- Email: [email protected]
Developments for 230 MeV superconducting cyclotrons for proton therapy and proton irradiation
Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Developments for 230 MeV superconducting cyclotrons for proton therapy and proton irradiation q Tianjue Zhang ⇑, Chuan Wang, Ming Li, Tao Cui, Zhiguo Yin, Bin Ji, Yinlong Lv, Fengping Guan, Tao Ge, Jiansheng Xing, Jianjun Yang, Xianlu Jia, Meng Yin, Suping Zhang, Xuelong Cao, Shizhong An, Sumin Wei, Jun Lin, Lei Cao, Dongsheng Zhang, Shigang Hou, Feng Wang, Pengfei Gong China Institute of Atomic Energy, P.O. Box 275(3), Beijing 102413, China
a r t i c l e
i n f o
Article history: Received 5 August 2016 Received in revised form 6 November 2016 Accepted 7 November 2016 Available online xxxx Keywords: Proton therapy Proton irradiation Superconducting cyclotron
a b s t r a c t There are very strong demands for mid-energy proton machine in recent years due to the surging cancer patients and fast progress of the space science in China. For the applications of proton therapy and proton irradiation, the energy range of proton beam is usually from 200 MeV to 250 MeV, or even higher for astronavigation. Based on the R&D starting from 2009, a construction project of a 230 MeV superconducting cyclotron (CYCIAE-230) has been launched recently at China Institute of Atomic Energy (CIAE). It was started in Jan 2015, for the program of proton therapy and space science launched by China National Nuclear Corporation (CNNC). In this paper, the designs for the superconducting (SC) cyclotron and its key components, including the main magnet, SC coils, internal ion source and central region, extraction system, etc, and the construction progress of the machine CYCIAE-230 will be presented. Ó 2016 Elsevier B.V. All rights reserved.
1. Introductions CANCER has become a globalized threat to public health. According to global cancer statistics published by WHO in late 2014, the deaths caused by cancer are 8.2 million, and the new cancer cases are 14 million in 2012 [1]. China has the largest number of both deaths caused by cancer and the new cancer cases (2.5 million and 3.5 million respectively), and these numbers are still growing [2]. However, only two proton therapy centers in China are currently operational, so there are very strong demands on proton machine for proton therapy. We noticed that, according to PTCOG statistics, over 60% of proton therapy machines in operation are cyclotrons [3]. Also due to the feasibility of producing CW proton beam and cheaper in construction cost, most of the leading proton irradiation test facilities, like PIT [4], TAMU [5], TRIUMF [6], LBL [7] are based on cyclotrons. Being aware of that, China Institute of Atomic Energy (CIAE) has started an R&D program on medium energy proton cyclotron since 2009. Based on this, a construction project of a 230 MeV superconducting cyclotron CYCIAE-230 for the program of proton therapy and space science has been launched recently (starting in Jan 2015) at CIAE, approved by the q This work was supported in part by the National Natural Science Foundation of China under Grant 11475269 and 11375274. ⇑ Corresponding author. E-mail address: [email protected] (T. Zhang).
superior agency of CIAE, China National Nuclear Corporation (CNNC).
2. Physics design of CYCIAE-230 2.1. General considerations of CYCIAE-230 For a 230 MeV proton cyclotron, the magnetic rigidity roughly equals to 2.3 (Tm). Combined with the maximum saturation magnetic field 2.1 T for iron, the pole radius of a 230 MeV room temperature cyclotron will be roughly 1.1 m and the total weight will be 200 tons. Due to the highly saturated iron, the required Ampere-turns of the coils are very high, and to reduce the magnetic reluctance, the pole gap has to be kept very small, which will limit the space for the design of the extraction system and thus increase the beam loss. Even the most popular medium energy room temperature cyclotron has a large power consumption on coil windings (200 kW) and not able to increase the extraction efficiency from 50% to 75% until recently after over ten years’ optimizations [8]. A superconducting cyclotron, on the contrary, could be comfortably working beyond the limit of the saturation of the irons by increasing the Ampere-turns. Due to the high magnetic field provided by the superconducting coils, the superconducting main magnet is very compact and the total weight of the magnet could
http://dx.doi.org/10.1016/j.nimb.2016.11.010 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: T. Zhang et al., Developments for 230 MeV superconducting cyclotrons for proton therapy and proton irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.11.010
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T. Zhang et al. / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
be reduced to less than 100 tons. The power consumption of the superconducting coils, will also be reduced to a very economical level, i.e., 50 kW including the power supply and cryo-coolers, which is only 1/4 of the room temperature cyclotron with the same extraction energy. More importantly, every other subsystem including the buildings could also be more compact and cheaper. And the pole gap could be increased and leave more space for the extraction system design which in turn makes an extraction efficiency larger than 80% easier to be achieved. So our goal is to design and construction of a superconducting cyclotron. The layout of the CYCIAE-230 superconducting cyclotron is shown in Fig. 1. The superconducting cyclotron consists of many subcomponents, mainly including the main magnet system; the superconducting coil system; the RF system; an internal PIG source; the extraction system; vacuum pumps and the lifting system. The overall parameters are listed in Table 1. Although the initial design goal is to extract proton at energy larger than 230 MeV, the extraction energy of final design has actually reached 240 MeV.
