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EP2410823B1 - Cyclotron for accelerating at least two kinds of particles - Google Patents

Cyclotron for accelerating at least two kinds of particles Download PDF

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Publication number
EP2410823B1
EP2410823B1 EP20100170531 EP10170531A EP2410823B1 EP 2410823 B1 EP2410823 B1 EP 2410823B1 EP 20100170531 EP20100170531 EP 20100170531 EP 10170531 A EP10170531 A EP 10170531A EP 2410823 B1 EP2410823 B1 EP 2410823B1
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EP
European Patent Office
Prior art keywords
cavity
transmission line
rod
frequency
intermediate portion
Prior art date
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EP20100170531
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German (de)
French (fr)
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EP2410823A1 (en
Inventor
Michel Abs
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Ion Beam Applications SA
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Ion Beam Applications SA
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Application filed by Ion Beam Applications SA filed Critical Ion Beam Applications SA
Priority to EP20100170531 priority Critical patent/EP2410823B1/en
Priority to JP2013520036A priority patent/JP5858300B2/en
Priority to PCT/EP2011/060835 priority patent/WO2012010387A1/en
Priority to CA2800290A priority patent/CA2800290C/en
Priority to US13/807,989 priority patent/US8823291B2/en
Priority to CN201180035515XA priority patent/CN103004292A/en
Publication of EP2410823A1 publication Critical patent/EP2410823A1/en
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    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05HPLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H7/00Details of devices of the types covered by groups H05H9/00, H05H11/00, H05H13/00
    • H05H7/14Vacuum chambers
    • H05H7/18Cavities; Resonators
    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05HPLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H13/00Magnetic resonance accelerators; Cyclotrons
    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05HPLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H7/00Details of devices of the types covered by groups H05H9/00, H05H11/00, H05H13/00
    • H05H7/02Circuits or systems for supplying or feeding radio-frequency energy

Definitions

  • the present invention relates to the field of cyclotrons, and in particular to cyclotrons capable of accelerating several types of charged particles having different charge (q) / mass (m) ratios, such as for example protons (equal q / m ratio at 1), alpha particles (ratio q / m equal to 1 ⁇ 2) or deuterons (ratio q / m also equal to 1 ⁇ 2).
  • protons equal q / m ratio at 1
  • alpha particles ratio q / m equal to 1 ⁇ 2
  • deuterons ratio q / m also equal to 1 ⁇ 2
  • such a cyclotron comprises acceleration electrodes 28, commonly called dice, each coupled to a vertical pillar 29 also called stem. Said die 28 and said pillar 29 are surrounded by a conductive enclosure which together constitute a resonant cavity.
  • the resonant cavities are generally excited by an RF power source and the successive passage of the charged particles in the accelerator gap consisting of dice and sectors brought to different potentials produces the acceleration of said particles.
  • a cyclotron can also operate in harmonic mode: in this case several oscillations of the RF voltage occur while the particles still circulate inside the die.
  • the pillar forms an axial transmission line essentially having an inductance for compensating the capacitive impedance of the die to minimize reactive RF power.
  • the cavities are arranged asymmetrically or symmetrically with respect to the median plane of circulation of the particles.
  • the two plates constituting the die are mechanically and electrically integral and constitute a single assembly carried by the pillar.
  • the lower and upper pillars respectively support the lower half-die and the upper half-die. These are electrically connected to each other at a few points in their perimeter as soon as the cyclotron is closed.
  • the die is part of a resonant cavity 5 as schematically shown in FIG. figure 1 at.
  • This cavity comprises the actual dice 10, a vertical cylindrical pillar 20 and a conducting enclosure 40.
  • figure 1c represents an equivalent circuit diagram of the cavity, in which the inductance L represents the pillar 20 and the capacitance C is that formed at the space between the die 10 and the conducting enclosure 40.
  • the . VS a parallel LC circuit
  • the present invention aims to solve at least partially the aforementioned difficulties.
  • the present invention relates to a resonant cavity for accelerating charged particles in a cyclotron, comprising a die, a pillar and a conductive enclosure at least partially including said pillar and said die, an end of said pillar supporting the die. , the conductive enclosure and the pillar thus forming a transmission line, an opposite end of said pillar being integral with a base of the conductive enclosure, characterized in that the linear capacitance of an intermediate portion of said transmission line located between said ends of the pillar is substantially greater than the linear capacity of the other portions of said transmission line.
  • Such a configuration makes it possible to make the cavity resonate according to two modes thus producing two distinct frequencies, without having to make use of mobile elements such as, for example, sliding shorts or moving plates, which resolves many of the problems mentioned. previously.
  • the linear capacitance of the intermediate portion of the transmission line is greater than twice the linear capacitance of the other portions of said transmission line. More preferably, the linear capacity of the intermediate portion of the transmission line is greater than ten times the linear capacity of the other portions of said transmission line.
  • the characteristic impedance of the intermediate portion and the characteristic impedances of the other portions of the transmission line are such that the cavity is able to resonate in two modes to produce two distinct frequencies in a substantially double ratio.
  • substantially double it is necessary to understand a frequency ratio lying between 1.7 and 2.3.
  • Such a cavity makes it possible to accelerate, in the same cyclotron, particles having values of q / m in a ratio of two, such as for example protons and alpha particles or protons and deuterons.
  • the pillar comprises a plurality of superimposed cylinders, one of these cylinders corresponding to said intermediate portion of the transmission line and having a mean diameter substantially greater than the average diameter of the other cylinders.
  • the conductive enclosure comprises a plurality of superposed hollow cylinders, one of these hollow cylinders corresponding to said intermediate portion of the transmission line and having a mean diameter substantially smaller than the average diameter of the other hollow cylinders.
  • the invention relates to a method for designing a dual-frequency resonant cavity as claimed.
  • the figure 1a represents a section of an asymmetric resonant cavity of a cyclotron of the prior art
  • the figure 1b represents a section of a symmetrical resonant cavity of a cyclotron of the prior art
  • the figure 1c represents a simplified equivalent electrical diagram of the resonant cavity of the Figure 1a or 1b ;
  • the figure 2a schematically represents a section of a cavity according to the invention with indication of the circulation of the current and the magnetic field during resonance at the low frequency;
  • the figure 2b represents the evolution of the voltage and the current along the pillar during the operation of the cavity of the figure 2a in mode ⁇ 4 ;
  • the Figure 2c represents a simplified equivalent electrical diagram of the resonant cavity of the figure 2a ;
  • the figure 3a schematically represents a section of a resonant cavity according to the invention with indication of the circulation of currents and magnetic fields during resonance at the high frequency;
  • the figure 3b represents the evolution of the voltage and the current along the pillar during the operation of the cavity of the figure 3a in mode 3 ⁇ ⁇ 4 ;
  • the figure 3c represents a simplified equivalent electrical diagram of the resonant cavity of the figure 3a ;
  • the figure 4a represents a real geometrical shape and a distribution of the equipotentials of a static electric field of a cavity of the prior art
  • the figure 4b schematizes a cavity of the prior art in the form of a coaxial transmission line whose characteristic impedance is a function of the diameters d and D;
  • the figure 5 represents a graph illustrating the power dissipated in a resonant cavity according to the invention for each of the two resonance frequencies as a function of the value of the capacitance of the characteristic low-impedance line portion;
  • the figure 6a represents an impedance diagram of a pillar in an embodiment of the invention.
  • the figure 6b schematically represents a section of the cavity according to the invention, to be related to the impedance diagram of the figure 6a ;
  • the figure 7 represents a section of a bi-frequency cyclotron equipped with four cavities according to the invention.
  • the figure 8 schematically represents a graph showing the two distinct frequencies in a double ratio obtained by frequency scanning of a cavity according to the invention.
  • the figure 2a schematically represents an exemplary embodiment of a dual-frequency cavity according to the invention.
  • This is a symmetrical cavity relative to the median plane of the cyclotron (represented by a mixed dotted line in the figure), but it is obvious that an asymmetrical cavity would also be suitable.
  • the cavity 6 comprises two half-dice 10 and 10 'electrically connected together and between which will circulate the particles to accelerate, two pillars each comprising three portions 20a, 20b and 20c (20a', 20b 'and 20c'), and two conductive speakers 40 and 40 'surrounding the whole.
  • the speakers have a cross section which, in this example, is substantially constant over the height of the pillars.
  • Each pillar respectively supports a half-die at one end, the opposite ends being respectively electrically connected to the bases 45 and 45 'of the conductive speakers 40 and 40' to constitute a short circuit from the radiofrequency point of view.
  • the different portions of the pillar are superimposed and preferably aligned along the same axis.
  • Said portions consist, in this example, of cylindrical tubes of different diameters, examples of dimensions of which will be given below when a method of designing a cavity according to the invention will be described.
  • the diameter of the intermediate portion 20b is substantially greater than the diameter of the other two portions 20a and 20c, so that the linear capacity (in Farad per meter) of this intermediate portion 20b is substantially greater than the linear capacity of the other two portions 20a and 20c. Consequently, the intermediate portion 20b will have a substantially capacitive behavior while the other portions 20a and 20c will have an essentially inductive behavior, in the operating frequency range of the cavity (which is in the megahertz).
  • a first type of operation is obtained by exciting the cavity in ⁇ 4 ( ⁇ being the wavelength), which makes it possible to obtain a first resonance frequency (hereinafter "the low resonance frequency", for example 33 MHz).
  • the figure 2b represents the evolution of the voltage (U x ) and the current (I x ) in this mode as a function of an axial position x along the pillar. The voltage is maximum at the die while the corresponding current is zero or very low. This is reversed when one comes back to the foot of the pillar.
  • This voltage configuration is particularly suitable for accelerating particles moving in the median plane of a cyclotron.
  • the magnetic field B is oriented identically on either side of the intermediate portion 20b (hereinafter "the low impedance line 20b").
  • the resulting current i 1 of this mode circulates axially and is distributed radially around the pillar as shown in FIG. figure 2a .
  • FIG 3a A second type of operation is illustrated in figure 3a .
  • the physical structure is identical to that of the figure 2a but we excite the mode 3 ⁇ ⁇ 4 , which makes it possible to obtain a second resonance frequency (hereinafter referred to as the "high resonance frequency", for example 66 MHz), higher than the first frequency.
  • the figure 3b represents the evolution of the voltage (U x ) and of the current (I x ) in this mode and, like the first resonance mode, the voltage is always maximum at the dice level, while the corresponding current is zero or very weak. Furthermore, the current is reversed at an intermediate point about halfway up the low impedance line 20b, which has the effect of dividing the capacitive effect of this portion of line 20b in two.
  • the figure 3c represents a simplified equivalent electrical diagram with the circulation of currents i 2 and i 3 respectively present in the upper and lower part of the half-cavity. They are distributed radially around the pillar, in opposition to a virtual horizontal plane transversely sharing the low impedance line 20b, in which they cancel each other out.
  • an intermediate portion of the cavity has a linear capacity substantially greater than the linear capacity of the other portions, preferably greater than twice the linear capacity of the other portions, even more preferably greater than ten times the linear capacity of the other portions.
  • a calculation method for designing and dimensioning a structure of a cavity according to the invention is provided below.
  • the characteristic impedance of the known pillar is evaluated, for example using the Tricomp program of Field Precision LLC. This program solves the electric field by the finite element method.
  • Z vs 1 VS . vs 0 from which a value of Z c is obtained, which is 90.1 ohms in the case of the example.
  • the surface currents in the cavity are then determined so as to evaluate the dissipated power and the quality factor. This can for example also be done using the Wavesim program.
  • the power dissipated in a cavity known according to the example provided is 1300 W and the quality factor Q is 10600. These values will serve as benchmarks for subsequent steps.
  • the numerical values obtained during these first five steps then make it possible to calculate the structure of a two-frequency cavity according to the invention.
  • the following steps of the calculation method according to the invention concern, by way of example, a cavity according to the Figures 2a and 3a and exploiting two resonant modes: a first mode to ⁇ 4 for a low frequency of about 33 MHz and a second mode to 3 ⁇ ⁇ 4 for a high frequency of about 66 MHz. It will be obvious to those skilled in the art to adapt what is necessary to adapt to these next steps for other frequencies and / or other frequency ratios.
  • a final optimization of the two-frequency cavity is preferably carried out by 2D electromagnetic simulation, for example using the Wavesim program. It examines the variation of the resonant frequency as a function of the variation of the geometrical characteristics of the different portions of the pillar. In particular, the most delicate point is the optimization of the low impedance line 20b. Indeed, if its capacity is chosen too low, the dissipation at the high frequency (eg at 66MHz) is important, as is the voltage developed at this point, in some cases as important as that present on the die. By increasing the value of the capacitance, the voltage decreases as well as the power dissipated in the bottom of the cavity.
  • an optimum point C opt is preferably determined.
  • the dissipated power is almost identical to the two resonance frequencies, as illustrated by figure 5 .
  • FIG. 6a is an impedance diagram of the different line portions constituting the pillar and the figure 6b being a schematic view in longitudinal section of a corresponding physical embodiment of the preferred cavity example according to the invention (only half of the cavity is shown).
  • the total length of the cavity is 1355 mm, of which 600 mm out of the cylinder head 60 of the cyclotron.
  • the low and high resonance frequencies are evaluated at 33.094 MHz and 66.486 MHz, respectively.
  • the dissipated powers are of the order of 2768 W at 33 MHz for a dc voltage of 25 kV and 2699 W at 66 MHz for a dc voltage of 50 kV.
  • the quality factors are 6700 to 33 MHz and 10000 to 66 MHz.
  • FIG. figure 7 A practical embodiment of a cavity according to the invention and its implantation in a cyclotron is illustrated in FIG. figure 7 .
  • the vertical section of this cyclotron makes it possible to distinguish four cavities according to the invention, only one of which has been annotated for clarity and comprehension.
  • the resonance frequencies of the cavity can be verified by performing a frequency sweep ("wobbulation"). This provides a curve of variation of the impedance as a function of the frequency revealing two distinct peaks. According to the preferred example provided, there is a peak at substantially 33 MHz and a second peak at substantially 66 MHz, as shown schematically in FIG. figure 8 .
  • the cavity 6 comprises a tuning capacitor 50 comprising a movable electrode electrically connected to the conducting enclosure 40 and placed opposite the pillar and substantially at the intermediate portion 20b. of the transmission line. This tuning capacitor 50 is visible on the figure 7 .
  • a resonant cavity (6) bi-frequency cyclotron which comprises a die (10), a pillar (20) and a conductive enclosure (40) encompassing said pillar and said die , one end of the pillar being integral with the base of the conductive enclosure and an opposite end of said pillar (20) supporting the die (10).
  • the conducting enclosure and the pillar form a transmission line comprising at least three portions (20a, 20b, 20c) each having a characteristic impedance (Z c1 , Z c2 , Z c3 ).
  • the characteristic impedance Z c2 of the intermediate portion (20b) is substantially lower than the characteristic impedances Z c1 and Z c3 of the two other portions (20a, 20b), which makes it possible to make the cavity resonate according to two modes in order to produce two frequencies separate without having to use moving parts such as for example sliding shorts or moving plates.

