Cyclotron

Abstract

A cyclotron 14 is described comprising: a pair of poles 1 & 11 each positioned on opposite sides of an acceleration plane 15; and a pair of coils 16 each surrounding one of the poles and configured to be coolable to cryogenic temperatures. Each pole is formed of an iron alloy, is operable at above cryogenic temperatures, and includes a surface (3, Fig. 1a) facing away from the acceleration plane, wherein the surface has back-cuts 4 forming a stepped profile. The stepped profile may comprise: discrete steps defined by pointed (Fig. 1a), chamfered (Fig. 1b) and / or rounded (Fig. 1c) corners and outer edges; and / or smoothed steps (Fig. 1d) defining a curvilinear profile. The stepped profile may ascend and / or descend towards the centre of the pole. The cyclotron may be a superconducting, isochronous cyclotron.

Description

Field of the invention The present invention relates to iron alloy poles, a cyclotron comprising the iron alloy poles, and a method of operating the cyclotron. Background The cyclotron is a well-known device used to accelerate charged particles such as protons. The first cyclotron was invented by Ernest Lawrence and disclosed in the 1932 patent US1948384A. Today, cyclotrons are used mainly for medical applications (for example, the production of radioisotopes or to provide particle beams directly for radiotherapy), but also for industrial applications and for research. The isochronous cyclotron was disclosed in US2872574A, filed in the name of E. McMillan and D. Judd. As described by F. A. Heyn and K. K. Tat in 1958 (Rev. Sci. Instrum. 29, 662), it is a means to obtain simultaneously high extraction energy and intensity, achieved by shaping the magnetic poles facing the beam such that the average field increases as the accelerated particles spiral outwards. A conventional cyclotron comprises several important parts that typically (but not always) include: poles to define the accelerated particle paths; current-carrying coils around the poles that drive the generation of the magnetic field seen by the particles; an outer yoke to confine the magnetic field; and suitable structures to provide the electric / electrostatic field that accelerates the particles. The poles are typically divided into sectors to achieve the varying focusing that confines the particles both horizontally and vertically during acceleration. The extracted particle energy from a cyclotron is larger for larger pole radius. For a given pole radius, the size of a cyclotron is typically dictated by the average magnetic field seen by the accelerated particles, and doubling that average field may reduce the yoke volume by as much as a factor of 8. Average fields above around 1.7 T may be achieved using superconducting rather than resistive coils. Typical cyclotrons are large and expensive machines. Reducing cyclotron size is advantageous for various reasons; it reduces manufacturing costs, reduces facility size, and reduces running costs. Furthermore, there is a particular need to reduce cyclotron size to maximise the number of hospitals / clinics that have access to the most effective imaging and beam treatment techniques. Thus, there is an ever-present drive in the cyclotron industry to design more compact and, thus, more cost-efficient machines. However, it is important that such cyclotrons still operate with sufficiently strong magnetic fields to achieve the required extracted particle energy. For isochronous cyclotrons, the stronger the magnetic field, the greater the variations in the magnetic field must be to focus particles for stable acceleration. In the art, multiple solutions have been proposed for generating sufficient magnetic field variations to focus particles at higher magnetic fields. These solutions include using superconducting flutter coils (for example, as described in US2018 / 0161598 Al and US2014 / 0371076 Al), using a high magnetic saturation rare-earth metal for the magnet poles (for example, as described in WO2020 / 257652 Al), and using bulk superconductor pole sectors (for example, as described in WO2021 / 198164 Al). However, these solutions have drawbacks. For example, these solutions require that the poles be contained in a cryostat or other form of cooling apparatus. This leads to an additional power requirement for these cyclotrons, additional design complexity, and additional cost. Thus, these cyclotrons do not adequately address the need for a compact cyclotron design. Other cyclotrons in the art are described in WO2013 / 006182 Al and US4943781, and in paper J. Kelly et al.: "Compact Rare-Earth Superconducting Cyclotron" arXiv: 1906.07642vl [physics.acc-ph], 2019 and report J. Minervini "High Intensity Superconducting Cyclotron" Tech. Rep. DTRA-TR-12-40, Defense Threat Reduction Agency, 2012. Summary According to a first aspect of the invention, there is provided a cyclotron comprising a pair of poles each positioned on opposite sides of an acceleration plane, and a pair of coils each surrounding one of the poles and configured to be coolable to cryogenic temperatures during operation of the cyclotron. Each pole is formed of an iron alloy, includes a surface facing away from the acceleration plane, the surface having back-cuts forming a stepped profile, and is operable at temperatures above cryogenic temperatures. Thus, the cyclotron according to the present application can operate without the need for pole cooling apparatus. This can reduce the manufacturing and running costs of the cyclotron. As used herein, "stepped profile" Is taken to mean any profile having at least two profile sections of differing height / level (corresponding to "steps"). The variation in the height / level of the profile sections is a function of pole radius. The stepped profile may have discrete steps defined by pointed, chamfered, and / or rounded inner corners and outer edges. The stepped profile may have smoothed steps ( / . e. steps having a smooth transition therebetween) such that the profile is curvilinear. As used herein, the term "iron alloy" refers to any iron-based alloy. Examples of iron alloys include iron-based alloys containing carbon, such as steel. "Steel" may refer to alloy steel, stainless steel, or other suitable types of steel. The steel used herein may include additional elements, for example chromium. Other examples of Iron alloys include permendur and its variants. The surface of each pole may include two or more back-cuts. The surface of each pole may include between 40 and 70 back-cuts. In one example, the surface of each pole includes 64 back-cuts, each back-cut with a radial width of 4 mm and a depth of cut of at least 100 mm. As used herein, "acceleration plane" refers to the plane equidistant between the poles. A charged particle may be accelerated within the acceleration plane. A charged particle may be accelerated outside the acceleration plane, for example in a plane substantially parallel to the acceleration plane. The cyclotron may be operable at a magnetic flux density of at least 3 T. Wherein the pair of poles and the pair of coils define a magnet subsystem, the magnet subsystem may be configured to generate a magnetic field having a magnetic flux density of at least 3 T. The magnetic flux density of the magnetic field may be between 3 T and 12 T. The upper limit of the magnetic flux density depends on the type of coil material and pole used. When the pole is incorporated into the magnetic subsystem, the variation in height / level within the stepped profile is such that the magnetic field is significantly influenced by the variation. Each pole may be operable above a threshold temperature, wherein the threshold