WO2020239992A1

WO2020239992A1 - Mechanism for adjusting the field of a multipole magnet

Info

Publication number: WO2020239992A1
Application number: PCT/EP2020/065013
Authority: WO
Inventors: Norbert Collomb
Original assignee: United Kingdom Research And Innovation
Priority date: 2019-05-30
Filing date: 2020-05-29
Publication date: 2020-12-03
Also published as: GB201907686D0

Abstract

There is provided a mechanism for adjusting the field of a multipole magnet, such as a quadrupole magnet. The mechanism comprises at least one support and a leaf spring, the leaf spring being deformable between first and second states. In the first state, the leaf spring and the at least one support are not in contact with one another, so that the leaf spring exhibits a first load-deformation response. In the second state, the leaf spring and the at least one support are in contact with one another, so that the leaf spring exhibits a second load-deformation response.

Description

TECHNICAL FIELD

The invention relates to a mechanism for adjusting the magnetic field of a multipole magnet, particularly a multipole magnet for altering the characteristics of a beam of charged particles in a particle accelerator. The invention also relates to a multipole magnet for deflecting a beam of charged particles.

BACKGROUND

Multipole magnets comprise a plurality of magnetic poles and, among other things, are used to deflect, focus or otherwise alter the characteristics of beams of charged particles in particle accelerators. Multipole magnets may be used to change the overall direction of a beam, focus or defocus a beam, or correct aberrations in a beam. The suitability of a multipole magnet for performing these tasks is determined largely by the number of magnetic poles present. Quadruple magnets having four magnetic poles are particularly suitable for focusing and defocusing a beam of charged particles. Magnets used in multipole magnets are typically electromagnets, comprising a current carrying wire coiled around a ferromagnetic pole. In modern particle accelerator drive beams, thousands of multipole magnets comprising electromagnets may be employed along a single drive beam.

The drive beam of the proposed Compact Linear Collider (CLIC) accelerator is expected to require approximately 42,000 quadruple magnets. As such, the CLIC accelerator will likely suffer from near-prohibitive power consumption, with a total estimated usage of approximately 580 MW. This represents a problem with regards to power generation and delivery capabilities, as well as accelerator power and cooling infrastructure, environmental impact and significant running costs tied to energy prices. A significant portion of the predicted energy consumption, approximately 124 MW, is expected to arise from dissipation in normal conducting electromagnets, which will be compounded by efficiency of the delivery system and energy consumption of water cooling and pumping systems. To counter to this, it has been proposed to replace at least some of the electromagnets with permanent magnets that are capable of adjusting their field by moving permanent magnet material relative to an associated pole. Such permanent magnets are described in earlier patent application published as WO 2012/046036 A1 , the content of which is incorporated herein by reference.

It is anticipated that the use of permanent magnets will have several advantages relevant to the CLIC accelerator, including no power draw during normal use, a small power draw when adjusting the field, reduced infrastructure, as there will be no requirement for large power supplies or cooling, and no vibration from water cooling systems or a need to extract excess heat. However, as the skilled reader will appreciate, movement of the permanent magnet material is made against very large forces of magnetic attraction, i.e. the attraction of the permanent magnet material to the opposing pole. Moreover, the movement is required to be very accurate. Indeed, it is envisaged that the required accuracy of the position of the permanent magnet material may be less than 10 microns. Achieving the required accuracy makes known arrangements very expensive.

It is an object of embodiments of the invention to at least mitigate one or more problems associated with known arrangements.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a mechanism for adjusting the field of a multipole magnet, the mechanism comprising: at least one support; and a leaf spring deformable between first and second states, wherein in the first state the leaf spring and the at least one support are not in contact with one another so that the leaf spring exhibits a first load-deformation response and in the second state the leaf spring and the at least one support are in contact with one another so that the leaf spring exhibits a second load-deformation response. The first load deformation response and the second load-deformation response are distinct from one another. The at least one support inhibits the deflection of the leaf spring, effectively increasing the stiffness thereof. The invention produces a step change in the load-deformation response of the leaf spring, between the first and the second load-deformation responses.

