FR2962622A1 - Accelerator i.e. fixed field alternating gradient accelerator for use in e.g. preparation room of hospital to accelerate hadrons for treatment of cancer of patient, has electromagnet applying magnetic field to packets of particles - Google Patents

Abstract

The accelerator (1) has an injector (2) e.g. cyclotron, injecting packets (3) of particles into an acceleration ring (4). An accelerator cavity (5) provides acceleration to each packet for increasing energy until extraction energy. An electromagnet (6) applies magnetic field to the packets, where the field includes amplitude at radial variation in the ring and temporally constant during acceleration. A distribution controller successively distributes the packets to each of extraction lines (7) at extraction energy with a frequency equal to a repletion frequency divided by a number of lines.

Description

GENERAL TECHNICAL FIELD The invention relates to a particle accelerator.

STATE OF THE ART Particle accelerators are electromagnetic devices intended for the high energy acceleration of elementary particles, such as for example protons or ions. Particle accelerators produce particle beams whose power depends on the energy of the particles and the amount of particles extracted from the accelerator per unit time, related to the accelerator repetition frequency, ie say to their ability to frequently repeat the particle acceleration cycle to the required high energy. These accelerators can be in particular linear or circular.

The need has been felt for several years to have a circular particle accelerator capable of producing multiple and distinct particle beams, and the high average current, equal to the average number of particles emitted per unit of time. In the field of circular particle accelerators, pulsed synchrotrons and cyclotrons are among others. In a pulsed synchrotron, the particles are injected at low energy into a ring, and accelerated to achieve the desired energy, via an accelerator field, which is an electric field. A magnetic field is applied to the particles, said field being pulsed at the rate of acceleration of the particles, in order to confine the particles on a quasi-circular trajectory. A generally unique extraction line makes it possible to extract the particles having reached the high energy required for the application in question. The technological solution used in synchrotrons requires the increase of the magnetic field applied to the particles. Thus, in order to have a beam of high energy particles emitted with a high repetition frequency, it is necessary to rapidly increase this magnetic field. Nevertheless, the rate of rise in the field is limited by the existence of eddy currents in the magnets of the ring, typically at 4 Tesla / second. Consequently, the repetition frequency is limited, and is of the order of a few Hz. Thus, in order to obtain an acceptable average current, the synchrotrons are most often limited to a single extraction line. In addition, the synchrotron itself is not very compact, this does not encourage the installation of several extraction lines, which would tend to further increase these machines and pose new technological difficulties. Another known type of accelerator is the cyclotron, in which the particles are injected continuously in the center of the device. The particles are provided with an alternating accelerating field, and an orthogonal magnetic field, which gives the particles a spiral-shaped path to the outside of the cyclotron, where they are extracted through an extraction line. In cyclotrons, it is almost impossible to obtain the emission of several particle beams. Indeed, each extraction causes a loss of about one third of the particles and therefore reduces the average current output of the accelerator. Cyclotrons are therefore most often limited to a single line of extraction. It is therefore necessary to provide a particle acceleration device configured to meet the needs expressed.

PRESENTATION OF THE INVENTION The invention proposes to overcome these disadvantages.

For this purpose, the invention proposes a particle accelerator comprising: an acceleration ring, 2 an injector of particle packets, capable of successively injecting particle packets with an injection energy into the ring, at least one accelerating cavity, adapted to give each injected pack an acceleration in the ring to increase its energy up to an extraction energy, said acceleration being repeated from one injected pack to the other at a repetition frequency greater than 10 Hz, at least one electromagnet for applying a magnetic field to the packets, the amplitude of which is variable radially in the ring and is temporally constant during the acceleration, said accelerator being characterized in that it comprises in in addition to a number N, greater than or equal to 2, packet extraction lines, a distribution controller, configured to distribute successively to each of the lines the packets at the extraction energy, with a frequency equal to the repetition frequency divided by the number N of lines.

According to a variant, the N extraction lines are arranged regularly on a periphery of the ring in a symmetry of angle 2rr / N. According to another variant, the distribution controller comprises at least one extraction magnet capable of applying a magnetic field for extracting the packets towards the lines.

According to another variant, the extraction magnet is positioned in the ring at a radial position corresponding to a desired energy or range of packet extraction energy.

