Superconducting RF module for particle accelerators and method for manufacturing a superconducting RF module for particle accelerators

Abstract

The invention relates to a superconducting RF module for particle accelerators, comprising at least one superconducting RF cavity (1) and means for cooling (2) the cavity (1). According to the invention, the superconducting RF module for particle accelerators is characterized in that at least one heating element (4, 5, 6, 9) for heating the cavity (1) is arranged within the module. In a second aspect, the invention relates to a method for manufacturing a superconducting RF module for particle accelerators according to the invention, characterized in that at least one heating element (4, 5, 6, 9) is arranged inside the module during its assembly, after which, following the evacuation of at least one cavity (1) at the accelerator site, the cavity (1) is heated and can be put into operation without interrupting the final evacuation, i.e., without refilling it with gas after heating.

Description

[0001] The present invention relates to a superconducting RF module for particle accelerators, such as those used, for example, in free-electron lasers or synchrotron radiation sources for accelerating charged particles, usually electrons or positrons. Instead of "module," the terms "cryomodule" or "structure" are also used in the literature, which are considered synonymous in the context of this application. A superconducting RF module within the meaning of the invention always comprises at least one superconducting RF cavity, the latter giving the module its name.A superconducting RF module is a device that, in addition to a cavity, comprises at least all the means required for operating the cavity, namely at least one vacuum chamber, which is part of the module and forms a chamber for accommodating all other parts of the module, means for cooling the cavity, which are usually helium baths in the form of helium chambers, evacuation means, magnetic shielding, input couplers, HOM absorbers, and devices for fine-tuning the cavities (tuners). A detailed description of superconducting RF modules within the meaning of the invention can be found in J. Knobloch (Superconducting RF: Enabling Technology for Modern Light Sources, in EJ Jaeschke et al. (eds.): Synchrotron Light Sources and Free-Electron Lasers, Springer International Publishing Switzerland 2016, 505-559).A superconducting RF cavity within the meaning of the invention is a device in which the "cavity," in its literal sense of forming a chamber, acts as a resonator for RF radiation (between 50 and 10,000 MHz). Superconducting cavities are usually made of solid niobium. Alternative materials include Nb3 Sn, NbN, and MgB2, as alternative materials for an RF layer of the cavity, e.g., on niobium or copper. Instead of "cavity," the literature also uses the terms "module" or "structure," as well as "resonator," which are to be considered synonymous in the context of this application. The use of the term "cavity" often includes at least one helium bath in which the cavity, i.e., at least the part comprising the cavities of the cavity, is located. A helium bath consists of a helium chamber equipped for filling with liquid helium and usually also capable of evacuation.The helium chamber is not permanently filled with liquid helium for cooling, but can also be filled or purged with helium or other, preferably inert gases, or evacuated. The term "helium chamber" will be used below, which always includes the chamber's function as a helium bath.

[0002] A superconducting RF resonator according to the invention is usually equipped with a helium chamber for cooling, as described, for example, by A. Schmidt and A. Matheisen (1.3 GHz Cavity Weld to Helium Vessel, TESLA Report 2007-02, 1-8). Since the cavities referred to here are superconducting at low temperatures (≤ 5 K or ≤ 18 K depending on the material), the helium chambers or similar cooling systems, such as conduction cooling with helium tubes (i.e., cooling by heat conduction) or similar, are a prerequisite for the function of the cavities and are always part of a superconducting RF module. For cavities with superconducting parts made of, for example, Nb3Sn or NbN, operation at temperatures down to 7-8 K (or even higher, since their T cclose to 18 K), which allows the use of conduction cooling (see above). The cavities are located in the helium chambers or are equipped with conduction cooling. It is assumed that the helium chambers are filled with liquid helium for cooling, but additional evacuation must always be provided to maintain a clean helium circuit. Examples of helium chambers equipped according to the invention are described, for example, by A. Schmidt and A. Matheisen (see above) and T.J. Peterson et al. (A survey of pressure vessel code compliance methods for superconducting radio frequency cryomodules, AIP Conference Proceedings 1434, 2012, 1575-1582).The cavities in question are often manufactured to form assemblies or components in which two or more cavities form a row. These assemblies are sometimes referred to collectively as a "cavity" in the singular. In this document, the term "cavity" is understood to mean a device comprising one or more cavities, and the term "cavities" also includes the singular. The latter arrangements of two or more cavities forming a row are sometimes referred to as "x-cell cavities," where "x" indicates the number of cavities in the row.

