Manufacturing method for radio-frequency cavity resonators and corresponding resonator

Abstract

A method of manufacturing a radio frequency cavity resonator, wherein said radio frequency cavity resonator comprises a tubular structure extending along a longitudinal axis, said tubular structure comprising a circumferential wall structure surrounding said longitudinal axis, one or more tubular elements and a first and a second support structure associated with each of said tubular elements, wherein said first and second support structures are provided on opposite sides of each tubular element and extend radially along a diameter of the tubular structure, wherein the method comprises producing the resonator by additive manufacturing in a manufacturing direction that is parallel to said diameter.

Claims

1. A method of manufacturing a radio frequency cavity resonator, wherein said radio frequency cavity resonator comprises: a tubular structure extending along a longitudinal axis, said tubular structure comprising a circumferential wall structure surrounding said longitudinal axis; one or more tubular elements arranged within said tubular structure, each having a bore and arranged such that the respective bore is aligned with said longitudinal axis of said tubular structure; and a first and a second support structure associated with each of said tubular elements, wherein said first and second support structures are provided on opposite sides of each tubular element and include one or more edges which extend radially along a diameter of the tubular structure between the tubular element and a corresponding one of two opposite wall structure portions of said tubular structure, wherein: the method comprises producing the entire resonator, or at least longitudinal sections thereof that are subsequently assembled to form the resonator, by additive manufacturing in a manufacturing direction that is parallel to said diameter, wherein said first support structure is produced first and said second support structure is produced thereafter; said additive manufacturing comprises forming said support structures such that: in a cross-sectional plane that is perpendicular to the longitudinal axis and includes the diameter, the width of at least said second support structure increases in radially outward direction, wherein in this cross-sectional plane, said width is the width in a direction perpendicular to the diameter of the tubular structure and such that in said cross-sectional plane, the edges of said second support structure have an average angle with respect to the diameter that is at least 25; and in a longitudinal sectional plane that includes the longitudinal axis and the diameter, at least said second support structure is formed to have a radially outer portion, in which the width increases in radially outward direction, wherein in this longitudinal sectional plane, said width is the width in longitudinal direction.

2. The method of claim 1, wherein in said longitudinal sectional plane that includes the longitudinal axis and the diameter, at least one of said support structures has a middle portion in which the width of said support structure assumes its minimum value, wherein in this longitudinal sectional plane, said width is the width in longitudinal direction.

3. The method of claim 1, wherein in said longitudinal sectional plane that includes the longitudinal axis and the diameter, at least said first support structure has a radially inner portion, in which the width increases in radially inward direction, wherein in this longitudinal sectional plane, said width is the width in longitudinal direction.

4. The method of claim 1, wherein at the radially outward end of the radially outer portion of at least said second support structure, where the second support structure reaches said circumferential wall of said tubular structure, the longitudinal width is such that an adjacent support structure associated with an adjacent tubular element in the finished resonator touch each other or are less than 5 mm apart from each other.

5. The method of claim 1, wherein a continuous transition is formed between the radially outward ends of the radially outer portions of at least adjacent second support structures, wherein in said longitudinal sectional plane, the transition forms a transition edge, and wherein the radius of curvature of said transition edge at the position where the tangent is parallel to the longitudinal axis is 8 mm or less.

6. The method of claim 1, wherein in said cross-sectional plane, the edges of at least said second support structure have an average angle with respect to the diameter that is at most 60.

7. The method of claim 1, wherein in said cross-sectional plane, the edges of one or both of said first and second support structures are straight along at least 70% of their length.

8. The method of claim 1, wherein in said longitudinal sectional plane the minimum value of the width of one or both of said first and second support structures is less than 40% of the longitudinal length of the corresponding tubular element.

9. The method of claim 3, wherein the radial length of said radially outer portion of one or both of said first and second support structures is longer than the radial length of their respective radially inner portion.

10. The method of claim 3, wherein in said longitudinal sectional plane, the edges of the radially inner portions of one or both of said first and second support structures are straight or concave.

11. The method of claim 1, wherein in said longitudinal sectional plane, the edges of the radially outer portions of one or both of said first and second support structures are straight or convex.

12. The method of claim 1, wherein a duct for carrying cooling fluid is formed in said support structures.

13. The method of claim 12, wherein the ducts of two support structures associated with a same tubular element are connected with each other, and wherein each of said support structures comprises a first duct and a second duct, wherein the first ducts and the second ducts of the support structures are connected with each other via a first cavity and a second cavity provided in said tubular element, respectively, wherein said first and second cavities are arranged on opposite sides of said bore.