According to another variant, the extraction magnet has a radial thickness corresponding to a desired energy or packet extraction energy range. According to another variant, the controller comprises for each extraction line at least one first extraction magnet, and at least one second extraction magnet positioned near an input of the line.

According to another variant, the accelerator comprises for each extraction line at least one extraction magnet, whose radial position and / or the radial thickness is different from one extraction line to the other, for the 10 selecting energies or energy ranges for extracting different packets from one extraction line to another.

According to another variant, the injector is able to inject the packets into the ring at a variable injection energy according to the injected pack. According to another variant, the injector is able to inject the packets into the ring at a variable injection radius according to the injected pack.

The invention also relates to a treatment building, cylindrical to circular base, comprising an accelerator as defined above and N treatment rooms connected to the N extraction lines.

According to another variant, the N treatment rooms are arranged at an angle of symmetry 2rr / N.

The invention has many advantages. An advantage of the invention is to provide an accelerator capable of providing a multiplicity of particle beams with a high average current. Another advantage of the invention is to allow the emission of a multiplicity of particle beams almost simultaneously. Yet another advantage of the invention is to provide high operating stability. Finally, another advantage of the invention is to provide a multiplicity of particle beams in different and adjustable energy ranges. PRESENTATION OF THE FIGURES Other features, objects and advantages of the invention will emerge from the description which follows, which is purely illustrative and nonlimiting, and which should be read with reference to the accompanying drawings, in which: FIG. is a schematic representation of the principle of a particle accelerator according to the invention; FIG. 2 is a representation of an embodiment of an accelerating cavity that can be used in a particle accelerator according to the invention; FIG. 3 is a schematic view from above of a treatment building using the accelerator according to the invention; - Figure 4 is a sectional view of the building of Figure 3.

DETAILED DESCRIPTION FIG. 1 schematizes a particle accelerator 1 on which the invention is based. Accelerator 1 is a fixed field accelerator and alternating gradients, also known by the acronym "FFAG" (Fixed Field Alternating Gradient). The accelerator 1 comprises an injector 2 of particle packets 3, able to successively inject packets 3 of particles into an acceleration accelerator ring 1. The term "packets 3 of particles" refers to a plurality of particles grouped together. It is most often billions of particles grouped together, but on a distance less than the circumference of the ring 4. The particles in question can be of very different natures according to the envisaged applications: protons, hadrons, ions Injector 2 is for example a cyclotron or other similar injection device. The injector 2 injects each packet 3 of particles into the ring 4 of the accelerator with an injection energy E n.

An order of magnitude of this injection energy is 10 MeV, but this value depends on the applications and particles envisaged. Ring in the context of the present description refers to a generally annular shape. The insertion of each packet 3 of particles takes place at the inner periphery 9 of the ring 4. Once injected, the packets 3 are driven into the ring 4. Accelerating cavities are provided along the outside. These cavities 5 apply an accelerating field to the packets 3, in order to accelerate them and thus increase their energy. This is typically a radiofrequency electric field. An embodiment of an accelerating cavity is shown in FIG. 2. The accelerating cavities known as "VITROVAC", which is an amorphous magnetic material used in these accelerating cavities, are particularly known from the state of the art. The packs 3 follow in the ring 4 a spiral-shaped trajectory, which tends to move them away from the inner periphery 9 of the ring 4 and to bring them closer to the outer periphery 10. The accelerator 1 furthermore comprises at least one electromagnet 6 adapted to apply a magnetic field to the packets 3 in the ring 4. The invention relates to the two main classes of FFAG, which are FFAG "variant" or "invariant" focusing. In "invariant" FFAGs, the accelerated particle package 3 undergoes invariant focusing, i.e., the optical functions and the wavenumber of the packets are unchanged during acceleration, and that the chromaticism is null. To this end, the electromagnets 6 are configured to apply a nonlinear magnetic field, of the form B ^ Bo r, where r is the radial ro position in the ring, taken from the center of the ring , and ro, k constants. In addition, the Bo field is temporally constant during the acceleration of the particle packets. However, it can vary from one acceleration cycle to another. In "variant" FFAGs, the accelerated particle package 3 undergoes a variant focusing, that is to say that the optical functions and the wavenumber of the packets 3 vary with the energy of the particle packets 3. during acceleration, and that the chromatism is non-zero. In this case, the electromagnets 6 are configured to apply a linear magnetic field, of the form B ^ Bo.r, where r is the radial position in the ring. In addition, the Bo field is temporally constant during acceleration of the particle packets. However, it can vary from one acceleration cycle to another. In the two classes of FFAG mentioned above concerned with the invention, the amplitude B of the magnetic field applied by the electromagnets 6 is at radial variation (r) in the ring 4 and is temporally constant during the acceleration. An important advantage of applying a magnetic field of constant amplitude during the acceleration of the particles is to obtain a great stability of operation, be it the magnetic field, the power supply of the electromagnets, or conditions of injection and extraction of particles. This contrasts with pulsed synchrotrons requiring a rise in the magnetic field, which imposes limitations on the repetition frequency. In addition, the accelerating cavities are simpler to implement than those used in pulsed synchrotrons, since they do not require a feedback system, since the accelerator does not require a ramp up. field of magnets. The electromagnets 6 serve both to guide the packets 3 in the ring 4, and to ensure a confinement of the spiral trajectory of the packets 3 inside the ring 4, and thus to avoid divergence and loss of packets 3.