So, the first two resonances, which are driven by field errors, have little effect on the beam quality. However, the Walkinshaw resonance, which is driven by the main field and is crossed at 234 MeV, will cause significant increasing of beam profile in the case of ±2 mm off centered beam, as is shown in Fig. 3. Compared to K250 design and optimized K250 design for ACCEL, a faster cross of the resonance in CYCIAE-230 is achieved. The beam losses are very crucial for proton therapy superconducting cyclotron, as harmful extra heat load on cryostat will be generated by the neutrons that come from proton beam losses on the metallic surfaces. In order to focus the beam in vertical direction, the tz > 0:2is maintained for energy larger than 20 MeV.
2.2. Beam dynamics The tune diagrams of the CYCIAE-230 and other medium energy superconducting cyclotron design, i.e., the MSU K250 design and the optimized version [9] are shown in Fig. 2. Fig. 2 shows that the beam mainly crosses tr tz ¼ 1, 2tz ¼ 1 and Walkinshaw resonance (tr ¼ 2tz ). Actually, the high extraction efficiency requires that only part of the particles with radial amplitude smaller than 1.5 mm can be accepted from the central region.
Fig. 2. The comparison of tune diagram between the design results of CYCIAE-230 and a K250 superconducting cyclotron [9].
Fig. 1. The layout of superconducting cyclotron CYCIAE-230.
Table 1 The overall parameters of the CYCIAE-230. Parameter
Value
Extraction energy Extraction current Injection/Extraction field Radius of return yoke Ampere turns Coil type RF frequency RF voltage
240 MeV 300 nA 2.35 T/2.95 T (Average field) 3200 mm 1.1 MA T NbTi/Cu superconducting coil 71.3 MHz 70 kV/110 kV
Fig. 3. Vertical oscillation for centered beam, ±1 mm off centered beam and ±2 mm off centered beam of CYCIAE-230.
Please cite this article in press as: T. Zhang et al., Developments for 230 MeV superconducting cyclotrons for proton therapy and proton irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.11.010
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more dedicated pure iron forging pieces from domestic supplier with specified chemical composition, as listed in Table 2, are used to fabricate the main magnet of CYCIAE-230. The B–H curve measurements show comparable magnetic properties of the pure iron forging pieces of CYCIAE-230 with the imported rolled plate used as the magnet pole of CYCIAE-100 from abroad. In order to restrict the circulating emittance growth less than 50%, the 1st harmonic tolerable should not exceed 2.5 Gauss for CYCIAE-230. This combined with the ±1 mm tolerance for off centered beam as suggested in Fig. 3, gives the standard for the quality control (QC) during the magnet construction and installation. The main magnet is now under construction at the constant temperature workshop of the Beijing No. 1 Machine Tool Plant, the parent company of WALDRICH COBURG GmbH, as is shown in Fig. 5. The construction process is supposed to be finished at the end of this year.