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Description

DOMAINE TECHNIQUETECHNICAL AREA

La présente invention se rapporte au domaine des cyclotrons, et en particulier aux cyclotrons capables d'accélérer plusieurs types de particules chargées présentant des rapports charge(q)/masse(m) différents, tels que par exemple des protons (rapport q/m égal à 1), des particules alpha (rapport q/m égal à ½) ou des deutons (rapport q/m également égal à ½).The present invention relates to the field of cyclotrons, and in particular to cyclotrons capable of accelerating several types of charged particles having different charge (q) / mass (m) ratios, such as for example protons (equal q / m ratio at 1), alpha particles (ratio q / m equal to ½) or deuterons (ratio q / m also equal to ½).

DESCRIPTION DE L'ART ANTÉRIEURDESCRIPTION OF THE PRIOR ART

On connaît par le document WO8606924 un cyclotron. En référence à la figure 2 de ce document, un tel cyclotron comporte des électrodes d'accélération 28, communément appelées dés, couplées chacune à un pilier vertical 29 aussi appelé stem. Ledit dé 28 et ledit pilier 29 sont entourés d'une enceinte conductrice qui, ensemble, constituent une cavité résonante.We know from the document WO8606924 a cyclotron. With reference to the figure 2 of this document, such a cyclotron comprises acceleration electrodes 28, commonly called dice, each coupled to a vertical pillar 29 also called stem. Said die 28 and said pillar 29 are surrounded by a conductive enclosure which together constitute a resonant cavity.

Les cavités résonantes sont généralement excitées par une source de puissance RF et le passage successif des particules chargées dans le gap accélérateur constitué des dés et des secteurs portés à des potentiels différents produit l'accélération des dites particules. La fréquence de la tension RF appliquée doit être égale à la « fréquence cyclotron » exprimée par l'équation suivante: f RFcyc = q . B 2 * π . m

Figure imgb0001
où q est la charge de la particule à accélérer, m sa masse et B le champ magnétique principal, normal au plan médian de circulation des particules. Un cyclotron peut également fonctionner en mode harmonique : dans ce cas plusieurs oscillations de la tension RF se produisent alors que les particules circulent encore à l'intérieur du dé.The resonant cavities are generally excited by an RF power source and the successive passage of the charged particles in the accelerator gap consisting of dice and sectors brought to different potentials produces the acceleration of said particles. The frequency of the applied RF voltage shall be equal to the "cyclotron frequency" expressed by the following equation: f RFcyc = q . B 2 * π . m
Figure imgb0001
where q is the charge of the particle to accelerate, m its mass and B the main magnetic field, normal to the median plane of circulation of the particles. A cyclotron can also operate in harmonic mode: in this case several oscillations of the RF voltage occur while the particles still circulate inside the die.