The accelerator 1 further comprises a number N, greater than or equal to 2, lines 7 for extracting the packets 3. In Figure 1, three lines 7 are shown. These lines 7 are generally arranged near the outer periphery 10 of the ring 4.

These N lines can be arranged in a regular manner around the ring 4, and advantageously with an angle symmetry 2rr / N. The accelerator field applied by the accelerating cavities causes an increase in the energy of the packets 3. As the packets 3 increase in energy, they move towards the outer periphery 10 of the ring. 4. The FFAG type accelerators are characterized in that they allow a high repetition frequency, that is to say that they accelerate the packets 3 rapidly so as to increase their energy energy. injection E; n to the extraction energy Eout to which the packets 3 are extracted from the accelerator. The acceleration of the injection energy E; n to the required extraction energy Eout is thus repeated from one pack 3 injected to the other at a frep repetition frequency of at least greater than 10 Hz. Most often, this repetition frequency may be several hundreds of Hz. This is made possible because the acceleration of the packets and their increase in energy are not limited by the rate of rise in the field of the magnets, as in the synchrotrons. pulsed. The repetition frequency is limited only by the amount of radiofrequency of the accelerating cavities. The acceleration and therefore the increase in energy are very fast. The extraction energy may for example be 100 MeV, and is generally between 30 and 250 MeV. This extraction energy depends on the application and the particles. The accelerator 1 further comprises a distribution controller 8, configured to successively distribute to each of the lines 7 the packets 3, with a frequency equal to the frequency of repetition frep divided by the number N of lines, when said packets 3 have reached the extraction energy Eout. This controller is very schematically represented in FIG. 1. The function of the controller 8 is therefore to successively distribute the accelerated packets 3 to the different lines 7, when these have reached the required extraction energy Eout. Thus, each extraction line 7 receives a packet 3 with a frequency equal to the repetition frequency divided by the number N of lines 7, ie frep / N (which corresponds to a period of 1 / (free / N)) . This is very advantageous, since the frequency frep / N is high, since the accelerator 1 is able, by its structure, to accelerate very quickly the energy injection packets E n to the energy extraction Eout, that is to say to achieve a high frequency frep repetition (the accelerator therefore repeats the acceleration cycles with a high repetition rate). As stated above, the frequency of repetition frep is greater than 10 Hz, and is of the order of several hundreds of Hz in many applications. Therefore, at the outlet of each of the extraction lines 7, a high average particle beam is available, since the high energy particle packets 3 are extracted from the ring 4 through each line 7 at a high frequency. The emission frequency of the particle beams at the output of the lines 7 is, for example, between 10 and 200 Hz, ie an average current of several tens of nanoamperes. Thus, the invention provides a multiplicity of beams with a high average current that can be used independently at the output of each of the extraction lines 7. Since the frequency of the particle beam emitted at the output of each extraction line 7 is high, it turns out that from the point of view of a user, it has in fact a continuous beam. There is therefore a multiplicity of beams of particles emitted almost simultaneously and at high frequency, which is very advantageous compared to the techniques of the prior art, and offers many applications. The invention thus exploits the high repetition frequency of FFAG type accelerators.

