Field of the invention

[0001] The invention relates to a synchrocyclotron for accelerating charged particles in a median plane of the accelerator and in particular to an extraction device for a synchrocyclotron.

[0002] Such a synchrocyclotron comprises

• a circular or annular acceleration region located in the median plane of the synchrocyclotron;

• a magnet structure for generating a magnetic field across the acceleration region whereby the magnetic field is parallel with a central axis perpendicular to the median plane.

[0003] The synchrocyclotron is configured such that, when in operation, accelerated charged particles are spiralling in the acceleration region in a rotational direction around an axis of rotation being parallel with the central axis.

[0004] Such a synchrocyclotron further comprises

• a magnetic regenerator located at a radial distance from the central axis and extending azimuthally over an angular width going from a first angular position being the angular begin position and a second angular position being the angular end position, the magnetic regenerator being configured for locally increasing the magnetic field such that particles traversing azimuthally the regenerator from the angular begin position to the angular end position are having their axis of rotation being displaced, • a magnetic extraction channel for extracting the accelerated charged particles out of the synchrocylotron, the magnetic extraction channel being located radially outward of the magnetic regenerator and having an entrance starting at a given azimuthal angle .

[0005] Such a magnetic regenerator is configured such that a charged particle spiralling in the acceleration region is traversing azimuthally the magnetic regenerator one or a multiple times before entering said magnetic extraction channel. The present invention is related to such a synchrocyclotron.

Description of prior art

[0006] Some synchrocyclotrons are known from Wu and Gordon in "Magnetic channel design for a 250 MeV superconducting synchrocyclotron", Proceedings of the 12the Int. Conf. On Cyclotrons and their applications, 1989.

[0007] As shown in this document, the extraction device is based on a magnetic regenerator followed by a passive magnetic channel as shown in Fig. 1 of the referenced document. This extraction device is discussed in more detail by Wu in "Conceptual design and orbit dynamics in a 250 MeV superconducting synchrocyclotron", Dissertation 1990. The regenerator has azimuthally an angular width in the median plane that is ranging from 162° to 198° and the passive magnetic channel comprises nine magnetic elements Ml to M9 located at azimuthal angles ranging from 103° to 192°.

[0008] Such an extraction device is also known from older documents such as "A modified regenerative extraction system for a synchro-cyclotron", Proceedings of the International conference on sector-focused Cyclotrons and Meson factories, Geneva, 1963 of De Kruiff and Verster. In this document a similar geometry of the magnetic regenerator and a magnetic extraction channel is shown in Fig. 1.

[0009] However, these passive extraction devices have some drawbacks. First the magnetic extraction channels in the prior art documents are covering a large angular span, i.e. a too long extraction path. This has a major drawback that the devices disclosed are less compact and that for superconducting synchrocyclotrons, the impact on the design of the cryostate is larger. Secondly it has also been pointed out by Antaya and Feng in "High field synchrocyclotron extraction proof of principle demonstration with a small compensated axial bump", 2007 that regenerator only extraction devices have an intrinsic low extraction efficiency.

[0010] More in general, for compact high-field superconducting cyclotrons (e.g. 5 Tesla, 250 MeV) the orbits are much smaller than conventional synchrocyclotrons where the field is lower. The fact that the orbits are closely together creates a difficulty to extract beams with high efficiency from high-field superconducting synchrocyclotron. A high extraction efficiency is needed to reduce activation inside the accelerator.

Hence, there is room for an improved extraction concept for synchrocyclotrons .

[0011] Document US 3,024,379 describes a cyclotron for accelerating particles comprising a device for extracting the accelerated particles. This extraction device comprises a regenerator of ferromagnetic material and a compressor for restricting the paths of the particles in the axial direction. Said regenerator being defined as a short regenerator covering a small azimuthal angle of 12,5°.