temperature is between 2 K and 10 K. Each pole may be operable above a threshold temperature, wherein the threshold temperature is 200 K. As used herein, the term "cryogenic temperatures" refers to any temperature below 120 K. Each pole may be configured to operate at room temperature, for example between 288 K and 298 K. As used herein, the term "room temperature" may refer to the temperature of one or more components of the cyclotron which are not being actively cooled, e.g. by a cryostat or other cooling apparatus. The stepped profile may comprise discrete steps defined by pointed, chamfered, and / or rounded inner corners and outer edges and / or smoothed steps defining a curvilinear profile. The back-cuts may form an ascending stepped profile towards the centre of the pole, a descending stepped profile towards the centre of the pole, or a combination thereof. The surface of each pole may have additional back-cuts forming one or more additional stepped profiles. The one or more additional stepped profiles may be ascending or descending stepped profiles towards the centre of the pole, or a combination thereof. The cyclotron may be a superconducting cyclotron. The cyclotron may be an isochronous cyclotron. The cyclotron may be configured to provide continuous wave ("CW") acceleration and extraction of particles. The cyclotron may be a classical cyclotron, an isochronous cyclotron, or a synchrocyclotron. The cyclotron may be another type of cyclotron. Each pole may be sectored to provide a plurality of sectors, wherein each sector comprises a hill and a valley. Each pole may comprise three sectors. Each pole may comprise four sectors. Each hill may have back-cuts forming a stepped profile and, optionally, each valley may have back-cuts forming a stepped profile. Wherein each sector extends from a central region to a peripheral region of the pole in a plane parallel to the acceleration plane, each sector may follow a spiral profile or a substantially straight profile. Each hill may have five back-cuts. Each hill may have back-cuts forming an ascending stepped profile towards the centre of the pole, a descending stepped profile towards the centre of the pole, or a combination thereof. The hills may have the same stepped profile or different stepped profiles. Each hill may have three back-cuts forming an ascending stepped profile towards the centre of the pole, two back-cuts forming a descending stepped profile towards the centre of the pole, and two back-cuts forming an ascending stepped profile towards the centre of the pole. Each valley may be an empty region of the pole. Each valley may be an indented region of the pole relative to the hills. Each valley may have back-cuts forming a stepped profile. A distance between an orbit-facing side of a hill of one pole and an orbit-facing side of a hill of the opposite pole may be less than 40 mm. The cyclotron may further comprise cooling apparatus arranged to cool the pair of coils. The cooling apparatus may be a liquid cooling system or a dry cooling system. The cooling apparatus may exclusively cool the pair of coils. The cooling apparatus is not arranged to cool the poles. The cooling apparatus may be a cryostat. The pair of coils may be formed of a superconductor, such as niobium-titanium or niobium-tln. The cyclotron may be configured to accelerate at least one charged particle such that the charged particle has a kinetic energy of at least 30 MeV. The at least one charged particle may have a kinetic energy from 30 MeV to 250 MeV, for example 70 MeV. The at least one charged particle may have a kinetic energy of at least from 200 MeV to 250 MeV. The one or more charged particles may exit the cyclotron having a kinetic energy of at least 30 MeV, for example from 30 MeV to 250 MeV, or at least from 200 MeV to 250 MeV. The charged particle(s) may be a proton or other positively-charged ion. The cyclotron may further comprise a yoke surrounding the pair of poles and the pair of coils, wherein, optionally, the yoke is formed of steel. If present, the yoke may be included in the magnet subsystem. The yoke may not be actively cooled, e.g. by the cooling apparatus. The yoke may be operable at room temperature. Each pole may be formed of steel. Each pole may be formed of permendur. According to a second aspect of the present invention, there Is provided a method of operating a cyclotron according to the first aspect. The method comprises introducing at least one charged particle into the centre of an acceleration region defined by the acceleration plane, applying: i) an alternating electric field within the acceleration region, and II) a magnetic field substantially perpendicular to the acceleration region, such that the at least one charged particle is induced into an outwardly spiralling orbit within the acceleration region. The method further comprises extracting the at least one charged particle from the acceleration region, wherein, during operation of the cyclotron, each coil is maintained at cryogenic temperatures and each pole is maintained at temperatures above cryogenic temperatures. The acceleration region may include a pair of electrodes configured to generate the alternating electric field. As used herein, "acceleration region" refers to the region In which the charged particle can orbit. The acceleration plane is within the acceleration region. The charged particle may orbit within the acceleration plane. The at least one charged particle may be introduced into the acceleration region by being injected into the cyclotron during operation. A bundle of charged particles may be injected simultaneously or substantially simultaneously into the cyclotron during operation. The at least one charged particle may be introduced into the acceleration region by being generated within the acceleration region using an internal particle source, such as a Penning Ionisation Gauge. The cyclotron of the method according to the second aspect may be an isochronous cyclotron, such as a superconducting isochronous cyclotron. The strength of the magnetic field may vary along an orbital path of the at least one charged particle. The magnetic flux density may be at least 3 T at the centre of the acceleration region. The at least one charged particle may have kinetic energy of at least 70 MeV on extraction from the acceleration region. The cyclotron may be used in radioisotope generation. The cyclotron may be used in particle therapy, for example proton beam therapy. The cyclotron may be used in positron emission tomography or photon emission computed tomography. According to a third aspect of the present invention, there is provided a pole for a cyclotron, wherein the pole: i) is formed of an iron alloy, ii) includes a surface having back-cuts forming a stepped profile, and iii) when incorporated into a cyclotron, is operable at temperatures above cryogenic temperatures. Brief Description of Drawings Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure la schematically illustrates a pole for a cyclotron; Figure lb schematically illustrates a pole for a cyclotron; Figure 1c schematically illustrates a pole for a cyclotron; Figure Id schematically illustrates a pole for a cyclotron; Figure 2a schematically illustrates a first sectored pole structure; Figure 2b schematically illustrates a second sectored pole structure; Figure 3a shows a view of a sectored pole; Figure 3b shows a view of a sectored pole; Figure 3c shows a view of a sectored pole; Figure 3d shows a view of a sectored pole; Figure 3e shows a view of a sectored pole; Figure 4 shows a view of a sectored pole inside a yoke; Figure 5 schematically illustrates a cyclotron; Figure 