14. The method of claim 1, wherein said resonator is made from high purity copper having a copper content of 99.9% or more.

15. The method of claim 1, wherein said resonator has between 3 and 10 tubular elements.

16. The method of claim 1, wherein said resonator is a resonator for or in a drift-tube linear accelerator (DTL), a side coupled DTL, a coupled cavity DTL, a coupled cavity linear accelerator or a buncher.

17. The method of claim 1, wherein said additive manufacturing is based on one of electron beam melting, selective laser sintering, and selective laser melting.

Description

SHORT DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a perspective view of a section of a prior art drift tube accelerator.

(2) FIG. 2a-c show various views of a pair of prefabricable annular segments of a prior art drift tube accelerator.

(3) FIG. 3a is a sectional view of a tubular structure with two enlarged portions schematically illustrating the limitation of additive manufacturing of overhangs.

(4) FIG. 3b-c show to schematic illustrations of overhanging individually printed layers.

(5) FIG. 3d-e shows schematic illustrations explaining the limitation of additive manufacturing of arcuate structures

(6) FIG. 4a-d show various views of virtual slices of an RF cavity resonator according to an embodiment of the invention.

(7) FIG. 5a-d show various views of virtual slices of an RF cavity resonator according to another embodiment of the invention, including ducts for cooling fluid.

(8) FIG. 6a-e show various views of an RF cavity resonator according to an embodiment of the invention.

(9) FIG. 7a-e show various views of an RF cavity resonator of an alternative design.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) It is to be understood that both the foregoing general description and the following description are exemplary and explanatory only and are not restrictive of the methods and devices described herein. In this application, the use of the singular may include the plural unless specifically stated otherwise. Also, the use of or means and/or where applicable or unless stated otherwise. Those of ordinary skill in the art will realize that the following description is illustrative only and is not intended to be in any way limiting. Other embodiments will readily suggest themselves to such skilled persons having the benefit of this disclosure. Reference will now be made in detail to various implementations of the example embodiments as illustrated in the accompanying drawings. The same reference signs will be used to the extent possible throughout the drawings and the following description to refer to the same or like items.

(11) With reference to FIG. 4a to FIG. 6e, a RF cavity resonator 42 and its manufacturing method will be described. The full RF cavity resonator 42 is only shown in FIG. 6a-e, while FIG. 4a-d and FIG. 5a-d only show virtual slices of the RF cavity resonator 42 from which the structure of its elements or components are more apparent. The full RF cavity resonator 42 is shown to have only three drift tubes 28, corresponding to a comparatively short prototype that was recently built, but it goes without saying that larger numbers, for example 6 to 8 or even 10 or more drift tubes 28 could be included in a resonator of very similar design but larger longitudinal length. The present invention is not limited to any number of drift tubes or other tubular elements.

(12) Moreover, the entire resonator 42 is monolithically 3D-printed, and it is therefore apparent that there is no physical boundary between any of the individual components or portions thereof that will be discussed below. Reference to individual components or portions is mainly made for explaining the geometric structure of the resonator. In the drawings, individual portions/components may be delimited from each other in the figures by dashed lines.

(13) The resonator 42 has a tubular structure extending along a longitudinal axis 26 and comprising a circumferential wall structure 44 surrounding said longitudinal axis 26. The inner circumferential shape of the wall structure 44 is circular, whereas the outer circumferential shape is octagonal.

(14) Within the tubular structure of the resonator 42, three tubular elements, in the shown embodiment drift tubes 28 are arranged. Each drift tube 28 has a bore 30 which is aligned with the longitudinal axis 26 of the tubular structure. Two support structures 32 are associated with each of said drift tubes 28. The support structures 32 are provided on opposite sides of each drift tube 28 and extend radially along a diameter 34 of the tubular structure between the drift tube 28 and a corresponding one of two opposite portions of the circumferential wall structure 44 of said tubular structure. The diameter 34 also indicates the manufacturing direction for the additive manufacture of the RF cavity resonator 42. In the embodiment shown in FIG. 4a to FIG. 6e, said two support structures 32 associated with each drift tube 28 are symmetrical with respect to the longitudinal axis 26, such that no distinction between first and second support structures will be made in the following description. It is, however, to be understood that the first support structure, i.e. the support structure that is arranged lower in vertical manufacturing direction and which is hence manufactured first could have a simplified geometry, for example the geometry shown with reference to an alternative design at reference sign 78 in FIG. 7a-e.

