Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H13/00—Magnetic resonance accelerators; Cyclotrons
  • H05H13/02—Synchrocyclotrons, i.e. frequency modulated cyclotrons
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H7/00—Details of devices of the types covered by groups H05H9/00, H05H11/00, H05H13/00
  • H05H7/04—Magnet systems, e.g. undulators, wigglers; Energisation thereof

Definitions

• the present invention relates to a synchrocyclotron. DESCRIPTION OF THE STATE OF THE ART
• Synchrocyclotrons like cyclotrons, are particle accelerators comprising a magnet structure comprising two magnetic induction coils radially surrounding a cavity intended for particle acceleration, between two poles, the cavity comprising a central axis and in which extends a median plane perpendicular to said central axis.
• the particles are produced in a source of particles located in the cavity in the vicinity of the central axis, and are extracted from the source to be accelerated, in the median plane along a spiral-shaped path, by accelerating electrodes fed by a high frequency alternating voltage generator.
• Such synchrocyclotrons are increasingly used for hadron therapy.
• the frequency of the electric field applied to the accelerating electrodes is modulated so as to compensate for the increase in relativistic mass when the particle velocity increases.
• synchrocyclotrons To reduce the size of a cyclotron, it is necessary to increase the magnetic field that guides the ions during acceleration.
• isochronous cyclotrons where the vertical focusing of the beam is obtained by magnetic sectors placed in the gap, it is difficult to increase the average magnetic field above 5 Tesla, because the vertical focus becomes insufficient.
• synchrocyclotrons the magnetic field level can in principle be increased without limits. Synchrocyclotrons are also more compact than cyclotrons, the size of a synchrocyclotron decreasing proportionally with respect to to the magnetic field generated between the two poles.
• a second disadvantage of magnetic fields greater than 6 Tesla is that the realization of the extraction device becomes very complex.
• a third disadvantage of magnetic fields greater than 6 Tesla in the center of the cyclotron is that, for such magnetic fields, the magnetic field in the coils exceeds the magnetic field for which a Niobium-Titanium alloy can be used for the coils. One must then use a Nb 3 Sn alloy, which is much more expensive.
• the synchrocyclotron described above comprises two poles whose profile allows a low focusing of accelerated particles in the median plane and a phase stability so that the charged particles acquire enough energy to maintain the acceleration in the air gap of the poles.
• a charged and accelerated particle oscillates radially and axially around an equilibrium orbit.
• r is the radius of the particle's orbit, the origin of the ray passing through a point of the central axis, and B is the magnetic field in this radius.
• the synchrocyclotron must therefore have a scalable field profile that decreases with the radius so as to satisfy the conditions set by the field focusing index.
• it is arranged to have a pole profile whose field focusing index is less than 0.2 in the cavity for accelerating particles.
• the magnetic field decreases more rapidly as a function of the radius, the magnetic field index increases, the radial frequency v r decreases and the axial frequency v z increases.
• the energy of radial oscillations can be transferred to axial oscillations.
• This increases the axial size of the beam and generally causes the loss of the majority of the accelerated ions.
• the synchrocyclotron includes pole wings located on the edge of the poles, causing a reduction of the air gap before the field index is equal to 0.2, so as to locally increase the magnetic field and to prevent the loss of particles.
• the profile of the poles must evolve from a region around the central axis where the air gap is sufficiently narrow to produce enough magnetic field, to a region located near the pole wings where the air gap is maximum and whose height is at least twice greater than that of the zone of the air gap near the central axis.
• the poles comprise beveled surfaces so as to progressively widen the gap of the poles, the region of the poles where the gap is maximum is between two surfaces forming an acute angle between them.
• the junction between flange 134 and surface 130 has an acute angle.
• Such a pole profile including a deep and narrow region is quite difficult to machine accurately.
