Superconducting RF module for particle accelerators

Abstract

Superconducting RF module for radiation between 50 and 10000 MHz for particle accelerators, comprising at least one superconducting RF cavity (1) and a means for cooling (2) for the RF cavity (1), characterized in that at least one heating element (4, 5, 6, 9), formed by a resistance heater made of heating wires, in particular made of tungsten or molybdenum, is arranged inside the RF module at the superconducting RF cavity (1) for heating the RF cavity (1).

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. In the literature, the terms "cryomodule" or "structure" are also used instead of "module" and 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 necessary for the operation of the cavity, namely at least one vacuum chamber, which is part of the module and forms a chamber for receiving 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 (tuning) the cavities. A detailed description of superconducting RF modules according to 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, acts as a chamber forming a resonator for RF radiation (between 50 and 10,000 MHz). Superconducting cavities are usually made of solid niobium. Alternative materials are Nb3Sn, NbN, and MgB2, as alternative materials for an RF layer of the cavity, e.g., on niobium or copper. The terms "module" or "structure," as well as "resonator," are also used in the literature instead of "cavity" and are to be considered synonymous in the context of this application. The use of the term "cavity" often includes at least a 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 is usually also evacuatable.The helium chamber is not constantly filled with liquid helium for cooling, but can also be filled or purged with helium or other, preferably inert, gases, or be evacuated. In the following text, the term "helium chamber" will be used to encompass 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), 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 up to 7-8 K (or even higher, since their Tc) is possible. c(near 18 K) is possible, 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 an evacuation system 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 TJ 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 in such a way that assemblies or components are formed in which two or more cavities form a row. These assemblies are sometimes also referred to collectively as a "cavity" in the singular. In this document, the term "cavity" is to be understood as 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 also referred to as an "x-cell cavity," where "x" indicates the number of cavities in the row.

[0003] The quality of the superconductivity parameters at the inner surface of the cavity, which is essential for the function of the cavities as RF resonators in particle accelerators, is related to the surface condition of the superconducting material from which the cavities are made. Impurities and factors that lead to defects strongly affect the superconductivity and must therefore be strictly controlled to prevent a deterioration of the superconductivity. Various influences on the quality with respect to these latter factors, as well as the influence of cavity treatment on its 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 this conditions the surface of the niobium, directly affecting the quality, i.e., the internal surface area of ​​the cavity. 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)) discusses the influence of baking on cavity quality from various perspectives.

[0004] The object of the invention is to provide a superconducting RF module for particle accelerators which enables a less complex manufacturing process compared to the prior art, while simultaneously improving the quality of the cavities.

[0005] The problem is solved by the features of claim 1. The main advantage of the invention lies in the reduction of the work steps required to install a module with the cavities or the cavities together with the helium chambers, since the cavities can be baked out after their assembly at the place of use and thus after they have already been evacuated for use, which reduces the possibilities of contamination - further filling with gases and evacuation of the cavities after baking out is not necessary.

[0006] The aspect of the invention is a superconducting RF module for particle accelerators, comprising at least one superconducting RF cavity and a means for cooling the cavity, and characterized in that at least one heating element for heating the cavity, formed by a resistance heater made of heating wires made of tungsten or molybdenum, is arranged inside the superconducting RF module for particle accelerators.

[0007] The invention eliminates a manufacturing step in which the cavity must be placed in a furnace for baking and then removed again before installation in a superconducting RF module for particle accelerators. With the invention, baking can be carried out when the cavity is already assembled on-site at the accelerator with the cooling system and all other components. Furthermore, the cavity can be evacuated and filled or purged with various atmospheres before baking, particularly an inert atmosphere, in any case helium, but also other gases such as argon or similar. Evacuating the cavity before baking has the greatest impact on the quality. According to the invention, the vacuum is maintained after baking, and further filling with gas and evacuation are unnecessary. For all the latter manufacturing steps, such as...The evacuation of the cavities before heating requires the necessary equipment to be available on site at the accelerator, where the cavities or modules are located, and in the vicinity of the accelerator.