6 schematically illustrates a subsection of a cyclotron; Figure 7 shows simulation data of a magnetic flux density distribution; Figure 8 shows a plot of magnetic flux density for simulation data and theoretically-predicted data; Figure 9a schematically illustrates a subsection of a cyclotron; 5 Figure 9b shows a resonant frequency structure; and Figure 10 is a process flow diagram of a method of operating a cyclotron. Detailed Description of Certain Embodiments In the following, like parts are denoted by like references. The present application is concerned with a cyclotron comprising a pair of iron alloy poles, each having back-cuts on the pole surface facing away from an acceleration plane of the cyclotron. The pole design according to the present application allows the cyclotron to operate without the need for pole cooling apparatus. Referring to Figure la, a pole 1 according to the present application is shown. Figure la is a cross-sectional view of the pole 1 in the x-y plane. The pole 1 is for use in a cyclotron, such as an isochronous cyclotron, as will be hereinafter described. The pole 1 is formed of an iron alloy, for example steel or permendur. The steel used herein may include additional elements, for example chromium. The pole 1 has a first surface 2 and an opposite, second surface 3. The first surface 2 may be substantially flat (in the x-z plane in Figure la). When incorporated into a cyclotron, the first surface 2 faces an acceleration plane (not shown) of the cyclotron, which is the plane in which a charged particle may be accelerated by the cyclotron. The acceleration plane is also commonly known as the median plane. The charged particle may be accelerated outside the acceleration plane. The region in which the charged particle can orbit is herein referred to as the "acceleration region". Thus, the acceleration plane is within the acceleration region. The second surface 3 faces away from the acceleration plane (not shown). A plurality of back-cuts 4 are formed within the second surface 3. Each back-cut 4 forms a depression in the second surface 3 having a base surface 5 and a side surface 6. The base surface 5 may be substantially perpendicular to the side surface 6. At least two back-cuts 4 are formed in the second surface 3. For example, between 40 and 70 back-cuts 4 may be formed, such as 64 back-cuts 4. The back-cuts 4 are positioned radially relative to each other between the perimeter and the centre of the pole 1. In some examples, an inner back-cut 4 is formed into the base surface 5 of an outer back-cut 4. Thus, in the cross-sectional view shown in Figure la, the plurality of back-cuts 4 form two stepped profiles descending towards the centre of the pole 1 (herein "descending stepped profile"). In other examples, the back-cuts 4 may form a stepped profile ascending towards the centre of the pole 1 ("ascending stepped profile"). In the ascending stepped profile, an inner back-cut 4 protrudes from the base surface 5 of an outer back-cut 4. In other examples, the back-cuts 4 may form a stepped profile having both ascending and descending stepped profiles (herein "combined stepped profile"), as shown in Figures 3a and 3b, described hereinafter. The pole 1 may comprise ascending stepped profiles only, descending stepped profiles only, combined stepped profiles only, or a combination thereof. The pole 1 further comprises a particle injection region 100, which will be described in more detail with reference to a specific example of the pole 1 hereinafter. Alternative stepped profiles As hereinbefore explained, the stepped profiles of the pole 1 are formed by back-cuts 4 formed into the second surface 3. In the example pole 1 shown in Figure la, neighbouring base surfaces 5 and side surfaces 6 meet to form either a pointed outer edge 5o or pointed inner corner 5i. In the cross-sectional view shown in Figure la, neighbouring base and side surfaces 5, 6 meet perpendicularly or substantially perpendicularly to form a pointed outer edge 5o / pointed inner corner 5i. In other examples, neighbouring base and side surfaces 5, 6 may meet at other acute or obtuse angles. Other types of edges and corners for the steps are possible. For example, the edges / corners may be chamfered (Figure lb) or rounded (Figure lc). A given stepped profile according to the present application may have pointed outer edges 5o and / or inner corners 5i only, chamfered or rounded outer edges and / or inner corners only, or a combination thereof. Figure lb shows an example of the pole 1 according to the present application in which the stepped profiles have chamfered outer edges 52 and chamfered inner corners 5s. As with Figure la, Figure lb is a cross-sectional view of the pole 1 in the x-y plane. Each chamfered outer edge 52 and chamfered inner corner 5s has a chamfer surface 54 which is angled with respect to the x and y axes. Each chamfer surface 54 may be at 45° with respect to the x axis. In the pole 1 shown in Figure lb, all outer edges and inner corners of the stepped profiles are chamfered. However, in other examples, only one or some of the outer edges and Inner corners may be chamfered. Figure 1c shows an example of the pole 1 according to the present application In which the stepped profiles have rounded outer edges 5s and rounded inner corners 5e. The rounded outer edges 5s and rounded inner corners 5e are shaped such that the neighbouring base and side surfaces 5, 6 meet without forming a point (in other words, they form a continuous profile). In the pole 1 shown in Figure 1c, all outer edges and inner corners of the stepped profiles are rounded. However, in other examples, only one or some of the outer edges and inner corners may be rounded. The chamfered outer edges 52 and chamfered inner corners 53, as well as the rounded outer edges 5s and rounded inner corners 5e, may be manufactured from the pointed outer edges 5o / pointed inner corners 5i using appropriate processing techniques. The design of the corners and edges may be intentional or may simply be the result of processing techniques used. The design of the corners and edges may be deliberately selected to obtain a desired magnetic field profile and / or to correct mistakes in the initial manufacturing process. Referring now to Figures la to 1c, some or all other edges and corners of the pole 1 may be shaped (e.g., pointed, chamfered, or rounded) - for example as a result of manufacturing techniques used or to meet specific technical requirements. For example, the perimeter edge 2o of the first surface 2 may be pointed (Figure la), chamfered (Figure lb), or rounded (Figure lc). Likewise, the perimeter edge 3o of the second surface 3 may be pointed (Figure la), chamfered (Figure lb), or rounded (Figure lc). Furthermore, an outer rim or edge 4o of each stepped profile (defined by the outermost back-cut 4) and / or an inner rim or edge 4i of each stepped profile (abutting the particle injection region 100) may be pointed (Figure la), chamfered (Figure lb), or rounded (Figure lc). Any edges or rims lOOo occupying the particle injection region 100 may be pointed (Figure la), chamfered (Figure lb), or rounded (Figure 1c). For example, an inner or outer edge of a channel pole piece (hereinafter described) may be pointed, chamfered, or rounded. An inner edge or rim of the pole piece tube (hereinafter described) may be pointed, chamfered, or rounded. In other examples, the stepped profile of the pole 1 may be smoothed to form a curvilinear stepped profile. An example of a curvilinear stepped profile according to the present application is shown in Figure Id. Figure Id shows a cross-sectional view of the pole 1 in