case on both sides of the cut surface between the respective disks. It is therefore conceivable that alternatively first dumbbell-shaped hollow bodies are formed, which are then joined together to form the resonator 9.

In this way, a single-crystal resonator 9 having improved electrical properties can be produced. These have the effect of significantly improving the quality of superconductivity under suitable environmental conditions, such as a suitable temperature. Furthermore, the advantage lies in the use of a monocrystalline resonator 9 The fact that a much better surface quality (smoothness) can be achieved even by the simple chemical pickling process, even compared to electropolishing.

This means that it is possible by means of a monocrystalline resonator 9, on the one hand to get higher acceleration field strengths and on the other hand to simplify the preparation.

Claims

claims

A method of manufacturing hollow bodies for resonators, comprising the following steps:

Providing a substrate (1) having a monocrystalline region, defining a cut surface (2) through the substrate

(D,

Applying markings (3, 3 ^Λ ) on both sides of the cut surface (2),

Producing two disks (4, 4 ^Λ ) by cutting along the cutting surface (2), wherein the disks (4, 4 ^λ ) are completely removed from the monocrystalline region,

- Forming the discs (4, 4 ^Λ ) to half-cells (5, 5 ^λ ), wherein the half-cells (5, 5 ^λ ) have a joining surface (6, 6 '),

Joining the half-cells (5, 5 ^λ ) to a hollow body (8), wherein the joining surfaces (6, 6 ^λ ) abut each other and wherein the markings (3, 3 ^λ ) on the half-cells (5, 5 ^λ ) on both sides of Joining surface (6, 6 ^λ ) are oriented to each other, as on both sides of the cut surfaces (2, 2 ^Λ ).

2. The method of claim 1, wherein the half-cells (5, 5 ^Λ ) have a closing surface (7, 7 ^λ ), which runs parallel to the joining surfaces (6, 6 ').

3. The method according to any one of claims 1 or 2, wherein the substrate 1 comprises a superconducting material.

4. The method according to any one of claims 1 to 3, wherein the substrate 1 comprises niobium.

5. The method according to any one of claims 1 to 4, wherein the monocrystalline region is cylindrical.

6. The method according to any one of claims 1 to 5, wherein the markings (3, 3 ^λ ) are punched or embossed.

7. The method according to any one of claims 1 to 6, wherein the discs (4, 4 ^λ ) are about 5 mm thick and have an extension in the plane of the cut surface (2, 2 ^λ ) of 200 mm.

8. The method according to any one of claims 1 to 7, wherein the surface of the discs (4, 4 ^λ ) is increased after cutting.

9. The method according to any one of claims 1 to 8, wherein the deformation is carried out by pressing, deep drawing and optionally rolling.

10. The method of claim 9, wherein the forming comprises creating a hollow truncated cone having two parallel open end surfaces.

11. The method of claim 1, wherein forming comprises creating a hollow cone.

12. The method of claim 11, wherein the largest diameter of the hollow cone is greater than or equal to the outer diameter of the half-cells (5, 5 ^λ ).

13. The method according to any one of claims 1 to 12, wherein the half-cells (5, 5 ^λ ) are rotationally symmetric.

14. The method according to any one of claims 1 to 13, wherein the joining is carried out by electron beam welding.

15. The method according to any one of claims 1 to 14, wherein the joining surfaces (6, 6 ^λ ) and / or the end surfaces (7, 7 ^λ ) are cleaned before joining.

16. The method according to claim 15, wherein the joining surfaces (6, 6 ^λ ) and / or the end surfaces (7, 7 ^λ ) are chemically pickled.

17. The method according to any one of claims 1 to 16, wherein a heat treatment of the hollow body (8) is performed.

18. The method of claim 17, wherein the heat treatment subsequently comprises a heating over a period of two to six hours at 400 ⁰ C to 500 ⁰ C and a heating over a period of one to three hours at 75O ⁰ C to 85O ⁰ C.

19. The method of claim 17, wherein the heat treatment subsequently comprises a heating over a period of two to six hours at 400 ⁰ C to 500 ⁰ C and a heating over a period of one to three hours at 75O ⁰ C to 800 ⁰ C.

20. A method of manufacturing a resonator (9), comprising the steps of:

Producing a plurality of hollow bodies (8, 8 ^λ , 8 ^{λ Λ} ...) according to one of claims 2 to 19, - joining the hollow body (8, 8 \ 8 ^{Λ λ} ) along the end surfaces (7, 7 \ 7 '\ 7 ^{λ λ} \ 7 ^{λ Λ λ λ} ), wherein half cells (5 \ 5 ^λ \ 5 ^{λ λ} \ 5 ^{> λ λ λ} ) originally adjacent slices in the substrate (1) are connected and wherein the to the end surfaces (7, 7 7 ^{> λ} , 7 ^{λ λ} \ 7 ^{λ Λ Λ} \ 7 ^{Λ Λ λ> λ} ) adjacent to one another. are arranged, as on both sides of the cut surface (2, 2 ^y ) between the discs (4, 4 ^Λ ).

21. The method of claim 20, wherein the resonator (9) is cleaned.

22. The method of claim 21, wherein the resonator (9) is chemically pickled.