The invention may counterbalance the attractive forces against which the permanent magnet material is moved, so that such movement requires less force. Consequently, the above-mentioned high level of accuracy required may be obtained with simpler and easier to manufacture components, providing a cost reduction in the manufacture of mechanisms for adjusting the field of a multipole magnet. Optionally, the first load-deformation response may be substantially, or at least approximately, linear. Additionally, or alternatively, the second load-deformation response may be substantially, or at least approximately, exponential.

In certain embodiments, the leaf spring may comprise at least one first region of reduced thickness, relative to the remainder of the leaf spring. The first region may in part determine the first load-deformation response. The first region may at least substantially determine the first load-deformation response. In certain embodiments, the leaf spring may comprise at least one second region of increased thickness, relative to the remainder of the leaf spring. The second region may in part determine the second load-deformation response. The second region may at least substantially determine the second load-deformation response.

The at least one support may comprise a pair of spaced apart supports between which, in the second state, a length of the leaf spring extends. The length of the leaf spring may vary in thickness between the spaced apart supports, which may in part determine the second load-deformation response. The varying thickness of the leaf spring may at least substantially determine the second load-deformation response. The pair of spaced apart supports may be shaped to vary the length of the leaf spring, upon increasing deformation of the leaf spring, to in part determine the second load- deformation response. In certain embodiments, the leaf spring may be an elliptical leaf spring. As such, the leaf spring may vary in thickness over each of a pair of opposing vertices of the leaf spring to at least in part determine the first load-deformation response. The varying thickness over the opposing vertices may at least substantially determine the first load- deformation response. Additionally, or alternatively, the leaf spring may vary in thickness over each of a pair of opposing co-vertices of the leaf spring to in part determine the second load-deformation response. The varying thickness over the opposing co-vertices may at least substantially determine the second load- deformation response.

In certain embodiments, the leaf spring and the at least one support may be in contact with one another, in the second state, at opposing first and second ends of the at least one support. According to another aspect of the invention, there is provided a multipole magnet for deflecting a beam of charged particles, the multipole magnet comprising: a plurality of ferromagnetic poles positioned about a pole plane; at least one magnet cap assembly comprising a permanent magnet material for supplying magnetomotive force for producing a magnetic field over the pole plane, the at least one magnet cap assembly being moveable relative to the ferromagnetic poles to adjust the magnetic field; and at least one mechanism for adjusting the field of a multipole magnet as above-described, the leaf spring being deformable by movement of the at least one magnet cap assembly to exert a force against the at least one magnet cap assembly acting away from the ferromagnet poles.

In certain embodiments, the at least one magnet cap assembly may comprise first and second magnet cap assemblies, each being moveable symmetrically about the ferromagnetic poles to adjust the magnetic field. The leaf spring may be an elliptical leaf spring, deformable by movement of the first and second magnet cap assemblies to exert simultaneously opposing forces against each of the first and second magnet cap assemblies, each of the opposing forces acting away from the ferromagnet poles.

In certain embodiments, the multipole pole magnet may comprise at least one electric motor configured to move at least one of the at least one magnet cap assembly. The at least one electric motor may be configured to move each of the first and second magnet cap assemblies.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures, in which:

Figure 1 is a perspective view of a multipole magnet, specifically, a quadrupole magnet, according to an embodiment of the invention;

Figure 2 is a front view of the multipole magnet shown in Figure 1 , in which the four poles of the quadrupole magnet are visible;

Figure 3 is a plot of the stroke-force relationship for a magnet cap assembly according to an embodiment of the invention;

Figure 4 is a front view of an elliptical leaf spring according to an embodiment of the invention; and

Figure 5 is a partial front view of the leaf spring shown in Figure 4, in which the leaf spring is in contact with a pair of supports.