the x-y plane. In curvilinear stepped profiles, the base surfaces 5 and side surfaces 6 are less clearly defined compared to the previously described stepped profiles due to the smoothed inner corners and outer edges. However, the curvilinear stepped profile still includes profile sections of varying height as a function of pole radius. In a similar way to the stepped profiles having rounded or chamfered edges and corners, the curvilinear stepped profile may be formed from the stepped profile having pointed outer edges and inner corners 5o, 5i (see Figure la) by polishing or using other appropriate processing techniques. The design of the curvilinear stepped profile may be deliberately selected to obtain a desired magnetic field profile and / or to correct mistakes in the initial manufacturing process. Merely by way of illustration, the example pole 1 in Figure Id has a curvilinear stepped profile and includes rounded perimeter edges 2o, 3o of the first and second surfaces 2, 3, and rounded edges lOOo within the particle injection region 100. However, the edges / corners of the pole 1 having the curvilinear stepped profile may take other forms. In other examples, the stepped profile may include a combination of curvilinear stepped profiles and stepped profiles having pointed, chamfered, and / or rounded edges and corners. Sectored pole examples As will be hereinafter described, the pole 1 may be sectored. In sectored pole examples, each back-cut 4 may define an annular negative space of the pole 1 which is in the form of a sector of a hollow cylinder, wherein the radii of the sector may be curved or straight. In the case of curved radii, the form of the curve may be substantially an Archimedean spiral. For non-sectored pole examples, the negative space of each back-cut 4 may be in the form of a ring torus having a quadrilateral cross-section. In other examples (both sectored and non-sectored), each of the back-cuts 4 may define a more complex negative space that is not substantially annular. Referring now to Figures 2a and 2b, different examples of sectored pole structures 7, 7i, 72 are shown. The pole 1 according to the present application may take the form of one of these sectored pole structures 7. Each sectored pole structure 7 includes several pairs of hills 8 and valleys 9. Typically, there are three pairs of hills 8 and valleys 9. Each hill 8 is a region of the pole 1 which extends from near the centre of the pole 1 to the perimeter. As will be hereinafter shown, when incorporated into a cyclotron, the hills 8 protrude relative to a yoke (not shown) towards the acceleration plane. Valleys 9 are typically empty regions of the pole 1 or indented regions relative to the hills 8. Herein, a hill 8 / valley 9 pair forms part of a "sector" 10 of the pole 1. Typically, during operation, hills 8 generate regions of higher magnetic field and valleys 9 generate regions of lower magnetic field. Figure 2a schematically shows a first sectored pole structure 7i in which three sectors 10 are shown. Each sector 10 follows a substantially straight profile. In other words, the edges 11 of the hills 8 / valleys 9 extending between the centre and the perimeter of the pole 1 follow substantially straight lines. Figure 2b schematically shows a second sectored pole structure 72 in which three sectors 10 are shown. Each sector 10 follows a substantially spiral profile. In other words, the edges 11 of the hills 8 / valleys 9 extending between the centre and the perimeter of the pole 1 follow substantially curved lines. The shape of the substantially spiral profile is determined by a mathematical function for obtaining suitable acceleration of the charged particle(s), and is typically in the form of an Archimedean spiral. Referring now to Figures 3a to 3d, a sectored example of the pole 1, li according to the present application (herein "sectored example pole") will now be described. Figures 3a to 3d each show different perspective views of the sectored example pole li. The sectored example pole li has the second sectored pole structure 72 hereinbefore described and is suitable for use In an isochronous cyclotron. The sectored example pole li has three sectors 10, but may have two or more. The valleys 9 are empty regions of the pole 1. Referring specifically to Figures 3a and 3b, two perspective views of the second surface 3 side ( / .e. the side facing away from the acceleration plane) of the sectored example pole li are shown. The second surface 3 takes the form of three hills 8, each hill 8 having five back-cuts 4: an outermost back-cut 4, 4i (herein "first back-cut"), and subsequent second, third, fourth, and fifth back-cuts 4, 42, 43, 44, 4s. In other examples, a different number of back-cuts 4 may be formed in each hill 8, for example two to ten back-cuts 4. The perimeter 12 of the sectored example pole li is in the form of an enclosed, curved strip which encompasses the sectors 10. Each hill 8 of the sectored example pole li has a combined stepped profile. For each hill 8, the first back-cut 4i and the subsequent second and third back-cuts 4, 42, 4s form an ascending stepped profile. The third back-cut 43 and the fourth back-cut 44 form a descending stepped profile towards the centre of the pole li. The fourth back-cut 44 and the innermost back-cut 4, 4s (or "fifth back-cut") form an ascending stepped profile. It should be appreciated that other example stepped profiles are possible. Furthermore, In other examples, the pole 1 may have any combination of hills 8 having ascending, descending, or combined stepped profiles. Figure 3c shows a plan view of the first surface 2 side of the sectored example pole li. As hereinbefore described, the sectored example pole li has three sectors 10 and has the second sectored pole structure 72. Figure 3d shows a perspective view of the first surface 2 side of the sectored example pole li. The first surface 2 is in the form of the underside of the hills 8 and is substantially flat. In other examples which will be hereinafter described, the valleys 9 may be indented regions relative to the hills 8 and may be formed of the same pole material as the hills 8. In other examples, the valleys 9 may be formed of a different pole material to the hills 8. In other examples, the pole 1 may have four sectors 10, rather than three sectors 10. The pole 1 may have some other number of sectors 10. Referring now to each of Figures 3a to 3d and Figures la to Id, the pole 1 further comprises the particle injection region 100 disposed at the centre of the pole 1. The particle injection region 100 comprises a particle injection channel 110 extending between the first and second surfaces 2, 3. When in operation, particles are injected into the acceleration region, such as the acceleration plane, via the particle injection channel 110. If present in the pole 1, the hills 8 and valleys 9 extend between the perimeter 12 and the particle injection region 100. In the sectored example pole li, each hill 8 comprises a channel pole piece 120 which occupies the particle injection region 100. In other words, each channel pole piece 120 is incorporated into a respective hill 8. Thus, in the present example, the sectored example pole li has three channel pole pieces 120. Each channel pole piece 120 has a substantially columnar shape and extends out of the first surface 3 and away from the acceleration plane. The pole pieces 120 are parallel and eguidistant to each other and define the particle injection channel 110. The particle injection region 100 may further comprise a pole piece tube 130 positioned at the first surface 2 side ( / .e. the side facing the acceleration plane) of