3. Design and progress of main magnet The main magnet consists of upper yoke/poles, bottom yoke/poles and a return yoke. To simplify the routine operation, the fixed magnetic field using multiple shimming plates and shimming bars instead of complicated trim coils are adopted for CYCIAE-230. Its design is starting from the isochronous Eq. (1) and oscillation frequency equations summarized in [10],
hB0 ðRÞi ¼ Bcenter cðRÞ ¼ Bcenter
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi c2
ð1Þ
c2 ðx0 RÞ2
where hB0 ðRÞi is the average field at radius R, Bcenter is the magnetic field in the center of the magnet, x0 is the proton cyclotron frequency, c is the speed of light. To be far away from mr ¼ N2 resonance in extraction region, 4 magnet poles are chosen other than 3 poles. The 2D magnetic stray field calculation indicates that sensitive electronic equipments should be installed outside 7.2 m (the 5 Gauss line) from the center of the cyclotron. 3D magnet models are used to perform detailed pole shape and shimming plates optimization to reduce the phase slip (the integral phase slip within ±16° is achieved in design stage) and to optimize the spiral angles to improve beam-focusing properties as is shown in Fig. 2. The optimized average magnetic field is shown in Fig. 4. From the construction experience of CYCIAE-100 [11], a room temperature 100 MeV H-cyclotron with 416 ton main magnet,
4. Superconducting coils and cryogenic system To maintain a stable operation of the superconducting cyclotron for proton therapy, the liquid helium zero-boiling cooling combined with cryo-coolers and the high copper/Sc ratio wire-inchannel NbTi wires, are used in the superconducting coils of
Fig. 4. The calculated and theoretical average magnetic field.
Fig. 6. The load line of the selected superconducting wire.
Table 2 The comparison of chemical composition of pure iron forging pieces between CYCIAE-230 and CYCIAE-100.
230 MeV (%) 100 MeV (%)
C
Si
P
S
Mn
Cr
Ni
Al
<0.01 0.02
0.13 0.19
0.003 0.005
0.005 0.003
0.15 0.30
0.029 0.06
0.032 0.09
0.044 0.047
Fig. 5. Fabrication of poles (left) and yokes (right).
Please cite this article in press as: T. Zhang et al., Developments for 230 MeV superconducting cyclotrons for proton therapy and proton irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.11.010
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Fig. 7. Fabrication of superconducting coil and cryostat.
CYCIAE-230 cyclotron. These technologies have been proven extremely stable and suitable for medical applications by thousands of MRIs that have been installed in hospitals worldwide. The superconducting coil system contains a pair of NbTi superconducting coils, a cryostat, support links and a liquid Helium recondenser with cryo-coolers. With the maximum of the magnetic field 2.9 T in coils and the critical values of NbTi material [12], the load line for the selected superconducting wire is shown in Fig. 6. There is a 3.4 K temperature margin between the critical temperature 7.6 K and the nominal working temperature 4.2 K, which indicates a large safety margin from quench. Beam tracking shows that a 0.1 mm deviation of the superconducting coil from the median plane of the main magnet will cause 3 mm 4 mm vertical deviation of beam profile. Thus, load cells have been equipped on each support links to precisely adjust the position of superconducting coils. The superconducting coils and cryogenic system is under construction, and will be finished within the third quarter of this year, as is shown in Fig. 7.
5. Design and progress of RF system Fig. 8. RF system design comparison between Varian [13] and CIAE.
The main characteristics of the CYCIAE-230 cyclotron RF system are tabulated as following, The CYCIAE-230 cyclotron has its own unique characteristics in the RF system design. The RF system has four cavities. Each opposing pairs of cavities are mechanically connected in/underneath the central region. The two pairs of cavities are joint together by distributed capacitance in the central region. The coupled four Dees are driven by two separated 75 kW power supplies through two independent RF couplers located in two valleys of the cyclotron. The voltage and the phase of the Dees are controlled by one set of LLRF controller, as shown in Fig. 8b. As can be seen from the following Fig. 8, the major difference between CIAE’s design and Varian’s design [13] is the usage of hard bridge to connect the two satellite resonators underneath the central region. The first reason to do so is that by this measure, the design eliminates a parasitic resonance mode identified by Lukas Stingelin [14]. This mode is site between the push–pull mode and pushpush mode, in which the three Dees are in phase while only one of the satellite Dees is in opposite phase. The secondary reason is that the two set of cavities can now be driven by two set of 75 kW amplifiers. The symmetrical generator driven scheme of this RF system gives the advantage of better phase balance between the main cavities group and the satellite cavities group. In another word, the coupling capacitance in existing design [15] will inevitably generate certain amount of phase shift while carrying power
from main cavities group. The third reason is that the two small amplifiers are considered easier to be manufactured for us. The reported RF system, as is listed in Table 3, is considered as a natural evolvement of CYCIAE-100 high intensity cyclotron RF system. For example, the two 75 kW RF amplifiers are identical to those used in CYCIAE-100 cyclotron RF system, the later can be considered as engineering prototypes for the new design. Besides the amplifiers, a new set of LLRF control also has been designed based on the previous experience with RF control of CYCIAE-100 cyclotron. The hardware of the new LLRF control box has been successfully tested with a 162 MHz double Dees resonator system at low power level. The mechanical design of the cavities of CYCIAE-230 cyclotron has been finished and is ready for manufacture.