Summary of the invention

[0012] It is an object of the invention to develop an extraction device for use in a synchrocyclotron which overcomes the aforementioned problems of the known extraction devices. To this end, a synchrocyclotron for accelerating charged particles in a median plane of the synchrocyclotron is provided. This synchrocyclotron comprises :

• a circular or annular acceleration region located around a central axis in said median plane) of the synchrocyclotron;

• a magnet structure for generating a magnetic field across the acceleration region so that said magnetic field being parallel with a central axis perpendicular to said median plane and passing through the centre of said circular or annular acceleration region.

[0013] Said synchrocyclotron is configured in such a way that, when in operation, accelerated charged particles are spiralling in said acceleration region in a rotational direction around an axis of rotation being parallel with said central axis.

[0014] Said synchrocyclotron further comprises:

• a magnetic regenerator located at a radial distance from said central axis and extending azimuthally over an angular width going from an angular begin position to an angular end position) , said magnetic regenerator being configured for locally increasing said magnetic field such that particles traversing azimuthally the regenerator from the begin position to the end position are having their axis of rotation being displaced;

• a magnetic extraction channel for extracting the accelerated charged particles out of the synchrocylotron, said magnetic extraction channel being located radially outward of said magnetic regenerator and azimuthally having an angular entrance position .

[0015] Preferably, the said angular width of the magnetic regenerator is larger than 20°.

[0016] The magnetic extraction channel is preferably located radially outward of the magnetic regenerator and azimuthally has an angular entrance position. The magnetic regenerator is further configured such that a charged particle spiralling in the acceleration region is traversing azimuthally the magnetic regenerator one or a multiple times before entering the magnetic extraction channel .

[0017] The synchrocyclotron is characterized in that the entrance position of the magnetic extraction channel is located less than 90° downstream from said angular begin position of the magnetic regenerator. Downstream is here defined with respect to the rotational direction of the spiralling particles in the acceleration region.

[0018] In this way, during the last turn before extraction, the particle beam is travelling for a large part in a region near or even outside de pole radius where there is a strong field gradient. As a result of this field gradient, there is a further separation of the trajectory of the last turn from the trajectory of the previous turn. Due to this additional turn separation effect, a regenerator with a less strong separation action can advantageously be used. The use of regenerator having a weaker separation action results in less beam perturbations and hence better beam quality and better extraction efficiency .

[0019] Preferably, the magnetic extraction channel comprises a septum and an anti-septum extending azimuthally over an angular width. The septum and anti-septum are radially separated and the anti-septum is located radially outward of the septum.

[0020] Preferably, the angular begin position of the septum is located more than 2° downstream of the angular begin position of the anti-septum.

[0021] Preferably, the septum has at the angular begin position a radial thickness larger than 5 mm.

[0022] Preferably, the magnetic regenerator comprises iron elements mounted on the upper and lower pole.

Preferably, a gradient corrector is positioned downstream of the magnetic extraction channel.

Short description of the drawings

[0023] These and further aspects of the invention will be explained in greater detail by way of example and with reference to the accompanying drawings in which: Fig.l shows a schematic of the magnetic structure of a synchrocyclotron according to the invention; Fig.2 shows a schematic of a sectional view of an exemplary synchrocyclotron according to the invention ;

Fig.3 shows a schematic of major elements of the extraction device according to the invention; Fig.4 shows a magnetic field as function of radius; Fig.5 shows some elements of an extraction device according to the invention;

Fig.6 shows elements of an extraction device according to an embodiment of the invention;

Fig.7 shows a preferred embodiment according to the invention comprising a gradient corrector;

Fig.8 shows a sectional view of a preferred embodiment of the invention comprising a gradient corrector; Fig.9 shows the effect of a gradient corrector on the magnetic field.

Fig.10 shows the radial position of the particles for the last four turns in a synchrocyclotron using an extraction device according to the invention.

[0024] The figures are not drawn to scale. Generally, identical components are denoted by the same reference numerals in the figures.