• a synchrocyclotron comprising an air gap in which a 5.5 Tesla magnetic field is generated is described in the Wu X document. "Conceptual Design and Orbit Dynamics in a 250 MeV Superconducting Synchrocyclotron” (PhD dissertation, Michigan State University, 1990). The losses of particles at the outlet of the source are less important for such a magnetic field. Nevertheless, the gap between the poles of this synchrocyclotron is relatively narrow, as in the previously described synchrocyclotron, which requires the drilling of a hole in the cylinder head along the central axis of the cylinder head for the introduction of a source of particles in the central region.
• the hole in the bolt hole locally changes the magnetic field at the center of the accelerating cavity, where the magnetic field in the vicinity of the source initially increases with the radius to a maximum, then falls slightly with the radius.
• the field focusing index is therefore initially negative, which causes a defocusing of the trajectory of the particles over a short radius. This effect increases with the radius of the source, hence the need to minimize the diameter of the hole in the cylinder head and the diameter of the source, which reduces the particle production capacity. Also, it is necessary to insert coins circular metal magnetic field compensation, commonly called "shims".
• synchrocyclotron whose gap between the two poles allows the easy insertion of a source and a high frequency oscillation circuit so as to avoid problems as encountered in synchrocyclotrons of the prior art.
• the present invention relates to a synchrocyclotron comprising a ferromagnetic structure, a cold mass structure and a source of particles.
• the ferromagnetic structure generally comprises: two disc-shaped cylinder heads located coaxially with respect to a central axis, parallel and substantially symmetrical with respect to a median plane; a pair of poles that have a section of generally circular shape, of radius R, which are arranged on either side of said median plane, centered on the central axis and separated from an air gap forming a cavity; flux returns that surround the poles and join the two plates of the cylinder heads.
• the cold mass structure generally comprises at least two magnetic induction coils, and is surrounded by flux returns and surrounds the poles.
• the source of particles is generally located in the cavity in a first circular zone of radius R1, less than the radius R of the cavity, its origin being a point of the central axis.
• the gap of the cavity normally has a substantially symmetrical profile with respect to the median plane, its height varying radially.
• the profile of the air gap comprises successively from said central axis: a first portion, of circular section with a radius R2, centered on the central axis, the height of the air gap in the center is equal to H this ntre, and which comprises an annular sub-portion (also called: first annular zone) in which the height increases gradually to a maximum height H max at the radius R2; and a second portion of annular section (also called: second annular zone), which surrounds the first portion, and wherein the height of the air gap gradually decreases to a height H b ords at the edges of the poles.
• the height H of this etween the air gap is greater than 10 cm, and the ratio of the maximum height H max of the height H that etween is between 1, 1 and 1, 5, advantageously between 1, 2 and 1, 5, and preferably between 1, 2 and 1, 4. It will be noted that with this profile of the gap, the average magnetic field produced in the cavity by the coils and the ferromagnetic structure can be between 4 and 7 Tesla.
• the first portion comprises a central sub-portion (also called central zone 6) of radius R1 less than R2, centered on the central axis, where the height of the air gap is constant and of height H this ntre-
• the poles advantageously comprise a succession of beveled annular surfaces centered on the central axis, each of these surfaces forming with its adjacent surface an angle a strictly greater than 90 °, preferably greater than 120 °, and even more preferably greater than 140 °.
• the central sub-portion extends over a radius R1 less than 20% of the radius R of the cavity
• the annular sub-portion extends between the radius R1 and a radius R2 less than 95% of the radius R of the cavity 9.
• the central sub-portion extends over a radius R1 of the order of 10% of the radius R of the cavity and the first annular sub-portion extends between the radius R1 and a radius R2 of the order of 70% of the radius R of the cavity.
• the source is advantageously located in the central sub-portion and held by a support inserted into the cavity substantially parallel to said median plane.
• the poles can be advantageously full, because the source of particles can be introduced radially in the central zone of the air gap.
• the magnetic induction coils can be made in NbTi.
