WO2018211282A1

WO2018211282A1 - Radio frequency cavities

Info

Publication number: WO2018211282A1
Application number: PCT/GB2018/051343
Authority: WO
Inventors: Graeme Burt; Samantha PITMAN; Anastasiya SOLODKO
Original assignee: Lancaster University
Current assignee: Lancaster University
Priority date: 2017-05-17
Filing date: 2018-05-17
Publication date: 2018-11-22
Anticipated expiration: 2019-11-17
Also published as: EP3626032A1; GB201707914D0

Abstract

The present invention relates to a device comprising a plurality of side-coupled radio frequency cavities. The device comprises multiple sections, each section having a main axial cavity portion and the sections being stacked in an axial direction such that the main axial cavity portions are axially aligned to form a main axial cavity through the device. A plurality of the sections comprise a partition extending across the main axial cavity such that the partitions are axially spaced along the main axial cavity of the device. The device comprises a side cavity for each partition and an aperture coupling each side cavity to the main axial cavity, each aperture being adjacent its respective partition and interfaces between the sections of the device are aligned through the apertures.

Description

Title - RADIO FREQUENCY CAVITIES

The present invention relates to improvements in the manufacture of side-coupled radio frequency (RF) cavities.

In conventional cavities, each cell is aligned around a common central axis. In side-coupled cavities, alternate cells are offset from the central axis. Side-coupled cavities operate in a standing wave configuration with a ninety-degree phase shift between adjacent cells so that the offset cells have zero amplitude. In this configuration there are equal numbers of modes above and below the resonance mode, such that their contributions cancel out. Side-coupled cavities may be used in particle accelerators, for example for security and medical applications.

It may be recalled from electromagnetic theory that the magnetic field strength adjacent to a convex corner (or convex edge) of a conductor becomes stronger as the radius of curvature is reduced, so that sharp edges and corners imply high field strength. Thus, in side-coupled RF cavities the magnetic fields are larger around the point at which the axial cavities and side cavities are coupled, and this limits the maximum electric field. Theoretically, this problem may be addressed by careful rounding any corners or sharp edges during manufacture. However, using current manufacture techniques this is difficult to implement.

Cavities are normally manufactured from pieces of copper, either circular or rectangular slabs or disks. These are machined, often using computer-controlled cutting equipment to make a series of sections, and then these are brazed together.

Another known technique is separate manufacture of the axial cavities and side cavities, and then attaching them via multiple brazing steps.

Higher frequencies require smaller cavities, and current designs and means of manufacture impose limits on operational frequency and power. Using current techniques, it is virtually impossible to machine the coupling slots with sufficiently rounded edges to high precision. This may result in sharp edges which enlarge the surface magnetic fields leading to undesirable surface heating and hence damage. Alternatively, low-precision manufacture may result in inaccurate dimensions leading to perturbation of the electromagnetic field and consequentially each cell would have a different resonant frequency making the field unbalanced in each cell.

It is one objective of the present invention to overcome deficiencies of the prior art, whether or not mentioned here.

The present invention concerns improvements to the manufacture of side-coupled RF cavities. A new organisation of component parts is proposed, such that the component parts may be manufactured to high precision, e.g. so as to reduce the radius of curvature at corners, and/or more easily join the component parts together to make a finished device.

According to a first aspect of the present invention, there is provided a device comprising a plurality of side-coupled radio frequency cavities where the device is manufactured in sections and the boundaries between sections are aligned through the apertures/slots coupling the side cavities to the axial cavities.

The device may comprise a main axial cavity, and one or more side cavity. Each of the cavities, or the cavity portions within each section, may be substantially circular in plan/section and/or cylindrical in form.

A plurality of side cavities may be arranged such that they are located on different/opposing sides of the main axial cavity. A first and second side cavity may be arranged such that they both have a side that at least partially overlaps a portion of different/opposing longitudinal sides of the main axial cavity. Any, any combination, or all of the side cavities may have parallel central axes, e.g. parallel with the axis of the main cavity. The device may comprise a plurality of cells stacked/arranged in series. Each cell may comprise at least three sections stacked/arranged in series to form the cell, where each of the sections comprises a cross section of the main and one or more side cavities. The at least three sections may comprise two cell end portions and at least one centre section. Each of the plurality of cells comprises a partition and opposing main cavity portions on each side thereof coupled by at least one side cavity.