(15) As was explained in the summary of the invention above, the support structures 32 have a special geometry that enables the printability of the RF cavity resonator 42 as a whole. FIGS. 4a-d and 5a-d show two embodiments with slightly different support structures 32, which however mainly agree in the general features discussed above.

(16) FIGS. 4a and 5a show a cross-sectional view of a slice of the resonator 42 in a cross-sectional plane that is perpendicular to the longitudinal axis 26 and includes the diameter 34. As is seen in both FIGS. 4a and 5a, the width of the support structures 32 increases in radially outward direction, wherein in this cross-sectional plane, said width is the width in a direction perpendicular to the diameter of the tubular structure.

(17) As is further seen FIGS. 4a and 5a, in this cross-sectional plane, the edges 46 of the support structures 32 are straight and have an angle with respect to the diameter 34 that is approximately 45.

(18) This angle can also be chosen differently, but for the reasons given above, it should preferably be at least 25, more preferably at least 30 and most preferably at least 35. The angle , which is defined with respect to the diameter 34 and hence the manufacturing direction, is complementary to the overhang angle that is likewise shown in FIGS. 4a and 5a. The angle should be chosen large enough such as to support a sufficiently large circumferential part at the top of the circumferential wall structure 44, where the term top again is understood with respect to the vertically upward manufacturing direction, and is also the top in FIGS. 4a and 5a. At the same time, the angle should not be too large such that the overhang angle does not become too small, to ensure that the support structure 32 can be printed with sufficient precision. A further reason why the angle should not be too large is that the support structure 32 would otherwise block too much of the cavity space and reduce the Q factor. Accordingly, the angle should preferably be at most 60, more preferably at most 52 and most preferably at most 45.

(19) FIG. 4b and FIG. 5b each show a longitudinal sectional view of a slice of the resonator 42 in a longitudinal sectional plane that includes the longitudinal axis 26 and the diameter 34. In this longitudinal sectional plane, each of the support structures 32 has a middle portion 48 in which the width of the support structure 32 assumes its minimum value, a radially inner portion 50, in which the width increases in radially inward direction, and a radially outer portion 52, in which the width increases in radially outward direction. In this longitudinal sectional plane, the width is the width in longitudinal direction.

(20) As is seen from the figures, with this geometry, the minimum value of the width of the support structure 32 can be much less than the longitudinal length of the corresponding drift tube 28. This reduced width of the support structure 32 in longitudinal direction allows limiting the space occupied by the support structure, to thereby increase the fraction of the unoccupied cavity and allows for an increased Q factor, as is readily seen in FIG. 4b and FIG. 5b. The precise dimensions can be somewhat different from what is shown in the figures, but in preferred embodiments, the minimum value of the width of the support structure 32 is less than 50%, preferably less than 40%, more preferably less than 30% and most preferably less than 20% of the longitudinal length of the corresponding drift tube 28.

(21) As is further seen in FIG. 4b, at the radially outward ends of the radially outer portions 52 of said support structures 32, where the support structures 32 reach said circumferential wall structure 44 of said tubular structure, the longitudinal width is such that adjacent support structures 32 associated with adjacent drift tubes 28 in the resonator 42 touch each other. The same would apply for the radially outward portions 52 of the embodiment of FIG. 5b, but in this figure, only one virtual slice is shown. It is therefore seen that the upper part of the tubular wall structure 44, i.e. the part corresponding to the part that is schematically emphasized with the upper thick line 40 in FIG. 3a, is supported by the radially outer portions 52 of the support structures 32 over its entire longitudinal length.

(22) With further reference to FIG. 4b, it is seen that a continuous transition is formed between the radially outward ends of the radially outer portions 52 of adjacent support structures 32. In the longitudinal sectional plane depicted in FIG. 4b, the transition forms an arcuate transition edge 54. At the apex of the arc formed by said arcuate transition edge 54, or in other words, at the position where the tangent of the transition edge 54 is parallel to the longitudinal axis 26, a comparatively small radius of curvature is formed. This radius of curvature should be chosen small enough to allow for the formation of the desired structure without deformations of the type schematically illustrated in FIG. 3e. In preferred embodiments, this radius of curvature is 8 mm or less, but preferably it is even smaller, such as 6 mm or less or even 4 mm or less.

