WO2002100133A2

WO2002100133A2 - Superconductor accelerator cavity with multiple layer metal films

Info

Publication number: WO2002100133A2
Application number: PCT/US2002/018482
Authority: WO
Inventors: Louis N. Hand; William R. Frisken
Original assignee: Cornell Research Foundation, Inc.
Priority date: 2001-06-06
Filing date: 2002-06-06
Publication date: 2002-12-12
Also published as: WO2002100133A3

Abstract

A first film of niobium is formed on a cavity shaped mandrel, followed by formation of a layer of molybdenum over the niobium. High temperature anneals are utilized for recyrstallization of the niobium and/or molybdenum. The mandrel material is then removed to create a superconducting cavity with the molybdenum forming a supporting structure, and the niobium forming an inner superconducting layer. In one embodiment, the mandrel is formed in the shape of a series of cavities for use in forming an accelerator following formation of the metal layers.

Description

SUPERCONDUCTOR ACCELERATOR CAVITY WITH MULTIPLE

LAYER METAL FILMS

Cross Reference to Related Applications

This application claims the benefit of priority under 35 U.S. C. 119(e) to U.S. Provisional Patent Application Serial Nos. 60/296,288 and 60/296,290, both filed June 6, 2001, both of which are incorporated herein by reference in their entireties.

Field of the Invention

The present invention relates to forming superconductor cavities, and in particular to methods of forming superconductor cavities having multiple layer metal films.

Background of the Invention

Niobiu is widely used as a superconductor cavity for accelerator applications because it has a high critical field and the highest critical temperature of any element. However, in the superconducting phase it has low thermal conductivity unless in a very highly purified form. If microscopic areas of the niobium reach too high a temperature during use, these areas lose their superconductivity and create a resistive instability which leads to catastrophic failure through excessive heating.

At present, a major barrier to large-scale accelerator applications is the cost of ultra-high purity niobium. Presently cavities are fabricated using electron beam welding of two cavity halves, which can be spun, hydroformed or otherwise fabricated as two separate pieces. The surface is smoothed using electropolishing. The cost of the material alone is currently very high, and the final product costs several times the cost of the material. Despite the very high cost, solid niobium cavities are still the only successful cavities for high energy accelerator applications. Another approach has been to sputter niobium films on the inside surface of a copper cavity. Due to the necessary limitation on the thickness of the niobium film caused by different coefficients of expansion, these cavities do not reach satisfactory accelerating fields.

Summary of the Invention A first film of niobium is formed on a cavity shaped mandrel, followed by formation of a layer of molybdenum over the niobium. The mandrel material is then removed to create a superconducting cavity with the molybdenum forming a supporting structure, and the niobium forming an inner superconducting layer. In one embodiment, the mandrel is formed in the shape of a series of cavities for use in forming an accelerator following formation of the metal layers. h one embodiment, the mandrel is formed of very highly polished material such as nickel. A thin material, such as titanium dioxide or titanium nitride is formed on the mandrel to function as a buffer layer. In one embodiment, one or more high temperature anneals of the niobium/molybdenum layers is performed at temperatures of approximately 1150 °C or higher.

In a further embodiment, niobium is deposited in a thin layer on the buffer film, and the temperature is raised above approximately 1150 °C to form an epitaxial surface parallel to the buffer film. Continued growth of the niobium film remains epitaxial for larger thickness. In still a further embodiment, lithium or calcium doping is used to form the niobium film. The niobium film is optionally heated to approximately 800 °C, removing hydrogen dissolved in the niobium.

Brief Description of the Drawings

Figure 1 is a cross section representation of a superconducting cavity used in accelerators of high energy electrons. Figure 2 is a magnified cross section of films forming the superconducting cavity of Figure 1. Figure 3 is a perspective view of a set of superconducting cavities used as an accelerator. Detailed Description of the Invention

In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims.

Figure 1 is a cross section representation of a single cell superconducting test cavity 110 used in accelerators of high energy electrons. The cavity comprises a first tube section 115, and cavity portion 120, and a second tube section 130. Electrons follow a course along a horizontal central axis 140 of the cavity, and are increased in energy as they pass through each cavity by an alternating electric field in the cavity, provided by a superconducting layer.