The location of successive side cavities along the device may alternate between opposing sides of the main axial cavity.

Adjacent cells may share a portion of a common aperture and/or side cavity.

Each section or cell may comprise at least one abutment surface arranged to abut a corresponding abutment surface on the adjacent section or cell. The opposing abutment surfaces may be correspondingly shaped, e.g. having the same sectional profile. The sectional profile of the abutment surface may define a profile of the side cavity. Where the section does not comprise a partition, the section may comprise a shoulder formation adjacent its abutment surface (i.e. adjacent the interface with the adjacent section) so as to define an extent of the aperture.

Each of the portions may be joined by braze joints and/or diffusion bonded joints.

The device may be a particle accelerator. The particle accelerator may be a linear particle accelerator. An electromagnetic field may be provided to each of the cells, or to the whole device in use. The electromagnetic field may be an oscillating electromagnetic field. The electromagnetic field may oscillate at radio frequency. The device may operate so as to accelerate a beam of electrically charged particles. The particles may be accelerated to energies between 1 MeV and 500 MeV. According to a second aspect of the present invention, there is provided a device comprising at least two adjacent cavity portions of a common main cavity coupled by at least one side cavity to form at least one side-coupled radio frequency cavity, wherein the at least one side-coupled radio frequency cavity is manufactured in sections, and wherein at least one of the boundaries between the sections divides at least one slot at which the adjacent cavity portions are coupled by the at least one side cavity.

According to a third aspect of the invention, there is provided a section of a linear accelerator device according to the first or second aspect.

Practicable embodiments of the invention will now be described in further detail, with reference to the accompanying drawings, of which: Figure 1 is a simplified schematic section (not to scale) through a series of side- coupled cavities.

Figure 2 is an enlarged view of the schematic of Figure 1 . Figure 3 is a device assembled according to the schematic of Figure 1 .

Figure 1 illustrates a simplified schematic section (not to scale) through a series of side-coupled cavities in an embodiment of the present invention. The machine is made by subtractive manufacture, starting with disks or slabs of metal 105, preferably copper. The axial cavity portions 101 are aligned along the axis of the machine. Each adjacent pair of axial cavity portions 101 is connected via a side cavity 103 at coupling apertures in the form of slots 1 15. Also aligned along the axis of the machine, and separating the axial cavity portions 101 are partitions 109, which take the form of solid walls in this example having a central opening 1 13, e.g. aligned with a common axis of the cavities 101 .

The partitions 109, 1 1 1 are shaped and arranged such that the electromagnetic field is maximised at partition opening/hole 1 13. Whilst a constant thickness of the partitions is shown by way of the simplified schematic of Figure 1 , the thickness may vary in different examples towards the central opening 1 13 and/or the free edge portion 1 14 adjacent the side cavity 103. Although not shown, each of the side cavities 103 may additionally comprise a tuning member configured to deform the walls of the cavity to tune the frequency of the electromagnetic field. The tuning member may be a tuning stub, a tuning pin, or a tuning peg. The dashed vertical lines 121 in the figure show the boundaries (e.g. joints and/or abutment faces) between the separately manufactured component parts. Each of the boundaries 121 splits a coupling slot 1 15.

The side cavities 103 may be provided on alternating opposing sides of the main cavity formed by the axial cavity portions 101 . As shown in Figure 1 , the side cavities alternate between the top and bottom of the main cavity. Only a portion of the device is shown in Figure 1 and the sequence of sections may be repeated as required in order to build up the compete device. A series of sections as shown in Figure 1 may define a cell. Each cell may have a central section comprising the entirety of the partition 109 and a pair of flanking sections on either side thereof, which are devoid of a partition. The flanking sections are shaped to define the open axial cavity portions 101 and may also define a portion of the side cavity 103 radially outside of the free edge 1 14 of the partition.

Any, or all, of the cavities may be substantially circular in cross section.

Figure 2 illustrates an enlarged view of the schematic of Figure 1 . As a result of the boundaries 121 splitting each of the coupling slots 1 15, the corners 107 (both concave and convex) are all easily accessible for machining. These corners 107 may thus be accurately machined with sharp or rounded edges as appropriate for the electromagnetic design. In particular, the adjacent section to each side of the partition 109 comprises a shoulder formation which defines an extremity of the slot. The shoulder formation has a convex and a concave corner, either or both of which may be rounded by machining when accessible using the boundary 121 arrangement shown in Figure 2. Similarly, the corners 107 of the free edge 1 14 of the partition itself may be rounded as required.