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

[0009] The heating elements used for the invention are advantageously formed as resistance heaters made of tungsten or molybdenum heating wire. All materials that are magnetic, i.e., that possess at least very small residual magnetization moments, are to be avoided, since superconductivity is severely impaired by magnetic fields. The heating elements can be arranged directly on the outer surface of the cavities, on the outer surface of the helium chamber, or on the portion of the cavity that projects beyond the helium chamber (if present), or, for example, on the inner surface of the helium chamber. 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 obstruct cooling during normal operation of the cavity.If the heating elements are mounted on the surface of the cavity, their positioning must be carefully chosen to ensure that cooling efficiency during normal operation is not compromised. Combinations of positions for the heating elements are possible, as are other mounting locations, such as the inner surface of the helium chamber. The heating element power must be sufficient to heat 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 heating capacity must be estimated based on the size and volume of the cavities to be heated.A particularly advantageous arrangement is to place the heating element directly on the outer surface of the cavity, which can be located within the helium chamber (if present). This position places the heating element in a location where the space occupied by the heater is free, and the efficiency of heating the cavity is increased. Furthermore, heating the cavity after its assembly in the module has the advantage of conditioning its outer surface for optimal heat transfer when the cavity is cooled by liquid helium in the container during operation. This is because the natural oxide on the outer surface of the cavity (niobium pentoxide in the case of Nb cavities) presents additional thermal resistance and reduces the cooling effect of the helium.Reducing or completely removing this oxide during the bake-out process improves the thermal conductivity through the outer surface of the cavity.

[0010] According to the prior art, the cavities are baked out separately before being installed in the superconducting RF modules. After baking, they must be filled with, for example, air or nitrogen, because flanges and other parts connecting the accelerator elements to the "beam" vacuum system (i.e., the system that evacuates the accelerator) must be arranged for the assembly of the modules and at least their installation in an accelerator. This is necessary because the entire system must be evacuated before the accelerator can be started up. During evacuation, the effect of the baking out diminishes. The invention advantageously eliminates the need to fill and re-evacuate the cavities after baking out—they remain under constant vacuum after baking and during operation. Thus, the effect of the baking out is fully utilized.

[0011] The invention relates to a method for manufacturing a superconducting RF module for particle accelerators, comprising the following steps: - Provision of a superconducting RF cavity as part of the superconducting RF module; - Provision of a means for cooling and all other parts for the construction of a superconducting RF module; - Assembly of all parts for the construction of a superconducting RF module, including the arrangement of 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 for carrying out the procedure. In the first step, a superconducting RF cavity is prepared. This cavity can be, for example, one of those described above. The cavity to be prepared in the first step is then prepared for bake-out; that is, all preparatory steps for treating the inner surface of the cavity, such as cleaning, pre-oxidation, or the like, are carried out, so that only the final bake-out step remains.To fully exploit the advantages of the invention, it is necessary to provide all further components for the construction of at least the superconducting RF module. These components, in addition to the superconducting RF cavity, include at least all means necessary for the operation of the cavity, namely at least one vacuum chamber, which is part of the module and serves as a chamber for all other parts of the module; cooling means for the cavity, which are usually helium chambers; evacuation devices; magnetic shielding; input couplers; HOM absorbers; and fine-tuning devices (tuners) for the cavity. 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, in its vicinity, or on the helium chamber. It should be noted that the sequence of steps does not necessarily have to correspond to that outlined here.It is only necessary that all parts, including the cavity and the heating elements, are present after the complete module has been assembled. Afterwards, the module, or at least the cavity, can be evacuated before the cavity is baked out, so that the cavity does not need to be refilled or evacuated afterwards.

[0013] It can be assumed that the expert is aware that many other parts such as flanges, pipes, pumps, connections, etc. are required for the complete assembly of the module or the entire accelerator; these are omitted here for the sake of clarity and can be read about, for example, in the article by J. Knobloch.

[0014] With the proposed superconducting RF module according to the invention and the demonstrated method for its production, it could be demonstrated that, in addition to reducing the preparation steps of the module, the performance of the cavity is improved compared to the prior art. Example of implementation

[0015] The invention is explained by means of an example and a figure.