DETAILED DESCRIPTION

Figures 1 and 2 show a quadrupole magnet 10 according to an embodiment of the invention. The quadrupole magnet 10 has four ferromagnetic poles 12a-d arranged about a pole plane, such that each of the poles 12a-d are symmetrical about the pole plane. A coordinate system is indicated in Figure 1 and includes an x-axis and a y- axis. The x-axis and the y-axis define the two-dimensions of the pole plane. A z-axis extends orthogonally to each of the x-axis and the y-axis. As shown in Figure 1 , the four poles 12a-d define a beamline space therebetween. In use, a beam of charged particles, such as electrons or positrons, may travel substantially orthogonally to the pole plane, i.e. substantially parallel to the z-axis, through the beamline space. The four poles 12a-d may be supported by a yoke 14. The yoke 14 and/or the four poles 12a-d may be supported by a frame 16. The magnet 10 further comprises first and second magnet cap assemblies 18, 20 arranged on opposing sides of the yoke 14. As shown in the illustrated embodiment, the first magnet cap assembly 18 may be positioned above the yoke 14. The second magnet cap assembly 20 may be positioned below the yoke 14. The magnet cap assemblies 18, 20 are moveable relative to the frame 16 and the yoke 14, as well as relative to one another, parallel to the y-axis. The frame 16 may support each of the magnet cap assemblies 18, 20 and/or facilitate movement thereof. To this end, the magnet cap assemblies 18, 20 may be moveable along guides 22 coupled or integral to the frame 16. The guides 22 may limit movement of the magnet cap assemblies 18, 20 to movement parallel to the y-axis.

Each of the magnet cap assemblies 18, 20 comprises a permanent magnet material to supply magnetomotive force to the ferromagnetic poles 12a-d. The magnetomotive force produces a magnetic field that extends over the pole plane and into the beamline space to deflect, focus or otherwise alter one or more characteristics of a beam of charged particles passing therethrough. Movement of the magnet cap assemblies 18, 20 parallel to the y-axis varies the magnet flux in the poles 12a-d, which consequently varies the magnetic flux across the beamline space. Therefore, the magnetic field strength within the beamline space is variable by movement of the magnet cap assemblies 18, 20 parallel to the y-axis. As the skilled reader will appreciate, movement of the magnet cap assemblies 18, 20 is symmetrical about the beamline space. Throughout the description and claims of this specification, the range of movement of each of the magnet cap assemblies 18, 20 parallel to the y-axis is referred to as its respective stroke.

With the stroke at a minimum value, after which no further movement towards the beamlines space is possible, an attractive force, due to fields produced by the magnetomotive force of the permanent magnet material, against which the respective magnet cap assembly 18, 20 must be moved will be at a maximum value, with the stroke-force relationship being at a maximum gradient. Conversely, with the stroke at a maximum value, after which no further movement away from the beamline space is possible, the attractive force against which the respective magnet cap assembly 18, 20 must be moved will be at a minimum value, with the stroke-force relationship being at a minimum gradient. Over the full length of the stroke, the force varies approximately linearly for a first portion of the stroke and thereafter approximately exponentially for a second portion of the stroke. To aid understanding, Figure 3 shows the stroke-force relationship for one of the magnet cap assemblies 18, 20 according to an exemplary embodiment of the invention. For the exemplary embodiment, it is apparent that the first portion extends from 32mm (at 1.36kN) to approximately 8mm (at 4.74kN) and the second portion extends thereafter to 0mm (at 17.21 kN). Therefore, for the exemplary embodiment, the maximum attractive force against which each of the magnet cap assemblies is moved is 17.21 kN.

Embodiments of the invention seek to counterbalance the attractive forces of the permanent magnet material of the respective magnet cap assemblies 18, 20, as these forces act in opposing directions parallel to the y-axis. To this end, the quadruple magnet 10 further comprises at least one leaf spring 24. The leaf spring 24 is a curved length of elastically, i.e. resiliently, deformable material. As shown in the illustrated embodiment, the leaf spring 24 may form an ellipse. As such, in certain embodiments, the leaf spring 24 may be referred to as an elliptical leaf spring. As such, the leaf spring 24 may have a pair of opposing vertices 26, 28 and a pair of opposing co-vertices 30, 32 (best shown in Figure 4). The vertices 26, 28 are regions of the leaf spring 24 coincident with the major axis of the ellipse. The co-vertices 30, 32 are regions of the leaf spring 24 coincident with the minor axis of the ellipse. The leaf spring 24 is disposed such that it may be deformed in compression by movement of at least one of the magnet cap assemblies 18, 20 over the length of its respective stroke, i.e. movement parallel to the y-axis. The leaf spring 24 may be coupled to at least one of the magnet cap assemblies 18, 20. Thus, with the leaf spring 24 coupled to, or at least contacting, at least one of the magnet cap assemblies 18, 20, movement of at least one of the magnet cap assemblies 18, 20 parallel to of the y-axis causes deformation of the leaf spring 24.