Several different size cavities may be formed. In one embodiment of a TESLA type cavity, the diameter of the tubes 115 and 130 is approximately 7.8 cm. An equator at the widest part of cavity portion 120 is approximately 20.6 cm. The cavity portion has a length of approximately 11.4 cm, and the length of the entire cavity 110 is approximately 37.0 cm. Other designs include DESY shape from the German High Energy physics Laboratory near Hamburg, CERN 87-03, and others. The invention is applicable to many different cavity and accelerator designs.

The cavity 110 comprises multiple metal layers indicated at 200 in a cross section of Figure 2. The layers 200 are formed over a self-supporting substrate made of a metal such as nickel. This serves as a mandrel 205, and is in the shape of the inside dimensions of the desired cavity 110 in Figure 1. The mandrel is a solid self-supporting piece. For example, a nickel mandrel can be produced by chemical vapor deposition from nickel carbonyl onto a glass mold. The nickel is atomically flat without subsequent polishing. In a further embodiment, the mandrel 205 has a surface that is polished to a very high degree of smoothness, such as atomic-level smoothness. A smooth surface should lead to significant reduction of the heating of the cavity wall by electrical currents flowing during operation.

The nickel mandrel 205 is optionally coated by evaporating a thin layer of a material such as titanium dioxide, titanium nitride or Al₂O₃ approximately 2000 angstroms or less thick. This serves as a buffer layer 210 to isolate the niobium from the nickel and prevent alloy formation. It also reduce reduces adhesion of the superconductor to the mandrel. The buffer layer 210 also serves as a template for formation of a niobium film 220. This approach produces a "first surface" niobium film. The microstructure of the niobium film can be controlled by the atomic structure of the buffer layer and of the nickel mandrel surface. Smoother niobium films such as those having a surface roughness of approximately less than 30nm, can be produced as first surfaces by other means, giving a choice of a wide variety of mandrels. Nickel is chosen because it can be formed or polished to atomic-level smoothness by well-developed techniques. The buffer layer will serve as a template for the niobium film. It also prevents direct exposure of the niobium to the nickel surface. Niobium deposition is then continued to a thickness of approximately 5-10 microns. A structural support layer of molybdenum 230 is then formed on top of the niobium film. The niobium and molybdenum films are produced in different ways, such as by physical vapor deposition (PVD) or by chemical vapor deposition (CVD). When the desired thickness of niobium film has been reached (perhaps in two or more stages to promote epitaxy), the niobium is optionally graded into the molybdenum over a short distance, of one or two microns in one embodiment. Grading is performed by co-evaporating both materials for a short time, then turning off an electron gun heating the source of niobium and continuing the molybdenum evaporation to a desired final thickness. The molybdenum film is deposited to approximately 1 mm thick or other thickness to provide support. The molybdenum is then in intimate contact with the niobium, ensuring good heat transfer. Physical strength of the cavity is provided by the molybdenum. If necessary for greater strength, more molybdenum can be added using plasma spraying or CVD.

After the cavity structure is completed, the mandrel is removed without damaging the cavity surface, hi the case of a nickel mandrel, this can be done with aqua regia, reverse elecfrodeposition, or melting in a vacuum furnace. The buffer layer can remain. It will play a passive role in cavity operation, much as the native oxide layer does in the more expensive solid niobium cavities, with the further advantage of preventing corrosion in moist air and absorption of hydrogen. h a further embodiment, the mandrel is formed in the shape of several serially connected cavities. It is possible to construct several meters of accelerator this way, consisting of approximately nine cells per meter. The serially connected cavities are fabricated as a unit, further reducing costs. Sections of such serially connected cavities can be coupled together to form longer accelerators.