[0003] The quality of the superconducting parameters at the inner surface of the cavity, which is essential for the cavities' function as RF resonators in particle accelerators, depends on the surface state of the superconducting material from which the cavities are made. Impurities and factors that lead to defects strongly influence superconductivity and must therefore be strictly controlled to prevent deterioration. Various influences on the quality of the latter factors, as well as the influence of cavity treatment on their quality, are discussed, for example, by H. Padamsee (The science and technology of superconducting cavities for accelerators, Superconductor Science and Technology, Vol. 14, 2001, R28-R51).An important manufacturing step for superconducting RF cavities is the "baking" of the cavities, as it conditions the surface of the niobium, which directly affects the quality, i.e., the inner surface of the cavity. In the article by H. Ito et al. (Influence of furnace baking on QE behavior of superconducting accelerating cavities, Progress of Theoretical and Experimental Physics, 2021, 071G01 (8 pages)), the influence of annealing on the quality of the cavities is discussed from various perspectives.

[0004] The object of the invention is to provide a superconducting RF module for particle accelerators which enables a less complex production and a corresponding process compared to the prior art while at the same time improving the quality of the cavities.

[0005] The object is achieved by the features of claims 1 and 4. The main advantage of the invention lies in the reduction of the work steps required until the installation of a module with the cavities or the cavities together with the helium chambers, since the baking of the cavities can be carried out after their assembly at the site of use and thus after their evacuation for use has already taken place, which reduces the possibility of contamination - further filling with gases and evacuation of the cavities after baking is not required.

[0006] In a first aspect of the invention, a superconducting RF module for particle accelerators is claimed, which comprises at least one superconducting RF cavity and a means for cooling the cavity and is characterized in that at least one heating element for heating the cavity is arranged in the interior of the superconducting RF module for particle accelerators.

[0007] The invention eliminates a manufacturing step in which the cavity is placed in a furnace for annealing and then removed again before installation in a superconducting RF module for particle accelerators. The annealing can be carried out with the invention when the cavity has already been assembled with the cooling means and all other parts on site at the accelerator. Furthermore, the cavity can be evacuated before annealing and filled or purged with various atmospheres, in particular with an inert atmosphere, preferably with helium, but also with other gases such as argon or similar. Evacuating the cavity before annealing has the greatest influence on quality. According to the invention, the vacuum is maintained after annealing; further filling with gas and evacuation is not necessary. For all the latter manufacturing steps, such asFor the evacuation of the cavities before baking, the necessary devices are available on site at the accelerator, where the cavities or modules are located, in the vicinity of the accelerator.

[0008] The superconducting RF cavities used in the invention, with the exception of the inventive equipment, are known from the prior art of particle accelerators, as are the helium chambers and all other parts of the superconducting RF modules. No further adaptation to the invention, other than the inventive arrangement of at least one heating element, is required.