With the leaf spring 24 deformed, the resiliency of the leaf spring 24 may exert a force against at least one of the magnet cap assemblies 18, 20, i.e. against whichever of the magnet cap assemblies 18, 20 the leaf spring 24 is coupled to or contacts. As shown in the illustrated embodiment, if the leaf spring 24 is elliptical, the leaf spring 24 may contact both of the magnet cap assemblies 18, 20, thus exerting an equal and opposite force against each. For any value of the stroke, the leaf spring 24 may provide a counterbalancing force, which acts to oppose the attractive force against which the respective magnet cap assembly 18, 20 must be moved. Consequently, the counterbalancing force reduces the force required to move at least one of the magnet cap assemblies 18, 20 and hence reduces the energy required to adjust the magnetic field of the quadruple magnet 10. This reduction in the energy required to adjust the magnetic field may permit, or at least facilitate, the development of particle accelerators that would otherwise likely suffer from near-prohibitive power consumption.

For the first portion of the stroke, the leaf spring 24 may be configured to have a first load-deformation response, i.e. spring rate, at least substantially equal to the stroke- force relationship over the first portion of the stroke. As such, for the exemplary embodiment described with reference the Figure 3, the spring rate of the leaf spring 24 during the first portion of the stroke may be expressed as y = 0.0046x² - 0.3218x + 6.9537 kN/mm. For each embodiment, determination of the configuration of the leaf spring 24, i.e. material selection and establishing the shape of the leaf spring 24, to provide the first load-deformation response may be carried out by finite element analysis. The skilled reader will appreciate that many finite element analysis software packages are available that would be suitable for this purpose, e.g. ANSYS (RTM). Using finite element analysis software, the leaf spring 24 may be modelled for a given material and load-deformation response to automatically optimise the size and shape of the leaf spring 24. In certain embodiments, the leaf spring 24 may comprise one or more first regions that are configured to determine the first load-deformation response. In particular, each of the first regions may have a lesser thickness than at least a portion of the remainder of the leaf spring 24 to determine the first load-deformation response. As shown in the illustrated embodiment, if the leaf spring 24 is elliptical, the first regions may be provided at the vertices 26, 28, over which the leaf spring 24 may vary in thickness.

During the first portion of the stroke, the leaf spring 24 has been found to work well on its own, in exerting the above-described equal and opposing force, as the leaf spring 24 may be configured for an at least approximately linear stroke-force relationship. However, as shown for the exemplary embodiment in Figure 3, the stroke-force relationship is not approximately linear over the entire length of the stroke. The leaf spring 24 alone cannot adequately provide the counterbalancing force for the second portion of the stroke. In other words, while a substantially linear spring rate may assist in counterbalancing the attractive forces of the permanent magnet material of the respective magnet cap assemblies 18, 20 for the linear part of the stroke, the exponential part of the stroke requires a different approach.

The quadrupole magnet 10 further comprises a pair of supports 34. The supports 34 are disposed to selectively contact the leaf spring 24, upon a predetermined deformation of the leaf spring 24 being reached. Upon contact with the leaf spring 24, the supports 34 inhibit continued deformation of the leaf spring 24 to cause a change in the load-deformation response thereof. As such, prior to contact of the leaf spring 24 against the supports 34 the leaf spring 24 is in a first state. In the first state, deformation of the leaf spring 24 occurs uninhibited by either of the supports 34 so exhibits the first load-deformation response. Thereafter, deformation of the leaf spring 24 may continue inhibited by the supports 36 so exhibits a second load-deformation response that is distinct from the first-load deformation response. Embodiments of the invention produce a step change in the load-deformation response of the leaf spring 24, between the first and the second load-deformation responses.