Dans le cas de ces cavités connues, on est en présence d'un résonateur λ 4

Figure imgb0002
chargé par la capacité de dé à une extrémité, son autre extrémité étant court-circuitée. Le pilier forme une ligne de transmission axiale se comportant essentiellement comme une inductance destinée à compenser l'impédance capacitive du dé afin de minimiser la puissance RF réactive. En fonction de la configuration du cyclotron, les cavités sont disposées asymétriquement ou symétriquement par rapport au plan médian de circulation des particules. Dans le cas d'une topologie asymétrique (figure 1a de la présente demande), les deux plaques constituant le dé sont mécaniquement et électriquement solidaires et constituent un seul ensemble porté par le pilier. Dans le cas d'une topologie symétrique (figure 1b de la présente demande), les piliers inférieur et supérieur supportent respectivement le demi-dé inférieur et le demi-dé supérieur. Ces derniers sont reliés électriquement entre eux en quelques endroits de leur périmètre dès que le cyclotron est fermé.In the case of these known cavities, it is in the presence of a resonator λ 4
Figure imgb0002
loaded by the die capacity at one end, its other end being short-circuited. The pillar forms an axial transmission line essentially having an inductance for compensating the capacitive impedance of the die to minimize reactive RF power. Depending on the configuration of the cyclotron, the cavities are arranged asymmetrically or symmetrically with respect to the median plane of circulation of the particles. In the case of an asymmetric topology ( figure 1a of the present application), the two plates constituting the die are mechanically and electrically integral and constitute a single assembly carried by the pillar. In the case of a symmetric topology ( figure 1b of the present application), the lower and upper pillars respectively support the lower half-die and the upper half-die. These are electrically connected to each other at a few points in their perimeter as soon as the cyclotron is closed.

Le dé fait partie d'une cavité résonante 5 tel que représentée schématiquement à la figure 1 a. Cette cavité comporte le dé proprement dit 10, un pilier cylindrique vertical 20 et une enceinte conductrice 40. La figure 1c représente un schéma électrique équivalent de la cavité, dans lequel l'inductance L représente le pilier 20 et la capacité C est celle formée au niveau de l'espace compris entre le dé 10 et l'enceinte conductrice 40. La fréquence de résonance propre d'un tel circuit LC parallèle est donnée par l'expression : f 0 = 1 2. π . L . C

Figure imgb0003
The die is part of a resonant cavity 5 as schematically shown in FIG. figure 1 at. This cavity comprises the actual dice 10, a vertical cylindrical pillar 20 and a conducting enclosure 40. figure 1c represents an equivalent circuit diagram of the cavity, in which the inductance L represents the pillar 20 and the capacitance C is that formed at the space between the die 10 and the conducting enclosure 40. The resonance frequency such a parallel LC circuit is given by the expression: f 0 = 1 2. π . The . VS
Figure imgb0003

Afin de pouvoir accélérer plusieurs types de particules de rapports q/m différents dans un même cyclotron, le champ B étant déterminé, plusieurs solutions se présentent :

  1. a) utiliser des modes harmoniques différents tout en conservant la fréquence RF identique
  2. b) utiliser le même mode harmonique tout en variant la fréquence RF
La première solution comporte les désavantages suivants:
  • une complexité accrue de la région centrale du cyclotron
  • à haut courant, des pertes de faisceau à l'intérieur de la machine provoquant l'activation de pièces mécaniques.
En revanche, la seconde solution présente les avantages suivants:
  • un même centrage des particules de masse différentes qui suivront donc une trajectoire similaire, au moins dans les premiers tours à basse énergie
  • moins de pertes de faisceau réduisant ainsi l'activation des pièces mécaniques situées à proximité de la trajectoire du faisceau
  • un meilleur gain par tour pour les particules de rapport q/m=1
  • un meilleur isochronisme.
In order to accelerate several types of particles of different ratios q / m in the same cyclotron, the field B being determined, there are several solutions:
  1. a) use different harmonic modes while maintaining the same RF frequency
  2. b) use the same harmonic mode while varying the RF frequency
The first solution has the following disadvantages:
  • increased complexity of the central cyclotron region
  • at high current, beam losses inside the machine causing the activation of mechanical parts.
On the other hand, the second solution has the following advantages:
  • the same centering of the different mass particles that will follow a similar trajectory, at least in the first low energy rounds
  • fewer beam losses, thus reducing the activation of mechanical parts near the beam path
  • a better gain per revolution for particles of ratio q / m = 1
  • better isochronism.

La mise en oeuvre de cette seconde solution impose de pouvoir modifier la fréquence de résonance de la cavité constituée par le dé, le pilier et l'enceinte conductrice. De telles solutions ont été proposées par M. Eiche et al. (« Dual Frequency resonator system for a compact cyclotron », Proc. XIII Intern. Conf. on Cyclotrons and Their Applications, (World Scientific, Singapore, 1992, p. 515 ), par P. Lanz et al. ("A dual Frequency Resonator", Proceedings of the 1993 IEEE Particle Accelerator Conférence, 17-20 May 1993, Washington, DC ; 15th IEEE Particle Accelerator Conférence, p.1151 ), et par Miura Iwao et al. (« Accelerating Résonance Cavity » , JP07-066877B, 1995 ).
Les deux premiers auteurs réalisent le changement de fréquence RF à l'aide de courts-circuits glissants, actionnés à l'aide de pistons, destinés à modifier la longueur du résonateur. Le dernier auteur procède au changement de fréquence RF grâce à des plaques mobiles pivotant de 90° qui modifient la capacité des électrodes et donc la fréquence de résonance.
The implementation of this second solution requires the ability to modify the resonant frequency of the cavity formed by the die, the pillar and the conductive enclosure. Such solutions have been proposed by M. Eiche et al. ("Dual Frequency Resonator System for a Compact Cyclotron", Proc.XIII Intern, Conf., Cyclotrons and Their Applications, (World Scientific, Singapore, 1992, 515). ), by P. Lanz et al. ("A Dual Frequency Resonator," Proceedings of the 1993 IEEE Particle Accelerator Conference, 17-20 May 1993, Washington, DC; 15th IEEE Particle Accelerator Conference, p.1151 ) and by Miura Iwao et al. ("Accelerating Resonance Cavity" , JP07-066877B, 1995 ).
The first two authors perform the RF frequency shift using sliding shorts, actuated by means of pistons, to change the length of the resonator. The last author proceeds to the change of frequency RF thanks to movable plates pivoting of 90 ° which modify the capacity of the electrodes and therefore the frequency of resonance.

Cette modification de la fréquence de résonance requiert une structure RF relativement complexe et onéreuse, à laquelle s'ajoutent des problèmes de fiabilité. En effet, les dispositifs de l'art antérieur présentent certains désavantages listés ci-après :

  • a. pour les courts-circuits mobiles :
    • la taille du piston est en rapport avec celle du court-circuit car celui-ci exerce une force de friction non négligeable sur les parois du résonateur ;
    • l'usure causée par les mouvements linéaires répétés du court-circuit lors des changements de fréquence. A terme, la dégradation de l'état de surface des contacts et/ou de la paroi sur laquelle ils glissent entraîne l'apparition de points plus résistifs qui dès lors qu'ils sont parcourus par des courants RF provoquent un échauffement localisé ;
    • la destruction pure et simple du court-circuit lorsque la pression exercée par celui-ci sur les parois n'est plus suffisante. Le cas échéant, la résistance de contact étant devenue trop importante eu égard aux courants RF à transporter entraîne ainsi une élévation de température qui peut provoquer la fusion des contacts.
  • b. pour les plaques mobiles :
    • l'axe de rotation des plaques nécessite la traversée de la partie sous vide du cyclotron afin d'assurer sa connexion sur le piston ou sur le moteur qui l'entraîne. Si ces derniers étaient contenus dans le vide, il faudrait néanmoins les alimenter électriquement ce qui nécessite quand même une traversée de câble vers l'extérieur.
    • le facteur de qualité de la cavité dans la fréquence basse est assez mauvais dû aux courants RF importants passant dans cette capacité mobile. La stabilité en fréquence peut également être problématique.
This modification of the resonance frequency requires a relatively complex and expensive RF structure, to which reliability problems are added. Indeed, the devices of the prior art have certain disadvantages listed below:
  • at. for mobile short circuits:
    • the size of the piston is related to that of the short circuit because it exerts a significant frictional force on the walls of the resonator;
    • the wear caused by repeated linear movements of the short circuit during frequency changes. Eventually, the degradation of the surface state of the contacts and / or the wall on which they slide leads to the appearance of more resistive points which, when they are traversed by RF currents, cause a localized heating;
    • the pure and simple destruction of the short circuit when the pressure exerted by it on the walls is no longer sufficient. If necessary, the contact resistance having become too great with regard to the RF currents to be transported thus causes a rise in temperature which can cause the melting of the contacts.
  • b. for moving plates:
    • the axis of rotation of the plates requires the passage of the vacuum part of the cyclotron to ensure its connection to the piston or the motor that drives it. If the latter were contained in the vacuum, it should nevertheless feed them electrically which still requires a cable crossing to the outside.
    • the quality factor of the cavity in the low frequency is bad enough due to the large RF currents passing in this mobile capacity. Frequency stability can also be problematic.

RESUME DE L'INVENTIONSUMMARY OF THE INVENTION

La présente invention a pour but de résoudre au moins partiellement les difficultés précitées.The present invention aims to solve at least partially the aforementioned difficulties.

Selon un premier aspect, la présente invention concerne une cavité résonante pour l'accélération de particules chargées dans un cyclotron, comprenant un dé, un pilier et une enceinte conductrice englobant au moins partiellement ledit pilier et ledit dé, une extrémité dudit pilier supportant le dé, l'enceinte conductrice et le pilier formant ainsi une ligne de transmission, une extrémité opposée dudit pilier étant solidaire d'une base de l'enceinte conductrice, caractérisé en ce que la capacité linéique d'une portion intermédiaire de ladite ligne de transmission située entre lesdites extrémités du pilier est substantiellement plus grande que la capacité linéique des autres portions de ladite ligne de transmission.According to a first aspect, the present invention relates to a resonant cavity for accelerating charged particles in a cyclotron, comprising a die, a pillar and a conductive enclosure at least partially including said pillar and said die, an end of said pillar supporting the die. , the conductive enclosure and the pillar thus forming a transmission line, an opposite end of said pillar being integral with a base of the conductive enclosure, characterized in that the linear capacitance of an intermediate portion of said transmission line located between said ends of the pillar is substantially greater than the linear capacity of the other portions of said transmission line.