Table 3 The RF parameters of the CYCIAE-230 cyclotron. Parameter
Value
Frequency RF Amplifier Power Rating Dee Voltage Stability Frequency Stability Operating Mode
71.2 MHz 2 75 kW ±5 104 ±5 109 Generator driven resonators, CW and h = 2
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Fig. 9. Left: the beam trajectory in central region (dashed line: Vacc = 72 kV; solid line: Vacc = 80 kV); Right: Layout of central region.
Fig. 10. Left: Configuration of the extraction system; Right: Extraction efficiency, beam loss on the extraction system with different radial emittance at 20°phase acceptance.
6. Design of central region and extraction systems The ion source is an ultra compact internal PIG source. For the central region design of a cyclotron with internal ion source, the matching from ion source to RF phase, the longitudinal acceptance, vertical focusing and beam off centering are key issues need to be optimized. The beam trajectories in central region under different accelerating voltages and the layout of the elements of central region are shown in Fig. 9. To reduce sparks in central region, the accelerating voltage of 72 kV is more suitable for stable operation. Benefitted by the 50 mm pole gap and 30 mm radial distance between the pole and the inner surface of cryostat, it is feasible to design a complex but highly efficient extraction system for CYCIAE-230, as is shown in the left of Fig. 10. The extraction system contains two electrostatic deflectors (ESD1 and ESD2) with 7 mm gap/operation voltage less than 60 kV and magnetic channel (MC) consisting of multiple focusing and compensating iron bars with field gradient of 3 kGauss/cm. Beam tracking at the extraction region shows that the most of the beam are lost on the deflectors, less than 2% of beam are lost at iron bars, and 80% beam could be extracted, with 20°phase acceptance, as is shown in the right of the Fig. 10. Considering that, the turn separation in Deflector 1 is smaller, the density of heat power on Deflector 1 due to the beam loss is larger, a watercooling feed through is used for Deflector 1 to ensure a stable operation.
The design stage of other subsystems, including the vacuum system, control system, etc., have also been finished. Special attention has been paid to the design of control system, as design specifications for control system of proton therapy machine are quite different from a research machine like CYCIAE-100, especially in system redundancy, dose stability control and control of fast turn-on/off of beam. 7. Conclusions All the engineering designs of subsystems of CYCAIE-230 have been fixed. Key components with long construction period have already been under fabrication recently. All the fabrications will be finished by the end of 2018. This prototype machine will be installed at a hospital in a beautiful port city near Tianjin, as a demonstration proton therapy center. CNNC has coordinated a joint alliance of proton therapy center with potential users in more than 15 provinces in China. The creative development of the superconducting cyclotron CYCIAE-230 will greatly improve the applications of medium energy protons in proton therapy and space science in China. References [1] http://www.who.int/mediacentre/factsheets/fs297/en/. [2] Chen Wanqing et al., China Cancer 25 (1) (2016). [3] http://www.ptcog.ch/index.php/facilities-in-operation.
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[4] Proton Irradiation Facility at Laboratory For Particle Physics, http://pif.web. psi.ch/. [5] Radiation Effects Facility, Cyclotron Institute, Texas A&M University, MS #3366, College Station, TX 77843, http://cyclotron.tamu.edu/ref. [6] E. Blackmore, IEEE Radiation Effects Data Workshop Record, 2000, pp. 1–5. [7] M.A. McMahan, NIMB 241 (1) (2005) 409–413. [8] R.V. Galkina, S.V. Gurskiia, Y. Jongen, et al., Tech. Phys. 59 (6) (2014) 917–924. [9] J. Kim, H. Blosser, Proc. of ICC 2001, May. 13 – 17, 2007, East Lansing, Michigan, USA, 2007, pp. 345–347.
[10] S. Zaremba, Magnets for Cyclotrons, CAS, Zeegse, The Netherlands, 24 May –2 June 2005. http://cds.cern.ch/record/1005055/files/p253.pdf. [11] T.J. Zhang, C.J. Chu, J.Q. Zhong, et al., NIM A 261 (1) (2007) 25–30. [12] S.W. Kim, FermiLab, ‘Material Properties for Quench Simulation’, TD Note 00–041. [13] D.W. Krischel, et al., 1st Workshop HADRON BEAM THERAPY OF CANCER, ERICE – SICILY, 24 APRIL – 1 MAY 2009. [14] Lukas Stingelin, THÈSE NO 3169 (2005), ingénieur physicien diplômé EPF de nationalité suisse et originaore de Muttenz (BL). [15] A.E. Geisler, et al., Proc. of ICC 2007, Oct. 1–5 2007, Giardini Naxos, Italy, pp. 9–14.