In order to extract the packets 3 from the accelerator, the distribution controller 8 generally comprises at least one extraction magnet 15. This type of magnet is called a "kicker" by those skilled in the art and is very schematically represented Figure 1. The extraction magnet is controlled to apply a magnetic extraction field to deflect the package 3 from its path to the extraction line. In general, each extraction line 7 is associated with at least one extraction magnet. In some embodiments, a second extraction magnet 16 is positioned near an inlet of the extraction line 7. This second magnet is known to those skilled in the art as a "septum" and is very schematically shown in Figure 1. It also controls the trajectory of the packets 3 to direct them in the line 7 of extraction. The septum is possibly pulsed. Advantageously, each extraction line 7 is associated with a first ("kicker") and a second magnets 15, 16 of extraction ("septum"). As the packet increases in energy, the packet 3 moves away towards the outer periphery 10. There is therefore a correspondence between the radial position (r) of the trajectory of the packet 3 in the ring 4 and his energy. When the package 3 is moving near the outer periphery 10, it has reached the maximum energy obtainable with the accelerator 1 in its configuration, which depends in particular on the accelerating field, the magnetic field, and the injection energy. In one embodiment, it is desired to extract the particle packets with an extraction energy Eout equal to the maximum energy of the accelerator 1, that is to say at a radial position of maximum trajectory. The extraction magnet 15 is then advantageously located near the outer periphery 10 of the ring 4. The extraction magnet 15 applies a magnetic extraction field to the packets 3 to deflect them towards the extraction lines 7 .

A fine adjustment of the extraction energy Eout can be obtained by synchronizing the application of the extraction magnetic field by the extraction magnet 15 with the moment when the particle pack 3 is on the path whose Radial position corresponds to the desired extraction energy.

If it is desired to modify the maximum energy that can be obtained with the accelerator 1, it is necessary to modify the accelerator field applied by the cavities 5, but also the magnetic field applied by the electromagnets 6, without which the packets 3 of Particles will diverge and will no longer exit at the maximum radius (outer circumference 10) of the accelerator. In another embodiment, it is desired to be able to extract the packet 3 not at the maximum energy, corresponding to a trajectory whose radial position is located near the outer periphery 10, but at a corresponding lower energy or energy range. therefore in an intermediate radial position trajectory in the ring 4. In this case, the extraction can be performed using at least one extraction magnet ("kicker") whose magnetic field is controlled to deflect the packets 3 said magnet being located at the radial position corresponding to the required energy or extraction energy range Eout. Thus, the "kicker" is positioned near the ad hoc turn of the spiral package towards the outer periphery. Alternatively, it is possible to use an extraction magnet 15 having a radial thickness corresponding to a desired energy or range of energy extraction End of packets 3. This magnet, by its radial thickness ("radial impasto"), allows to divert only the desired energy packets to the lines 7, or whose energy is located in the required energy range.

The invention makes it possible to have a multiplicity of energy beams or variable and adjustable energy ranges according to the lines 7. For this purpose, each extraction line 7 comprises at least one extraction magnet 15, 16 whose radial position and / or the radial thickness is different from one extraction line 7 to the other. This allows the selection of energies or ranges of extraction energy Bout packets 3 different from a line 7 extraction to another. Alternatively, to obtain beams of variable energy, it is possible to vary the injection energy E1 for each packet 3 inserted by the injector 2. Since the packets 3 are injected at a different energy (for example lower), those This will follow a different trajectory in the ring 4 if the magnetic field applied by the electromagnets 6 is not modified accordingly.