For the second portion of the stroke, the leaf spring 24 may be configured such that the second load-deformation response is at least substantially equal to the stroke- force relationship over the second portion of the stroke. As for the first-load deformation response, for each embodiment, determination of the configuration of the leaf spring 24 to provide the second load-deformation response may be carried out by finite element analysis. In certain embodiments, the leaf spring 24 may comprise one or more second regions that are configured to determine the second load-deformation response. In particular, each of the second regions may have a greater thickness than at least a portion of the remainder of the leaf spring 24 to determine the second load- deformation response. As shown in the illustrated embodiment, if the leaf spring 24 is elliptical, the second regions may be provided at the co-vertices 30, 32, over which the leaf spring 24 may vary in thickness. Moreover, if the leaf spring 24 is elliptical, with the leaf spring 24 in contact with the supports 34, deformation of the leaf spring 24, or at least a length of the leaf spring 24 spanning between the supports 34, may be substantially like deformation of a beam. As such, a region of the leaf spring 24 may be configured by beam analysis to provide a non-linear load-deformation response required for the second portion of the stroke. For example, a section of the leaf spring 24 extending between the spaced apart supports 34 may act substantially like a simply supported beam.

Considering the leaf spring as a simply supported beam, the maximum deflection at the midspan of the region, i.e. where the force will be applied, may be approximated by b_max = PL³ / 48EI. b_max is the deflection at the midspan of the length of the leaf spring 24 spanning between the supports 34, P is the force, in this case the attractive force, L is the length of the leaf spring 24 spanning between the supports 34, E is the elastic modulus of the leaf spring 24 and I is the second moments of area of the region. Implementing this approach in a finite element analysis software package for the exemplary embodiment described with reference the Figure 3, the spring rate of the leaf spring 24 during the second portion of the stroke may be expressed as y = 0.2895x² - 3.8755x + 17.214 kN/mm. The distance between the supports 34 may also be varied with increasing deformation of the leaf spring, to further determine the second load-deformation response. This may be achieved by providing a curved contact surface at the ends 60 of the supports 34. As shown in the illustrated embodiment, if the leaf spring 24 is elliptical, the leaf spring 24 and the supports 34 may contact one another at opposing ends 60 of the supports 34, i.e. upon upper and lower inner surfaces of the leaf spring 24.

Together, the leaf spring 24 and the supports 34 provide a mechanism for adjusting the magnetic field of the quadruple magnet 10.

Movement of the magnet cap assemblies 18, 20 may be by any suitable actuation means. As shown in the illustrated embodiment, movement of the magnet cap assemblies 18, 20 may be by a motor 36. Moreover, the leaf spring 24 may not balance the attractive forces exactly, thus the motor 36 may account for at least some amount of variation between the between the actual attractive force and the spring force. The motor 36 may be connected to the each of the magnet cap assemblies 18, 20 by a respective bar 38, 40. Moreover, as is also shown in the illustrated embodiment, the magnet cap assemblies 18, 20 may hang from the respective bars 38, 40. The motor 36 may be connected to the each of the magnet cap assemblies 18, 20 by a worm gear 42 and a driveshaft 44. A sinusoidal disk 46 may be coupled to the driveshaft 44 and coupled to the bars 38, 40. The sinusoidal disk 46 converts the rotational movement of the driveshaft 44 to linear movement of the bars 38, 40 required to move the magnet cap assemblies 18, 20 parallel to the y-axis. The worm gear 42 may inhibit back driving, to maintain the position of the magnet cap assemblies 18, 20 with no power to the motor 36. As shown in the illustrated embodiment, a single motor 36 may control the stroke of both of the magnet cap assemblies 18, 20. The connection between the bars 38, 40 and the magnet cap assemblies 18, 20 may not be central to the magnet cap assemblies 18, 20, causing an acentric load on the connection therebetween and a desire for the magnet cap assemblies 18, 20 to rotate about the connection. As such, the guides 22 may help to maintain this the correct alignment of the magnet cap assemblies 18, 20.

The invention is not restricted to the details of any foregoing embodiments. For example, while the invention is described above in relation to a quadrupole magnet, the invention relates to multipole magnets having any number of poles. In certain embodiments, movement of the magnet cap assemblies 18, 20 may be by hydraulic means.