Please cite this article in press as: T. Zhang et al., Developments for 230 MeV superconducting cyclotrons for proton therapy and proton irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.11.010
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Developments for 230 MeV superconducting cyclotrons for proton therapy and proton irradiation q Tianjue Zhang ⇑, Chuan Wang, Ming Li, Tao Cui, Zhiguo Yin, Bin Ji, Yinlong Lv, Fengping Guan, Tao Ge, Jiansheng Xing, Jianjun Yang, Xianlu Jia, Meng Yin, Suping Zhang, Xuelong Cao, Shizhong An, Sumin Wei, Jun Lin, Lei Cao, Dongsheng Zhang, Shigang Hou, Feng Wang, Pengfei Gong China Institute of Atomic Energy, P.O. Box 275(3), Beijing 102413, China
a r t i c l e
i n f o
Article history: Received 5 August 2016 Received in revised form 6 November 2016 Accepted 7 November 2016 Available online xxxx Keywords: Proton therapy Proton irradiation Superconducting cyclotron
a b s t r a c t There are very strong demands for mid-energy proton machine in recent years due to the surging cancer patients and fast progress of the space science in China. For the applications of proton therapy and proton irradiation, the energy range of proton beam is usually from 200 MeV to 250 MeV, or even higher for astronavigation. Based on the R&D starting from 2009, a construction project of a 230 MeV superconducting cyclotron (CYCIAE-230) has been launched recently at China Institute of Atomic Energy (CIAE). It was started in Jan 2015, for the program of proton therapy and space science launched by China National Nuclear Corporation (CNNC). In this paper, the designs for the superconducting (SC) cyclotron and its key components, including the main magnet, SC coils, internal ion source and central region, extraction system, etc, and the construction progress of the machine CYCIAE-230 will be presented. Ó 2016 Elsevier B.V. All rights reserved.
1. Introductions CANCER has become a globalized threat to public health. According to global cancer statistics published by WHO in late 2014, the deaths caused by cancer are 8.2 million, and the new cancer cases are 14 million in 2012 [1]. China has the largest number of both deaths caused by cancer and the new cancer cases (2.5 million and 3.5 million respectively), and these numbers are still growing [2]. However, only two proton therapy centers in China are currently operational, so there are very strong demands on proton machine for proton therapy. We noticed that, according to PTCOG statistics, over 60% of proton therapy machines in operation are cyclotrons [3]. Also due to the feasibility of producing CW proton beam and cheaper in construction cost, most of the leading proton irradiation test facilities, like PIT [4], TAMU [5], TRIUMF [6], LBL [7] are based on cyclotrons. Being aware of that, China Institute of Atomic Energy (CIAE) has started an R&D program on medium energy proton cyclotron since 2009. Based on this, a construction project of a 230 MeV superconducting cyclotron CYCIAE-230 for the program of proton therapy and space science has been launched recently (starting in Jan 2015) at CIAE, approved by the q This work was supported in part by the National Natural Science Foundation of China under Grant 11475269 and 11375274. ⇑ Corresponding author. E-mail address: [email protected] (T. Zhang).
superior agency of CIAE, China National Nuclear Corporation (CNNC).