the particle injection channel 110. The channel pole pieces 120 are joined together by the pole piece tube 130. Thus, when in operation, an injected particle travels between the channel pole pieces 120 and through the pole piece tube 130. The pole piece tube 130 may protrude or be indented from the plane of the first surface 2. In other examples, the pole piece tube 130 may terminate at the plane of the first surface 2 (as shown in Figure 3d). In other examples, the pole piece tube 130 may not be present. As shown in Figures 3c and 3d, each channel pole piece 120 may have a cross-section such that, in plan view (Figure 3c), each hill 8 has a spiral shape. It should also be appreciated that, in other examples, the particle injection region 100 and particle injection channel 110 may take other forms. In other examples, the pole 1 may not comprise the particle injection region 100. The charged particles to be accelerated may be generated within the acceleration region (such as the acceleration plane 15) rather than injected, for example using a Penning Ionisation Gauge ("PIG"). In examples wherein the particle injection region 100 is absent, the channel pole pieces 120 may be substantially joined such that the particle injection channel 110 and pole piece tube 130 are absent. Referring now to Figure 3e, an alternative sectored example of the pole 1, 12 according to the present application (herein "alternative sectored example pole") will now be described. The alternative sectored example pole I2 is the same as the sectored example pole li hereinbefore described apart from with respect to the form of the valleys 9. In the alternative sectored example pole I2, the valleys 9 take the form of indented regions relative to the hills 8 (in other words, the valleys 9 are not empty regions). The valleys 9 may be formed of the same pole material as the hills 8 or may be formed of a different pole material. In other words, the hills 8 and valleys 9 may be formed of different iron alloys. Figure 3e shows the second surface 3 side ( / .e. the side facing away from the acceleration plane) of the alternative sectored example pole I2. Each hill 8 has the combined stepped profile hereinbefore described (as shown In Figures 3a and 3b). Each valley 9 may have the same combined stepped profile as the hills 8, as shown for example in Figure 3e. In the example shown, the back-cuts 4, 4i, 42, 43, 44, 4s are made at the same radii in both the hills 8 and valleys 9. In other examples, the back-cuts 4 may be made at different radii in the hills 8 to the back-cuts 4 made in the valleys 9. The valleys 9 may have the same or different stepped profiles to the hills 8. The valleys 9 may have any combination of ascending, descending, and / or combined stepped profiles hereinbefore described. Moreover, in other examples, the valleys 9 may be substantially flat on both the first and second surfaces 2, 3 (herein "substantially flat profile"). The alternative sectored example pole I2 may comprise valleys 9 having stepped profiles only, substantially flat profiles only, or any combination thereof. Each valley 9 of the alternative sectored example pole I2 may include a channel pole piece 120, as hereinbefore described with respect to the sectored example pole li. Thus, in such an example, the particle injection region 100 comprises the channel pole pieces 120 of both the hills 8 and valleys 9. In the example shown in Figure 3e, the alternative sectored example pole 12 has six channel pole pieces 120 occupying the particle injection region 100. These channel pole pieces 120 may be joined together by the pole piece tube 130. In other examples, the valleys 9 may not include channel pole pieces 120 and, thus, the valleys 9 may not be joined to the pole piece tube 130. Referring now to Figure 4 and Figures 3a to 3d, the sectored example pole li is placed within a yoke 13 when incorporated into a cyclotron (not shown). Figure 4 is a cross-sectional view of the yoke 13 and the pole li taken along the acceleration plane. As shown, the first surface 2 of the sectored example pole li is facing towards the acceleration plane. Cyclotron example Referring also to Figure 5, a superconducting isochronous cyclotron 14 (herein "cyclotron") is shown in a cross-sectional view along the x-y plane. The cyclotron 14 comprises a pair of sectored example poles 1, li according to the present application. The acceleration plane 15 hereinbefore described is equidistant between the sectored example poles li. When the cyclotron 14 is in operation, charged particles, such as protons, are injected into the cyclotron 14 towards the acceleration plane 15. The charged particles may orbit within or substantially within the acceleration plane 15. As hereinbefore mentioned, the charged particles may orbit outside the acceleration plane 15, for example substantially parallel to the acceleration plane 15. Examples of charged particles include protons and other positively-charged ions. Examples of suitable positively-charged ions include positively-charged molecular hydrogen ion, H2+, fully-stripped helium ion, He2+, and fully-stripped carbon ion, C6+. Figure 5 schematically shows the back-cuts 4 formed in the second surface 3 of each sectored example pole li and shows the acceleration plane along the x-z plane. In the example shown in Figure 5, the sectored example poles 1, li have descending stepped profiles. The cyclotron 14 further comprises a pair of coils 16, which, in the present example, are superconducting coils. Each coil 16 surrounds one of the sectored example poles li and is configured to drive a magnetic field perpendicular to the acceleration plane 15. Each coil 16 is enclosed by a coil support 17 which is configured to provide structural support against the magnetic hoop force (that acts radially outwards) and the attractive magnetic force between the coils 16 (that acts towards the acceleration plane 15). The current-carrying component of the coils 16 may be formed of or substantially formed of a suitable superconducting wire, such as niobium-titanium (NbTi), niobiumtin (NbsSn), or a high-temperature superconducting ("HTS") material such as rare-earth barium copper oxide (ReBCO), magnesium diboride (MgBz) or variants thereof, or bismuth strontium calcium copper oxide (such as Bi-2212). During operation of the cyclotron 14, the coils 16 are kept at cryogenic temperatures by cooling apparatus within the cyclotron 14. This is done to achieve temperature conditions to enable sufficient current within the coils 16. In the present example, the coils 16 are placed within a cryostat 18 to maintain their cryogenic temperature. In some examples, the temperature within the cryostat 18 may be achieved using a cryo-cooler ("dry cooling"). In other examples, the temperature within the cryostat 18 may be achieved using liquid or gaseous cryogens ("liquid cooling"). The yoke 13 surrounds the other components of the cyclotron 14 hereinbefore described. The yoke 13 may be formed of an iron alloy, for example steel. Unlike the coils 16, the yoke 13 may not be cooled by cooling apparatus. The yoke 13 may be kept at room temperature during operation. The space 19 formed between the second surface 3 and the inner surface of the yoke 13 for each pole 1 may be a void (air or vacuum) or it may contain a non-magnetic material. The non-magnetic material may be used as a mechanical support structure for the poles. A pair of channels 140 runs through the yolk 13 (in the y direction in Figure 5). Each channel 140 is perpendicular to and formed on opposite sides of the acceleration plane 15. Each channel 140 aligns with a respective sectored example pole li