Detailed description of preferred embodiments [0025] The present invention has been described in terms of specific embodiments, which are illustrative of the invention and not to be construed as limiting. More generally, it will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and/or described hereinabove. The invention resides in each and every novel characteristic feature and each and every combination of characteristic features described in one or several embodiments.

[0026] Reference numerals in the claims do not limit their protective scope.

[0027] Use of the verbs "to comprise", "to include", "to be composed of", or any other variant, as well as their respective conjugations, does not exclude the presence of elements other than those stated.

Use of the article "a", "an" or "the" preceding an element does not exclude the presence of a plurality of such elements.

[0028] With reference to Fig. 1, a schematic of an exemplary synchrocyclotron 1 is shown (not all parts of the accelerator are shown) .

[0029] The synchrocyclotron comprises a magnetic structure for creating a magnetic field for circulating the particles during the acceleration process. The magnetic structure comprises two circular or annular superconducting magnetic coils 20, 25. These coils, having an annular shape, are symmetrically superimposed as illustrated in Fig.l. In this example, the coils have an outer radius of about 68.5 cm and an inner radius of about 55.4 cm. The magnet structure further comprises a magnetic yoke structure 30 (dashed area in Fig. 1) having an upper pole 33 and a lower pole 34. In this example, the poles have a radius of about 49.7 cm and the external radius of the yoke structure is about 125 cm.

[0030] The superconducting coils 20, 25, together with the magnetic yoke structure 30 are forming the magnetic structure of the synchrocyclotron and are generating a combined magnetic field between the two poles of the magnetic structure. This magnetic field is parallel with an axis 50 perpendicular to the median plane 60 and passing through the centre of the circular or annular acceleration region. This axis 50, as indicated in Fig. 1, is also going axially through the centres of the coils and this axis 50 is forming a central axial axis for the entire magnetic structure. In this example, a 250 MeV proton synchrocyclotron is provided having a magnetic structure designed for providing a total magnetic field of about 5.6 Tesla for bending protons during a circular or annular acceleration process.

[0031] In a synchrocyclotron the particles are accelerated in a circular or annular region 10 in the median plane 60 of the accelerator. This acceleration region 10 is located between the poles 33, 34 of accelerator. An ion source 40 is installed at or near the centre of the circular or annular acceleration plane 10.

[0032] The superconducting coils 20, 25 with their supporting structure are called the cold-mass structure of the magnet structure as these parts are kept below a temperature where the conductors of the coils are becoming superconducting. The cold-mass structure is encapsulated in a cryostat (not represented) .

[0033] With reference to Fig. 2, a schematic representation of a sectional view in a median plane of the exemplary synchrocyclotron having an extraction device according to the invention is shown (only a limited number of components are schematically shown on Fig. 2) . The synchrocyclotron is configured such that, when in operation, the accelerated charged particles are spiralling in the acceleration region 10 in a rotational direction 90 around an axis of rotation which is parallel with the central axis 50.

[0034] The extraction device is based on a so-called regenerative method. The extraction device comprises a regenerator 70 and a magnetic extraction channel 80. The magnetic regenerator 70 is located at a given radial distance from the central axis 50 of the magnetic structure and configured for locally increasing the magnetic field in a region of an azimuthal angular width. The regenerator 70 is located close to the pole radius 36, e.g. as is schematically shown in Fig. 2. The pole radius 36, defined as the distance from the centre of the accelerator region to the radial edge of the pole, is shown in Fig. 2 as a dotted circle. The magnetic regenerator 70 extends azimuthally over an angular width going from an angular begin position 71 to an angular end position 72. The configuration of the regenerator 70 and the magnetic extraction channel 80 are also shown in Fig. 3 in more detail .