Une telle configuration permet en effet de faire résonner la cavité selon deux modes produisant ainsi deux fréquences distinctes, sans devoir faire usage d'éléments mobiles tels que par exemple des courts-circuits glissants ou des plaques mobiles, ce qui résout bon nombre des problèmes évoqués précédemment.Such a configuration makes it possible to make the cavity resonate according to two modes thus producing two distinct frequencies, without having to make use of mobile elements such as, for example, sliding shorts or moving plates, which resolves many of the problems mentioned. previously.

De préférence, la capacité linéique de la portion intermédiaire de la ligne de transmission est plus grande que deux fois la capacité linéique des autres portions de ladite ligne de transmission. De manière plus préférée, la capacité linéique de la portion intermédiaire de la ligne de transmission est plus grande que dix fois la capacité linéique des autres portions de ladite ligne de transmission.Preferably, the linear capacitance of the intermediate portion of the transmission line is greater than twice the linear capacitance of the other portions of said transmission line. More preferably, the linear capacity of the intermediate portion of the transmission line is greater than ten times the linear capacity of the other portions of said transmission line.

De manière encore plus préférée, l'impédance caractéristique de la portion intermédiaire et les impédances caractéristiques des autres portions de la ligne de transmission sont telles que la cavité est apte à résonner selon deux modes pour produire deux fréquences distinctes dans un rapport substantiellement double.
Par substantiellement double, il faut comprendre un rapport de fréquences se situant entre 1,7 et 2,3.
Une telle cavité permet en effet d'accélérer, dans un même cyclotron, des particules ayant des valeurs de q/m dans un rapport de deux, tel que par exemple des protons et des particules alpha ou des protons et des deutons.
Even more preferably, the characteristic impedance of the intermediate portion and the characteristic impedances of the other portions of the transmission line are such that the cavity is able to resonate in two modes to produce two distinct frequencies in a substantially double ratio.
By substantially double, it is necessary to understand a frequency ratio lying between 1.7 and 2.3.
Such a cavity makes it possible to accelerate, in the same cyclotron, particles having values of q / m in a ratio of two, such as for example protons and alpha particles or protons and deuterons.

De manière encore plus préférée, le pilier comporte plusieurs cylindres superposés, un de ces cylindres correspondant à ladite portion intermédiaire de la ligne de transmission et possédant un diamètre moyen substantiellement supérieur au diamètre moyen des autres cylindres. Alternativement ou conjointement, l'enceinte conductrice comporte plusieurs cylindres creux superposés, un de ces cylindres creux correspondant à ladite portion intermédiaire de la ligne de transmission et possédant un diamètre moyen substantiellement inférieur au diamètre moyen des autres cylindres creux. Une telle configuration cylindrique du pilier et/ou de l'enceinte conductrice permet en effet d'obtenir une bonne rigidité mécanique de l'ensemble, de faciliter sa construction et d'assurer une bonne répartition des équipotentielles du champ électrique depuis le pilier.Even more preferably, the pillar comprises a plurality of superimposed cylinders, one of these cylinders corresponding to said intermediate portion of the transmission line and having a mean diameter substantially greater than the average diameter of the other cylinders. Alternatively or jointly, the conductive enclosure comprises a plurality of superposed hollow cylinders, one of these hollow cylinders corresponding to said intermediate portion of the transmission line and having a mean diameter substantially smaller than the average diameter of the other hollow cylinders. A such cylindrical configuration of the pillar and / or the conductive enclosure makes it possible to obtain good mechanical rigidity of the assembly, to facilitate its construction and to ensure a good distribution of equipotentials of the electric field from the pillar.

Selon un second aspect, l'invention concerne une méthode de conception d'une cavité résonante bi-fréquence telle que revendiquée.According to a second aspect, the invention relates to a method for designing a dual-frequency resonant cavity as claimed.

Ces aspects ainsi que d'autres aspects de l'invention seront clarifiés dans la description détaillée de modes de réalisation particuliers de l'invention.These and other aspects of the invention will be clarified in the detailed description of particular embodiments of the invention.

BRÈVE DESCRIPTION DES FIGURESBRIEF DESCRIPTION OF THE FIGURES

Les figures sont données à titre indicatif et ne constituent pas de limitation de la présente invention. Par ailleurs, les proportions des dessins ne sont pas respectées. Des composants identiques ou analogues sont généralement désignés par les mêmes numéros de référence parmi l'ensemble des figures.The figures are given for information only and do not constitute a limitation of the present invention. Moreover, the proportions of the drawings are not respected. Identical or like components are generally designated by the same reference numerals among all the figures.

La figure 1a représente une coupe d'une cavité résonante asymétrique d'un cyclotron de l'art antérieur ;The figure 1a represents a section of an asymmetric resonant cavity of a cyclotron of the prior art;

La figure 1b représente une coupe d'une cavité résonante symétrique d'un cyclotron de l'art antérieur ;The figure 1b represents a section of a symmetrical resonant cavity of a cyclotron of the prior art;

La figure 1c représente un schéma électrique équivalent simplifié de la cavité résonante de la figure 1a ou 1b ;The figure 1c represents a simplified equivalent electrical diagram of the resonant cavity of the Figure 1a or 1b ;

La figure 2a représente schématiquement une coupe d'une cavité selon l'invention avec indication de la circulation du courant et du champ magnétique lors de la résonance à la fréquence basse ;The figure 2a schematically represents a section of a cavity according to the invention with indication of the circulation of the current and the magnetic field during resonance at the low frequency;

La figure 2b représente l'évolution de la tension et du courant le long du pilier lors du fonctionnement de la cavité de la figure 2a en mode λ 4

Figure imgb0004
;The figure 2b represents the evolution of the voltage and the current along the pillar during the operation of the cavity of the figure 2a in mode λ 4
Figure imgb0004
;

La figure 2c représente un schéma électrique équivalent simplifié de la cavité résonante de la figure 2a ;The Figure 2c represents a simplified equivalent electrical diagram of the resonant cavity of the figure 2a ;

La figure 3a représente schématiquement une coupe d'une cavité résonante selon l'invention avec indication de la circulation des courants et des champs magnétiques lors de la résonance à la fréquence haute ;The figure 3a schematically represents a section of a resonant cavity according to the invention with indication of the circulation of currents and magnetic fields during resonance at the high frequency;

La figure 3b représente l'évolution de la tension et du courant le long du pilier lors du fonctionnement de la cavité de la figure 3a en mode 3 λ 4

Figure imgb0005
;The figure 3b represents the evolution of the voltage and the current along the pillar during the operation of the cavity of the figure 3a in mode 3 λ 4
Figure imgb0005
;

La figure 3c représente un schéma électrique équivalent simplifié de la cavité résonante de la figure 3a ;The figure 3c represents a simplified equivalent electrical diagram of the resonant cavity of the figure 3a ;

La figure 4a représente une forme géométrique réelle et une répartition des équipotentielles d'un champ électrique statique d'une cavité de l'art antérieur ;The figure 4a represents a real geometrical shape and a distribution of the equipotentials of a static electric field of a cavity of the prior art;

La figure 4b schématise une cavité de l'art antérieur sous forme d'une ligne de transmission coaxiale dont l'impédance caractéristique est fonction des diamètres d et D ;The figure 4b schematizes a cavity of the prior art in the form of a coaxial transmission line whose characteristic impedance is a function of the diameters d and D;

La figure 5 représente un graphe illustrant la puissance dissipée dans une cavité résonante suivant l'invention pour chacune des deux fréquences de résonance en fonction de la valeur de la capacité de la portion de ligne à basse impédance caractéristique ;The figure 5 represents a graph illustrating the power dissipated in a resonant cavity according to the invention for each of the two resonance frequencies as a function of the value of the capacitance of the characteristic low-impedance line portion;

La figure 6a représente un diagramme d'impédance d'un pilier dans un mode de réalisation de l'invention ;The figure 6a represents an impedance diagram of a pillar in an embodiment of the invention;

La figure 6b représente schématiquement une coupe de la cavité suivant l'invention, à mettre en rapport avec le diagramme d'impédance de la figure 6a ;The figure 6b schematically represents a section of the cavity according to the invention, to be related to the impedance diagram of the figure 6a ;

La figure 7 représente une coupe d'un cyclotron bi-fréquence équipé de quatre cavités selon l'invention ;The figure 7 represents a section of a bi-frequency cyclotron equipped with four cavities according to the invention;

La figure 8 représente schématiquement un graphe montrant les deux fréquences distinctes dans un rapport double obtenues par balayage en fréquence d'une cavité selon l'invention.The figure 8 schematically represents a graph showing the two distinct frequencies in a double ratio obtained by frequency scanning of a cavity according to the invention.