such that the channel 140 forms part of a respective particle injection channel 110. It should be appreciated that the alternative sectored example poles 12 may be incorporated into the cyclotron 14 instead of the sectored example poles li. It should also be appreciated that the cyclotron 14 hereinbefore described is merely an example cyclotron in which the poles 1 according to the present application may be incorporated. In other examples, the charged particles to be accelerated may be generated within the acceleration region (such as the acceleration plane 15) and, thus, the cyclotron 14 may not comprise the particle injection region 100. Referring now to Figure 6, a cross-sectional view in the x-y plane of a subsection of the cyclotron 14 is shown. The subsection focusses on the pair of sectored example poles li and the acceleration plane 15 therebetween. This subsection of the cyclotron 14 has been projected onto the x-y plane such that the view of the poles 1 is "unwrapped" at a constant orbital radius, r, from the centre of the cyclotron 14, and follows the varying azimuthal angle, 0, from 0° to 360° for one complete orbit of a particle. As will be hereinafter described, the cyclotron 14 comprises a resonant frequency structure surrounding the acceleration plane 15. This structure has been omitted from Figure 6. As a particle travels within the acceleration plane 15 in Figure 6, the particle experiences a higher magnetic field in regions between the hills 8 of each pole li and experiences a lower magnetic field in regions between the valleys 9 of each pole li. As hereinbefore explained, the azimuthal variations around the average magnetic field strength created by the hills 8 and valleys 9 (the "flutter") are part of the forces that focus the charged particle beams being accelerated. The distance, dl, between the hills 8 of the separate poles li may be less than 40 mm. In other examples, the distance, dl, is greater than 40 mm. The distance, dl, may be a constant as a function of orbital radius. This condition does not apply in the particle injection region 100 for examples in which the pole piece tube 130 protrudes or is indented from the plane of the first surface 2. By contrast, the distance between the second surface 3 and the yoke 18 in the space 19 varies as a function of orbital radius due to the back-cuts 4. The pole 1 according to the present invention is operable at temperatures above cryogenic temperatures, for example room temperature. A typical operational temperature of the pole 1 when incorporated into a superconducting isochronous cyclotron may be near to 293 K. By use of the back-cuts 4, a pair of the poles 1 according to the present application can provide a magnetic field of sufficient variation when in use in a cyclotron. In other words, the magnetic field generated by the pair of poles 1 can sustain stable acceleration of the charged particles in the plane in which the particles are being accelerated, for example the acceleration plane 15. This can be achieved without the poles 1 being cooled to cryogenic temperatures. Thus, cooling apparatus for the poles 1 according to the present application can be dispensed with, reducing the required cold mass (the mass to be cooled) and lowering manufacturing costs. This also helps to reduce the running costs of the cyclotron; the superconducting coils 16 have a lower power requirement and the cyclotron has more compact radio frequency cavities. Referring specifically to the sectored example pole li and the alternative sectored example pole 12, the back-cuts 4 on these poles li, I2 can generate a radially-increasing isochronous magnetic field in the acceleration plane 15. This keeps the orbital frequency of particles constant during acceleration and thereby allows for a larger circulating current (particle beam current) than would otherwise be possible. Furthermore, the second sectored pole structure 72 of the poles li, I2 generates variations in the magnetic field strength which focuses the particles. Cooling apparatus for the poles li, I2 is not needed to generate this radially-increasing isochronous magnetic field. Pole simulation data Figure 7 shows a simulation of the distribution of the magnetic flux density of a magnetic field generated by a pair of sectored example poles li incorporated into the superconducting isochronous cyclotron 14 hereinbefore described. The magnetic flux density distribution is in the plane in which the charged particle(s) orbits (expressed in the x-y plane in Figure 7). This simulation was generated using a software package for performing simulations of electromagnetic / electromechanical systems in two or three dimensions. As shown in Figure 7, regions of higher magnetic flux density (approximately 5.0 T to 5.4 T) substantially correspond to regions between opposing hills 8 of the sectored example poles li ("hill regions", indicatively marked "Hl" and "H2" in Figure 7). Whereas regions of lower magnetic flux density (approximately 3.9 T to 4.4 T) substantially correspond to regions between opposing valleys 9 ("valley regions", indicatively marked "VI" and "V2"). The magnetic flux density rapidly decreases at the boundaries between hill and valley regions (indicatively marked "Bl" and "B2"). A subsection of the distribution is shown in panel "a" of Figure 7. The central region of this subsection (indicatively marked "C" in panel "a") substantially corresponds to the region between opposing pole piece tubes 130. In some examples, a constant magnetic field exists in this central region. In the simulation shown in Figure 7, the central region "C" has a magnetic flux density of approximately 4.5 T. The field variation in intermediate regions of the subsection (indicatively marked "CH" and "CV" in panel "a") is less than at larger particle orbit radii (for example, at regions "H2" and "V2" respectively). The intermediate regions "CH" substantially correspond to regions between opposing hills 8 and the intermediate regions "CV" substantially correspond to regions between opposing valleys 9. Thus, the simulation data in Figure 7 illustrates that a simulated magnetic field generated by the pair of sectored example poles li is a substantially isochronous field. This is further illustrated in Figure 8. Figure 8 shows the simulated magnetic flux density of Figure 7 ("Simulation B(r,9)") plotted against a theoretically-calculated isochronous magnetic flux density ("Isochronous y(r)Bo") as a function of radius, r, from the centre of the acceleration plane 15, averaged over all azimuthal angles, 0, for a given radius, r. The Simulation B(r,0) plot closely follows the Isochronous y(r)Bo plot. The simulation data in Figure 7 further shows that the simulated magnetic field has a magnetic flux density of approximately 4.5 T at the centre of the acceleration plane 15 (indicatively marked "C" in panel "a"). The simulation data also shows the simulated magnetic field having an average magnetic flux density of at least 4.5 T. A cyclotron according to the present application (in other words, incorporating the poles 1 according to the present application) may have a magnetic flux density at the centre of the acceleration plane 15 of at least 3 T. The cyclotron according to the present application may have an average magnetic flux density of at least 3 T, for example at least 4.5 T. The superconducting isochronous cyclotron 14 hereinbefore described is capable of generating a magnetic field in the acceleration plane 15 with an average magnetic flux density of at least 4.5 T. The cyclotron according to the present application is capable of generating these magnetic fields using non-cooled poles 