Figure 3 is a perspective simplified representation of a set of superconducting cavities used as an accelerator. A plurality of cavities 310, 315, 320, 325 and 330 are coupled in series to form one unit of an accelerator having a first end 335 and second end 337. The first end 335 of the accelerator unit includes an RF drive port 345, and the second end 337 includes an RF pickup port. In some embodiments, the accelerator comprises hundreds or even thousands of cavities, and extends for many meters or even, in the case of a linear collider, many kilometers. An outer cooling shell 340 surrounds the cavities, and contains liquid helium 17. (superfluid liquid helium) at a temperature of approximately 1.8 °K. This maintains the temperature of the niobium film well below the critical temperature Tc for optimum performance. The inside of the cavity is held at a vacuum during operation.

A typical PVD system for physical vapor deposition of both metal and ceramic films is described in general terms, followed by a description of a process which uses the PVD system to produce the multiple layer films.

Fabrication of the niobium/molybdenum bilayers for superconducting cavities is useful at least in particle accelerators. A CVD process may also be used in conjunction with PVD. One electron beam physical vapor deposition system (EB-PVD) utilized has six electron beam guns, four of which are used to evaporate the coating materials and two of which are used to preheat the substrate. Each gun has a 45 KW capacity. The maximum diameter of the substrate is about 400 mm, and the substrate can be rotated at a speed up to 110 rpm. Continuous rotation is possible during the deposition process. Deposition rates up to 150 microns/minute are possible, with an evaporation rate of 10-15 Kg/hour. Much lower rates are used for niobium film deposition due to the need for only 5-10 microns of the film. An ion beam can provide dense coatings with improved microstructure. The ion beam can also be used to clean the surface prior to deposition. Substrate temperatures can be controlled up to and above 1000°C. Additional heat sources may be utilized to obtain higher temperatures.

In addition to significant cavity cost reduction, the present invention allows improved cavity performance. For example, if the accelerating electric field is improved from 25 MeV/meter up to 40 MeV/meter, the length of a 1 TeV linear collider can be reduced from 40 kilometers to 25 kilometers-an estimated cost reduction of 37.5%. The possibility of improving the performance comes from the ability to heat the metal films both before during and after the deposition process. Recrystallization of the film microstructure occurs if the films are heated above the recyrstallization temperature of approximately 1150°C in a vacuum furnace. The desired temperature in one embodiment is 1200°C. The grain size and film density can be altered. This affects the electrical properties such as residual resistance (heating of the cavity walls), and field emission (breakdown at high electric fields). The length of time of the anneal is approximately a couple hours at temperatures close to the recrystallization temperature, and less for higher temperatures. The amount of time for the anneal is limited to prevent significant diffusion of the metal films into each other.

In a further embodiment, highly textured or even nearly epitaxial films are created. A thin (« 20 nanometers) precursor layer of niobium is first deposited on an oriented buffer layer film, and the temperature is raised above approximately 1150°C. The niobium will "wet" the surface of the buffer film, forming an epitaxial 110 surface (surface of minimum energy) parallel to the buffer film surface. A lattice match is not necessary between the buffer film and the niobium. The resultant niobium film is nearly epitaxial to thicknesses up to at least 3 microns. A molybdenum film deposited on top of the niobium may also be epitaxial, using the same type of a thin precursor layer. A highly textured superconducting film should improve the superconducting properties at very high currents (> 1000 amperes/centimeter), which are present when the accelerator is operating. If surface defects can be eliminated through high temperature annealing, ftuxon entry into the film at high fields will be suppressed. This is thought to be the mechanism currently limiting the maximum accelerating electric fields in present-day cavities. h a further embodiment the grain boundaries in the niobium film are doped with another element, such as lithium or calcium. Experiments with the high Tc superconductors have shown that it is possible to increase the critical current by an order of magnitude or more, using calcium doping. A precursor film could be a niobium/lithium or niobium/calcium alloy. Upon heating, these lithium or calcium ions will diffuse preferentially into the grain boundaries of the niobium film. This can have a profound effect on the electrical properties in the superconducting state. According to theoretical speculation, the "weak links" in the grain boundaries contribute substantially to residual resistance and critical current limits.

In still a further embodiment, heating the films above 800°C essentially eliminates hydrogen dissolved in the niobium. Very small quantities of hydrogen degrade the performance of the superconductor. Although most of the hydrogen can be removed by heating to 350°C, some hydrogen remains trapped on oxygen accumulated in grain boundaries and other types of defects. Heating to higher temperatures will eliminate all trace amounts of hydrogen. This is not possible in niobium/copper cavities, since the melting point of copper is too low.