2. Physics design of CYCIAE-230 2.1. General considerations of CYCIAE-230 For a 230 MeV proton cyclotron, the magnetic rigidity roughly equals to 2.3 (Tm). Combined with the maximum saturation magnetic field 2.1 T for iron, the pole radius of a 230 MeV room temperature cyclotron will be roughly 1.1 m and the total weight will be 200 tons. Due to the highly saturated iron, the required Ampere-turns of the coils are very high, and to reduce the magnetic reluctance, the pole gap has to be kept very small, which will limit the space for the design of the extraction system and thus increase the beam loss. Even the most popular medium energy room temperature cyclotron has a large power consumption on coil windings (200 kW) and not able to increase the extraction efficiency from 50% to 75% until recently after over ten years’ optimizations [8]. A superconducting cyclotron, on the contrary, could be comfortably working beyond the limit of the saturation of the irons by increasing the Ampere-turns. Due to the high magnetic field provided by the superconducting coils, the superconducting main magnet is very compact and the total weight of the magnet could
http://dx.doi.org/10.1016/j.nimb.2016.11.010 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: T. Zhang et al., Developments for 230 MeV superconducting cyclotrons for proton therapy and proton irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.11.010
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be reduced to less than 100 tons. The power consumption of the superconducting coils, will also be reduced to a very economical level, i.e., 50 kW including the power supply and cryo-coolers, which is only 1/4 of the room temperature cyclotron with the same extraction energy. More importantly, every other subsystem including the buildings could also be more compact and cheaper. And the pole gap could be increased and leave more space for the extraction system design which in turn makes an extraction efficiency larger than 80% easier to be achieved. So our goal is to design and construction of a superconducting cyclotron. The layout of the CYCIAE-230 superconducting cyclotron is shown in Fig. 1. The superconducting cyclotron consists of many subcomponents, mainly including the main magnet system; the superconducting coil system; the RF system; an internal PIG source; the extraction system; vacuum pumps and the lifting system. The overall parameters are listed in Table 1. Although the initial design goal is to extract proton at energy larger than 230 MeV, the extraction energy of final design has actually reached 240 MeV.
So, the first two resonances, which are driven by field errors, have little effect on the beam quality. However, the Walkinshaw resonance, which is driven by the main field and is crossed at 234 MeV, will cause significant increasing of beam profile in the case of ±2 mm off centered beam, as is shown in Fig. 3. Compared to K250 design and optimized K250 design for ACCEL, a faster cross of the resonance in CYCIAE-230 is achieved. The beam losses are very crucial for proton therapy superconducting cyclotron, as harmful extra heat load on cryostat will be generated by the neutrons that come from proton beam losses on the metallic surfaces. In order to focus the beam in vertical direction, the tz > 0:2is maintained for energy larger than 20 MeV.
2.2. Beam dynamics The tune diagrams of the CYCIAE-230 and other medium energy superconducting cyclotron design, i.e., the MSU K250 design and the optimized version [9] are shown in Fig. 2. Fig. 2 shows that the beam mainly crosses tr tz ¼ 1, 2tz ¼ 1 and Walkinshaw resonance (tr ¼ 2tz ). Actually, the high extraction efficiency requires that only part of the particles with radial amplitude smaller than 1.5 mm can be accepted from the central region.
Fig. 2. The comparison of tune diagram between the design results of CYCIAE-230 and a K250 superconducting cyclotron [9].
Fig. 1. The layout of superconducting cyclotron CYCIAE-230.
Table 1 The overall parameters of the CYCIAE-230. Parameter
Value
Extraction energy Extraction current Injection/Extraction field Radius of return yoke Ampere turns Coil type RF frequency RF voltage
240 MeV 300 nA 2.35 T/2.95 T (Average field) 3200 mm 1.1 MA T NbTi/Cu superconducting coil 71.3 MHz 70 kV/110 kV
Fig. 3. Vertical oscillation for centered beam, ±1 mm off centered beam and ±2 mm off centered beam of CYCIAE-230.
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more dedicated pure iron forging pieces from domestic supplier with specified chemical composition, as listed in Table 2, are used to fabricate the main magnet of CYCIAE-230. The B–H curve measurements show comparable magnetic properties of the pure iron forging pieces of CYCIAE-230 with the imported rolled plate used as the magnet pole of CYCIAE-100 from abroad. In order to restrict the circulating emittance growth less than 50%, the 1st harmonic tolerable should not exceed 2.5 Gauss for CYCIAE-230. This combined with the ±1 mm tolerance for off centered beam as suggested in Fig. 3, gives the standard for the quality control (QC) during the magnet construction and installation. The main magnet is now under construction at the constant temperature workshop of the Beijing No. 1 Machine Tool Plant, the parent company of WALDRICH COBURG GmbH, as is shown in Fig. 5. The construction process is supposed to be finished at the end of this year.