1, for example poles 1 at room temperature. As hereinbefore described, the size of the cyclotron according to the present application can be reduced by omitting the cooling apparatus for the poles 1. Thus, the cyclotron according to the present application has a reduced size compared to cyclotrons that use cooled poles whilst providing a suitable magnetic field for particle acceleration of comparable or greater strength. Radio-frequency cavities The superconducting isochronous cyclotron 14 is configured to generate an alternating electric field during operation. The alternating electric field is generated by means of a resonant radiofrequency structure, for example a resonant cavity or cavities of conventional design. Referring also to Figure 9a, a cross-sectional view in the x-y plane of a subsection of the cyclotron 14 is shown. The subsection shows an example of a suitable resonant frequency structure with a pair of opposing hills 8; the example shown is that of an example vertical X / 2 coaxial resonator. The other hills 8 of the sectored example pole li have been omitted from Figure 9a. Referring also to Figure 9b, a perspective view of the resonant frequency structure is shown. The cross-section defined by the curved dashed line a-b in Figure 9b substantially corresponds to the cross-sectional view shown in Figure 9a. The line a-b is curved within a plane parallel to the acceleration plane 15. The resonant frequency structure comprises, for each sector 10, a dee 23 and stem 24 within a liner 25. The liner 25 is a casing surrounding the dee 23 and stem 24. The stem 24 is arranged to support the dee 23 within the liner 25. The liner 25 extends into the region defined by the valleys 9 of the sectored example poles li and conforms to the footprint of the poles li (which, in the example shown in Figure 9b, follows the second sectored pole structure 7z). The dee 23 and stem 24 are arranged within this region of the liner 24. The liner 25 further extends into the region defined by an opposing pair of hills 8. The part of the liner 25 occupying this region takes the form of a dummy dee 26. Thus, the resonant frequency structure comprises, for each sector 10, a dee 23 and a neighbouring dummy dee 26. The acceleration plane 15 is defined by the aligning cavities within each dee 23 and its neighbouring dummy dee 26, as shown in Figure 9b. The dummy dees 26 are not indicated in Figure 9b. The alternating electric field forms substantially within the acceleration plane 15. In the present example, the alternating electric field is formed across the gap between the dee 23 and neighbouring dummy dee 26. Thus, a potential difference is induced between the dee 23 and dummy dee 26. In some examples, there are three dee-dummy dee pairs and, thus, six gaps (such as in the example shown in Figure 9b). In other examples, there may be at least two gaps, from two to eight gaps, or at least eight gaps. The number of gaps depends on the number of dee-dummy dee pairs. In some examples, the frequency of the alternating electric field may be equal to the orbit frequency of the accelerating particles so as to keep isochronism of the accelerating particles. In other examples, the frequency of the alternating electric field may be an integer harmonic of the orbit frequency, or it may be another frequency. In one example, the frequency of the alternating electric field is 205.8 MHz, and the voltage generated across each gap is 25 kV. The frequency of the alternating electric field may be adjusted by means of a moveable tuner 27 (shown in Figure 9b) so as to match the ratio of resonant frequency to orbit frequency to obtain stable acceleration. The voltage across each gap may be likewise varied to obtain suitable acceleration of the particles. In other examples, the frequency of the alternating electric field may be between 50 MHz and 300 MHz, or may be lower or higher than those frequencies. Likewise, the gap voltage may be between 5 kV and 100 kV, or may be lower or higher than those voltages. In other examples, the alternating electric field may be generated by means of a A / 2 vertical resonator, by a A / 4 horizontal resonator, or by another resonant cavity structure. Cyclotron operation Referring also to Figure 10, an example method of operating a cyclotron will now be described. Merely by way of illustration, this example method describes operating the superconducting isochronous cyclotron 14 hereinbefore described and shown in Figure 5. In step Sl.l, a charged particle (such as a proton) is injected into the acceleration region of the cyclotron 14 via the particle injection channel 110. In other examples, more than one charged particle may be injected during step Sl.l, such as a bunch of charged particles. In other examples, a charged particle or charged particles may be generated within the acceleration region during step Sl.l by means of an Internal particle source, such as the PIG source hereinbefore described. As hereinbefore described, the acceleration region is the region between the sectored example poles li in which the charged particle can orbit. The charged particle can orbit within or proximate to the acceleration plane 15. Thus, the acceleration plane 15 defines the acceleration region within the cyclotron 14. Preceding, during, or after step Sl.l, the cyclotron 14 applies an alternating electric field across the acceleration region (SI.2) and a magnetic field substantially perpendicular to the acceleration region (step SI.3). The alternating electric field may be generated by means of the resonant frequency structure hereinbefore described. Steps SI.2 and SI.3 occur simultaneously such that a given particle is gradually accelerated. The magnetic field is generated by the sectored example poles li in response to the induction of a current in the superconducting coils 16. This magnetic field is a radially-increasing isochronous magnetic field. As a result of application of steps SI.2 and SI.3, the charged particle is induced Into an outwardly spiralling orbit within the acceleration region, for example within the acceleration plane 15. The orbital frequency of the charged particle is constant or substantially constant as the orbital radius and kinetic energy of the particle increases. After a pre-set time or after a threshold kinetic energy of the charged particle is reached, the particle is extracted from the acceleration region (step SI.4). The charged particle may have a kinetic energy of at least 30 MeV on extraction from the acceleration region. Cyclotron use The cyclotron according to the present invention may be configured to produce radioisotopes, proton beams, and / or other positively-charged ion beams. Thus, the cyclotron may be used in medical Imaging and / or in clinical or hospital environments for treating cancer patients. Conventional cyclotrons which are configured to produce radioisotopes, proton beams, and / or other positively-charged ion beams are either large and expensive or small but not producing sufficient particle energy. The present invention provides a compact and cheap cyclotron of high extraction energy (for example, approximately 70 MeV). Use of this cyclotron will enable a greater array of isotope-types to be available to hospitals and clinics for imaging and treatment, leading to more accurate and earlier diagnoses and improved radiotherapy treatment. Modifications It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known. Features of one embodiment may be replaced or supplemented by features of another embodiment. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicant hereby gives notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

