1MeV强流中子发生器供电系统研究
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摘要
用低能加速器通过T(d,n)~4He或D(d,n)~3He反应分别产生14MeV或2.5MeV左右中子的中子发生器被广泛应用于中子物理及其应用研究。由于核聚变反应堆和战略武器材料抗辐射研究及快中子治疗癌症研究的需要,中子发生器向“强流”方向发展,通常把中子产额达到或超过10~(12)n/s量级的称为强流中子发生器。
兰州大学研制中的强流中子发生器的预定指标为14MeV中子产额达到8×10~(12)n/s。为了提高靶寿命,采用分析氘束和较高的1MeV氘束能量,同时希望D(d,n)~3He反应产生的2.5MeV中子产额亦能达到10~(12)n/s量级。
论文评述了各种放射治疗手段(包括γ与X射线、带电粒子束、后装机、中子俘获和快中子治癌)的物理效应和生物效应、优缺点及其发展历史和现状;结合快中子治癌的要求,给出了兰州大学快中子治疗机拟达到的主要技术指标,并定性或定量地分析了各项指标的确定依据及其可行性。
根据强流中子发生器各主要组成部分的工作原理及其对中子发生器性能的影响,分析了各部分对供电电源的具体要求。
论文主要研究了强流中子发生器的供电系统。从ECR离子源引出的混合氘束预加速到120keV,经90°磁分析器分析出单原子离子D~+束,而后经加速管被加速到1MeV。离子源及预加速系统所需电源不仅功率大,性能要求高,而且处在接近1MV的高电位端,不能采取地电位直接供电,这成为供电系统的第一个难题。为了尽可能减小供电系统尺寸并保证电源性能和降低噪音,高压端供电用20kHz中频。用5路耐压1MV的隔离变压器将5台20kHz中频电源产生的中频电输送到1MV高电位端。隔离变压器用我们自主创新的强耦合隔离变压技术,使由空载到满载时的输出电压降不超过输出电压的15%。在输出端电压取样,通过电光转换—光纤传输—光电转换闭环调制中频电源输出电压,可使高压端中频电压空载与满载时的稳定度达到5%以下。对于某些稳定度要求高的直流电源,如预加速电源和分析磁铁电源,可对直流电压或电流取样,通过电光—光电—中频电源大闭环调制稳压。采用先进的光纤光电耦合技术,完成信号的隔离传递,从而实现在地电位对高电位上各电源的调整和控制。简单探讨了中频供电情况下的若干问题,并提出了相应的解决措施。
The neutron generators which produce neutrons with energies of 14MeV or around 2.5MeV through T(d, n)~4He or D(d, n)~3He reactions by low-energy accelerators are widely used in the researches of neutron physics and its applications, furthermore, they have been being developed toward the "intense current" direction because of the needs of the anti-radiation researches on the materials of fusion reactors and strategic weapons, and cancer therapies by fast neutrons. Neutron generators whose neutron yields reach or exceed 10~(12)n/s are generally called "intense neutron generators".The intense neutron generator being developed in Lanzhou University is designed to generate 8×10~(12)14MeV neutrons per second. Analytical deuterium beam with higher energy of up to lMeV will be adopted in order to prolong the life of the T-Ti target and to get a 10 n/s yield of 2.5MeV neutrons produced by D(d, n) He reaction.In this paper, the physical and biological effects, the advantages and disadvantages, the developing histories and actualities of various kinds of cancer therapy means, are reviewed, including gamma and X rays, charged particle beams, intra-cavitary radiation by radioisotopes, neutron capture and fast neutrons. Combined with the technical needs of cancer therapies by fast neutrons, the main designing performances of the cancer therapy machine of Lanzhou University are given, and the bases and feasibilities of each index are analyzed qualitatively or quantitatively.The concrete demands for power supplies of each main components of the neutron generator are analyzed according to their working principles and influences to the capabilities of the neutron generator.This thesis has focused on the power supplying system of the intense neutron generator. The mixed deuterium beam extracted from the ECR ion source is first pre-accelerated to 120keV. The single-atomic ion D~+ beam is analyzed from the mixed beam in the 90° magnetic analyzer and then accelerated to lMeV in the accelerating tube. The first difficulty of the power supplying system lies in the fact that the power supplies used in the ion source and pre-accelerating system have high powers and rigorous performance demands and, what is