• the present invention relates to a method for producing a synchrocyclotron comprising the steps of: fixing the height of the air gap between the poles in the vicinity of the central axis H that etween such that the height It is greater than 10 cm;
• FIG. 1 is a simplified section of a synchrocyclotron according to an embodiment of the present invention; the section plane containing the central axis of the synchrocyclotron, and the section illustrating above all a ferromagnetic structure of the synchrocyclotron;
• FIG. 2 is a section identical to the section of FIG. 1, also schematically showing a source of particles.
• Figs. 1 and 2 schematically show a synchrocyclotron according to the present invention. It should be noted that some parts of the synchrocyclotron are not shown so as not to clutter the figures.
• the synchrocyclotron shown in the figures to illustrate the invention in a non-limiting manner comprises:
• a ferromagnetic structure 4 comprising: o two base plates, also called disc-shaped cylinder head plates 16, 16 'arranged coaxially with respect to a central axis 1 of the synchrocyclotron, parallel and substantially symmetrical with respect to a median plane 2; a pair of poles 5, 5 ', having a section of generally circular shape, of radius R, arranged on either side of the median plane 2 of the synchrocyclotron, centered on the central axis 1 and separated from a gap forming a cavity 9; and
• a cold mass structure comprising at least two magnetic induction coils 3, surrounded by the flux returns 17 and surrounding the poles 5, 5 ';
• a source of particles 1 1 located in the cavity 9 in a first zone 6 of circular section, of radius R1 less than the radius R of the cavity 9 and whose origin is a point of said axis central 1;
• a high frequency voltage generator 14 located outside the flow returns 17;
• this accelerating electrode (see FIG 2) coupled to the high frequency voltage generator 14; this accelerating electrode comprising a pair of parallel plates, substantially semicircular and separated from one another by a gap, located inside the cavity 9, extending parallel and symmetrically on both sides the middle plane 2 facing the source; and
• a transmission line 13 surrounding the accelerating electrode 12 and located at a distance from the electrode 12.
• the magnetic field generated in the gap between the poles 5, 5 'of the synchrocyclotron is chosen:
• the magnetic field generated in the gap between the poles is advantageously between 4 and 7 Tesla, preferably between 4 and 6 Tesla. It will be appreciated that the production of such a magnetic field does not require the use of Nb 3 Sn superconducting coils. Indeed, NbTi superconducting coils are suitable for the production of a field between 3 and 5 Tesla, which is combined the magnetic field generated by the ferromagnetic structure 4, which is generally of the order of 2 Tesla. NbTi superconducting coils are less expensive and easier to implement than Nb 3 Sn coils.
• the cavity 9 formed by the poles 5 has a radius R whose origin passes through a point of the central axis 1 and whose end coincides with the edges 10 of the poles 5.
• the height of the gap varies according to the radius so as to satisfy the conditions set by the field focusing index n.
• the gap comprises three zones 6, 7 and 8, starting from the central axis towards the edge of the poles:
• a second annular zone 8 between a circle of radius R2 and the edges 10 of the poles, in which the gap between the poles decreases progressively to a minimum height H min at the edges of the poles, so as to increase again the magnetic field and decrease the field focusing index n before the field focusing index n reaches a limit value at which the particles oscillating axially around an equilibrium orbit resonate with the particles oscillating radially around the same equilibrium orbit.
• the ratio between the maximum height H max of the air gap and the height H that etween the air gap in the vicinity of the central axis is strictly greater than 1 and less than 1, 5, of in order to facilitate the machining of the inside of the poles, while satisfying the conditions set by the field focusing index. More preferably, the ratio H max / Hcentre is between 1, 2 and 1, 5.
• the zone comprising the first annular zone 7 and the second annular zone 8 is characterized by a succession of bevelled annular surfaces, centered on the central axis 1, each of these surfaces forming with its adjacent surface an angle a strictly greater than 90 °, preferably greater than 120 °, and even more preferably greater than 140 °.