DESCRIPTION DÉTAILLÉE DE MODES DE RÉALISATIONDETAILED DESCRIPTION OF EMBODIMENTS

La figure 2a représente schématiquement un exemple de réalisation d'une cavité bi-fréquence selon l'invention. Il s'agit ici d'une cavité symétrique par rapport au plan médian du cyclotron (représenté par une ligne pointillée mixte sur la figure), mais il est évident qu'une cavité asymétrique conviendrait également. La cavité 6 comporte deux demi-dés 10 et 10' reliés ensemble électriquement et entre lesquels circuleront les particules à accélérer, deux piliers comportant chacun trois portions 20a, 20b et 20c (20a', 20b' et 20c'), et deux enceintes conductrices 40 et 40' entourant le tout. Les enceintes ont une section transversale qui, dans cet exemple, est substantiellement constante sur la hauteur des piliers. Chaque pilier supporte respectivement un demi-dé à une extrémité, les extrémités opposées étant respectivement connectées électriquement aux bases 45 et 45' des enceintes conductrices 40 et 40' pour constituer un court-circuit du point de vue radiofréquence. Les différentes portions du pilier sont superposées et de préférence alignées suivant un même axe. Lesdites portions sont constituées, dans cet exemple, de tubes cylindriques de différents diamètres dont des exemples de dimensions seront donnés ci-après lorsque sera décrite une méthode de conception d'une cavité selon l'invention. Le diamètre de la portion intermédiaire 20b est substantiellement plus grand que le diamètre des deux autres portions 20a et 20c, de sorte que la capacité linéique (en Farad par mètre) de cette portion intermédiaire 20b est substantiellement plus grande que la capacité linéique des deux autres portions 20a et 20c. En conséquence, la portion intermédiaire 20b aura un comportement essentiellement capacitif alors que les autres portions 20a et 20c auront un comportement essentiellement inductif, dans la gamme de fréquences de fonctionnement de la cavité (qui se situe dans les mégahertz).The figure 2a schematically represents an exemplary embodiment of a dual-frequency cavity according to the invention. This is a symmetrical cavity relative to the median plane of the cyclotron (represented by a mixed dotted line in the figure), but it is obvious that an asymmetrical cavity would also be suitable. The cavity 6 comprises two half-dice 10 and 10 'electrically connected together and between which will circulate the particles to accelerate, two pillars each comprising three portions 20a, 20b and 20c (20a', 20b 'and 20c'), and two conductive speakers 40 and 40 'surrounding the whole. The speakers have a cross section which, in this example, is substantially constant over the height of the pillars. Each pillar respectively supports a half-die at one end, the opposite ends being respectively electrically connected to the bases 45 and 45 'of the conductive speakers 40 and 40' to constitute a short circuit from the radiofrequency point of view. The different portions of the pillar are superimposed and preferably aligned along the same axis. Said portions consist, in this example, of cylindrical tubes of different diameters, examples of dimensions of which will be given below when a method of designing a cavity according to the invention will be described. The diameter of the intermediate portion 20b is substantially greater than the diameter of the other two portions 20a and 20c, so that the linear capacity (in Farad per meter) of this intermediate portion 20b is substantially greater than the linear capacity of the other two portions 20a and 20c. Consequently, the intermediate portion 20b will have a substantially capacitive behavior while the other portions 20a and 20c will have an essentially inductive behavior, in the operating frequency range of the cavity (which is in the megahertz).

Un schéma électrique équivalent simplifié d'une telle cavité est présenté à la figure 2c.A simplified equivalent electrical diagram of such a cavity is presented at Figure 2c .

Un premier type de fonctionnement est obtenu en excitant la cavité en mode λ 4

Figure imgb0006
(λ étant la longueur d'onde), ce qui permet d'obtenir une première fréquence de résonance (ci-après « la fréquence de résonance basse », par exemple 33 MHz).
La figure 2b représente l'évolution de la tension (Ux) et du courant (Ix) dans ce mode en fonction d'une position axiale x le long du pilier. La tension est maximale au niveau du dé tandis que le courant correspondant y est nul ou très faible. Cela s'inverse lorsqu'on se ramène au pied du pilier. Cette configuration de tension convient particulièrement bien pour accélérer des particules évoluant dans le plan médian d'un cyclotron.A first type of operation is obtained by exciting the cavity in λ 4
Figure imgb0006
(λ being the wavelength), which makes it possible to obtain a first resonance frequency (hereinafter "the low resonance frequency", for example 33 MHz).
The figure 2b represents the evolution of the voltage (U x ) and the current (I x ) in this mode as a function of an axial position x along the pillar. The voltage is maximum at the die while the corresponding current is zero or very low. This is reversed when one comes back to the foot of the pillar. This voltage configuration is particularly suitable for accelerating particles moving in the median plane of a cyclotron.

Le champ magnétique B est orienté identiquement de part et d'autre de la portion intermédiaire 20b (ci-après « la ligne à faible impédance 20b »). Le courant résultant i1 de ce mode circule axialement et se répartit radialement autour du pilier tel que représenté à la figure 2a.The magnetic field B is oriented identically on either side of the intermediate portion 20b (hereinafter "the low impedance line 20b"). The resulting current i 1 of this mode circulates axially and is distributed radially around the pillar as shown in FIG. figure 2a .

Un second type de fonctionnement est illustré à la figure 3a. La structure physique est identique à celle de la figure 2a mais on excite le mode 3 λ 4

Figure imgb0007
, ce qui permet d'obtenir une deuxième fréquence de résonance (ci-après « la fréquence de résonance haute », par exemple 66 MHz), plus élevée que la première fréquence. La figure 3b représente l'évolution de la tension (Ux) et du courant (Ix) dans ce mode et, de manière identique au premier mode de résonance, la tension est toujours maximale au niveau du dé tandis que le courant correspondant y est nul ou très faible. Par ailleurs, le courant s'inverse en un point intermédiaire situé environ à mi-hauteur de la ligne faible impédance 20b, ce qui a pour effet de diviser l'effet capacitif de cette portion de ligne 20b en deux.A second type of operation is illustrated in figure 3a . The physical structure is identical to that of the figure 2a but we excite the mode 3 λ 4
Figure imgb0007
, which makes it possible to obtain a second resonance frequency (hereinafter referred to as the "high resonance frequency", for example 66 MHz), higher than the first frequency. The figure 3b represents the evolution of the voltage (U x ) and of the current (I x ) in this mode and, like the first resonance mode, the voltage is always maximum at the dice level, while the corresponding current is zero or very weak. Furthermore, the current is reversed at an intermediate point about halfway up the low impedance line 20b, which has the effect of dividing the capacitive effect of this portion of line 20b in two.

Compte tenu de ce qui précède, le champ magnétique B est en opposition de part et d'autre de ce point intermédiaire. La figure 3c représente un schéma électrique équivalent simplifié avec la circulation des courants i2 et i3 respectivement présents dans la partie supérieure et inférieure de la demi-cavité. Ils se répartissent radialement autour du pilier, en opposition par rapport à un plan horizontal virtuel partageant transversalement la ligne faible impédance 20b, dans lequel ils s'annulent.In view of the above, the magnetic field B is in opposition on both sides of this intermediate point. The figure 3c represents a simplified equivalent electrical diagram with the circulation of currents i 2 and i 3 respectively present in the upper and lower part of the half-cavity. They are distributed radially around the pillar, in opposition to a virtual horizontal plane transversely sharing the low impedance line 20b, in which they cancel each other out.

Il apparaîtra évident pour l'homme du métier que bien d'autres configurations géométriques de la cavité sont possibles, du moment qu'une portion intermédiaire de la cavité présente une capacité linéique substantiellement supérieure à la capacité linéique des autres portions, de préférence supérieure à deux fois la capacité linéique des autres portions, de manière encore plus préférée supérieure à dix fois la capacité linéique des autres portions. On pourrait ainsi alternativement prévoir un pilier de section constante sur sa hauteur et une enceinte conductrice présentant une portion intermédiaire de section substantiellement plus petite que celle des autres portions. On pourrait aussi prévoir une combinaison de ces deux solutions, à savoir une enceinte comportant un rétrécissement intermédiaire et un pilier comportant un élargissement intermédiaire, comme illustré par exemple aux figures 6b et 7, ou toute autre combinaison.It will be obvious to one skilled in the art that many other geometric configurations of the cavity are possible, provided that an intermediate portion of the cavity has a linear capacity substantially greater than the linear capacity of the other portions, preferably greater than twice the linear capacity of the other portions, even more preferably greater than ten times the linear capacity of the other portions. One could alternatively provide a pillar of constant section over its height and a conductive enclosure having an intermediate portion of section substantially smaller than that of the other portions. One could also provide a combination of these two solutions, namely an enclosure having an intermediate narrowing and a pillar having an intermediate expansion, as illustrated for example by the figures 6b and 7 , or any other combination.

Une méthode de calcul permettant la conception et le dimensionnement d'une structure d'une cavité selon l'invention est fournie ci-après.A calculation method for designing and dimensioning a structure of a cavity according to the invention is provided below.