3. Design and progress of main magnet The main magnet consists of upper yoke/poles, bottom yoke/poles and a return yoke. To simplify the routine operation, the fixed magnetic field using multiple shimming plates and shimming bars instead of complicated trim coils are adopted for CYCIAE-230. Its design is starting from the isochronous Eq. (1) and oscillation frequency equations summarized in [10],
hB0 ðRÞi ¼ Bcenter cðRÞ ¼ Bcenter
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi c2
ð1Þ
c2 ðx0 RÞ2
where hB0 ðRÞi is the average field at radius R, Bcenter is the magnetic field in the center of the magnet, x0 is the proton cyclotron frequency, c is the speed of light. To be far away from mr ¼ N2 resonance in extraction region, 4 magnet poles are chosen other than 3 poles. The 2D magnetic stray field calculation indicates that sensitive electronic equipments should be installed outside 7.2 m (the 5 Gauss line) from the center of the cyclotron. 3D magnet models are used to perform detailed pole shape and shimming plates optimization to reduce the phase slip (the integral phase slip within ±16° is achieved in design stage) and to optimize the spiral angles to improve beam-focusing properties as is shown in Fig. 2. The optimized average magnetic field is shown in Fig. 4. From the construction experience of CYCIAE-100 [11], a room temperature 100 MeV H-cyclotron with 416 ton main magnet,
4. Superconducting coils and cryogenic system To maintain a stable operation of the superconducting cyclotron for proton therapy, the liquid helium zero-boiling cooling combined with cryo-coolers and the high copper/Sc ratio wire-inchannel NbTi wires, are used in the superconducting coils of
Fig. 4. The calculated and theoretical average magnetic field.
Fig. 6. The load line of the selected superconducting wire.
Table 2 The comparison of chemical composition of pure iron forging pieces between CYCIAE-230 and CYCIAE-100.
230 MeV (%) 100 MeV (%)
C
Si
P
S
Mn
Cr
Ni
Al
<0.01 0.02
0.13 0.19
0.003 0.005
0.005 0.003
0.15 0.30
0.029 0.06
0.032 0.09
0.044 0.047
Fig. 5. Fabrication of poles (left) and yokes (right).
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Fig. 7. Fabrication of superconducting coil and cryostat.
CYCIAE-230 cyclotron. These technologies have been proven extremely stable and suitable for medical applications by thousands of MRIs that have been installed in hospitals worldwide. The superconducting coil system contains a pair of NbTi superconducting coils, a cryostat, support links and a liquid Helium recondenser with cryo-coolers. With the maximum of the magnetic field 2.9 T in coils and the critical values of NbTi material [12], the load line for the selected superconducting wire is shown in Fig. 6. There is a 3.4 K temperature margin between the critical temperature 7.6 K and the nominal working temperature 4.2 K, which indicates a large safety margin from quench. Beam tracking shows that a 0.1 mm deviation of the superconducting coil from the median plane of the main magnet will cause 3 mm 4 mm vertical deviation of beam profile. Thus, load cells have been equipped on each support links to precisely adjust the position of superconducting coils. The superconducting coils and cryogenic system is under construction, and will be finished within the third quarter of this year, as is shown in Fig. 7.
5. Design and progress of RF system Fig. 8. RF system design comparison between Varian [13] and CIAE.
The main characteristics of the CYCIAE-230 cyclotron RF system are tabulated as following, The CYCIAE-230 cyclotron has its own unique characteristics in the RF system design. The RF system has four cavities. Each opposing pairs of cavities are mechanically connected in/underneath the central region. The two pairs of cavities are joint together by distributed capacitance in the central region. The coupled four Dees are driven by two separated 75 kW power supplies through two independent RF couplers located in two valleys of the cyclotron. The voltage and the phase of the Dees are controlled by one set of LLRF controller, as shown in Fig. 8b. As can be seen from the following Fig. 8, the major difference between CIAE’s design and Varian’s design [13] is the usage of hard bridge to connect the two satellite resonators underneath the central region. The first reason to do so is that by this measure, the design eliminates a parasitic resonance mode identified by Lukas Stingelin [14]. This mode is site between the push–pull mode and pushpush mode, in which the three Dees are in phase while only one of the satellite Dees is in opposite phase. The secondary reason is that the two set of cavities can now be driven by two set of 75 kW amplifiers. The symmetrical generator driven scheme of this RF system gives the advantage of better phase balance between the main cavities group and the satellite cavities group. In another word, the coupling capacitance in existing design [15] will inevitably generate certain amount of phase shift while carrying power
from main cavities group. The third reason is that the two small amplifiers are considered easier to be manufactured for us. The reported RF system, as is listed in Table 3, is considered as a natural evolvement of CYCIAE-100 high intensity cyclotron RF system. For example, the two 75 kW RF amplifiers are identical to those used in CYCIAE-100 cyclotron RF system, the later can be considered as engineering prototypes for the new design. Besides the amplifiers, a new set of LLRF control also has been designed based on the previous experience with RF control of CYCIAE-100 cyclotron. The hardware of the new LLRF control box has been successfully tested with a 162 MHz double Dees resonator system at low power level. The mechanical design of the cavities of CYCIAE-230 cyclotron has been finished and is ready for manufacture.