[0009] The heating elements used for the invention are advantageously designed as resistance heaters made of, for example, tungsten, molybdenum, etc., or ceramic heating elements such as those made of silicon carbide, boron nitride, graphite, etc. All materials that are magnetic, i.e., those that carry at least very small residual magnetization moments, should be avoided, since superconductivity is severely impaired by magnetic fields. The heating elements can be arranged directly on the outer surface of the cavities, as well as on the outer surface of the helium chamber, or on the part of the cavity that protrudes beyond the helium chamber—if present—or, for example, on the inner surface of the helium chamber, as is also the case with embodiments of the invention. The heating elements can also be arranged near the surface of the cavities, but without direct contact with it, so that the heating elements do not block cooling during normal operation of the cavity.If the heating elements are attached to the surface of the cavity, their positioning must be carefully chosen so that cooling efficiency is not compromised during normal operation. Combinations of heating element positions are possible, as are other heating element mounting locations, such as the inner surface of the helium chamber. The heating element power must be large enough to heat at least the cavity to a temperature in the range of 100°C to 400°C, although temperatures between 100°C and 300°C are sufficient for most applications. Thermal sensors, such as thermocouples, are advantageously placed on the cavity and possibly on other parts of the modules, such as the helium chamber—if present—to monitor the temperature during bakeout. The heater capacity must be estimated based on the size and volume of the cavities to be heated.The arrangement directly on the outer surface of the cavity, which can be located in the interior of the helium chamber - if present - is particularly advantageous, as the heating element is then placed where the space occupied by the heater is free, and with the heating elements directly on the cavity, the efficiency of heating the cavity is increased. Bakeout after the cavity has been mounted in the module also has the advantage that the outer surface of the cavity can be conditioned to be optimized for heat transfer when the cavity is cooled by liquid helium in the container while the cavity is in use. This is due to the fact that the natural oxide on the outer surface of the cavity (niobium pentoxide in Nb cavities) has additional thermal resistance and reduces the cooling effect of the helium.Reducing or completely removing this oxide during annealing improves thermal conductivity through the outer surface of the cavity.

[0010] According to the prior art, the cavities are baked separately before installation in the superconducting RF modules. After baking, they must be filled with air or nitrogen, for example. This is because assembling the modules and at least installing them in an accelerator requires the installation of flanges and other parts that connect the accelerator elements to the "beam" vacuum system (i.e., the system that evacuates the accelerator). Before the accelerator can be put into operation, the entire system must be evacuated. During evacuation, the baking effect diminishes. The invention advantageously eliminates the need to fill and re-evacuate the cavities after baking – they remain under constant vacuum after baking and during operation. This allows the baking effect to be fully utilized.

[0011] A second aspect of the invention is a method for manufacturing a superconducting RF module for particle accelerators, comprising the following steps: - Providing a superconducting RF cavity as part of the superconducting RF module; - Providing a means of cooling and all other parts for the construction of a superconducting RF module; - Assembling all parts for the construction of a superconducting RF module and arranging at least one heating element for heating the cavity inside the module; - Evacuation of at least the superconducting RF cavity; - Heating the cavity with the heating elements.

[0012] The following instructions must be observed when carrying out the process. In the first step, a superconducting RF cavity is created. This cavity can, for example, be one of those described above. The cavity to be created in the first step is prepared for annealing, i.e., in particular, all preparatory steps for treating the inner surface of the cavity, such as cleaning, pre-oxidation, or the like, are performed, leaving only the final step of annealing.To fully exploit the advantages of the invention, it is necessary to provide all other components for the construction of at least the superconducting RF module, which, in addition to the superconducting RF cavity, comprise at least all the means necessary for operating the cavity, namely at least one vacuum chamber, which is part of the module and represents a chamber for all other parts of the module, means for cooling the cavity, which are usually helium chambers, evacuation devices, magnetic shielding, input couplers, HOM absorbers, and devices for fine-tuning the cavities (tuners). Either a helium chamber or conduction cooling can be provided for cooling the cavity. The heating elements can, for example, be arranged either on the outer surface of the cavity, near it, or on the helium chamber. It should be noted that the sequence of steps does not necessarily have to correspond to that outlined here.All that's required is that all parts, including the cavity and heating elements, are present after the complete module is assembled. After that, the module, or at least the cavity, can be evacuated before baking, eliminating the need to refill or evacuate the cavity afterwards.

[0013] It can be assumed that the person skilled in the art is aware that many additional parts such as flanges, pipes, pumps, connections, etc. are required for a complete assembly of the module or the complete accelerator, which are omitted here for reasons of clarity and can be found, for example, in the article by J. Knobloch.

[0014] With the proposed superconducting RF module according to the invention and the method for its production shown, it was demonstrated that, in addition to the reduction of the preparation steps of the module, the performance of the cavity is improved compared to the state of the art. Example

[0015] The invention is explained by an example each for the first and second aspects of the invention and a figure illustrating the first and second aspects of the invention.

