WO2012104636A1

WO2012104636A1 - Multipole magnet

Info

Publication number: WO2012104636A1
Application number: PCT/GB2012/050215
Authority: WO
Inventors: Neil Marks
Original assignee: The Science And Technology Facilities Council
Priority date: 2011-02-03
Filing date: 2012-02-01
Publication date: 2012-08-09
Also published as: GB201101859D0

Abstract

A multipole magnet for deflecting a beam of charged particles, comprising a first pair of magnetic poles having opposing polarities, a second pair of magnetic poles having opposing polarities and a beamline space for the passage of a beam of charged particles; wherein the magnitude of magnetic scalar potential at the ends of the poles of the first pair of magnetic poles is less than the magnitude of magnetic scalar potential at the ends of the poles of the second pair of magnetic poles.

Description

Multipole Magnet

The present invention relates to a multipole magnet, and in particular, although not exclusively, to a multipole electromagnet, to a fixed field alternating gradient accelerator using steel cored magnets, or to any other type of accelerator or electromagnetic device which requires one or more multipole steel cored magnets where the beam is asymmetrically located with respect to the magnetic centre of those magnets.

Multipole magnets, such as multipole electromagnets, are used in many particle accelerators to accelerate, steer and/or focus a beam of charged particles. A known kind of particle accelerator that utilises multipole magnets is a fixed field alternating gradient accelerator (FFAG). In a typical FFAG, a beam of charged particles is steered and focused using an electromagnetic field which does not vary with time but has a field gradient which causes the field amplitude to vary with position, either in a linear or nonlinear way. In order to achieve the magnetic field gradient needed to hold the charged particles in an FFAG, a plurality of electromagnets are arranged in a ring. A vacuum tube is provided within the ring, within which charged particles may be guided and focused by the magnetic fields. The field gradient focuses the beam of charged particles as it passes around the vacuum tube within the FFAG. Charged particles located away from a centre of the magnet will also follow a curved trajectory and will consequently be steered on a circular orbit.

Typically, electromagnets used in an FFAG have a plurality of cores, each formed from a ferromagnetic material and a current carrying coil arranged to produce a magnetic field and magnetise each of the cores. A strong magnetic field, of the order of IT for example, is required to satisfactorily bend (i.e. steer) or focus the beam of charged particles within an FFAG. The fabrication and operating costs of electromagnets of this specification can be considerable. For example, an electromagnet having large cores will require a significant amount of ferromagnetic material, such as soft iron, and there will be high energy costs associated with supplying the surrounding coils with sufficient current to produce the desired field. In addition, due to the size of the electromagnets, there will be a considerable amount of heat generated in use due to energy losses which will require an active cooling system. Both the setup and operating costs of the cooling system contribute to the overall cost of the electromagnet system.

It is an object of the invention to provide a multipole magnet which overcomes or mitigates disadvantages of the prior art.

Aspects of the present invention are defined in the appended claims.

A specific embodiment of the invention will now be described by way of example only with reference to the accompanying Figures, in which:

Figure 1 schematically shows a quadrupole electromagnet which is known from the prior art;

Figures 2 and 3 each show, schematically, a sextupole electromagnet which is known from the prior art;

Figure 4 schematically shows a modified four pole electromagnet which is providing a field distribution normally obtained from a sextupole electromagnet, which is known from the prior art;

Figure 5 schematically shows a four pole electromagnet according to an embodiment of the present invention;

Figure 6 graphically shows an electromagnet which is known from the prior art; and

Figure 7 schematically shows an electromagnet according to an embodiment of the invention.

Figure 1 shows schematically, in cross-section, a quadrupole electromagnet 2 which may for example form part of a synchrotron, FFAG, or other device for transmitting a focused beams of charged particles. The electromagnet 2 comprises four poles 4 which extend inwardly from a square frame 6 and are each equidistant from a 'magnetic centre' of the electromagnet, which is defined below. The poles 4 and frame 6 are formed from one or more pieces of ferromagnetic material. Wires (not shown) are coiled around the poles 4 and energise the electromagnet 2 when conducting an electric current. The electromagnet 2 is arranged to produce a magnetic field that is capable of influencing (e.g. steering, focussing or otherwise deflecting) a beam of charged particles passing through a beamline space of the electromagnet 2.

At the magnetic centre, indicated in Figure 1 by the intersection of the dashed lines 10 (x and y directions), the magnetic scalar potential is substantially zero due to the arrangement of the poles 4. The poles 4 are arranged such that the magnetic field increases as a function of radial distance (i.e. a direction having an x and/or y component) from the magnetic centre. This magnetic field profile is achieved using poles 4 which are identical in size and shape and which have magnetic scalar potentials of the same magnitude but of alternating polarity (i.e. two pairs of N and S poles). The ends of the poles 4 may, for example, be shaped so as to approximate to a hyperbola, as the lines of magnetic scalar potential of a perfect quadrupole are parabolic.