Table 3 The RF parameters of the CYCIAE-230 cyclotron. Parameter
Value
Frequency RF Amplifier Power Rating Dee Voltage Stability Frequency Stability Operating Mode
71.2 MHz 2 75 kW ±5 104 ±5 109 Generator driven resonators, CW and h = 2
Please cite this article in press as: T. Zhang et al., Developments for 230 MeV superconducting cyclotrons for proton therapy and proton irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.11.010
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Fig. 9. Left: the beam trajectory in central region (dashed line: Vacc = 72 kV; solid line: Vacc = 80 kV); Right: Layout of central region.
Fig. 10. Left: Configuration of the extraction system; Right: Extraction efficiency, beam loss on the extraction system with different radial emittance at 20°phase acceptance.
6. Design of central region and extraction systems The ion source is an ultra compact internal PIG source. For the central region design of a cyclotron with internal ion source, the matching from ion source to RF phase, the longitudinal acceptance, vertical focusing and beam off centering are key issues need to be optimized. The beam trajectories in central region under different accelerating voltages and the layout of the elements of central region are shown in Fig. 9. To reduce sparks in central region, the accelerating voltage of 72 kV is more suitable for stable operation. Benefitted by the 50 mm pole gap and 30 mm radial distance between the pole and the inner surface of cryostat, it is feasible to design a complex but highly efficient extraction system for CYCIAE-230, as is shown in the left of Fig. 10. The extraction system contains two electrostatic deflectors (ESD1 and ESD2) with 7 mm gap/operation voltage less than 60 kV and magnetic channel (MC) consisting of multiple focusing and compensating iron bars with field gradient of 3 kGauss/cm. Beam tracking at the extraction region shows that the most of the beam are lost on the deflectors, less than 2% of beam are lost at iron bars, and 80% beam could be extracted, with 20°phase acceptance, as is shown in the right of the Fig. 10. Considering that, the turn separation in Deflector 1 is smaller, the density of heat power on Deflector 1 due to the beam loss is larger, a watercooling feed through is used for Deflector 1 to ensure a stable operation.
The design stage of other subsystems, including the vacuum system, control system, etc., have also been finished. Special attention has been paid to the design of control system, as design specifications for control system of proton therapy machine are quite different from a research machine like CYCIAE-100, especially in system redundancy, dose stability control and control of fast turn-on/off of beam. 7. Conclusions All the engineering designs of subsystems of CYCAIE-230 have been fixed. Key components with long construction period have already been under fabrication recently. All the fabrications will be finished by the end of 2018. This prototype machine will be installed at a hospital in a beautiful port city near Tianjin, as a demonstration proton therapy center. CNNC has coordinated a joint alliance of proton therapy center with potential users in more than 15 provinces in China. The creative development of the superconducting cyclotron CYCIAE-230 will greatly improve the applications of medium energy protons in proton therapy and space science in China. References [1] http://www.who.int/mediacentre/factsheets/fs297/en/. [2] Chen Wanqing et al., China Cancer 25 (1) (2016). [3] http://www.ptcog.ch/index.php/facilities-in-operation.
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[10] S. Zaremba, Magnets for Cyclotrons, CAS, Zeegse, The Netherlands, 24 May –2 June 2005. http://cds.cern.ch/record/1005055/files/p253.pdf. [11] T.J. Zhang, C.J. Chu, J.Q. Zhong, et al., NIM A 261 (1) (2007) 25–30. [12] S.W. Kim, FermiLab, ‘Material Properties for Quench Simulation’, TD Note 00–041. [13] D.W. Krischel, et al., 1st Workshop HADRON BEAM THERAPY OF CANCER, ERICE – SICILY, 24 APRIL – 1 MAY 2009. [14] Lukas Stingelin, THÈSE NO 3169 (2005), ingénieur physicien diplômé EPF de nationalité suisse et originaore de Muttenz (BL). [15] A.E. Geisler, et al., Proc. of ICC 2007, Oct. 1–5 2007, Giardini Naxos, Italy, pp. 9–14.
Please cite this article in press as: T. Zhang et al., Developments for 230 MeV superconducting cyclotrons for proton therapy and proton irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.11.010