[0035] The magnetic regenerator 70 comprises iron elements. The effect of the magnetic regenerator 70 on the magnetic field is that when viewing the magnetic field in the median plane 60 as function of the radius a positive magnetic field dump increasing the main magnetic field at the location of the regenerator is observed. This is illustrated in Fig. 4 where the magnetic field in the median plane of the exemplary synchrocyclotron is shown along a line starting at the centre of the acceleration region and going azimuthally through the middle of the regenerator. At the location of the regenerator, i.e. at a radius of about 470 mm the field dump is observed. At a higher radius the magnetic field drops when passing the pole radius 36 (located around 497 mm) . The magnetic field dump extends azimuthally over an interval Delta_REG related to the azimuthal size of the regenerator. The magnetic regenerator used for the exemplary synchrocyclotron has an azimuthal width of 29°.

[0036] The function of the generator is to increase the radius-gain per turn. The separation between the turns must be larger than the opening of the magnetic extraction channel 80 in order to let the beam to enter the magnetic extraction channel 80 for extraction the beam successfully out of the accelerator. The magnetic regenerator 70 is configured such that a charged particle spiralling in the acceleration region 10 is traversing the magnetic regenerator 70 one or multiple times before entering said magnetic extraction channel 80.

[0037] As shown in Fig. 2, according to the invention, the magnetic extraction channel 80 is, as viewed in the direction 90 of the spiralling particles, located downstream of the magnetic regenerator 70 such that a particle traversing the magnetic regenerator 70 for the last time is further spiralling in the median plane over an azimuthal angle of at least 360° before entering the magnetic extraction channel.

[0038] In this way, before the particle enters the magnetic extraction channel 80 the particle is travelling in the fringe field of the poles 33, 34 located at the pole edge 36 or just outside the pole edge where the magnetic field is dropping (as further discussed below and illustrated in Fig. 4) . The effect of this strongly reduced magnetic field on the beam particles is that the separation between the last turn and the previous turn is further increasing .

[0039] According to the geometry of the synchrocyclotron of the present invention, where optimum use is made of the additional beam separation effect due to the magnetic fringe field, the necessary separation action of the regenerator 70 can be reduced (e.g. by reducing it radial size or by reducing it thickness) . As the separation action of the regenerator 70 is reduced, the effect of increasing the size of the beam is also reduced which leads to less beam losses in the extraction channel 80 and hence an overall gain in extraction efficiency. A calculation of the last four turns (N, N-l, N-2, N-4) in the synchrocyclotron according to the invention is shown in Fig. 10. This figure represents the radius R of the particles as function of their angular position Θ in the accelerator for the last four turns in the accelerator. In this illustration, the particles are circulating in the accelerator from 0° to 360°. After the last turn N, the beam exits the accelerator through the magnetic extraction channel 80. The angular positions of the magnetic regenerator 70 and the magnetic extraction channel 80 are indicated on the figure (dashed areas) . As shown on the figure, the angular entrance position 85 of the magnetic extraction channel 80 is in this example located less than 50° downstream from the angular begin position 71 of the magnetic regenerator 70. With this geometry of the regenerator 70 and extraction channel 80 according to a configuration of the invention and shown e.g. in Fig. 2 and Fig. 3, an extraction efficiency of about 50 % is obtained which is to be compared with an extraction efficiency of about 25% obtained with a classical geometry where the entrance of the magnetic channel starts upstream from the begin location of the regenerator (see also the geometry disclosed by Wu and Gordon as discussed above) .

[0040] According to the invention, the entrance position 85 of the magnetic extraction channel 80 is located less than 90° downstream from said angular begin position 71 of the magnetic regenerator 70, whereby downstream is being defined with respect to the rotational direction 90 of the spiralling particles in the acceleration region 10.

[0041] Preferably, the entrance position 85 of the magnetic extraction channel 80 is located less than 45° downstream from said angular begin position 71 of the magnetic regenerator 70, whereby downstream is being defined with respect to the rotational direction 90 of the spiralling particles in the acceleration region 10. [ 0042 ] More preferably, the entrance position 85 of the magnetic extraction channel 80 is located less than 35° downstream from said angular begin position 71 of the magnetic regenerator 70, whereby downstream is being defined with respect to the rotational direction 90 of the spiralling particles in the acceleration region 10.

