Patent Publication Number: US-2015084548-A1

Title: Circular accelerator

Description:
BACKGROUND 
     The present invention relates to a circular accelerator that accelerates a charged particle beam to a high energy by way of a high frequency electric field while circulating the charged particle beam in a magnetic field and extracting the accelerated charged particle beam externally. 
     Circular accelerator types include cyclotrons and synchrocyclotrons, etc. In cyclotrons, a charged particle beam emitted for example from an ion source operating in a high vacuum is accelerated with circular motion in a magnetic field. In order to accelerate the charged particle beam, the cyclotron inputs the charged particle beam from the ion source in a circular orbit on a plane that is perpendicular to the magnetic field, applies a high frequency alternating current voltage to a so-called dee electrode mounted along that circular orbit, accelerates the passing charged particle beam, and increases the energy of that charged particle beam. 
     A synchrocyclotron disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2008-507826 has the main features of firstly frequency modulating the acceleration frequency from a high frequency voltage applicator electrode at approximately 1 kHz operating cycles during beam acceleration; secondly focusing the beam in the longitudinal direction based on the principle of high frequency acceleration phase stability, and thirdly focusing the beam vertical direction in a weak focus magnetic field. In the device disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2008-507826, the frequency modulation of resonance frequency at 1 KHz level is extremely difficult from a technical standpoint. 
     In a cyclotron disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. Hei5 (1993)-501632, a charged particle beam extracted from an ion source is gradually accelerated after each passage by way of an acceleration electrode while circulated by a bending magnetic field. The diameter of the circular orbit of the charged particle beam gradually becomes larger as the charged particle beam is accelerated and its energy increases, and achieves a spiral orbit and after the accelerated charged particle beam reaches the maximum energy level, is extracted by an extraction deflector and transported outside the accelerator. The operation thus far is the same as the synchrocyclotron. 
     In order for a cyclotron to stably accelerate the charged particle beam, a specified high frequency acceleration electric field must first of all be applied in the beam longitudinal direction at a timing that matches the passage of the charged particle beam; secondly a specified focusing force must be applied in the beam vertical direction, moreover thirdly there is no focusing force in the beam longitudinal direction. 
     The cyclotron disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. Hei5 (1993)-501632 differs from the synchrocyclotron in that there is no need to modulate the frequency of the high frequency acceleration electric field since a magnetic field distribution for the bending magnet is generated to prevent the changes occurring along with acceleration of the charged particle beam circulating frequency. This magnetic field is called an isochronous magnetic field. The isochronous magnetic field has no focusing force acting on the longitudinal direction of the beam, so has accuracy as high as 1×10 −6  for shaping the magnetic field from the magnet and moreover increases the acceleration voltage with a structure of several hundred turns for extracting the beam. Providing an isochronous magnetic field requires a magnetic field distribution that becomes intense in the direction having a larger radius, causing a large defocusing force to occur in the vertical direction. To obtain a focusing force in vertical direction capable of cancelling out this defocusing force, the bending magnet is a sector concentration cyclotron or AVF cyclotron structure having alternately repeating large valley gaps of magnetic pole and small hill gaps of magnetic pole along the circulation direction of the charged particle beam, and the magnetic pole is a spiral shape. 
     Japanese Patent Application No. 2011-244298 discloses an idea for varying the energy of the extraction beam by changing the bending magnetic field so that the amount of change in the bending magnet between 0.7% and 24.7%. However, changing the intensity of the magnetic field requires time, and instantaneously changing the energy of the extraction beam is impossible. 
     SUMMARY 
     Changing the energy of the extraction beam in synchrocyclotrons and cyclotrons of the related art requires changing the intensity of the magnetic field or changing the frequency of the high frequency acceleration electric field. However, making a large change in the magnetic field intensity also causes a large change in the spatial distribution in the isochronous magnetic field, causing the problem that a long time is required for adjusting this isochronous magnetic field. Also, making a large change in the acceleration frequency requires making a large change in the resonance frequency of RF acceleration cavity so the problem occurs that a long time is needed to adjust this resonance frequency. Due to the aforementioned reasons, making a large instantaneous change in the energy of the synchrocyclotron and cyclotron extraction beam is impossible. 