Préalablement au calcul de la cavité bi-fréquence suivant l'invention, une modélisation d'une cavité connue telle que décrite dans le document WO8606924 - c'est-à-dire une cavité dont le pilier et l'enceinte conductrice présentent une section constante - est réalisée suivant une méthode décrite ci-après afin de déterminer précisément le diamètre d'un dé équivalent, supposé circulaire, et l'impédance du pilier d'une telle cavité connue :

  1. 1. calcul de la capacité linéique du pilier d'une cavité dont le pilier et l'enceinte conductrice présentent une section constante, permettant de déduire l'impédance caractéristique de la ligne de transmission ainsi formée par ledit pilier et l'enceinte conductrice ;
  2. 2. calcul de l'impédance caractéristique pour différents diamètres de pilier ;
  3. 3. détermination du diamètre extérieur moyen équivalent de l'enceinte conductrice ;
  4. 4. simulation électromagnétique 2D de la cavité s'appuyant sur les dimensions trouvées précédemment et détermination du diamètre d'un dé équivalent, supposé circulaire, produisant la même fréquence de résonance que ladite cavité de l'art antérieur ;
  5. 5. calcul des paramètres intrinsèques de la cavité, tels que le facteur de qualité Q, la puissance dissipée, l'énergie stockée et comparaison des résultats avec des valeurs mesurées.
Prior to the calculation of the dual-frequency cavity according to the invention, a modeling of a known cavity as described in the document WO8606924 - that is to say a cavity whose pillar and the conductive enclosure have a constant section - is carried out according to a method described below to precisely determine the diameter of an equivalent die, supposed circular, and the impedance of the pillar of such a known cavity:
  1. 1. calculation of the linear capacity of the pillar of a cavity, the pillar and the conductive enclosure have a constant section, to deduce the characteristic impedance of the transmission line thus formed by said pillar and the conductive enclosure;
  2. 2. Calculation of the characteristic impedance for different pillar diameters;
  3. 3. determination of the equivalent mean outside diameter of the conductive enclosure;
  4. 4. 2D electromagnetic simulation of the cavity based on the dimensions previously found and determination of the diameter of an equivalent die, assumed circular, producing the same resonance frequency as said cavity of the prior art;
  5. 5. Calculation of the intrinsic parameters of the cavity, such as the quality factor Q, the dissipated power, the stored energy and comparison of the results with measured values.

Détails de l'étape 1Details of step 1

On évalue l'impédance caractéristique du pilier connu, par exemple à l'aide du programme Tricomp de la société Field Precision LLC. Ce programme résout le champ électrique par la méthode des éléments finis.
La figure 4a montre par exemple la répartition des équipotentielles de champ électrique obtenues en appliquant une tension de 1 V sur un pilier connu de diamètre d = 90 mm, tandis que l'enceinte conductrice est au potentiel de la masse. On obtient une énergie stockée de 18,53 pJ/m.
The characteristic impedance of the known pillar is evaluated, for example using the Tricomp program of Field Precision LLC. This program solves the electric field by the finite element method.
The figure 4a shows for example the distribution of electric field equipotentials obtained by applying a voltage of 1 V on a known pillar of diameter d = 90 mm, while the conducting enclosure is at the potential of the mass. A stored energy of 18.53 pJ / m is obtained.

On obtient ensuite la valeur de la capacité C à partir de l'expression : E = C . V 2 2

Figure imgb0008
, ce qui donne C = 37,06 pF/m dans le cas de l'exemple.The value of the capacitance C is then obtained from the expression: E = VS . V 2 2
Figure imgb0008
which gives C = 37.06 pF / m in the case of the example.

Ensuite, en combinant les deux expressions suivantes: Z c = L / C et c 0 = 1 L . C avec c 0 = vitesse de la lumière ,

Figure imgb0009
Then, combining the two following expressions: Z vs = The / VS and vs 0 = 1 The . VS with vs 0 = speed of light ,
Figure imgb0009

l'expression de l'impédance caractéristique Zc peut se réécrire sous la forme : Z c = 1 C . c 0

Figure imgb0010
, de laquelle on obtient une valeur de Zc, qui vaut 90,1 ohms dans le cas de l'exemple.the expression of the characteristic impedance Z c can be rewritten in the form: Z vs = 1 VS . vs 0
Figure imgb0010
from which a value of Z c is obtained, which is 90.1 ohms in the case of the example.

Détails de l'étape 2Details of step 2

Le même calcul d'impédance caractéristique est effectué pour d'autres diamètres de pilier. On obtient ainsi par exemple les valeurs suivantes :

  • pour d = 100mm : C = 39,88pF/m et Zc = 83,58 ohms
  • pour d = 80mm : C = 34,36pF/m, Zc = 97,01 ohms
The same characteristic impedance calculation is performed for other pillar diameters. For example, the following values are obtained:
  • for d = 100mm: C = 39.88pF / m and Zc = 83.58 ohms
  • for d = 80mm: C = 34.36pF / m, Zc = 97.01 ohms

Détails de l'étape 3Details of step 3

L'enceinte conductrice connue n'ayant pas nécessairement une section circulaire (comme on le voit par exemple sur la figure 4a qui montre une section plus ou moins triangulaire pour l'exemple d'enceinte connue), on détermine ensuite un diamètre moyen équivalent D (voir figure 4b) de cette enceinte conductrice par l'expression suivante : D = d . e Z C 60

Figure imgb0011

Pour l'exemple fourni, cela donne : D = 404,02 mm pour un diamètre de pilier de d = 90 mm.
En effectuant par ailleurs le même calcul avec les données obtenues à l'étape 2, on remarque que D ne varie quasi pas pour différents diamètres de pilier. On obtient en effet que D = 402,69 pour d = 100 mm, et que D = 402,97 pour d = 80 mm. Nous choisissons dans cet exemple la valeur de D= 400mm (+/- 3mm sont alloués à l'épaisseur de cuivre de l'enceinte conductrice).The known conducting enclosure does not necessarily have a circular section (as seen for example on the figure 4a which shows a more or less triangular section for the example of known enclosure), an equivalent mean diameter D is then determined (see figure 4b ) of this conducting enclosure by the following expression: D = d . e Z VS 60
Figure imgb0011

For the example provided, this gives: D = 404.02 mm for a pillar diameter of d = 90 mm.
Moreover, by performing the same calculation with the data obtained in step 2, it is noted that D does not vary substantially for different pillar diameters. We obtain in fact that D = 402.69 for d = 100 mm, and that D = 402.97 for d = 80 mm. We choose in this example the value of D = 400mm (+/- 3mm are allocated to the copper thickness of the conductive enclosure).

Détails de l'étape 4Details of step 4

Afin de déterminer le diamètre d'un dé équivalent, supposé circulaire, produisant la même fréquence de résonance que ladite cavité de l'art antérieur, on procède à une simulation électromagnétique 2D de la cavité s'appuyant sur les dimensions trouvées précédemment, par exemple au moyen du programme Wavesim de la société Field Precision LLC. On procède ainsi par approximations successives jusqu'à obtenir la bonne fréquence de résonance.In order to determine the diameter of an equivalent die, assumed to be circular, producing the same resonance frequency as said cavity of the prior art, a 2D electromagnetic simulation of the cavity based on the dimensions found previously, for example, is carried out. using the Wavesim program from Field Precision LLC. This is done by successive approximations until the good resonance frequency.

Dans le cas de l'exemple, on trouve un diamètre équivalent du dé de 378 mm pour une fréquence de résonance de f0 = 66 MHz (mesurée sur la cavité réelle).In the case of the example, there is an equivalent diameter of the dice of 378 mm for a resonance frequency of f 0 = 66 MHz (measured on the actual cavity).

Détails de l'étape 5Details of step 5

On détermine ensuite les courants de surface dans la cavité de manière à évaluer la puissance dissipée et le facteur de qualité. Ceci peut par exemple aussi s'effectuer au moyen du programme Wavesim.The surface currents in the cavity are then determined so as to evaluate the dissipated power and the quality factor. This can for example also be done using the Wavesim program.

Pour une tension de 50 kV présente dans le gap accélérateur, la puissance dissipée dans une cavité connue selon l'exemple fourni est de 1300 W et le facteur de qualité Q est de 10600.
Ces valeurs serviront de points de repère pour les étapes ultérieures.
For a voltage of 50 kV present in the accelerator gap, the power dissipated in a cavity known according to the example provided is 1300 W and the quality factor Q is 10600.
These values will serve as benchmarks for subsequent steps.

Les valeurs numériques obtenues lors de ces cinq premières étapes permettent ensuite le calcul de la structure d'une cavité bi-fréquence selon l'invention. Les étapes suivantes de la méthode de calcul selon l'invention concernent, à titre d'exemple, une cavité selon les figures 2a et 3a et exploitant deux modes résonants : un premier mode à λ 4

Figure imgb0012
pour une fréquence basse d'environ 33 MHz et un second mode à 3 λ 4
Figure imgb0013
pour une fréquence haute d'environ 66 MHz. Il sera évident pour l'homme du métier d'adapter ce qu'il est nécessaire d'adapter à ces étapes suivantes pour d'autres fréquences et/ou d'autres rapports de fréquence.The numerical values obtained during these first five steps then make it possible to calculate the structure of a two-frequency cavity according to the invention. The following steps of the calculation method according to the invention concern, by way of example, a cavity according to the Figures 2a and 3a and exploiting two resonant modes: a first mode to λ 4
Figure imgb0012
for a low frequency of about 33 MHz and a second mode to 3 λ 4
Figure imgb0013
for a high frequency of about 66 MHz. It will be obvious to those skilled in the art to adapt what is necessary to adapt to these next steps for other frequencies and / or other frequency ratios.