[ 0043] The magnetic extraction channel 80 comprises a septum 81 and an anti-septum 82 as show in Fig. 3. The radial distance between the septum 81 and anti-septum 82 defines the opening for extracting the beam through the channel between septum 81 and anti-septum 82. The arrow 100 in Fig. 3 indicates the beam being extracted out of the accelerator .

[ 0044 ] Preferably, the septum 81 and anti-septum 82 having angular begin positions that are different from each other .

[ 0045] Preferably, the angular begin position of the septum 81 is located more than 2° downstream of the angular begin position of the anti-septum. This separation in angular begin position of septum and anti-septum contributes to an improved beam focussing in the extraction channel .

[ 0046] Preferably, the components of the regenerator 70 and the components of the magnetic extraction channel 80 are mounted on aluminium support plates 85 as shown in Fig. 5 and 6. The aluminium support plates 85 with its components are then mounted on the poles of the accelerator. On Fig 6 it is shown that the anti-septum 82 and the regenerator 70 comprises an upper and a lower part, installed on the upper and lower pole, respectively. The septum 81 is made out of a single component installed in the median plane of the accelerator. Preferably, additional iron correction bars 75 are mounted on the upper and lower aluminium support plates 85. These correction bars 75 are used to correct for distortions of the magnetic field. On Fig. 5, not all components are shown in order not to overload the drawing.

[0047] More preferably, the septum 81 has at its angular begin position a radial thickness larger than 5 mm. A broader septum has the advantage that the magnetic field at a radial distance outside of the septum 81 is more reduced and as a result a stronger field gradient inside the channel between septum and anti-septum is obtained. This contributes to reducing the extraction path length out of the cyclotron. In prior art synchrocyclotrons, the septum is in general much thinner (e.g. 3 to 4 mm) in order to have not too many beam losses on the septum. With the extraction device of the invention, the beam separation at the level of the angular begin position of the septum is already so large that a thicker septum can be used.

[0048] More preferably, the synchrocyclotron according to the invention further comprises a gradient corrector 95 positioned downstream of said magnetic extraction channel 80. A three dimensional view of the gradient corrector 95 and its position with respect to the extraction channel is shown in Fig. 7. In this specific embodiment the entrance of the gradient corrector starts at about 11 cm from the exit of the magnetic extraction channel 80, which is at a position where there is still a substantial magnetic field (of the order of 1 T) . The function of this gradient corrector is to modify the transverse phase space of the extracted beam such that it is better adapted to the beam optical requirement of the external beam line. The gradient corrector 95 comprises three iron bars with optimized geometrical shape. In Fig. 8, a sectional view is illustrating the location of the gradient corrector 95 with respect to the coil 20 and yoke 30. This sectional view shows only one upper quarter of the accelerator and hence not all the parts of the gradient corrector 95 are shown on this figure. The iron of the gradient corrector 95 is magnetized by the external magnetic field. By this magnetization it induces a change of the magnetic field and the gradient of the magnetic field at the location where the beam passes through the gradient corrector. The effect of the gradient corrector 95 on the magnetic field is illustrated in Fig. 9 where the magnetic field without (dashed line) and with gradient corrector (full line) is shown. Clearly the gradient in the magnetic field has been reduced when using the gradient corrector. This change of the magnetic field represents a magnetic dipole effect that steers the beam in a horizontal plane. The change of gradient represents a quadrupole effect that changes the horizontal and vertical divergence of the beam envelope. More specifically, in this preferred embodiment, the gradient corrector 95 was optimized in such a way as to modify the vertical divergence of the beam at the exit of the cyclotron from strongly focused to almost parallel. This allows simplifying the design of the external beam line .