It should be noted that the whole of the side cavities are not shown in Figures 1 and 2, only the section at which the side cavity couples to the axial cavities. It should also be noted that Figures 1 and 2 simplify the structure of cavity devices in order to facilitate understanding of the invention. In practice, the cavities 101 , 103 may have more detailed/complex geometric structure, such as profiling, shaping and/or tapering designed to enhance their electromagnetic performance. The present invention may be used to improve manufacture of such structures by increasing the range of geometries that can be created.

Figure 3 illustrates a device assembled according to the schematic of Figure 1 . The view on the left illustrates the device assembled with each of the

manufactured component sections joined together. The view on the right illustrates each of the manufactured component parts separately, aligned as they are to be assembled. The hashed areas of the component parts in the left-hand and right- hand views represent regions that comprise solid material/metal, and the non- hashed areas of the component parts represent parts that are hollow.

The central view shows a plan view of the end of the left-hand view, i.e. showing the profile of the main cavity 101 and the side cavity 103 above it.

It can be seen from Figure 3 that the boundaries of the component parts run through the coupling slots so as to divide the point at which the side cavities 103 are coupled to the axial cavities 101 .

The cavities 101 , 103 and the partitions 109, 1 1 1 of Figure 3 are curved/profiled and/or otherwise shaped such that the electromagnetic field is maximised at the partition holes 1 13. In particular, the edges of the side cavities 103 have been sufficiently rounded.

The device comprises a plurality of cells arranged in series, each cell being made up of a centre section and two end/flanking sections as described above. Any two adjacent cells share a common flanking/end section, such that a right-end section for a first cell also acts as a left-end section for a second, adjacent cell when taken in sequence from left to right. Each end section of a cell is provided with two protruding abutment surfaces at opposite ends and on opposite longitudinal sides.

These abutment surfaces are arranged such that the two end sections of any cell will abut when the cell is joined together. This abutment creates the side cavity 103 which couples the two axial cavities 101 . A void behind the free edge portion of the partition 109, as the central section of a cell, defines the side cavity geometry, i.e. between the radially outer portions of adjacent flanking/side sections.

By facilitating accurate manufacturing, the present invention improves the operation of side-coupled cavities. In particular, by allowing precision machining of internal corners 107 it allows operation at higher voltages. This may provide higher output energies and/or reduce the total length of accelerator equipment (which may comprise several cavities each operating at tens of megavolts). Using a chain of cavities 101 , 103 according to the present invention, an ion beam (for example a proton beam) may be accelerated from tens of MeV up to several hundreds of MeV, as is desirable for hadron therapy equipment.

In some embodiments, the boundaries between portions may not be planes. The boundaries between portions may be substantially straight. Alternatively, the boundaries between portions may be of any shape, provided the boundaries of adjacent portions correspond to one another and/or may interconnect. Each of the boundaries between portions may be substantially parallel relative to each other. Each of the boundaries between portions may be substantially radially/perpendicularly aligned with respect to the central axis of the device. The sections thus take the form of a series of flat plates that can be stacked in the axial direction to form the device.

The partitions 109 may be aligned substantially parallel relative to the boundaries between sections and/or substantially radially aligned with respect to the central axis. The partitions may extend inwardly towards the central axis from opposing sides (e.g. from the perimeter) of the main cavity towards its centre, such that a hole through the partition couples the at least two adjacent cavity portions 101 on either side of the partition.

The partition hole may be arranged such that an electromagnetic field within the device is maximised at the hole.

A free edge section of the partition, i.e. adjacent the side cavity may be T-shaped. The side cavity may be behind the T-shaped edge. The edge of the partition may otherwise be described as having a lip or flange portion extending in an axial direction.

The side cavity may be a substantially cylindrical passage coupling the main cavity on opposing sides of a partition. The side passage may be formed by a

combination of some or all of two or three adjacent sections.