Therefore, it is necessary to vary the amplitude of the magnetic field applied by the electromagnets 6, that is to say the Bo field. The magnetic field change applied to the electromagnets 6 requires a duration which is of the order of 100 ms, which is compatible with an average current of the beams at the output of the accelerator 1 of several tens of nanoamperes per line, and is compatible with an irradiation rate greater than 5 Gy.1 / min in proton therapy applications. It is also conceivable to vary the injection radius of the packets 3 in the ring 4, for example by using a linear injector type LINAC ("linear accelerator") and a "stripping foil" in the accelerator 1. A Another solution for accessing beams of variable energy consists in using a beam degrader at the output of one or more extraction lines 7, which is well known to those skilled in the art, making it possible to modify the energy of the beams. at the output of the extraction lines 7. However, it is not necessary to use the bulky and expensive beam degraders of the prior art, such as those used in the state of the art for cyclotrons. In fact, the degraders used for cyclotrons are most often integrated into heavy installations called ESS ("Energy Selection System"). This is advantageous in terms of neutron environment, radiation protection measures, concrete volume, shielding volume, and accelerator cost. The invention has many applications, both in the fields of industrial irradiation, as research tools. It can for example be applied to the characterization of materials, or the coating of materials by deposition of layers. A non-limiting application of the accelerator according to the invention is radiotherapy, and in particular hadrontherapy, which is a method of cancer therapy using packets of hadrons. The fact of installing a patient treatment room at the exit of each of the N lines 7 of extraction of the accelerator 1 according to the invention makes it possible to feed the N treatment rooms in a quasi-continuous manner (at a frequency typically of the order of 100-200 Hz) by a medium high current hadron beam. In terms of irradiation, the dose is greater than 1 Gy.1 / min per line, and even greater than 5-10 Gy.1 / min in proton therapy. It should be noted that from the point of view of the operator in the treatment room, everything happens as if the beam was emitted continuously. Thus, it is possible to treat N patients simultaneously, which makes it possible to reduce the waiting times and thus the treatment costs. Unlike the solutions of the prior art, it is no longer necessary to wait until the end of the treatment performed in the adjacent rooms. On the contrary, the invention makes possible the simultaneous treatment of several patients. This multiplies the number of patients treated in a given period of time. In addition, the duration of treatment of patients can be increased without having to block the other treatment rooms. It is also possible to permanently devote a room to the beam qualification, while the other rooms are still available to treat patients. In the prior art, the beam qualification made the other treatment rooms unavailable. In addition, since the invention provides access to beams of variable energy, each treatment room may have a beam at a different energy or energy range. This is very advantageous for the operators of the treatment rooms and makes the invention very flexible. Each operator can treat localized tumors at different depths, i.e. with a different Bragg peak depth. The general cost of an accelerator-based treatment according to the invention is reduced compared to the prior art for a variety of reasons. The proposed technological solution is particularly less complex than pulsed synchrotrons, which require heavy pulsed magnets to implement. Moreover, cyclotrons impose many technical difficulties, such as the protection against radiation by thick layers of concrete. In general, the invention makes it possible to revolutionize the architecture of treatment buildings, and in particular hospitals specialized in the treatment of patients by hadrontherapy, carbotherapy or other.

Prior art treatment centers (hadrontherapy centers, proton therapy, etc.) use synchrotrons or cyclotrons based on a single line of extraction of particle packets, feeding contiguous treatment rooms. In view of the technological limitations of the accelerators of the prior art, these treatment rooms are not powered simultaneously, but one after the other. The geometry of the treatment building, which is generally rectangular, results therefrom, as well as the arrangement of the treatment rooms, arranged contiguously, and the patient preparation rooms, opposite the treatment rooms. As illustrated in FIGS. 3 and 4, the invention proposes a cylindrical building with a circular base, comprising the accelerator 1 with its N lines 7 for extracting packages 3, and N treatment rooms 21 placed at the output of the N lines 7 extraction. In general, transmission lines connect the N lines 7 extraction N treatment rooms. Some of the treatment rooms may be research rooms, for example rooms devoted to research and development in radiobiology. The treatment building also includes patient preparation rooms, consultation rooms and other rooms generally present in this type of building. Advantageously, the N lines 7 for extracting the accelerator 1 are arranged regularly around the periphery 10 of the ring 4, in a cylindrical symmetry, for example according to an angle symmetry 2rr / N. Similarly, the N treatment rooms are arranged in the building 20 regularly, in a cylindrical symmetry, for example according to a symmetry of angle 2rr / N. In general, the N treatment rooms are located on the periphery of the building. The building and its treatment rooms are therefore adapted to the symmetry of the extraction lines 7 accelerator 1. With this configuration, access to each N treatment rooms is possible from the center of the building.