     In presently utilized synchrocyclotrons and cyclotrons, the energy from the accelerator cannot be changed, so after extracting a beam at a specified energy, the degrader causes an energy loss that changes the energy level; however, this method has the problem that a larger device must be fabricated along with the beam performance gradually deteriorates. 
     According to one aspect of the present invention and in view of the aforementioned problems, the present invention has the object of providing a circular accelerator that allows freely varying the energy of the extraction beam while maintaining the acceleration frequency and bending magnetic field of the charged particle beam. 
     According to another aspect of the present invention to resolve the aforementioned problems, the circular accelerator of the present invention includes an electrode that applies a high frequency electric field for accelerating a charged particle beam, an electromagnetic device that bends the charged particle beam, and a direct current (DC) power supply device that applies a direct current electric field to the electrode. 
     According to another aspect of the present invention, the circular accelerator is capable of changing the energy of the charged particle beam extracted from the circular accelerator, and preventing deterioration in beam performance due to the emittance growth and so on that occurs along with the energy loss in the beam due to the degrader, and further requires no degrader of the related art in order to change the energy. Furthermore, the overall accelerator system can be achieved in a compact size and a lower cost because no degrader is required. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustrative diagram showing the overall structure of the energy-variable type circular accelerator according to the first embodiment of the present invention; 
         FIG. 2  is an illustrative diagram showing a perpendicular cross section of the overall structure of the energy-variable type circular accelerator according to the first embodiment of the present invention; 
         FIG. 3  is a drawing showing an overview of the beam drift orbit of the energy-variable type circular accelerator according to the first embodiment of the present invention; 
         FIG. 4  is a drawing showing the relation between the radius re applied DC voltage, the DC voltage Vdc, and the extracted beam energy when the present invention is applied to a cyclotron device for PET pharmaceutical manufacture; 
         FIG. 5  is a concept drawing showing the overall structure of the energy-variable type circular accelerator according to the second embodiment of the present invention; 
         FIG. 6  is an illustrative diagram showing the overall structure of the energy-variable type circular accelerator according to the third embodiment of the present invention; 
         FIG. 7  is an illustrative diagram showing the overall structure of the low-loss beam extraction system of the circulator accelerator according to the fourth embodiment of the present invention; and 
         FIG. 8  is a drawing showing the overall structure of the traditional circulator accelerator of the related art. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention can apply in general to all circular beam accelerator devices however the description given here is for an energy-variable type circular accelerator and an extracted beam energy changing method. 
     First Embodiment 
     A preferred embodiment of the circular accelerator of the present invention is described while referring to  FIG. 1 .  FIG. 1  is an illustrative diagram showing the overall structure of the perpendicular cross section of the circular accelerator  100  of the present embodiment. The extracted beam energy change in the charged particle beam by the circular accelerator  100  of the present embodiment is described next. 
     The beam energy-variable type circular accelerator  100  of the present embodiment contains a dee electrode  1 , the dummy dee electrodes  2   a ,  2   b ,  2   c ,  2   d ,  2   e  insulated and divided in a concentric circle shape by an insulator material  3 , a DC power supply  6 , an inductance  7  for maintaining a high impedance cavity resonator, multiple switches  8 , multiple resistors  9 , and an electrostatic deflector electrode  10 . The dummy electrodes are insulated and divided in the structure utilized in the present embodiment; however, the present invention is not limited to five separate insulations and for example a structure with multiple separate insulations may be utilized. The dee electrode as referred to here is defined as the high-voltage side electrode among the two electrodes for accelerating the particles by generating a high frequency electric field between the electrodes mounted along the circular orbit. The dummy dee electrode is defined as the ground voltage side electrode among the two electrodes for accelerating the particles. The charged particle beam generated in the ion source (not shown in drawing) is injected to the center section of the circular accelerator  100 , and is sequentially accelerated by a high-frequency acceleration electric field between the dummy dee electrodes  2   a ,  2   b ,  2   c ,  2   d ,  2   e  insulated and divided in a concentric circle shape by the insulator material  3 , and the dee electrode  1  to which high-frequency (RF) electric power is transmitted along the coaxial mode from the quarter-wave cavity resonator. A magnetic field is applied across the paper direction in the region occupied by the dee electrode  1  and the dummy dee electrodes  2   a  through  2   e  so that the beam orbit along which the charged particle beam passes acquires a spiral shape due to bending by the magnetic field; and the charged particle beam finally extracted by the electrostatic deflector electrode  10  mounted on the outer side of the dummy dee electrode  2   e . The circular accelerator  100  of the present embodiment differs from the traditional circular accelerator of the related art shown in  FIG. 8  in containing a DC power supply  6 ; the dummy dee electrode  2  is divided into multiple electrodes in a concentric circle shape and each of the divided dummy dee electrodes  2   a ,  2   b ,  2   c ,  2   d ,  2   e  are insulated by an insulator material  3 ; and a switch unit  8  provided for selecting whether or not to apply a DC voltage from the DC power supply  6  to each of the dummy dee electrodes  2   a ,  2   b ,  2   c ,  2   d ,  2   e.    