In a synchrotron or other device in which a beam of charged particles is focussed, a plurality of quadrupole electromagnets 2 of the type shown in Figure 1 are provided in a ring. A vacuum tube extends around the synchrotron arranged to pass between the poles of each of the electromagnets, within the beamline space, to contain the moving beam of charged particles. A suitable vacuum tube 8 is shown in cross-section in Figure 1. In the electromagnet 2 of Figure 1, the vacuum tube 8 is located centrally and symmetrically between the poles 4 and is centred on the magnetic centre. In use, the poles 4 focus the charged particles circulating around the synchrotron within the vacuum tube 8.

Conventionally, the poles of electromagnets used in synchrotrons have a symmetrical configuration, such as the configuration shown in Figure 1, for example. In other conventional configurations, an electromagnet having six poles may be used, such as the sextupole electromagnet 14 shown in Figure 2.

The construction of the sextupole electromagnet 14 is largely similar to that of the quadrupole electromagnet 2 of Figure 1. The sextupole electromagnet 14 comprises six poles 16 which extend radially inwardly from a rectangular frame 18 towards a magnetic centre (shown by the intersection of dashed lines in Figure 2), where the poles 16 and frame 18 are formed from one or more pieces of ferromagnetic material. A vacuum tube 8 passes through a central beamline space between the poles 16 and is centred on the magnetic centre of the electromagnet 14. The shape of the end of each pole 16 of the sextupole electromagnet 14 is described by third order equations and each generates a magnetic field distribution which follows a quadratic law. The sextupole electromagnet 14 of figure 2 therefore influences a beam of charged particles in a fundamentally different way in comparison to the quadrupole electromagnet 2 of Figure 1.

Sextupole electromagnets are essential components in many particle accelerators. Conventional sextupole electromagnets have a symmetric configuration, such as the configuration shown in Figure 2, for example. As in a quadrupole electromagnet, a conventional sextupole electromagnet has poles which have six-fold symmetry in size and shape and which have magnetic scalar potentials of the same amplitude but of alternating polarity. An example of another known electromagnet 14 is shown schematically in cross-section in Figure 3. The electromagnet 14 comprises six poles 16 which all have the same shape and are located at equal distances from the magnetic centre. The poles 16, and a frame 18 which supports the poles 16, are formed from one or more pieces of ferromagnetic material. The required magnetic fields are defined by lines of constant magnetic scalar potential and the ferromagnetic poles in conventional magnets follow such lines having equal magnitude but alternating polarity. At the magnetic centre of the magnet 14 the field is zero and this can be defined also as the point of zero magnetic scalar potential.

The electromagnet 14 of figure 3 is typical of the kind that might be used as part of an FFAG and has a vacuum tube 20 passing through a beamline space between the poles 16. In many, but not all, FFAGs, this vacuum tube 20 is located away from the zero magnetic field location (i.e. it is displaced in the x-direction). This is done so that the magnetic field within the vacuum tube 20 has non-zero amplitude in addition to having a gradient. A charged particle which is introduced into the FFAG with a low energy will initially circulate within the FFAG close to the side of the vacuum tube 20 which experiences a lower magnetic field (i.e. the left side of the vacuum tube in Figure 3). The gradient of the magnetic field will hold the charged particle within the FFAG. As the charged particle accelerates, and thereby increases in energy, it moves towards the side of the vacuum tube 20 which experiences a higher magnetic field (i.e. the right side of the vacuum tube in Figure 3). Once the charged particle has reached a predetermined energy, it will be extracted from the FFAG. The acceleration of charged particles provided by the FFAG is a continuous process, and it is essential that the amplitude and gradient of the magnetic field within the vacuum tube 20 holds the particles in a stable orbit. The acceleration of charged particles does not cycle in the way that occurs in a synchrotron. Due to the manner in which an FFAG operates, a large number of electromagnets are needed in order to provide the required magnetic field gradient with sufficiently high field strength to stabilise the orbits of the charged particles. The cost of fabricating the magnets, and the electricity required in order to power the magnets, is substantial. A known way in which the cost of fabricating and operating the electromagnets can be reduced is by making an electromagnet having the configuration shown schematically in cross-section in Figure 4. This electromagnet 30 has only four poles 32, but these poles are effectively configured as four poles of a sextupole electromagnet (c.f. the sextupole electromagnet 14 of Figure 3), with the correct cubic law defining the pole shape, as required in a sextupole magnet to generate a correct sextupole magnetic field. The other two poles of the sextupole electromagnet have been omitted. This is possible because the contribution of those poles to the magnetic field in the horizontally offset vacuum tube 20 (which contains the beam of particles) is negligibly small, so that removing these poles does not have a significant effect on the quality or magnitude of the sextupole magnetic field or on the particle trajectories within the vacuum tube 20. The four poles 32 are supported by a rectangular frame 34, and the poles 32 and the frame 34 are formed from one or more pieces of ferromagnetic material. Although the electromagnet of Figure 4 does not have a symmetric configuration, each of the poles 32 has the same size and shape, and each has the same magnitude of magnetic scalar potential, but with alternating polarity.

