GB2296372A - Bending accelerated charged particle beams - Google Patents

Abstract

A separated orbit cyclotron incorporates radio frequency cavities (16, Fig 1) synchronised to accelerate charged particles, and an array of permanent magnets (18, 21) arranged to bend the trajectory of the charged particles into the required cyclotron orbits in beam pipes (11, 19, 22). The magnets may be such that the magnetic field strength to which the particles are subjected increases with increasing radius in the cyclotron. The magnets (18, 21) may locate between successive turns of the spiral beam path followed by the particles, with iron formers (23, 24) being provided above and below the plane of the spiral. The radial separation between successive orbits can hence be arranged to be the same at all radii. Electromagnets may be provided for focussing and fine adjustment. The array of permanent magnets can be used to bend a accelerated particle beam in other applications, e.g. proton beam therapy. <IMAGE>

Description

Bending Accelerated Charted Particle Beams The invention relates to apparatus for providing magnetic bending fields for accelerated charged particle beams and more particularly to a separated orbit cyclotron.

The principle of a conventional cyclotron is well known. Electromagnetically excited sector magnets provide a magnetic field which bends moving charged particles into a curved orbit. The particles are accelerated from orbit to orbit by an applied radio frequency electric field. The accelerated particles remain in synchronism with the radio frequency field and follow a spiral path in which the separation of each successive orbit reduces as the particle energy increases, due to the combination of relativistic mass increase and falling magnetic field towards the edge of the cyclotron sector magnets. This bunching of outer orbits occurs as the phase of accelerated particles drifts to the point where no further accelerating field is seen. At this point, acceleration stops and the energy limit of the cyclotron is reached.

Such an acceleration limiting effect is avoided in a separated orbit cyclotron in which sector magnets are provided containing individual bending magnet channels for each orbit within each sector magnet. By arranging for an appropriate increase with increasing radius of the accelerating radio frequency fields and also an increase with increasing radius of the bending magnetic fields, it is possible in a separated orbit cyclotron to achieve orbits in which each successive orbit is separated by the same radial distance from the preceding orbit and the accelerated particles are maintained in synchronicity with the driving radio frequency field. In this way, higher accelerated particle energies can be reached than are possible in conventional cyclotrons.

The technical problems and both fabrication and running costs of providing individual electromagnet bending channels for each orbit and dealing with the defocusing effect created by radially increasing magnetic fields have presented such serious difficulties that little attention has been directed to development of separated orbit cyclotrons.

The present invention is based on an approach to construction of a separated orbit cyclotron using permanent magnets to provide the main bending field which overcomes or significantly reduces these difficulties.

The invention provides, in one of its aspects, a separated orbit cyclotron charged particle accelerator comprising means for applying appropriately synchronised radio frequency fields for accelerating charged particles, and an array of permanent magnets arranged to provide magnetic field for bending the trajectory of charged particles into the required cyclotron orbits.

In a preferred configuration, the array of permanent magnets is divided into a plurality of sectors with radio frequency electrodes or cavities positioned between each sector.

Preferably the magnetic field strength is increased with increasing radius in the cyclotron by an appropriately graded increase in the size of the permanent magnets in dependence upon the radial location thereof. Conveniently the said increase in size is provided by an increase in both width and length of the permanent magnets, whilst the depth remains constant, the depth being the dimension separating the north and south poles of the permanent magnets.

A spiral beam tube is positioned to accommodate a beam of charged particles accelerated in the cyclotron and electrically conducting coils are located around portions of beam tube so that fine adjustments, e.g. for focusing, can be made to the magnetic field by passing controlled electrical currents through the coils.

To facilitate assembly, each sector array of permanent magnets is made up of a plurality of subsectors. Conveniently, the angular extent of each subsector is such as accommodates the length of a single permanent magnet in each magnet channel.

The invention provides in another of its aspects an accelerated particle beam bending device comprising a sectored array of permanent magnets, provided, preferably, with electromagnetic coils for effecting fine adjustments to the magnetic field strength produced by the permanent magnets.

A specific construction of separated orbit cyclotron and accelerated particle beam bending device will now be described by way of example and with reference to the drawings filed herewith in which: Figure 1 is a diagrammatic perspective illustration of the essential components of a separated orbit cyclotron, Figure 2 is a diagrammatic plan view of part of a sector array of magnets, and Figure 3 is a vertical section, at a larger scale, on line 3-3 of Figure 2.

Figure 1 shows the essential elements of the separated orbit cyclotron. A spiral beam tube 11 is connected to a vacuum system (not shown). From the inner part of the spiral, the beam tube is connected via section 12 to a source of ions which are directly injected into the accelerator at 13. The necessary bending of the injected beam to follow the bends in the beam tube being provided in a conventional manner.