       FIG. 2  shows a perpendicular cross section of the overall structure of the beam energy-variable type circular accelerator  100  according to the first embodiment. As shown in  FIG. 2 , the beam energy-variable type circular accelerator  100  of the present embodiment in addition to the structural device in  FIG. 1  also contains a main magnetic field coil  12  for bending the charged particle beam, an auxiliary magnetic field coil  13  for adjusting the magnetic field, an electromagnet core  14 , and an RF power supply  15 . 
     The method employed by the circular accelerator  100  for changing the energy of the charged particle beam and for operation is described next. A DC power supply  6  is connected between the dee electrode  1  for transmitting the high frequency field and the dummy dee electrodes  2   a  through  2   e , and the DC voltage generated by the DC power supply  6  is connected to each of the dummy dee electrodes  2   a  through  2   e  by way of the multiple resistors  9  branching in parallel. The switch units  8   a ,  8   b ,  8   c ,  8   d ,  8   e  are usually closed towards the ground voltage side, and each of the dummy dee electrodes  2   a  through  2   e  is at ground voltage. The resistors  9   a ,  9   b ,  9   c ,  9   d ,  9   e  are used for limiting the current from the DC power supply  6 , and the inductance  7  is a device for preventing a drop in the shunt impedance of the RF cavity and for maintaining a high shunt impedance. The dummy dee electrode  2   a  is connected between the switch  8   a  and the resistor  9   a ; the dummy dee electrode  2   b  is connected between the switch  8   b  and the resistor  9   b ; and the dummy dee electrode  2   c  is connected between the switch  8   c  and the resistor  9   c ; the dummy dee electrode  2   d  is connected between the switch  8   d  and the resistor  9   d ; and the dummy dee electrode  2   e  is connected between the switch  8   e  and the resistor  9   e . Opening the switch units  8   a  through  8   e  connected to each of the dummy dee electrodes  2   a  through  2   e  applies a DC voltage Vdc from the DC power supply  6 . In other words, on and off control can be achieved by applying a DC voltage Vdc to each of the dummy dee electrodes  2   a  through  2   e  by separately controlling the opening and closing of each of the switch units  8   a  through  8   e . A DC electric field E is applied by way of the DC voltage Vdc between the dee electrode  1  and the dummy dee electrodes  2   a  through  2   e  so that an E×B drift is generated towards the perpendicular angle formed by the bending magnetic field B and the DC electric field E along the orbit of the charged particle beam. A certain DC voltage Vdc is applied to a radius re of any of the dummy dee electrodes  2   a  through  2   e , to adjust the generated E×B drift and change the energy of the extraction beam. 
     Here, taking a proton cyclotron for PET pharmaceutical manufacture as one example of a circular accelerator, the specifications required for the system are approximately an acceleration frequency to 50 MHz, a beam current peak value to 1.0 mA, an input energy 30 keV, and an extraction energy of 10 MeV. The high frequency (RF) acceleration voltage required in this case is up to 100 kV, and the beam bending magnetic field is up to 0.33 T.  FIG. 3  shows one example of the beam orbit in the present embodiment, when the charged particle beam is accelerated without applying a DC voltage inside the radius re of the dummy dee electrode, and when the charged particle beam is accelerated while applying a DC voltage to the dummy dee electrode from outside the radius re. In  FIG. 3 , an E×B drift is generated along the spiral-shaped orbit to the radius re by the beam bending magnetic field B and the electric field E from the DC voltage Vdc at the outer side from re, the beam orbit  11  drift from the center to the outer side, and the charged particle beam is extracted by the electrostatic deflector electrode  10 .  FIG. 4  shows one example of the relation of the DC voltage Vdc and the radius re for starting to apply a DC voltage for the beam extraction energy. As shown in  FIG. 4 , the beam extraction energy can be freely changed by adjusting the DC voltage Vdc and the radius re where application of the DC voltage starts. If insulation materials such as Mylar®, polycarbonate, and Kapton® are utilized for insulating the DC voltage from the DC power supply, the break down electric field will have a solid insulation performance (or insulation performance for solids) of 100 kV per millimeter or higher and so the design can provide a sufficient insulated structure. 