Conclusion A novel approach to fabricating superconducting accelerator cavities and/or sets of cavities in series has been proposed. Potential advantages include significant cost savings over conventional solid niobium cavities by using up to 100 times less of expensive niobium. The use of a "first surface" produces atomically flat and defect-free niobium surfaces. Use of a 1 mm molybdenum rather than copper cavity outer layer allows for cavity high temperature vacuum annealing, producing the ability to realize higher accelerating electric fields and enlianced performance as described above. The molybdenum theπnal expansion coefficient is much closer to that of niobium than is copper, allowing for thicker mobium films without fear of delamination and lower stress at cold temperatures. Delamination is also avoided by grading the niobium film into the molybdenum in a continuous transition, so that perfect adhesion between the films is achieved. Tungsten may be substituted for molybdenum in further embodiments.

The process also allows the formation of multiple series cavities at the same time. Such cavities save fabrication costs over previous methods of forming half cavities and electron beam welding each cavity.

Claims

1. A method of forming adjacent films on a superconducting accelerator cavity mandrel, the method comprising: depositing a first layer of metal on the cavity mandrel; annealing the first layer at or above the recyrstallization temperature of the first metal; depositing a second layer of metal on top of the first layer; annealing the second layer at or above the recyrstallization temperature of the second metal; and removing the mandrel.

2. The method of claim 1 wherein the first layer is a 5-10 micron thick film of niobium.

3. The method of claim 2 wherein the second layer is molybdenum.

4. The method of claim 1 wherein the mandrel is atomically smooth.

5. The method of claim 4 wherein the mandrel is formed of solid nickel.

6. The method of claim 5 wherein the mandrel is removed by at least one of aqua regia, reverse electrodeposition and melting in a vacuum furnace.

7. A superconductor for a superconducting accelerator, the superconductor comprising: a first film of niobium formed in the shape of a cavity; and a second film of molybdenum formed over the first film to provide support.

8. The superconductor of claim 7 wherein the first film is approximately 5-

10 microns thick.

9. The superconductor of claim 7 wherein the first film is atomically smooth.

10. The superconductor of claim 7 and further comprising a heat conducting layer formed on an exterior surface of the cavity.

11. The superconductor of claim 10 wherein the heat conducting layer formed on the exterior surface of the cavity comprises a cooling fluid container.

12. A method of forming a superconducting cavity, the method comprising: forming an atomically smooth mandrel; depositing a buffer layer on the mandrel; depositing a niobium layer of metal on the buffer layer; annealing the niobium at or above its recyrstallization temperature; depositing a second layer of metal on top of the niobium layer of metal; and removing the mandrel.

13. The method of claim 12 wherein depositing the niobium layer comprises forming a precursor layer prior to annealing, then forming a thicker layer of niobium following anealing.

14. The method of claim 13 wherein the precursor layer is approximately 20 Angstrom thick.

15. The method of claim 14 wherein the niobium layer is approximately 5-10 micron thick.

16. The method of claim 13 wherein the second layer comprises molybdenum.

17. A superconducting cavity for a RF accelerator comprising: an internal layer of niobium having a surface roughness of approximately less than 30nm; a second metal support layer having substantial lattice mismatch with niobium, wherein the niobium and second metal layer form a cavity.

18. The superconducting cavity of claim 17 wherein the second metal layer is molybdenum.

19. The superconducting cavity of claim 18 wherein the molybdenum and niobium form a graded bimetal layer.

20. A superconducting RF accelerator comprising: a plurality of cavities coupled in series, the cavities comprising: an internal layer of niobium having a surface roughness of approximately less than 30nm; a second metal layer having substantial lattice mismatch with niobium, wherein the niobium and second metal layer form a bimetal layer in the shape of a superconducting cavity; an RF drive port coupled to as first end of the series coupled cavities; a RF pick-up port coupled to a second end of the series coupled cavities; and a cooling container surrounding the series coupled cavities to keep the cavities at a temperature below a critical temperature for super conductivity.