The slots 1 15 may be defined as the locations at which the adjacent cavities couple to the side cavity. The slots may be located either side of at least one T- shaped partition edge. The slots may have rounded corners. Each side cavity may comprise two slots. Each slot may be between an end section and a centre section of each cell. Each boundary between sections thus divides at least one slot. Any, or any combination of the sections, including the partitions, may be made of metal. The metal may have a conductivity of greater than 5x10⁷ S/m at 20°C. The metal may comprise copper, such as oxygen-free high thermal conductivity (OFHC) copper.

While the present invention has been described in terms of several embodiments, those skilled in the art will recognize that the present invention is not limited to the embodiments described, but can be practised with modification and alteration within the scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.

Claims

1 . A device comprising a plurality of side-coupled radio frequency cavities wherein the device comprises multiple sections, each section having a main axial cavity portion and the sections being stacked in an axial direction such that the main axial cavity portions are axially aligned to form a main axial cavity through the device, wherein a plurality of the sections comprise a partition extending across the main axial cavity such that the partitions are axially spaced along the main axial cavity of the device, the device comprising a side cavity for each partition and an aperture coupling each side cavity to the main axial cavity, each aperture being adjacent its respective partition and interfaces between the sections of the device are aligned through the apertures.

2. A device as claimed in Claim 1 , wherein an aperture is provided on either side of each partition.

3. A device as claimed in Claim 2, wherein the aperture on either side of a common partition lead into a common side cavity.

4. A device as claimed in Claim 2, wherein two apertures are provided for each partition comprising an upstream and a downstream aperture with respect to an axial direction.

5. A device as claimed in any preceding claim, wherein opposing portions of each aperture are defined by adjacent sections of the device.

6. A device as claimed in any preceding claim, wherein the entirety of each partition is formed in a single section.

7. A device as claimed in any preceding claim, wherein each side cavity is formed by a plurality of adjacent sections, such that the adjacent sections each comprise a portion of each side cavity.

8. A device as claimed in Claim 7, wherein each side cavity is formed by three adjacent sections.

9. A device as claimed in Claim 8, wherein the three adjacent sections comprise a central section having a partition and a pair of flanking sections.

10. A device as claimed in Claim 9, wherein the flanking sections are devoid of a partition.

1 1 . A device as claimed in any preceding claim, wherein the aperture comprises a slot.

12. A device as claimed in any preceding claim, wherein each aperture extends in a lateral and/or circumferential direction with respect to the main cavity axis.

13. A device as claimed in any preceding claim, wherein alternate sections along the main cavity of the device comprise a partition.

14. A device according to any preceding claim, comprising intermediate sections between each partition, wherein each intermediate section comprises aperture portions on opposing sides of the main cavity axis.

15. A device according to any preceding claim, wherein at least a portion of each aperture is formed by a shoulder formation in a section adjacent a partition.

16. A device according to any preceding claim, wherein each partition comprises a free edge portion at a side of the main axial cavity.

17. A device according to Claim 16, wherein each side cavity is provided behind or radially outside the free edge portion of a respective partition.

18. A device according to Claim 16 or 17, wherein the free edge portion of each partition comprises a lip or head formation, e.g. extending in an upstream and/or downstream axial direction.

19. A device according to any preceding claim, wherein each partition has a central aperture, e.g. aligned with the main cavity axis.

20. A device according to any preceding claim, wherein the sections are platelike.

21 . A device according to any preceding claim, wherein the interfaces between sections are arranged in a plane which is substantially perpendicular to the main cavity axis.

22. A device according to any preceding claim, comprising joints at the interfaces between adjacent sections.

23. A device as claimed in Claim 22, wherein the joints comprise braze joints and/or diffusion bonded joints.

24. A particle accelerator comprising a device as claimed in any preceding claim.

25. A particle accelerator as claimed in Claim 24 for accelerating particles to energies between 1 MeV and 500 MeV.

26. A device section arranged to be provided in a stack of sections to form a side-coupled radio frequency cavity device, the section having a main axial cavity extending between opposing major faces of the section, and a partition extending across the main axial cavity and terminating at a free edge portion at a side of the main cavity, the section comprising a side cavity radially outside the free edge portion of the partition, wherein the free edge portion is recessed within the opposing major faces of the section so as to define an aperture between the free edge portion and the or each major face of the section, the aperture coupling the side cavity to the main axial cavity.