It is a feature of a separated orbit cyclotron that extraction is relatively straightforward because of the accessibility of the beam tube. Extraction can be effected by simple deflection of the beam from the beam tube and this can be achieved, if desired, at intermediate radial locations as well as at the periphery of the cyclotron. The deflection system for extraction is not shown as this can readily be achieved with conventional technology. Peripheral extraction is indicated diagrammatically at 14 in Figure 1.

The spiral path of the beam tube 11 passes through a sectored array of bending magnets, as at 15, with radially extending accelerating radio frequency cavities 16 positioned between each sectored array of bending magnets 15.

Figure 2 is a very diagrammatic representation in plan of part of a single sectored array of bending magnets 15. The sectored array of bending magnets 15 is made up of three sub-sectors each, in this example, of a suitable angular extent to accommodate the length of a single permanent magnet at each bending magnet channel defined by a single orbit of the beam tube 11.

In each magnetic channel of each sub-sector, the transverse magnetic field extending through the beam tube 11 derives from two magnets. Thus, the magnets 17 and 18 contribute to the main bending field experienced in the beam tube 11 at 19. The magnets 18 and 21 contribute to the field at 22. This magnetic field in the region of the beam pipe 11 (as at 19 and 22 for example) is in a direction perpendicular to the paper as seen in Figure 2 and in the plane of the paper as seen in Figure 3 parallel with the depth of the permanent magnets, that is parallel with the arrow Dm marked on Figure 3. Magnetic field lines have not been shown on the drawings to avoid confusion.

It will be seen from Figure 3 that the principal structural components of each sub-sector are provided by a pair of spaced apart iron or steel formers 23, 24.

Each magnet is received in a small recess in each former, the recesses serving to locate the magnets.

As seen in Figure 3, this structure provides a series of window apertures through which the spiralling beam tube 11 passes in each orbit of the cyclotron. The magnetic poles of the permanent magnets such as 18 and 21 are at the surfaces received in the recesses, that is so that the direction of magnetic field within the magnets coincides with the dimensional arrows Dm marked on Figure 3. The formers 23 and 24 have the effect of concentrating the magnetic field provided by the magnets and appearing in the gap represented by the window apertures through which each orbit of the beam tube 11 passes.

As shown in Figure 2, for a separated orbit cyclotron, it is preferred to arrange that the separation 6 between each successive orbit remains constant. To achieve this, it is necessary to control the accelerating radio frequency fields so that these increase radially and also to arrange for the bending magnetic fields to increase radially. This is achieved by using permanent magnets of different dimensions according to their radial position. The dimensions of the magnets are its length Lm as shown in Figure 2, its width Wm shown in Figure 3 and being the shorter dimension (not marked) in Figure 2, and the depth Dm shown in Figure 3. The depth Dm remains constant, but the width Wm and length Lm are both increased as the radial position of the magnet increases.

Thus, in Figure 3, Wm' is greater than Wm.

For a given permanent magnet material with a remanant field Br, the field in the "air" gap can be varied by an appropriate choice of the parameters Wm, Wa, Dm and La, where Wa is the width and La is the height of the air gap as marked on Figure 3. The field Ba in the air gap is then given approximately by:

The principal effect of the change in width (and length) of the magnets with increasing radius is to increase the value of Br. There will, however, be a second order influence from the changes in Wm and Wa.

In a conventional cyclotron, there is a natural focusing effect which results from the decrease in magnetic field with increasing radius. This effect is evidently lost in a separated orbit cyclotron so that it is necessary to provide a supplementary field for each path of the beam tube 11 through each sectored array of bending magnets 15 to keep the beam focused within the beam tube 11. In principle, the requirement is to provide a modification to the field distribution within each aperture window such that the magnetic field is stronger at the innermost radius of the beam tube in that channel and weaker at the outermost radius of the beam tube in that channel. Such a field configuration can be achieved by contouring the surfaces of the permanent magnets.However, a simpler and much more flexible approach is to provide electromagnet coils within each magnet channel in each sectored array of bending magnets.

To avoid confusion of the drawings, only one such coil is illustrated diagrammatically in Figure 2 at 25 in the innermost magnet channel of the part of a sector 15 shown in the figure. As may be seen from Figure 3 there are two coils in each window aperture respectively on opposite sides of the beam tube 11. As seen in cross section in Figure 3, the plane of each of the coils respectively 25 and 26 is inclined in the form of a V with the beam tube 11 contained within the arms of the V.

If we consider a single turn of coil 25, this runs along the length of the beam tube within the sector 15 close to the centre of the window aperture on the innermost side thereof. At the end of the run, the turn extends radially wrapping around the outside of the beam tube 11 to a return run, again parallel with the beam tube 11 and extending along the length of the magnet channel where it returns radially, again bending over the beam tube, to start the second turn. As will be clear from Figure 3, the radially outermost run of each turn of the coil 25 is at the uppermost corner (as seen in Figure 3) of the aperture window. Coil 26 is best described as a mirror image of coil 25 as seen in a mirror positioned in the plane defined by the spiral comprising the centre line of the beam tube 11.