[0016] The figure shows: Fig. 1: Schematic representation of a cross-section through a superconducting RF module for particle accelerators in which the 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 this example 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 are omitted for clarity. The heating elements are shown as cross-sections of heating wires and differ in their positioning, which is to be understood as an example: 1. on the outer surface of the cavity 1, which lies inside the helium chamber 2, represented by the strip-filled circles 5; 2. on the outer surface of the helium chamber 2, represented by the open circles 4; 3. at the parts of cavity 1 protruding from helium chamber 2, represented by circles 6 and 4 filled with dots.within helium chamber 2, represented by the circles 9 filled with squares. It should be noted that, although positioning the heating elements on the surface of cavity 1, which lies within helium chamber 2, is obviously the best position in terms of heating efficiency, the heating efficiency achieved by positions further away from cavity 1, such as on the outer surface of helium chamber 2, is not as affected as it would be in air or another gaseous medium due to the vacuum installed before baking. Consequently, positions other than those on the surface of cavity 1, which lies within helium chamber 2, can be considered from other perspectives, such as the simultaneous treatment of the surface of cavity 1 during the baking process and from aspects of space and installation.Different positions can be combined if necessary. Multiple heating elements can be arranged in one position if advantageous, or they can be arranged in different positions. The helium chamber is shown here as an example of a cooling means within the module.

[0018] The example involves an 8-cell elliptical 1.3 GHz RF cavity of type TESLA, 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. In this example, the outer surface of helium chamber 2 is free of other components and can be used for the installation of heating elements 4. Thermocouples are connected to helium chamber 2 and to other parts of cavity 1 as temperature sensors. Regarding the latter, it should be noted that temperature sensors can also be connected to other components to monitor their temperature during the bake-out process. Several resistance heaters (for more even heating) made of molybdenum or tungsten wire are wound around the helium chamber 2 in a fiberglass tube.Helium chamber 2, with its installed heating elements 4, is covered with a metal foil (e.g., aluminum) to reflect infrared radiation, increase heating efficiency, and minimize the impact on the temperature of surrounding components. While not strictly necessary, this is advantageous. After assembling the module (or installing it in the accelerator), the following steps are performed: evacuation of cavity 1; evacuation of helium chamber 2; and evacuation of the vacuum chamber (insulation). Following these three evacuation steps, cavity 1 is heated. During this final step, the heating elements are energized to maintain a temperature of 240 °C in helium chamber 2, thus heating cavity 1 for a period of 24 hours.Further details are described in the following example of a method for manufacturing the invention.

[0019] The following description of a method for manufacturing a superconducting RF module for particle accelerators is exemplary.

[0020] The example is explained using the following steps: 1. A provided superconducting RF cavity 1 is brought into a cleanroom. 2. Auxiliary components required for sealing the vacuum in cavity 1 and other items are installed at cavity 1 in the cleanroom (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 the assembly work can be carried out without the risk of contaminating the internal volume of cavity 1. The cavity is now ready for the final step of its manufacture: bake-out. 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's design. Cavity 1 can be connected directly or via other components (e.g., beam tubes with magnets, solenoids, or absorbers). At this stage, cavity 1 is usually placed on a frame. 6. The “strand” assembled in step 5 (or cavity 1 with all components if there is only one cavity 1 in the module) is installed with the frame in 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 heating elements 4, 5, 6, 9 is carried out in step 4 of the example, and in this example, the heating element is positioned at position 9 inside helium chamber 2. It should be noted that if helium chamber 2 has its own connection, for example, heating elements 4, 5, 6, 9 (heating elements inside helium chamber 2) can be installed later, once the cooling lines are connected. 8. Bake-out, the final step of 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 bake-out to achieve the best effect of the invention. Should a further bake-out of cavity 1 be necessary due to a vacuum breach after the module's commissioning, reassembly of the module is advantageously unnecessary according to the invention. Cavity 1 can be baked out again at its point of use.

Claims

[1] Superconducting RF module for radiation between 50 and 10000 MHz for particle accelerators, comprising at least one superconducting RF cavity (1) and a means for cooling (2) the RF cavity (1), characterized by , that at least one heating element (4, 5, 6, 9) formed by a resistance heater made of heating wires, in particular made of tungsten or molybdenum, is arranged for heating the RF cavity (1) inside the RF module at the superconducting RF cavity (1). [2] Superconducting RF module for radiation between 50 and 10000 MHz for particle accelerators according to claim 1, characterized by , that the means for cooling is a helium chamber (2).

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