(23) As was pointed out in the summary of the invention above, it is preferred but not necessary that the longitudinal width of the outermost portions of the radially outer portion 52 is large enough such that adjacent outermost portions 52 touch each other. Instead, small longitudinal gaps might be formed in between that are chosen small enough such that the upper portion of the circumferential wall structure 44 of the tubular structure is still sufficiently supported. The longitudinal width of these gaps should be no more than 5 mm, preferably no more than 2.5 mm, to still allow for manufacturing with desired precision.

(24) FIG. 5a to FIG. 5d show various views of a slice of resonator 42 similar to those of FIGS. 4a to 4d. With respect to the fundamental features discussed above, both embodiments are in agreement with each other. In particular, in both embodiments, in the longitudinal sectional plane shown in FIG. 4b and FIG. 5b, the edges 56 of the radially inner portions 50 are concave. This shape has been found particularly useful for obtaining a high Q factor. However, in other embodiments, this edge could also be straight.

(25) In the embodiment of FIG. 4, particularly seen in the longitudinal sectional plane shown in FIG. 4b, the edges 58 of the radially outer portions 52 are slightly convex, while in the embodiment of FIG. 5, they are straight along almost the entire length, i.e. up to the region of the continuous transition discussed above. Both variants have been found to give good results.

(26) The most pronounced difference between the embodiment of FIG. 4 and FIG. 5 is that in the embodiment of FIG. 5, in each of the support structures 32, a first duct 60a and a second duct 60b for carrying a cooling fluid is formed. It is one of the great advantages of the additive manufacturing that these ducts 60a, 60b can be readily formed in the manufacturing process. In contrast to this, forming these ducts 60a, 60b in prior art resonators in which individual slices are formed by machining would be much more cumbersome, particularly in case of small diameter RF cavity resonators 42, where the support structures 32 are rather delicate.

(27) In the embodiment shown, the first ducts 60a and the second ducts 60b of two support structures 32 associated with a same drift tube 28 are connected with each other via a a corresponding first cavity 62a and second cavity 62b, respectively, both of which being provided in said drift tube 28. The first and second cavities 62a, 62b are arranged on opposite sides of said bore 30, allowing for highly efficient cooling of the drift tube 28.

(28) As was indicated above, FIG. 6a to FIG. 6e show various views of a complete radiofrequency cavity resonator 42 including three drift tubes 28 with corresponding support structures 32 formed according to the virtual slices shown in FIG. 4a to FIG. 4d. Also shown in FIG. 6a are three openings 64 for coupling power into and out of the cavity and frequency tuning purposes, as is known to the skilled person.

(29) The resonator 42 shown in FIG. 6a-e has been made as a first prototype in one piece by additive manufacturing, in this case by selective laser sintering using powder of highly pure copper, with a copper content of 99.9% or more, and immediately gave a Q-factor of 6000. This is only moderately reduced over the theoretical value for a simulated structure with perfect surface quality, which provide a Q-factor of about 8000. The somewhat lower Q-factor is attributable to a certain degree of surface roughness due to the additive manufacturing process. The surface roughness can be improved by additional surface treatment, such as conventional surface polishing, but the experience of the inventors shows that this will typically be dispensable. Instead, even the Q-factor obtained in the prototype, where the manufacturing had not been optimized yet, was found to be already sufficient for its intended use as buncher or accelerator structure. Meanwhile, the inventors have found that with the same general geometry, but optimized parameters with regard to distance between drift tubes 28, diameter of the cavity, diameter of the drift tubes 28, specific choice of angles and radii of curvature, the Q-factor can be raised to values similar to those of the conventional manufacturing method.

(30) While the first prototype had only three drift tubes 28, a similar design can be used for a longer RF cavity resonator 42 having a larger number of drift tubes 28, such as 5 to 10 drift tubes 28. In principle, larger structures can likewise be printed in a single piece, as long as the size of the additive manufacturing apparatus allows for this. In the alternative, it is possible to print a number of longitudinal sections of the RF cavity resonator 42 separately and assemble them afterwards, for example by brazing or electron beam welding. These longitudinal sections should be made as large as possible, and preferably include at least two, preferably at least three tubular structures 28 and their corresponding support structures 32 each.