With the above described configuration, magnetic bending fields up to 0.8 Tesla can be achieved with permanent magnets now available. The electrical currents required in the electromagnetic coils for focusing and fine adjustments to the magnetic fields are one tenth or less of the currents required in electromagnets if these were to provide the full beam bending magnetic fields.

It will be appreciated that assembly of the structure illustrated in Figures 2 and 3 presents problems in that a well controlled jacking system is required to bring the components together when they are in register because of the very large magnetic forces involved. The approach in which each sectored array of bending magnets 15 is made up of a plurality of subsectors greatly facilitates the assembly, since the forces involved in bringing together the magnets and the formers 23, 24 for each sub-sector will be less than that which would be experienced if one attempted to assemble a whole sector 15 in one go.

For a given radial extent of cyclotron, electromagnetic bending magnets can provide higher fields than presently available permanent magnets. As consequence, a cyclotron of larger radius will be required, using permanent magnets, to achieve the same extracted beam energy. However, the permanent magnet arrangement of the above described example provides many compensating advantages. Thus, the problems of providing water cooling for the electromagnet coils are avoided as also are the problems of providing adequately firm, but electrically insulating, mountings for the electromagnetic coil windings to withstand the full magnetic forces involved. Furthermore, electromagnets, with their cooling require more space transverse to the radial dimension; that is the permanent magnet configuration of the above described example can be in the form of a relatively thin pancake.It is, in fact, possible to assemble two or more such cyclotron accelerators stacked one on top of the other and, further, to arrange for the extracted beams from each cyclotron to be coalesced into an approximation of a single beam.

A further advantage of the separated orbit cyclotron is the ability to accelerate positively charged particles directly from a source of positive ions. In a conventional cyclotron, the difficulty of extraction means that negative particles have to be accelerated and extracted by impingement upon a stripping foil which simultaneously converts the charge from negative to positive. Not only is there a loss of beam intensity in this stripping process but also the cross-section for loss of negative particles by collision with residual gas in the beam tube is higher than the corresponding crosssection for positive particles.

The above described example has applications for proton therapy where proton beams at the required level of about 1 microamp are readily achieved with energies in the range 70 MeV to 200 MeV. Accelerators for proton therapy have a low duty factor requirement.

Other applications are in radio isotope production and both fast and thermal neutron sources. For radio isotope production, beam energy in the range 3 MeV to 30 MeV are required at the rather greater intensity of the order of 5 milliamps.

It will be appreciated that a somewhat simplified form of sectored array of bending magnets coupled with electromagnets for fine adjustments is useful for any accelerated beam bending requirement such as, for example, in a beam transport gantry for proton beam therapy. The function of such a gantry is to change the direction of a beam extracted from an accelerator so that it is positioned conveniently for ease of location of a patient and ease of operation by the surgeon.

The invention is not restricted to the details of the foregoing example. For instance recesses need not necessarily be provided for locating the magnets.

Claims (11)

Claims

1. A separated orbit cyclotron charged particle accelerator comprising means for applying appropriately synchronised radio frequency for accelerating charged particles, and an array of permanent magnets arranged to provide magnetic field for bending the trajectory of charged particles into the required cyclotron orbits.

2. An accelerator as claimed in Claim 1, wherein the array of permanent magnets is divided into a plurality of sectors with radio frequency electrodes positioned between each sector.

3. An accelerator as claimed in Claim 1 or Claim 2, wherein the magnetic field strength is increased with increasing radius in the cyclotron by an appropriately graded increase in size of the permanent magnets in dependence upon the radial location thereof.

4. An accelerator as claimed in Claim 3, wherein the said increase in size is provided by an increase in both width and length of the permanent magnets, whilst the depth remains constant, the depth being the dimension separating the north and south poles of the permanent magnets.

5. An accelerator as claimed in any one of the preceding claims, wherein a spiral beam tube is positioned to accommodate a beam of charged particles accelerated in the cyclotron, and electrically conducting coils are located around portions of beam tube so that fine adjustments can be made to the magnetic field by passing controlled electrical currents through the coils.

6. An accelerator as claimed in Claim 2, wherein each sector is made up of a plurality of sub-sectors.

7. An accelerator as claimed in Claim 6, wherein the angular extent of each sub-sector is such as to accommodate the length of a single permanent magnet in each magnet channel.

8. A stacked array of two or more accelerators as claimed in any one of the preceding claims.

9. An accelerated particle beam bending device comprising a sectored array of permanent magnets.

10. An accelerated particle beam bending device as claimed in Claim 8, wherein electromagnetic coils located around a beam tube provide for fine adjustments to the magnetic field strength provided by the permanent magnets.

11. A separated orbit cyclotron charged particle accelerator substantially as hereinbefore described with reference to and illustrated in the drawings filed herewith.