more, they are located in the high potential terminal of near 1MV thus they may not be driven directly from ground. An intermediate frequency of 20kHz is adopted in the power supplying of the high voltage terminal in order to minish the dimension of the power supplying system, to assure the performances of the power supplies and to depress the noise. The powers generated by 5 20kHz power supplies are transported to 1MV high voltage terminal with 5 sets of isolation transformers, each of which have a insulation capability of 1MV. An intense-coupling technique we innovate is adopted in the isolation transformer so that the ratio of the voltage drop of the transformer between its non-load and full-load states to the output voltage can be reduced to less than 15%. This ratio can even be reduced to less than 5% by means of voltage sampling in the output terminal and modulation of the intermediate frequency output voltage through closed loop negative feed-back which is composed of electro-optical conversion, optical fiber transmission and photo-electric conversion. In some power supplies which require higher voltage or current stabilization, e.g., the pre-accelerating and the analytical magnet power supplies, dc voltage or current samplings and large closed loop negative feed-backs between the intermediate frequency transformers and the power supplies are introduced. Advanced photo-electrical coupling technique through optical fiber is adopted to transfer signals and carry out from ground the regulating and controlling of the power supplies standing on the high voltage potential. Some problems arisen from intermediate frequency supplying are simply discussed and the corresponding solving measures are put forward.Another difficulty of the power supplying system is the 1MV, 50mA dc high voltage power supply which requires a voltage stabilization of less than 0.1%. Techniques such as symmetrical voltage multiplier, RC filters, dual voltage stabilizing circuit, transformer compensation, fast voltage stabilization, and so on, as well as 20kHz intermediate frequency supplying, are adopted to guarantee the performances of this power supply.The neutron generator itself does not need a strict energy stabilization of the deuterium beam. The primary reason for the dc high voltage power supply to have a higher stabilization is that 4 experimental terminals are designed in this machine and deflection magnet is therefore needed. If the accelerating voltage is not stable, the excursion of the deuterium beam energy will result in an excursion of the beam trajectory. A method which can synchronize the
variations of the accelerating voltage and the exciting current of the deflection magnet is explored to assure the stability of the beam trajectory.The overvoltage and overcurrent protection is also of great importance. In this paper, the short circuit transient process of the Cockcroft-Walton circuit is studied thoroughly and systematically for the first time in the world. A simplified equivalent circuit based on some reasonable assumptions is given. Some formulae, which may enough be reasonably used toestimate the most possible maximum \i2(t)dt value of the rectifiers in the short circuitotransient process, and provide quantitative references for the selections of the parameters of the rectifiers and protective components in engineering design, are deducted. Conclusions that it is only necessary to connect protective resistors in series with the rectifiers situated in the locations of the highest and lowest potentials of the voltage multiplier and unnecessary to connect fast speed protective spark gap in parallel with the secondary winding of the step-up transformer, are drawn, which provide a theoretical basis for the simplification of the structure of the power supply.