     The circular accelerator  100  of the present embodiment is capable of changing the extraction beam energy, requires no degrader of the related art for changing the energy, and can therefore prevent degradation in beam performance due to the emittance growth and so on generated accompanying the energy loss in the beam due to the degrader. Moreover, the overall accelerator system can be achieved in a compact size and a lower cost because no degrader is required. 
     The present embodiment is capable of changing and adjusting the energy of the extraction beam from the circular accelerator  100  by adjusting the DC voltage and concentric radius with the multiple insulated and divided dummy dee electrodes  2  to which the DC voltage is applied. No changes are made to the magnetic field and the acceleration frequency so that the overall circular accelerator system including an energy change device for the extraction beam is capable of high performance, a compact size, and a lower cost, and also capable of extracting the beam with high efficiency and minimal beam loss, which contributes to even higher performance. 
     Second Embodiment 
     The circular accelerator of the second embodiment of the present invention is described next while referring to  FIG. 5 .  FIG. 5  is a concept drawing showing the overall structure of the vertical cross section of the circular accelerator  101  of the present embodiment. 
     The circular accelerator  101  of the present embodiment is a structure that applies a DC voltage Vdc from the DC power supply  6  between the dee electrode  1  and the dummy dee electrode  2 , to in this way generate an E×B drift by way of an electric field E and bend magnetic field B, to change the extracted beam energy and extract the charged particle beam the same as the circular accelerator  100  of the first embodiment. In the circular accelerator  100  of the first embodiment, the multiple dummy dee electrodes are each formed concentrically as multiple separate insulation structures. However, in the circular accelerator  101  of the present embodiment, the dummy dee electrode  2  is not separate insulation structures, and is a structure that is one dummy dee electrode. Comparing the circular accelerator  101  of the present embodiment with the circular accelerator  100  having dummy dee electrodes that are insulated and divided in a concentric state as in the first embodiment, shows that the circular accelerator  101  of the present embodiment utilizes only one dummy dee electrode structure so that the manufacturing cost can be reduced and consecutive changes can be made to the energy. However, a DC electric field is applied across the entire region around the dee electrodes so that an E×B drift is generated at the entire region, and therefore the case might occur where the particles are not capable of normally accelerating to the target energy due to disruption of the isochronism when drift occurs in the beam orbit. So in order to compensate the isochronous magnetic field, installing additional equipment such as a trim coil might become necessary. 
     The circular accelerator  101  of the present embodiment is capable of changing the extraction beam energy, requires no degrader of the related art for changing the energy, and can therefore prevent degradation in beam performance due to the emittance growth and so on generated accompanying the energy loss in the beam due to the degrader. Moreover, the overall accelerator system can be achieved a compact size and a lower cost because no degrader is required. 
     The present embodiment is capable of changing and adjusting the energy of the extraction beam from the circular accelerator  101  by adjusting the DC voltage and concentric radius with the dummy dee electrode  2  to which the DC voltage is applied. No changes are made to the magnetic field and the acceleration frequency so that the overall circular accelerator system including an energy change device for the extraction beam is capable of high performance, a compact size, and a lower cost, and also capable of extracting the beam with high efficiency and minimal beam loss, which contributes to even higher performance. 
     Third Embodiment 
     The circular accelerator of third embodiment of the present invention is described next while referring to  FIG. 6 .  FIG. 6  is a concept drawing showing the overall structure of the vertical cross section of the circular accelerator  102  of the present embodiment. 