• the height H of this etween the air gap in the vicinity of the central axis 1 is greater than 10 cm, more preferably greater than 15 cm, more preferably greater than 18.4 cm. It will be appreciated that the height H of this etween the air gap in the vicinity of the central axis higher relative to synchrocyclotrons of the prior art, allows an easier insertion of the source and the high-frequency oscillation circuit comprising the accelerating electrodes and the transmission line. The widening of the gap allows for example to increase the gap between the two plates 12 of the accelerating electrode so as to avoid a collision of particles with the plates 12. The widening of the air gap also allows to increase the distance between the accelerating electrode and the transmission line 13, which reduces the capacitance between these two components and allows the voltage generator 14 to supply a high frequency alternating voltage to the accelerating electrode with less power.
• the height H that etween high in the region of the air gap surrounding the central axis 1, allows the insertion of a source 1 1 laterally rather than axially (cf. Fig. 2).
• the insertion of the source 1 1 can be done, for example, by means of a support 15 coming from outside the cavity 9 and comprising conduits for the circulation of the gas in the source, as well as electrical connections for ignition of the source.
• the insertion of a source laterally makes it possible to dispense with the drilling of a hole in the cylinder head 16, 16 'and the poles 5, 5', which eliminates the negative variation of the field focusing index in the region of the air gap near the central axis 1 and also allows the use of a source of larger diameter than in the prior art synchrocyclotrons. In this way, the source can produce a higher particle current. Also, with the suppression of the negative variation of the field index in the region of the air gap near the central axis, the defocusing problems of the particles at the exit of the source are minimized, and the field compensation rings as used in the prior art synchrocyclotrons become optional, simplifying this region of the gap.
• the average magnetic field in the air gap between the two poles is 5.6 Tesla.
• the height of the gap between the poles in the region near the central axis H that etween is 18.4 cm and the height of the maximum gap H max is 25.3 cm.
• the ratio H max / Hcentre is therefore equal to 1 .375.
• the distance z (cm) separating the poles of the median plane as a function of the radius of the poles r (cm) is given in Table 1.
• the external radius and the height of the synchrocyclotron are respectively 125 cm and 156 cm.
• the dimensions of this embodiment according to the present invention are lower than the cyclotron described by Wu (magnetic field produced in the cavity: 5.53 Teslas, height of the synchrocyclotron: 173.4 cm, external radius of the synchrocyclotron: 132 , 3 cm). Still in this same embodiment according to the present invention, the gap between the plates of the accelerating electrode is 2 cm, and the gap between these plates and the transmission line is 7.4 cm.
• the skilled person can optimize the profile of the poles as a function of the position of the coils relative to the median plane, as well as the dimensions and shape of this coil, while placing itself in conditions where the height of the gap between the two poles in the first zone is greater than 10 cm, where the maximum air gap height ratio H max of the height of the minimum gap H ours that in the central zone (6) is between 1, 1 and 1, 5 more preferably between 1, 2 and 1.5.
• the coils have an inner radius of 55.4 cm centered on the central axis 1, a width of 13 cm and a height of 28.1 cm, and are spaced one from the another 20 cm.
• the present invention also relates to a method for manufacturing a synchrocyclotron comprising two poles separated by an air gap, the method comprising the following steps: fixing the height of the air gap in the vicinity of the central axis H such that such that the height H etween this is greater than 10 cm, preferably greater than 15 cm, preferably greater than 18.4 cm and less than 37 cm;

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• 2010
  • 2010-10-27 BE BE2010/0641A patent/BE1019557A3/fr not_active IP Right Cessation
• 2011
  • 2011-10-27 CN CN201180058890.6A patent/CN103493603A/zh active Pending
  • 2011-10-27 WO PCT/EP2011/068844 patent/WO2012055958A1/fr active Application Filing

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