Throughout the description and claims of this specification, "ferromagnetic" is to be understood as synonymous with "magnetically soft" and "magnetically permeable" and to refer to reasonably high permeability of at least 10mo, where po is the permeability of free space. For the invention, one suitable ferromagnetic material is steel. Flowever, other suitable ferromagnetic materials may be used. Throughout the description and claims of this specification, "magnetic field strength" is substantially equivalent to “magnetic flux density”, whatever its spatial distribution. The leaf spring 24 may be formed from a plain carbon steel. The leaf spring 24 may be heat treated after an initial forming process, to provide at least one of greater strength, greater load capacity, greater range of deflection and better fatigue properties. The leaf spring 24 may be laminated, i.e. comprising a plurality of leaves. Multiple leaf springs 24 may be used, for example one leaf spring 24 on opposing sides of the frame 16, or one or more leaf springs 24 associated with each of the magnet cap assemblies 18, 20.The leaf spring 24 may be coupled to the framel 6.

Specific values shown and described in relation to Figure 3 are to aid understanding of the invention and are no way to be considered to limit to the invention. All features disclosed in this specification (including any accompanying claims and drawings) may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings). The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.

Claims

1. A mechanism for adjusting the field of a multipole magnet, the mechanism comprising:

at least one support; and

a leaf spring deformable between first and second states,

wherein in the first state the leaf spring and the at least one support are not in contact with one another so that the leaf spring exhibits a first load-deformation response and in the second state the leaf spring and the at least one support are in contact with one another so that the leaf spring exhibits a second load-deformation response.

2. A mechanism according to claim 1 , wherein the first load-deformation response is substantially linear.

3. A mechanism according to either of claim 1 or 2, wherein the second load- deformation response is substantially exponential.

4. A mechanism according to any preceding claim, wherein the leaf spring comprises at least one first region of reduced thickness relative to the remainder of the leaf spring that at least in part determines the first load-deformation response.

5. A mechanism according to any preceding claim, wherein the leaf spring comprises at least one second region of increased thickness relative to the remainder of the leaf spring that at least in part determines the second load-deformation response.

6. A mechanism according to any preceding claim, wherein the at least one support comprises a pair of spaced apart supports between which, in the second state, a length of the leaf spring extends.

7. A mechanism according to claim 6, wherein the length of the leaf spring varies in thickness between the spaced apart supports.

8. A mechanism according to claim 6 or 7, wherein the pair of spaced apart supports are shaped to vary the length of the leaf spring upon increasing deformation of the leaf spring to at least in part determine the second load-deformation response.

9. A mechanism according to any preceding claim, wherein the leaf spring is an elliptical leaf spring.

10. A mechanism according to claim 9, wherein the leaf spring varies in thickness over each of a pair of opposing vertices of the leaf spring to at least in part determine the first load-deformation response.

1 1 . A mechanism according to claim 9 or 10, wherein the leaf spring varies in thickness over each of a pair of opposing co-vertices of the leaf spring to at least in part determine the second load-deformation response.

12. A mechanism according to any of claims 9 to 1 1 , wherein the leaf spring and the at least one support are in contact with one another, in the second state, at opposing ends of the at least one support.

13. A multipole magnet for deflecting a beam of charged particles, the multipole magnet comprising:

a plurality of ferromagnetic poles positioned about a pole plane;

at least one magnet cap assembly comprising a permanent magnet material for supplying magnetomotive force for producing a magnetic field over the pole plane, the at least one magnet cap assembly being moveable relative to the ferromagnetic poles to adjust the magnetic field; and

at least one mechanism according to any of claims 1 to 12, the leaf spring being deformable by movement of the at least one magnet cap assembly to exert a force against the at least one magnet cap assembly acting away from the ferromagnet poles.

14. A multipole magnet according to claim 13, wherein the at least one magnet cap assembly comprises first and second magnet cap assemblies being moveable symmetrically about the ferromagnetic poles to adjust the magnetic field.

15. A multipole magnet according to claim 14, wherein the leaf spring is an elliptical leaf spring deformable by movement of the first and second magnet cap assemblies to exert a force against each of the first and second magnet cap assemblies acting away from the ferromagnet poles.

16. A multipole magnet according to any of claims 13 to 15, wherein the multipole pole magnet comprises at least one electric motor configured to move at least one of the at least one magnet cap assembly.

17. A multipole magnet according to claim 16 when dependent upon claim 14, wherein the at least one electric motor is configured to move each of the first and second magnet cap assemblies.