兰州大学研制中的强流中子发生器的预定指标为14MeV中子产额达到8×10~(12)n/s。为了提高靶寿命,采用分析氘束和较高的1MeV氘束能量,同时希望D(d,n)~3He反应产生的2.5MeV中子产额亦能达到10~(12)n/s量级。
论文评述了各种放射治疗手段(包括γ与X射线、带电粒子束、后装机、中子俘获和快中子治癌)的物理效应和生物效应、优缺点及其发展历史和现状;结合快中子治癌的要求,给出了兰州大学快中子治疗机拟达到的主要技术指标,并定性或定量地分析了各项指标的确定依据及其可行性。
根据强流中子发生器各主要组成部分的工作原理及其对中子发生器性能的影响,分析了各部分对供电电源的具体要求。
论文主要研究了强流中子发生器的供电系统。从ECR离子源引出的混合氘束预加速到120keV,经90°磁分析器分析出单原子离子D~+束,而后经加速管被加速到1MeV。离子源及预加速系统所需电源不仅功率大,性能要求高,而且处在接近1MV的高电位端,不能采取地电位直接供电,这成为供电系统的第一个难题。为了尽可能减小供电系统尺寸并保证电源性能和降低噪音,高压端供电用20kHz中频。用5路耐压1MV的隔离变压器将5台20kHz中频电源产生的中频电输送到1MV高电位端。隔离变压器用我们自主创新的强耦合隔离变压技术,使由空载到满载时的输出电压降不超过输出电压的15%。在输出端电压取样,通过电光转换—光纤传输—光电转换闭环调制中频电源输出电压,可使高压端中频电压空载与满载时的稳定度达到5%以下。对于某些稳定度要求高的直流电源,如预加速电源和分析磁铁电源,可对直流电压或电流取样,通过电光—光电—中频电源大闭环调制稳压。采用先进的光纤光电耦合技术,完成信号的隔离传递,从而实现在地电位对高电位上各电源的调整和控制。简单探讨了中频供电情况下的若干问题,并提出了相应的解决措施。
The neutron generators which produce neutrons with energies of 14MeV or around 2.5MeV through T(d, n)~4He or D(d, n)~3He reactions by low-energy accelerators are widely used in the researches of neutron physics and its applications, furthermore, they have been being developed toward the "intense current" direction because of the needs of the anti-radiation researches on the materials of fusion reactors and strategic weapons, and cancer therapies by fast neutrons. Neutron generators whose neutron yields reach or exceed 10~(12)n/s are generally called "intense neutron generators".The intense neutron generator being developed in Lanzhou University is designed to generate 8×10~(12)14MeV neutrons per second. Analytical deuterium beam with higher energy of up to lMeV will be adopted in order to prolong the life of the T-Ti target and to get a 10 n/s yield of 2.5MeV neutrons produced by D(d, n) He reaction.In this paper, the physical and biological effects, the advantages and disadvantages, the developing histories and actualities of various kinds of cancer therapy means, are reviewed, including gamma and X rays, charged particle beams, intra-cavitary radiation by radioisotopes, neutron capture and fast neutrons. Combined with the technical needs of cancer therapies by fast neutrons, the main designing performances of the cancer therapy machine of Lanzhou University are given, and the bases and feasibilities of each index are analyzed qualitatively or quantitatively.The concrete demands for power supplies of each main components of the neutron generator are analyzed according to their working principles and influences to the capabilities of the neutron generator.This thesis has focused on the power supplying system of the intense neutron generator. The mixed deuterium beam extracted from the ECR ion source is first pre-accelerated to 120keV. The single-atomic ion D~+ beam is analyzed from the mixed beam in the 90° magnetic analyzer and then accelerated to lMeV in the accelerating tube. The first difficulty of the power supplying system lies in the fact that the power supplies used in the ion source and pre-accelerating system have high powers and rigorous performance demands and, what is
more, they are located in the high potential terminal of near 1MV thus they may not be driven directly from ground. An intermediate frequency of 20kHz is adopted in the power supplying of the high voltage terminal in order to minish the dimension of the power supplying system, to assure the performances of the power supplies and to depress the noise. The powers generated by 5 20kHz power supplies are transported to 1MV high voltage terminal with 5 sets of isolation transformers, each of which have a insulation capability of 1MV. An intense-coupling technique we innovate is adopted in the isolation transformer so that the ratio of the voltage drop of the transformer between its non-load and full-load states to the output voltage can be reduced to less than 15%. This ratio can even be reduced to less than 5% by means of voltage sampling in the output terminal and modulation of the intermediate frequency output voltage through closed loop negative feed-back which is composed of electro-optical conversion, optical fiber transmission and photo-electric conversion. In some power supplies which require higher voltage or current stabilization, e.g., the pre-accelerating and the analytical magnet power supplies, dc voltage or current samplings