     The circular accelerator  102  of the present embodiment is a structure that applies a DC voltage Vdc from the DC power supply  6  between the dee electrode  1  and the dummy dee electrode  2 , and in this way generates an E×B drift by way of electric field E and bend magnetic field B, to change the extracted beam energy and extract the charged particle beam the same as the circular accelerator  100  of the first embodiment. In the circular accelerator  100  of the first embodiment, the dummy dee electrode  2  is formed concentrically as multiple insulated and divided structure. However, in the circular accelerator  102  of the present embodiment, the dummy dee electrode  2  is arranged linearly in multiple insulated and divided structures. Comparing the circular accelerator  102  of the present embodiment with the circular accelerator  100  having dummy dee electrodes that are divided and insulated in a concentric state as in the first embodiment, shows that the circular accelerator  102  of the present embodiment has the insulated and divided dummy dee electrode structure in a linear shape so that the manufacturing cost can be reduced and changes can be made to the energy. In the circular accelerator  102  of the present embodiment, the DC electric field of the insulator sections for the divided dummy dee electrodes  2   a ,  2   b ,  2   c ,  2   d  is applied only in a fixed direction so that when drift occurs in the beam orbit of the charged particle beam due to the generation of E×B drift, the isochronism might be broken so that the case might occur where the particles are not capable of normally accelerating to the target energy. So installing additional equipment such as a trim coil might become necessary in order to compensate the isochronous magnetic field. 
     The circular accelerator  102  of the present embodiment is capable of changing the extraction beam energy, requires no degrader of the related art for changing the energy, and can therefore prevent deterioration in beam performance due to the emittance growth and so on generated accompanying the energy loss in the beam due to the degrader. Moreover, the overall accelerator system can be achieved in a compact size and a lower cost because no degrader is required. 
     The present embodiment is capable of changing and adjusting the energy of the extraction beam from the circular accelerator  102  by adjusting the DC voltage to the multiple insulated and divided electrodes to which a DC voltage is applied. No changes are made to the magnetic field and the acceleration frequency so that the overall circular accelerator system including an energy change device for the extraction beam is capable of high performance, a compact size, and a lower cost, and also capable of extracting the beam with high efficiency and minimal beam loss, which contributes to even higher performance. 
     Fourth Embodiment 
     The circular accelerator of fourth embodiment of the present invention is described next while referring to  FIG. 7 .  FIG. 7  is a concept drawing showing the overall structure of the vertical cross section of the circular accelerator  103  of the present embodiment. 
     The circular accelerator  103  of the present embodiment, the same as for the circular accelerator  100  of the first embodiment, is a structure in which a DC power supply  6  applies a DC voltage Vdc between a dee electrode  1  and a dummy dee electrode  2  that is insulated and divided in a concentric circle shape near the maximum radius of the orbit. An E×B drift is generated by the electric field E and bending magnetic field B from that applied DC voltage Vdc, expanding the gap between the last two turn orbits of the beam, and the beam conducts to the deflector electrode to reduce the amount of loss, and the beam is extracted with high efficiency. The point where the circular accelerator  103  of the present embodiment differs from the circular accelerator  100  of the first embodiment is that the objective is high-efficiency beam extraction rather than changing the extracted beam energy. 
     Generally, in circular accelerators for bending the orbit in a magnetic field B and accelerating particles in a high frequency electric field by way of an RF acceleration cavity, an E×B drift is generated by a DC electric field E and a bending magnetic field B, so that the above described first through the fourth embodiments can be applied in the same way as these extraction beam energy changing systems or high efficiency beam extracting methods, and can achieve high performance, compactness, and low cost in all systems including charged particle beam extraction and energy changing devices. 
     The circular accelerator  103  of the present embodiment is capable of changing the extraction beam energy, requires no degrader of the related art for changing the energy, and can therefore prevent degradation in beam performance due to the emittance growth and so on generated accompanying the energy loss in the beam due to the degrader. Moreover, the overall accelerator system can be achieved a compact size and a lower cost because no degrader is required. 
     The present embodiment is capable of changing and adjusting the energy of the extraction beam from the circular accelerator  103  by adjusting the DC voltage to the multiple insulated and divided dummy dee electrodes  2  to which a DC voltage is applied. No changes are made to the magnetic field and the acceleration frequency so that the overall circular accelerator system including an energy change device for the extraction beam is capable of high performance, a compact size, and a lower cost, and further capable of a high efficiency beam extraction with minimal beam loss, which contributes to even higher performance. 
     A charged particle beam was described in the first through the fourth embodiments however if a beam, even if of electrons or protons, then the energy varing method in the circular accelerator of the present invention can be achieved.