[0016] The figure shows: Fig. 1: Schematic representation of a cross-section through a superconducting RF module for particle accelerators in which different positions for the arrangement of heating elements according to the invention are shown.

[0017] In Fig.Figure 1 shows a simplified schematic cross-section of a superconducting RF module for particle accelerators according to the invention. It is simplified in that only the cavity 1 (in the example here a "3-cell cavity"), flanges 8a, 8b, the vacuum chamber 7, the helium chamber 2, the supply line 3 for the helium chamber 2 and four examples of the positioning of the heating elements 4, 5, 6, 9 are shown. All other parts have been omitted for the sake of clarity. The heating elements are shown as cross-sections of heating wires and differ in their positioning, which are to be understood as examples: 1. on the outer surface of the cavity 1, which lies within the helium chamber 2, represented by the circles filled with stripes 5; 2. on the outer surface of the helium chamber 2, represented by the open circles 4; 3. on the parts of the cavity 1 protruding from the helium chamber 2, represented by the dotted circles 6 and 4.within the helium chamber 2, represented by the square-filled circles 9. It should be noted that, although positioning the heating elements on the surface of the cavity 1 located inside the helium chamber 2 is obviously the best position in terms of heating efficiency, due to the vacuum installed before bakeout, the heating efficiency achieved by positions farther from the cavity 1, such as on the outer surface of the helium chamber 2, is not as affected as it would be in air or another gaseous medium. Consequently, positions other than the surface of the cavity 1 located inside the helium chamber 2 may be considered for other reasons, such as the simultaneous treatment of the surface of the cavity 1 during the step of baking the cavity 1, and for space and installation considerations.If necessary, different positions can be combined. Multiple heating elements can be arranged in one position, if advantageous, or multiple heating elements can be arranged in different positions. The helium chamber is shown here as an example of a cooling means in the module.

[0018] The example is an 8-cell elliptical 1.3 GHz TESLA-type RF cavity made of niobium. A titanium-helium chamber 2 is welded to the cavity. Cavity 1 is equipped with HOM absorber antennas, the field probe antenna, the main coupler, and other equipment required for the operation of Cavity 1. The tuner for the cavity (type E-XFEL) is also installed.

[0019] In this example, the outer surface of the helium chamber 2 is free of other components and can be used for the installation of the heating elements 4. Thermocouples are connected to the helium chamber 2 and other parts of the cavity 1 as thermal sensors. Regarding the latter, it should be noted that thermal sensors can also be connected to other components to monitor their temperature during the bakeout. Several resistance heaters made of molybdenum or tungsten wire are wound in a fiberglass tube around the helium chamber 2 (for more uniform heating). The helium chamber 2, with the heating elements 4 installed on it, is covered with a metal foil (e.g., aluminum) to reflect the infrared radiation, increase the heating power, and minimize the impact on the temperature of the surrounding components. The latter is not necessary but advantageous.After assembly of the module (or installation in the accelerator), the following steps are performed: - Evacuation of the volume of cavity 1; - Evacuation of the volume of helium chamber 2; - Evacuation of the volume of the vacuum chamber (insulation). After these three evacuation steps, the annealing of cavity 1 begins. During the latter step, the heating elements are supplied with power to maintain a temperature of 240°C in the helium chamber 2, in the example, and thus also to anneal cavity 1 over a period of 24 hours. Further details are described in the following example of a method according to the invention.

[0020] The following description of a method for manufacturing a superconducting RF module for particle accelerators is exemplary.