La forme d'une une cavité bi-fréquence selon l'invention est déterminée par plusieurs composants physiques dont les caractéristiques suivantes peuvent être obtenues, et de préférence optimisées, par exemple à l'aide du logiciel de simulation radiofréquence Genesys de la société Agilent :

  • l'impédance caractéristique et la longueur de la ligne 20c ;
  • l'impédance caractéristique et la longueur de la ligne faible impédance 20b, assimilable à un condensateur ;
  • l'impédance caractéristique et la longueur de la ligne 20a.
The shape of a two-frequency cavity according to the invention is determined by several physical components whose following characteristics can be obtained, and preferably optimized, for example using the Genesys radio frequency simulation software from Agilent:
  • the characteristic impedance and the length of line 20c;
  • the characteristic impedance and the length of the low impedance line 20b, comparable to a capacitor;
  • the characteristic impedance and the length of the line 20a.

Une fois ces éléments déterminés, on procède de préférence à une optimisation finale de la cavité bi-fréquence par simulation électromagnétique 2D, par exemple à l'aide du programme Wavesim. On y examine la variation de la fréquence de résonance en fonction de la variation des caractéristiques géométriques des différentes portions du pilier.
En particulier, le point le plus délicat est l'optimisation de la ligne à faible impédance 20b. En effet, si sa capacité est choisie trop faible, la dissipation à la fréquence haute (p.ex. à 66MHz) est importante, de même que la tension développée à cet endroit, dans certains cas aussi importante que celle présente sur le dé. En augmentant la valeur de la capacité, la tension diminue de même que la puissance dissipée dans le bas de la cavité. En tenant compte de la valeur maximale admissible du champ électrique ainsi que de la valeur de la capacité permettant d'obtenir le rapport de fréquence souhaité, par exemple un rapport double (point référencé Cmin), on détermine de préférence un point optimal Copt pour lequel la puissance dissipée est quasi identique aux deux fréquences de résonance, comme l'illustre la figure 5.
Once these elements have been determined, a final optimization of the two-frequency cavity is preferably carried out by 2D electromagnetic simulation, for example using the Wavesim program. It examines the variation of the resonant frequency as a function of the variation of the geometrical characteristics of the different portions of the pillar.
In particular, the most delicate point is the optimization of the low impedance line 20b. Indeed, if its capacity is chosen too low, the dissipation at the high frequency (eg at 66MHz) is important, as is the voltage developed at this point, in some cases as important as that present on the die. By increasing the value of the capacitance, the voltage decreases as well as the power dissipated in the bottom of the cavity. Taking into account the maximum acceptable value of the electric field as well as the value of the capacitance making it possible to obtain the desired frequency ratio, for example a double ratio (referenced point C min ), an optimum point C opt is preferably determined. the dissipated power is almost identical to the two resonance frequencies, as illustrated by figure 5 .

Une multitude de solutions existent. Cependant certains critères techniques ont guidés la conception d'une cavité plus préférée selon l'invention :

  1. i. avoir un pilier de diamètre supérieur ou égale à 80 mm dans la portion 20c pour des raisons de rigidité mécanique ;
  2. ii. avoir une longueur totale du pilier la plus courte possible ;
  3. iii. prolonger la ligne faible impédance 20b hors de la culasse du cyclotron, permettant ainsi une injection de la puissance RF et un accord cavité optimum ;
  4. iv. permettre une puissance RF d'excitation des cavités aussi basse que possible, en particulier à la fréquence haute (p.ex. à 66 MHz), afin d'avoir une réserve pour l'accélération du faisceau de particules.
A multitude of solutions exist. However, certain technical criteria have guided the design of a more preferred cavity according to the invention:
  1. i. have a pillar of diameter greater than or equal to 80 mm in the portion 20c for reasons of mechanical rigidity;
  2. ii. have a total length of the pillar as short as possible;
  3. iii. extend the low impedance line 20b out of the cyclotron breech, thus allowing RF power injection and optimum cavity tuning;
  4. iv. to allow RF excitation power of the cavities as low as possible, particularly at the high frequency (eg at 66 MHz), in order to have a reserve for the acceleration of the particle beam.

En appliquant la méthode ci-dessus, on obtient finalement les dimensions préférées suivantes pour le pilier :

  • portion 20c (en deux parties) :
    • ○ première partie : diamètre = 80 mm, longueur = 520 mm, Zc = 96,5 ohms ;
    • ○ deuxième partie : diamètre = 80 mm, longueur = 145 mm, Zc = 70 ohms ;
  • portion 20b : diamètre = 258 mm, longueur = 285 mm, Zc = 5 ohm (portion à faible impédance)
  • portion 20a : diamètre = 184 mm, longueur = 405 mm, Zc= 60 ohms.
Applying the above method, the following preferred dimensions for the pillar are finally obtained:
  • portion 20c (in two parts):
    • ○ first part: diameter = 80 mm, length = 520 mm, Zc = 96.5 ohms;
    • ○ second part: diameter = 80 mm, length = 145 mm, Zc = 70 ohms;
  • portion 20b: diameter = 258 mm, length = 285 mm, Zc = 5 ohm (low impedance portion)
  • portion 20a: diameter = 184 mm, length = 405 mm, Zc = 60 ohms.

Ce résultat est illustré aux figures 6a et 6b, la figure 6a étant un diagramme d'impédances des différentes portions de ligne constituant le pilier et la figure 6b étant une vue schématique en coupe longitudinale d'une réalisation physique correspondante de l'exemple de cavité préférée selon l'invention (seule une moitié de la cavité est représentée).This result is illustrated in Figures 6a and 6b , the figure 6a being an impedance diagram of the different line portions constituting the pillar and the figure 6b being a schematic view in longitudinal section of a corresponding physical embodiment of the preferred cavity example according to the invention (only half of the cavity is shown).

La longueur totale de la cavité est de 1355 mm, dont 600 mm hors de la culasse 60 du cyclotron. Les fréquences de résonance basse et haute sont évaluées respectivement à 33,094 MHz et à 66,486 MHz. Les puissances dissipées sont de l'ordre de 2768 W à 33 MHz pour une tension dé de 25 kV et de 2699 W à 66 MHz pour une tension dé de 50 kV. Les facteurs de qualité sont de 6700 à 33 MHz et de 10000 à 66 MHz.The total length of the cavity is 1355 mm, of which 600 mm out of the cylinder head 60 of the cyclotron. The low and high resonance frequencies are evaluated at 33.094 MHz and 66.486 MHz, respectively. The dissipated powers are of the order of 2768 W at 33 MHz for a dc voltage of 25 kV and 2699 W at 66 MHz for a dc voltage of 50 kV. The quality factors are 6700 to 33 MHz and 10000 to 66 MHz.

Une réalisation pratique d'une cavité selon l'invention et son implantation dans un cyclotron est illustrée à la figure 7. La coupe verticale de ce cyclotron permet de distinguer quatre cavités selon l'invention, dont une seule a été annotée pour la clarté et la compréhension.A practical embodiment of a cavity according to the invention and its implantation in a cyclotron is illustrated in FIG. figure 7 . The vertical section of this cyclotron makes it possible to distinguish four cavities according to the invention, only one of which has been annotated for clarity and comprehension.

Les fréquences de résonance de la cavité peuvent être vérifiées en effectuant un balayage en fréquence (« wobbulation »). Cela fournit une courbe de variation de l'impédance en fonction de la fréquence laissant apparaître deux pics distincts. Selon l'exemple préféré fourni, on retrouve un pic à substantiellement 33 MHz et un deuxième pic à substantiellement 66 MHz, tel que montré schématiquement à la figure 8.The resonance frequencies of the cavity can be verified by performing a frequency sweep ("wobbulation"). This provides a curve of variation of the impedance as a function of the frequency revealing two distinct peaks. According to the preferred example provided, there is a peak at substantially 33 MHz and a second peak at substantially 66 MHz, as shown schematically in FIG. figure 8 .

Lors de son fonctionnement, la fréquence de résonance de la cavité va dériver, principalement à cause de dérives thermiques modifiant ses dimensions. Suivant l'art antérieur il est connu de placer un condensateur d'accord motorisé et asservi dans le plan médian du cyclotron et destiné à ajuster la fréquence RF injectée dans la cavité. Cette configuration n'aurait toutefois que peu d'effet à la fréquence basse, par exemple à 33 MHz.
Selon une version préférée de l'invention, la cavité 6 comporte un condensateur d'accord 50 comprenant une électrode mobile reliée électriquement à l'enceinte conductrice 40 et placée en vis-à-vis du pilier et substantiellement au niveau de la portion intermédiaire 20b de la ligne de transmission. Ce condensateur d'accord 50 est visible sur la figure 7. Par simulation, on a en effet déterminé qu'un tel condensateur d'accord 50 placé à un tel endroit permet d'obtenir une amplitude de réglage très proche aux deux fréquences de résonance, à savoir, dans le cas d'une fréquence basse de 33 MHz et d'une fréquence haute de 66 MHz, une variation de 12,6 KHz/pF à 33 MHz et une variation de 12,2 KHz/pF à 66 MHz.
During its operation, the resonant frequency of the cavity will drift, mainly because of thermal drifts changing its dimensions. According to the prior art, it is known to place a tuning capacitor motorized and controlled in the median plane of the cyclotron and intended to adjust the RF frequency injected into the cavity. This configuration, however, would have little effect at low frequency, for example at 33 MHz.
According to a preferred version of the invention, the cavity 6 comprises a tuning capacitor 50 comprising a movable electrode electrically connected to the conducting enclosure 40 and placed opposite the pillar and substantially at the intermediate portion 20b. of the transmission line. This tuning capacitor 50 is visible on the figure 7 . By simulation, it has in fact been determined that such a tuning capacitor 50 placed at such a place makes it possible to obtain an adjustment amplitude very close to the two resonance frequencies, namely, in the case of a low frequency of 33 MHz and a high frequency of 66 MHz, a variation of 12.6 KHz / pF at 33 MHz and a variation of 12.2 KHz / pF at 66 MHz.