1. A cyclotron comprising:a pair of poles each positioned on opposite sides of an acceleration plane; anda pair of coils each surrounding one of the poles and configured to be coolable to cryogenic temperatures during operation of the cyclotron;wherein each pole:I) is formed of an iron alloy;ii) includes a surface facing away from the acceleration plane, the surface having back-cuts forming a stepped profile; andiii) is operable at temperatures above cryogenic temperatures.

2. A cyclotron according to claim 1, wherein the cyclotron is operable at a magnetic flux density of at least 3 T.

3. The cyclotron according to claims 1 or 2, wherein each pole is operable above a threshold temperature, wherein the threshold temperature is between 2 K and 10 K.

4. The cyclotron according to claims 1 or 2, wherein each pole is operable above a threshold temperature, wherein the threshold temperature Is 200 K.

5. The cyclotron according to any preceding claim, wherein the stepped profile comprises:discrete steps defined by pointed, chamfered, and / or rounded inner corners and outer edges; and / orsmoothed steps defining a curvilinear profile.

6. The cyclotron according to any preceding claim, wherein the back-cuts form an ascending stepped profile towards the centre of the pole, a descending stepped profile towards the centre of the pole, or a combination thereof.

7. A cyclotron according to any preceding claim, wherein the cyclotron is a superconducting cyclotron.

8. A cyclotron according to any preceding claim, wherein the cyclotron is an isochronous cyclotron.

9. A cyclotron according to claim 8, wherein each pole is sectored to provide a plurality of sectors, wherein each sector comprises a hill and a valley.

10. A cyclotron according to claim 9, wherein each pole comprises three sectors or four sectors.

11. A cyclotron according to claims 9 or 10, wherein:each hill has back-cuts forming a stepped profile and, optionally, each valley has back-cuts forming a stepped profile.

12. A cyclotron according to any preceding claim, wherein the cyclotron further comprises cooling apparatus arranged to cool the pair of coils.

13. A cyclotron according to claim 12, wherein the cooling apparatus is a liquid cooling system or a dry cooling system.

14. A cyclotron according to any preceding claim, wherein the pair of coils are formed of a superconductor, such as niobium-titanium or niobium-tin.

15. A cyclotron according to any preceding claim, wherein the cyclotron is configured to accelerate at least one charged particle such that the charged particle has a kinetic energy of at least 30 MeV.

16. A cyclotron according to claim 15, wherein the at least one charged particle has a kinetic energy from 30 MeV to 250 MeV, for example 70 MeV.

17. A cyclotron according to claims 15 or 16, wherein the at least one charged particle has a kinetic energy of at least from 200 MeV to 250 MeV.

18. A cyclotron according to any preceding claim, the cyclotron further comprising a yoke surrounding the pair of poles and the pair of coils, wherein, optionally, the yoke is formed of steel.

19. A cyclotron according to any preceding claim, wherein each pole is formed of steel.

20. A cyclotron according to any of claims 1 to 18, wherein each pole is formed of permendur.

21. A method of operating a cyclotron according to any preceding claim, the method comprising:introducing at least one charged particle into the centre of an acceleration region defined by the acceleration plane;applying:i) an alternating electric field within the acceleration region, andii) a magnetic field substantially perpendicular to the acceleration region,such that the at least one charged particle is induced into an outwardly spiralling orbit within the acceleration region;extracting the at least one charged particle from the acceleration region;wherein, during operation of the cyclotron:each coil is maintained at cryogenic temperatures; andeach pole is maintained at temperatures above cryogenic temperatures.

22. A method according to claim 21, wherein the strength of the magnetic field varies along an orbital path of the at least one charged particle.

23. A method according to claim 22, wherein the magnetic flux density is at least 3 T at the centre of the acceleration region.

24. A method of any one of claims 21 to 23, wherein the at least one charged particle has kinetic energy of at least 70 MeV on extraction from the acceleration region.

25. A pole for a cyclotron, wherein the pole:I) is formed of an Iron alloy;ii) includes a surface having back-cuts forming a stepped profile; andiii) when incorporated into a cyclotron, is operable at temperatures above cryogenic temperatures.

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