[0021] The example is explained using the following steps: 1. A prepared superconducting RF cavity 1 is brought into a clean room. 2. Auxiliary components required to seal the vacuum in Cavity 1 and others are installed at Cavity 1 in the clean room (e.g., antennas, parts of the main couplings, vacuum bellows and vacuum gate valves, etc.). 3. In this example, cavity 1 is sealed and evacuated (or filled with clean gas) so that assembly work can be carried out without the risk of contamination of the internal volume of cavity 1. The cavity is now ready for the final step of its manufacturing process, bakeout. 4. In the example, cavity 1 leaves the cleanroom to an assembly of the accelerator in which the module is to be operated and in which all components not connected to the cavity vacuum are installed (tuners, magnetic shields, temperature sensors, cooling lines, cables, etc.). 5. Next (this can also be done before step 4), cavity 1 can be connected to another cavity 1 to form a "string"—some modules have only one cavity inside, others several (up to 8 or more)—this depends on the accelerator design. Cavity 1 can be connected to another cavity 1 directly or via other components (e.g., beamlines with magnets, solenoids, or absorbers). At this stage, cavity 1 is usually placed on a rack. 6. The "strand" assembled under step 5 (or cavity 1 with all components if there is only one cavity 1 in the module) is installed with the frame into the helium chamber 2, which is located in the vacuum chamber 7 of a superconducting RF module. The cooling lines are connected. 7. The installation of the heating elements 4, 5, 6, 9 is performed in step 4 in the example, and in the example, the heating element is placed at position 9 within the helium chamber 2. Note that if the helium chamber 2 has its own connection, for example, the heating elements 4, 5, 6, 9 (heating elements within the helium chamber 2) can be installed later, once the cooling lines are connected. 8. Bakeout, the final step in the process for a superconducting RF module, can be performed either after assembly of the entire module or after installation of the module in the accelerator. It is important that the vacuum in cavity 1 is maintained after bakeout of cavity 1 to achieve the best effect of the invention. Should a break in the vacuum after commissioning of the module require further bakeout of cavity 1, the invention advantageously eliminates the need to reassemble the module. Cavity 1 can be baked again at its place of use. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited non-patent literature

[0000] J. Knobloch (Superconducting RF: Enabling Technology for Modern Light Sources, in EJ Jaeschke et al. (eds.): Synchrotron Light Sources and Free-Electron Lasers, Springer International Publishing Switzerland 2016, 505-559

[0001] A. Schmidt and A. Matheisen (1.3 GHz Cavity Weld to Helium Vessel, TESLA Report 2007-02, 1-8

[0002] T. J. Peterson et al. (A survey of pressure vessel code compliance methods for superconducting radio frequency cryomodules, AIP Conference Proceedings 1434, 2012, 1575-1582

[0002] H. Padamsee (The science and technology of superconducting cavities for accelerators, Superconductor Science and Technology, Vol. 14, 2001, R28-R51

[0003] H. Ito et al. (Influence of furnace baking on Q-E behavior of superconducting accelerating cavities, Progress of Theoretical and Experimental Physics, 2021, 071G01 (8 Seiten

[0003]

Claims

[1] Superconducting RF module for particle accelerators, comprising at least one superconducting RF cavity (1) and a means for cooling (2) for the cavity (1), characterized by that at least one heating element (4, 5, 6, 9) for heating the cavity (1) is arranged inside the module. [2] Superconducting RF module for particle accelerators according to claim 1, characterized by that the at least one heating element (4, 5, 6, 9) is arranged on the superconducting RF cavity (1). [3] Superconducting RF module for particle accelerators according to claim 1, characterized by that the cooling means is a helium chamber (2) and the at least one heating element (4, 5, 6, 9) is arranged on the helium chamber (2). [4] Superconducting RF module according to claim 1, characterized bythat the at least one heating element (4, 5, 6, 9) is formed by a resistance heater made of, for example, tungsten, molybdenum or ceramic heating elements such as those made of silicon carbide, boron nitride and graphite. [5] A method for producing a superconducting RF module for particle accelerators according to any one of claims 1 to 4, comprising at least the steps: - providing a superconducting RF cavity (1) as part of the superconducting RF module; - provision of a means for cooling (2) and all other parts for the construction of a superconducting RF module; - assembling all parts to construct a superconducting RF module and arranging at least one heating element (4, 5, 6, 9) for heating the cavity (1) inside the module; - evacuation of at least the superconducting RF cavity (1); - Heating the cavity (1) using the heating elements (4, 5, 6, 9).

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