A multipole magnet according to an embodiment of the invention is shown schematically in cross-section in Figure 5. The multipole magnet comprises an electromagnet having a first pair of poles 42a and a second pair of poles 42b each supported by a frame 44. The poles 42a,42b and the frame 44 are formed from one or more pieces of ferromagnetic material. The poles 42a,42b are each encircled by wires (not shown) which in use carry current and thereby energise the electromagnet to produce a magnetic field in a beamline space between the poles 42a,42b. The multipole magnet also comprises a vacuum tube 46, which may for example form part of an FFAG, within the beamline space, radially offset (in the x-direction) from the magnetic centre of the magnet.

The first pair of poles 42a of the electromagnet project closer to the magnetic centre of the magnet (indicated by the intersection of dashed lines x,y in Figure 5) than the corresponding poles 32 of the electromagnet of Figure 4. This is made possible by there being no vacuum chamber present at this position. In the multipole magnet of Figure 5, the poles of the first pair of poles 42a are each spaced from the magnetic centre of the magnet by a first distance and the poles of the second pair of poles 42b are each spaced from the magnetic centre of the magnet by a second distance that is greater than the first distance. This is in contrast to the electromagnet 32 of Figure 4 in which all four poles 32 are equidistant from the magnetic centre.

In order to generate the correct magnetic field distribution in the region of the vacuum tube 46, ends of the poles 42a must be shaped to follow lines of lower magnetic scalar potential than the ends of the poles of the second pair of poles 42b. Consequently, the current supplied to the coils (not shown) surrounding the first pair of poles 42a must be less than the current supplied to the second pair of poles 42b. The effect of the electromagnet shown in Figure 5, in terms of the magnetic field provided within the vacuum tube 46, can be understood by comparing it with the electromagnet 30 shown in Figure 4. The first pair of poles 42a are positioned closer to the magnetic centre of the magnet, and require lower electrical current than the equivalent poles in the electromagnet of Figure 4. However, they provide the same contribution to the magnetic field in the vacuum tube 46 as was provided by the equivalent poles 32 of the electromagnet of Figure 4. The net effect of the reduction in electrical current and closer pole location of the magnet shown in Figure 5 compared to the magnet of Figure 4 is no appreciable change to the magnetic field experienced by the particle beam; the arrangement of Figure 5 provides a magnetic field that is substantially the same as the magnetic field provided in the vacuum tube 20 of Figure 4.

The electromagnet of Figure 5 allows the desired sextupole magnetic field to be generated off axis, using an electromagnet which has a smaller number of poles, which uses less electrical current and electrical power than the known sextupole electromagnet shown in Figure 4. This is particularly advantageous since a lesser amount of ferromagnetic material is needed in order to fabricate the electromagnet (as evident from Figure 5)) and hence it is cheaper to make. In addition, it is cheaper to operate since less electrical power is needed to energise the poles. A further advantage of the multipole magnet of the present invention is that it allows a more compact FFAG to be constructed than is possible using the known electromagnets, such as the one shown in Figure 4.

The multipole magnet of the present invention goes against the conventionally held view that multipole magnets need to be constructed with each pole having the same physical size, the same radial position with respect to the magnet's magnetic centre and having the same magnitude of scalar magnetic potential.

A mathematical model has been used to determine an electromagnet arrangement which will provide a required magnetic field, using the known electromagnet configuration of Figure 4 and the new electromagnet configuration of Figure 5. The mathematical model was obtained using two dimensional, non-linear, magneto-static, finite-element modelling software, "OPERA2D", available from Cobham Technical Services (Vector Fields), 24 Bankside Kidlington, Oxford 0X5 1JE, UK.

The desired magnetic field corresponds to a 6 pole (sextupole) configuration, with lower order harmonics (2 pole and 4 pole) present at the vacuum tube, and a small non- dominant higher order field (8 pole) included in the pole configuration. A pole configuration determined using the mathematical model, in accordance with the arrangement shown in Figure 4, is shown graphically in Figure 6. Figure 6 shows only the-upper half of the magnet with the upper two of the four required poles 30. The upper half of the vacuum tube 20 is also shown in Figure 6. The remainder of the vacuum tube 20 and the two remaining poles are mirror images of what is shown in Figure 6 (reflected in the x-axis). The centre of the vacuum tube 20 is used to define the origin of the x and y coordinates shown in Figure 6. The magnetic centre of the magnet, where the dipole and quadrupole field components are zero, is at x = -39 mm and is marked with a dashed vertical line. A 60 degree line, emanating from the magnetic centre, marks the physical boundary between the two illustrated poles.