In an advantageous embodiment, and as illustrated in FIG. 4, the building comprises at least two floors. A first floor (for example the basement of the building) includes the accelerator 1, while a second floor, located above the first floor, comprises the N treatment rooms 21.

Advantageously, the accelerator 1 is disposed in the center of the building 20. The building 20 has many advantages over the buildings of the state of the art.

The ambulatory route of the patients is substantially reduced since all the treatment rooms are accessible from the center of the building. This results in a reduction in the staff needed to welcome and manage patients and reduce associated costs. In addition, the flow of patients is increased, since the accelerator makes it possible to feed the N treatment rooms almost simultaneously. With the configuration of the building described above, the accelerator and its radiative environment are away from the traffic areas of people, which optimizes the protection of people against radiation. The constraints of safety and protection against radiation are thus minimized. The particular layout of the building also allows many architectural optimizations: the minimization of the volumes of concrete entering the protection against ionizing radiation, the minimization of the cable runs and other distributions of fluids constituting the acceleration of the accelerator , the addition of natural light inputs in the building and in medical areas in particular (openings on the outside by the periphery and in the center by a skylight) etc. The applications of the accelerator according to the invention are very numerous. The examples cited in the present description can not be considered as limiting. 16

Claims (3)

REVENDICATIONS1. Accelerator (1) of particles comprising: an acceleration ring (4), an injector (2) of packets (3) of particles, adapted to successively inject packets (3) of particles with an injection energy (E; n) in the ring (4), at least one accelerating cavity (5), able to give each injected pack (3) an acceleration in the ring (4) to increase its energy up to an extraction energy (Eout), said acceleration being repeated of a packet (3) injected at the other at a repetition frequency (frep) greater than 10 Hz, at least one electromagnet (6) for the application of a magnetic field to packets (3), the amplitude (B) of which is radially varying (r) in the ring (4) and is temporally constant during acceleration, said accelerator (1) being characterized in that it further comprises a number N, greater than or equal to 2, lines (7) for extracting the packets (3), a controller (8) for distribution, configured to successively distribute to each of the lines (7) the packets (3) at the extraction energy (Eout), with a frequency equal to the repetition frequency (frep) divided by the number N of lines (7). 2. Accelerator according to claim 1, wherein the N lines (7) of extraction are regularly arranged on a periphery (10) of the ring (4) in a symmetry angle of 2rr / N. 3. Accelerator according to one of claims 1 or 2, wherein the controller (8) distribution comprises at least one extraction magnet (15) capable of applying a magnetic field extraction of packets (3) to the lines (7). 17. Accelerator according to claim 3, wherein the extraction magnet (15) is positioned in the ring (4) at a radial position corresponding to an energy or range of extraction energy (Eout) of the packets ( 3) desired. 5. Accelerator according to one of claims 3 or 4, wherein the extraction magnet (15) has a radial thickness corresponding to an energy or range of extraction energy (Eout) packets (3) desired. 6. Accelerator according to one of claims 1 to 5, wherein the controller (8) comprises for each extraction line (7) at least one first extraction magnet (15), and at least one second magnet ( 16) located near an entrance of the line (7). 7. Accelerator according to one of claims 1 to 6, comprising for each extraction line (7) at least one extraction magnet (15, 16), whose radial position and / or the radial thickness is different. from one extraction line (7) to the other, for the selection of energies or ranges of extraction energy (Eout) of packets (3) different from a line (7) of extraction to the other. 8. Accelerator according to one of claims 1 to 7, wherein the injector is able to inject the packets (3) in the ring (4) at an injection energy (E; n) variable according to the injected package. 9. Accelerator according to one of claims 1 to 8, wherein the injector is adapted to inject the packets (3) in the ring (4) to a variable injection radius according to the injected package. 10. Building (20) treatment, circular cylindrical, comprising the accelerator (1) according to one of claims 1 to 9, and N rooms (21) for processing connected to the N lines (7) of extraction. 18. The building (20) of claim 10, wherein the N treatment rooms (21) are arranged at 2rr / N angle symmetry. 19