and large closed loop negative feed-backs between the intermediate frequency transformers and the power supplies are introduced. Advanced photo-electrical coupling technique through optical fiber is adopted to transfer signals and carry out from ground the regulating and controlling of the power supplies standing on the high voltage potential. Some problems arisen from intermediate frequency supplying are simply discussed and the corresponding solving measures are put forward.Another difficulty of the power supplying system is the 1MV, 50mA dc high voltage power supply which requires a voltage stabilization of less than 0.1%. Techniques such as symmetrical voltage multiplier, RC filters, dual voltage stabilizing circuit, transformer compensation, fast voltage stabilization, and so on, as well as 20kHz intermediate frequency supplying, are adopted to guarantee the performances of this power supply.The neutron generator itself does not need a strict energy stabilization of the deuterium beam. The primary reason for the dc high voltage power supply to have a higher stabilization is that 4 experimental terminals are designed in this machine and deflection magnet is therefore needed. If the accelerating voltage is not stable, the excursion of the deuterium beam energy will result in an excursion of the beam trajectory. A method which can synchronize the
variations of the accelerating voltage and the exciting current of the deflection magnet is explored to assure the stability of the beam trajectory.The overvoltage and overcurrent protection is also of great importance. In this paper, the short circuit transient process of the Cockcroft-Walton circuit is studied thoroughly and systematically for the first time in the world. A simplified equivalent circuit based on some reasonable assumptions is given. Some formulae, which may enough be reasonably used toestimate the most possible maximum \i2(t)dt value of the rectifiers in the short circuitotransient process, and provide quantitative references for the selections of the parameters of the rectifiers and protective components in engineering design, are deducted. Conclusions that it is only necessary to connect protective resistors in series with the rectifiers situated in the locations of the highest and lowest potentials of the voltage multiplier and unnecessary to connect fast speed protective spark gap in parallel with the secondary winding of the step-up transformer, are drawn, which provide a theoretical basis for the simplification of the structure of the power supply.
引文
[1] 丁厚本和王乃彦,中子源物理,科学出版社,1984,p.155
[2] 苏桐龄,强流中子发生器及其应用.核技术,12(8,9)(1989)553-556
[3] Kong Xiangzhong, Wang Yongchang and Yang Jingkang et al., Communication of Nuclear data progress. 7(4)(1992)
[4] F. M. Bacon, R. J. Walko and D. F. Cowgill et al., E M. Bacon, R. J. Walko and D. F. Cowgill et al., Intense neutron source development for use in cancer therapy. IEEE Trans. Nucl. Sci., NS-28(2)(1981) 1902-1905
[5] 洪忠悌,加速器在快中子治癌中的应用.核物理动态,9(4)(1992)27-32
[6] Tong-Ling Su, Yi-Min Zhang and Shang-Wen Chen et al., A 600kV, 15mA Cockcroft-Walton high voltage power supply with high stability and low ripple voltage. Nucl. Instr. Meth. A, 560(2)(2006) 613-616
[7] Y. M. Zhang, X. D. Su and H. Su et al., Study on the short circuit transient process and overcurrent protection of Cockcroft-Walton high voltage power supply. Rev. Sci. Instrum., 77(4)(2006) 045101(6 pages)
[2] 苏桐龄,强流中子发生器及其应用.核技术,12(8,9)(1989)553-556
[3] Kong Xiangzhong, Wang Yongchang and Yang Jingkang et al., Communication of Nuclear data progress. 7(4)(1992)
[4] F. M. Bacon, R. J. Walko and D. F. Cowgill et al., E M. Bacon, R. J. Walko and D. F. Cowgill et al., Intense neutron source development for use in cancer therapy. IEEE Trans. Nucl. Sci., NS-28(2)(1981) 1902-1905
[5] 洪忠悌,加速器在快中子治癌中的应用.核物理动态,9(4)(1992)27-32
[6] Tong-Ling Su, Yi-Min Zhang and Shang-Wen Chen et al., A 600kV, 15mA Cockcroft-Walton high voltage power supply with high stability and low ripple voltage. Nucl. Instr. Meth. A, 560(2)(2006) 613-616
[7] Y. M. Zhang, X. D. Su and H. Su et al., Study on the short circuit transient process and overcurrent protection of Cockcroft-Walton high voltage power supply. Rev. Sci. Instrum., 77(4)(2006) 045101(6 pages)