(31) With reference to FIGS. 7a to 7e, a radio frequency cavity resonator 70 according to an alternative design is shown. Same reference signs are used for similar or like features as shown in the previous figures. The radio frequency cavity resonator 70 comprises a vessel structure extending along a longitudinal axis 26. The vessel structure comprises a circumferential wall structure 72 surrounding said longitudinal axis 26.

(32) FIG. 7a shows a perspective view of the entire resonator 70. FIG. 7b is a cross-sectional view of the resonator 70, wherein the cross-sectional paper plane is perpendicular to said longitudinal axis 26. Also shown in FIG. 7b are a longitudinal vertical sectional plane 74, which is a plane that is parallel to a vertically upward manufacturing direction and includes the longitudinal axis 26, and a longitudinal horizontal sectional plane 76, which is a plane that is perpendicular to the vertically upward manufacturing direction and includes the longitudinal axis 26.

(33) FIG. 7c shows a top view onto the resonator 70, in which elements covered by the upper part of the wall structure 70 are shown with hatched lines.

(34) FIG. 7d is a longitudinal horizontal sectional view along the arrows B-B in FIG. 7b. In other words, the longitudinal horizontal sectional plane 76 of FIG. 7b corresponds to the paper plane of FIG. 7d. FIG. 7e is a longitudinal vertical sectional view along the arrows A-A in FIG. 7c, i.e. the longitudinal vertical sectional plane 74 of FIG. 7b corresponds to the paper plane of FIG. 7e.

(35) As is seen in the Figures, three tubular elements 28, in the particular embodiment drift tubes 28, are arranged within the vessel structure, each having a bore 30 and arranged such that the respective bore 30 is aligned with said longitudinal axis 26. However, different from the previous embodiments, a single support structure 78 is associated with each of said drift tubes 28 only. Each support structure 78 has a first end 80 attached to a portion of said circumferential wall structure 72 and a second end 82 attached to said drift tube 28.

(36) The entire resonator 70 is suitable for producing by additive manufacturing in a vertically upward manufacturing direction, which is the upward direction in FIGS. 7b and 7e. The vessel structure has a bottom portion with respect to the vertically upward manufacturing direction, at which said first end 80 of said support structure 78 is formed, and an upper portion 84, in which inner surface portions of said wall structure 72 on both sides of a longitudinal vertical sectional plane 74 converge towards each other in vertically upward direction. The upper portion 84 is everything shown above the longitudinal horizontal sectional plane 76 shown in FIG. 7b. The converging portions of the wall structure 72 on both sides of the longitudinal vertical section plane 74 form what is referred to herein as a pitched roof-type structure.

(37) Note that below a further horizontal plane 86 shown in FIG. 7b, the inner surface of the wall structure 72 has a cylindrical shape. At the horizontal plane 86, the slope of the inner surface of the wall structure 72 with respect to the horizontal planes 76 or 84 reaches a lower boundary value, which in this embodiment is 45. This slope is shown as the overhang angle in FIG. 7b. Above the horizontal plane 86, this slope is kept constant, leading to the triangular or pitch roof-type structure. Note that this specific choice of the slope is only exemplary, and that the slope may change in a different manner, as long as it does not fall below a certain threshold value. In the shown preferred embodiment, this threshold value has been chosen to be 45, but in other embodiments, it may be 38, or even 30. This way, it is possible to produce the top portion 84 of the vessel structure 70 by additive manufacturing, without having to provide an additional, second support structure like the upper support structures 32 in the previous figures.

(38) It is seen in FIG. 7b that the width of the support structure 78 in the cross-sectional plane (paper plane of FIG. 7b) is constant. However, this is not mandatory, and it would e.g. be possible to provide a radially inner portion in which the width increases in radially inward direction and a radially outer portion in which the width increases in radially outward direction, similar as in the previous embodiments.

(39) In FIG. 7e, it is seen that at the second end 82 of the support structure 78, a radially inner portion is formed, in which the width in the vertically upward sectional plane increases in radially inward direction. However, similar to the support structures 32 shown in the previous embodiments, it will also be possible to provide for a radially outward portion, in which the width would increase in radially outward direction.

(40) While the present invention has been described in terms of specific embodiments, it is understood that variations and modifications will occur to those in the art, all of which are intended as aspects of the present invention. Accordingly, only such limitations as appear in the claims should be placed on the invention.