The outer surfaces of the poles 30 (i.e. their "ends") follow lines of constant magnetic scalar potential. The magnetic scalar potentials of the two poles shown in Figure 6 are equal in magnitude but have opposite polarities. The two poles which are not illustrated have magnetic scalar potentials which are equal in magnitude but are also opposite in polarity to each other and also to the poles above the x axis that are located at the same horizontal position.

An electromagnet pole configuration determined using the mathematical model, in accordance with an embodiment of the present invention is shown graphically in Figure 7. Each of the four poles 42a, 42b of the electromagnet are shown in Figure 7, together with a vacuum tube 46 which may for example form part of an FFAG. The portions of the poles 42a, 42b which provide a significant contribution to the magnetic field at the vacuum tube 46 are shown. Portions of the poles 42a, 42b which provide only a negligible contribution to the magnetic field at the vacuum tube 46 are not shown.

The second pair of poles 42b are substantially reduced in size (compared with those shown in Figure 6) and are closer to the x-axis (closer also to the vacuum tube 46). The reduction in distance is such that the magnetic scalar potential at the surface of these poles is only 2.8% of the magnetic scalar potential at the surface of the first pair of poles 42a. Since the magnetic scalar potential at the second pair of poles 42b is 2.8% of the magnetic scalar potential at the first pair of poles, the wire coils used to power the second pair of poles need only be excited with 2.8% of the Ampere-turns needed on the first pair of poles (i.e. the second pair of poles 42b require only 2.8% of the total electrical current required by the first pair of poles 42a). The total power required by the electromagnet of Figure 7 is therefore roughly half of the total power required by the electromagnet of Figure 6. The present invention provides a substantial reduction of the running cost of an multipole magnet by reducing the space between the poles and the magnetic centre of the magnet.

Additionally, the side and top and bottom poles can be terminated at circa y = ± 25mm (compared to circa y = 40 mm for the un-modified magnet), resulting in a substantial reduction in the overall size of the magnet and, consequentially, a significant reduction in capital cost.

Although the embodiment of the invention illustrated in Figure 5 is an electromagnet having four poles, in other embodiments, the multipole magnet may be any multipole magnet having other numbers of poles.

In general, the present invention provides a desired magnetic field using a smaller multipole magnet and using less electrical current, thereby offering potentially significant cost savings in both production and operation. Furthermore, the present invention allows for a multipole magnet that is more compact in volume compared to prior art arrangements, which is significantly advantageous when used in a tunnel environment, where many beamlines are disposed.

The shapes of the poles used in an multipole magnet according to the present invention may be determined by using lines of magnetic scalar potential.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

1. A multipole magnet for deflecting a beam of charged particles, comprising a first pair of magnetic poles having opposing polarities, a second pair of magnetic poles having opposing polarities and a beamline space for the passage of a beam of charged particles; wherein the magnitude of magnetic scalar potential at the ends of the poles of the first pair of magnetic poles is less than the magnitude of magnetic scalar potential at the ends of the poles of the second pair of magnetic poles.

2. A multipole magnet according to claim 1 , wherein the magnetic poles are arranged radially around a magnetic centre of the multipole magnet that has substantially zero magnetic scalar potential; and

each pole of the first pair of magnetic poles is spaced from the magnetic centre by a first distance, and each pole of the second pair of magnetic poles is separated from the magnetic by a second distance that is greater than the first distance.

3. A multipole magnet according to claim 1 or 2, wherein the ends of the magnetic poles follow lines of substantially constant magnetic scalar potential.

4. A multipole magnet according to any preceding claim, wherein the multipole magnet is a quadrupole magnet.

5. A multipole magnet according to any of claims 1 to 3, wherein the multipole magnet is a sextupole magnet.

6. A multipole magnet according to any of claims 1 to 3, wherein the multipole magnet produces combinations of two or more quadrupole, sextupole or higher order magnetic fields.

7. A multipole magnet according to any preceding claim, wherein the magnetic poles are electromagnetic poles each comprising a current carrying coil arranged around one or more pieces of ferromagnetic material.

8. A fixed field alternating gradient accelerator comprising a multipole magnet according to any preceding claim, and a vacuum tube in the beamline space for the passage of a beam of charged particles.

9. A fixed field alternating gradient accelerator according to claim 8, wherein the vacuum tube is radially offset relative to the magnetic centre of the multipole magnet.

10. A multipole magnet for deflecting a beam of charged particles substantially as hereinbefore described with reference to Figures 5 and 7.

11. A fixed field alternating gradient accelerator substantially as hereinbefore described with reference to Figures 5 and 7.