En résumé, l'invention peut également être décrite comme suit : une cavité résonante (6) bi-fréquence pour cyclotron qui comprend un dé (10), un pilier (20) et une enceinte conductrice (40) englobant ledit pilier et ledit dé, une extrémité du pilier étant solidaire de la base de l'enceinte conductrice et une extrémité opposée dudit pilier (20) supportant le dé (10). L'enceinte conductrice et le pilier forment une ligne de transmission comportant au moins trois portions (20a, 20b, 20c) ayant chacune une impédance caractéristique (Zc1, Zc2, Zc3). L'impédance caractéristique Zc2 de la portion intermédiaire (20b) est substantiellement inférieure aux impédances caractéristiques Zc1 et Zc3 des deux autres portions (20a, 20b), ce qui permet de faire résonner la cavité selon deux modes afin de produire deux fréquences distinctes sans devoir faire usage d'éléments mobiles tels que par exemple des courts-circuits glissants ou des plaques mobiles.In summary, the invention can also be described as follows: a resonant cavity (6) bi-frequency cyclotron which comprises a die (10), a pillar (20) and a conductive enclosure (40) encompassing said pillar and said die , one end of the pillar being integral with the base of the conductive enclosure and an opposite end of said pillar (20) supporting the die (10). The conducting enclosure and the pillar form a transmission line comprising at least three portions (20a, 20b, 20c) each having a characteristic impedance (Z c1 , Z c2 , Z c3 ). The characteristic impedance Z c2 of the intermediate portion (20b) is substantially lower than the characteristic impedances Z c1 and Z c3 of the two other portions (20a, 20b), which makes it possible to make the cavity resonate according to two modes in order to produce two frequencies separate without having to use moving parts such as for example sliding shorts or moving plates.

La présente invention a été décrite en relation avec des modes de réalisations spécifiques, qui ont une valeur purement illustrative et ne doivent pas être considérés comme limitatifs. D'une manière générale, il apparaîtra évident pour l'homme du métier que la présente invention n'est pas limitée aux exemples illustrés et/ou décrits ci-dessus. L'invention comprend chacune des caractéristiques nouvelles ainsi que toutes leurs combinaisons. Cependant, ce sont les revendication qui déterminent l'étendue de sa protection. La présence de numéros de référence aux dessins ne peut pas être considérée comme limitative, y compris lorsque ces numéros sont indiqués dans les revendications.
L'usage des verbes « comprendre », « inclure », « comporter », ou toute autre variante, ainsi que de leur conjugaison, ne peut en aucune façon exclure la présence d'éléments autres que ceux mentionnés. L'usage de l'article indéfini « un », « une », ou de l'article défini « le », « la », ou « l' », pour introduire un élément n'exclut pas la présence d'une pluralité de ces éléments.
The present invention has been described in relation to specific embodiments, which have a purely illustrative value and should not be considered as limiting. In general, it will be apparent to those skilled in the art that the present invention is not limited to the examples illustrated and / or described above. The invention includes each of the novel features as well as all their combinations. However, it is the claims that determine the extent of its protection. The presence of reference numbers to the drawings can not be considered as limiting, even when these numbers are indicated in the claims.
The use of the verbs "to understand", "to include", "to include", or any other variant, as well as their conjugation, can in no way exclude the presence of elements other than those mentioned. The use of the indefinite article "a", "an", or the definite article "the", "the", or "the", to introduce an element does not exclude the presence of plurality of these elements.

Claims (9)

  1. Resonant cavity (6), for accelerating charged particles in a cyclotron, comprising a dee (10), a rod (20), and a conductive chamber (40) at least partially surrounding said rod and said dee, one end of said rod (20) supporting the dee (10), the conductive chamber and the rod (20) thus forming a transmission line, one opposite end of said rod (20) being securely fastened to a base (45) of the conductive chamber (40), characterized in that the capacitance per unit length of an intermediate portion (20b) of said transmission line located between said ends of the rod is substantially higher than the capacitance per unit length of the other portions (20a, 20c) of said transmission line.
  2. Resonant cavity according to Claim 1, characterized in that the capacitance per unit length of the intermediate portion (20b) of the transmission line is twice as high as the capacitance per unit length of the other portions (20a, 20c) of said transmission line.
  3. Resonant cavity according to Claim 2, characterized in that the capacitance per unit length of the intermediate portion (20b) of the transmission line is ten times higher than the capacitance per unit length of the other portions (20a, 20c) of said transmission line.
  4. Resonant cavity according to any one of the preceding claims, characterized in that the characteristic impedance (Z c2 ) of the intermediate portion (20b) and the characteristic impedances (Z c1 , Z c3 ) of the other portions (20a, 20c) of the transmission line are such that the cavity (6) is able to resonate in two modes so as to produce two separate frequencies, one being substantially double the other.
  5. Resonant cavity according to any one of the preceding claims, characterized in that the rod (20) comprises a number of superposed cylinders (20a, 20b, 20c), one of these cylinders (20b) corresponding to said intermediate portion (20b) of the transmission line and having an average diameter that is much larger than the average diameter of the other cylinders (20a, 20c).
  6. Resonant cavity according to any one of the preceding claims, characterized in that the conductive chamber (40) comprises a number of superposed hollow cylinders, one of these hollow cylinders corresponding to said intermediate portion (20b) of the transmission line and having an average diameter substantially smaller than the average diameter of the other hollow cylinders.
  7. Resonant cavity according to any one of the preceding claims, characterized in that it furthermore comprises a matching capacitor (50) comprising a moveable electrode electrically connected to the conductive chamber (40) and placed facing the rod and substantially level with the intermediate portion (20b) of the transmission line.
  8. Method for designing a dual-frequency resonant cavity according to any one of the preceding claims, comprising the following steps:
    - calculating the capacitance per unit length of a cavity the rod and the conductive chamber of which have a constant cross section, allowing the characteristic impedance of the transmission line thus formed by said pillar and conductive chamber to be deduced;
    - calculating the characteristic impedance for various rod diameters;
    - determining the equivalent average outside diameter of the conductive chamber;
    - producing a 2D electromagnetic simulation of the cavity using the dimensions found beforehand and determining the diameter of an equivalent dee, hypothesized to be circular, producing the same resonant frequency as said cavity, the rod and the conductive chamber of which cavity have a constant cross section;
    - calculating the intrinsic parameters of the cavity, such as the quality factor Q, the power dissipated, the energy stored and comparing the results with measured values; and
    - characterizing, using a radiofrequency simulation, the various line portions forming the rod of a cavity, two resonant modes of which are used to produce two separate frequencies.
  9. Method for designing a dual-frequency resonant cavity according to Claim 8, furthermore comprising a final step of optimizing the dual-frequency cavity using 2D electromagnetic simulation.
EP20100170531 2010-07-22 2010-07-22 Cyclotron for accelerating at least two kinds of particles Active EP2410823B1 (en)

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EP20100170531 EP2410823B1 (en) 2010-07-22 2010-07-22 Cyclotron for accelerating at least two kinds of particles
JP2013520036A JP5858300B2 (en) 2010-07-22 2011-06-28 Resonant cavity used in cyclotron
PCT/EP2011/060835 WO2012010387A1 (en) 2010-07-22 2011-06-28 Cyclotron able to accelerate at least two types of particle
CA2800290A CA2800290C (en) 2010-07-22 2011-06-28 Cyclotron able to accelerate at least two types of particle
US13/807,989 US8823291B2 (en) 2010-07-22 2011-06-28 Cyclotron able to accelerate at least two types of particles
CN201180035515XA CN103004292A (en) 2010-07-22 2011-06-28 Cyclotron able to accelerate at least two types of particle

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CN102917529B (en) * 2012-10-24 2016-01-13 中国科学院近代物理研究所 Helical multi-gap high-frequency resonance device and pack and accelerated method
US9456532B2 (en) * 2014-12-18 2016-09-27 General Electric Company Radio-frequency power generator configured to reduce electromagnetic emissions
US9894747B2 (en) * 2016-01-14 2018-02-13 General Electric Company Radio-frequency electrode and cyclotron configured to reduce radiation exposure
CN106163072B (en) * 2016-07-29 2018-08-07 中国原子能科学研究院 A kind of isochronous cyclotron radio frequency cavity
US10306746B2 (en) * 2017-01-05 2019-05-28 Varian Medical Systems Particle Therapy Gmbh Cyclotron RF resonator tuning with asymmetrical fixed tuner
KR102165370B1 (en) * 2019-01-31 2020-10-14 성균관대학교산학협력단 Cyclotron having multifle cyclotron
JP7397622B2 (en) * 2019-10-29 2023-12-13 住友重機械工業株式会社 cavity and stem

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