【作者】 徐玉存；

【导师】 何多慧； 王相綦；

【作者基本信息】 中国科学技术大学 ， 核技术及应用， 2011， 博士

【摘要】 上世纪九十年代由日本学者重新启动研究的固定场交变梯度加速器(FFAG)由于其大的动力学孔径和较高的输出功率,不仅有可能应用于次临界堆的驱动源、散裂中子源,还有可能成为中微子工厂/μ介子对撞机的有效加速手段和癌症治疗、微型电子源的有效工具,因此也再次引起国际加速界的关注。另一方面,在核能技术的诸多领域使用的金属材料中普遍存在着氦的产生及其引起的脆化问题,尤其在反应堆领域,氦与包层材料作用机理和机制的研究,已成为未来核科学技术的重点研究方向之一。因此我们在国家同步辐射实验室借助于氦脆研究的契机,进行一台小型的氦离子FFAG加速器概念设计,并希望通过这一设计研究为将来FFAG在中国的研究发展建立基础。计划中小型氦离子FFAG加速器输出能量为36 MeV,采用径向三弯转组合扇形DFD设计,回旋周期在数百ns到微秒量级。本文针对氦离子FFAG输出能量和回旋频率较低的特点,提出利用感应加速方式取代通常FFAG加速器设计中的磁合金宽带高频腔加速方式以获得更大的束流流强的设计。设计这一感应加速系统的最主要技术难点包括快速可调整的调制器设计和跑道型的感应腔设计,本文主要针对这两项技术难点进行相关技术和概念研究。我们首先进行了快速调制器技术的研究。对于氦离子FFAG加速器,回旋周期的变化从数百ns到数μs量级,相应的要求感应腔驱动源调制器必须也跟随这个周期变化,这就需要对调制器输出脉冲格式不断调整和具有较高的重复频率,这对于传统的线性调制器是无法实现的。而目前国外大型粒子加速器实验室对于采用半导体器件作为开关的固态调制器技术研究已经能够实现MHz的连续波运行,国内对于快速的固态调制器技术研究还处于起步阶段,因此本篇论文首先进行了固态调制器技术的相关实验研究。由于功率开关MOSFET(金属氧化物半导体场效应管)具有较快的开启速度(约10 ns),因此我们的实验方案选择基于MOSFET开关的固态调制器样机研究。通过对于快速MOSFET调制器的理论分析,我们具体设计了基于FPGA快速同步时钟电路和MOSFET同步驱动电路,建成了一套由五单元感应叠加的调制器样机,并进行了相关负载的实验测试。我们获得了脉冲电压在3 kV、脉冲电流在118A,脉冲宽度在100 ns左右的放电波形,并通过增大负载阻抗获得了较好的脉冲上升速度,这种窄脉冲基本可以满足氦离子回旋最短周期内的加速需求。快速固态调制器技术不仅可以用于感应加速腔,还以用于加速器中快速冲击磁铁、雷达发射以及医用加速器等诸多领域。FFAG加速器的加速设计通常采用低品质因子软磁合金宽带RF腔,由于受到频率调谐速率的限制,加速重复频率不高,另外RF腔也具有较强的尾场效应,容易造成电场崩溃,因此输出平均流强受到限制。为此对于氦离子FFAG加速器,我们提出利用感应腔取代磁合金宽带高频腔的概念设计。由于FFAG加速器粒子轨道半径随能量变化的范围相对较大,因此真空盒尺寸在径向较大,为此我们设计一个截面为跑道型的感应腔结构,并通过建立感应腔的集中参数模型和分布参数模型来计算腔的基本分布参数和分析模拟加速电压波形。通过分析计算我们给出了满足平均输出流强为10 mA的感应腔设计方案,其全环共有4个感应腔,单腔设计加速电压为5 kV,脉冲宽度为200 ns,总功耗180 kW。为保证氦离子不因加速能量差异而丢失,我们对调制器输出脉冲模式提出了要求,并通过能量补偿和延长加速周期两种方案来对粒子进行约束,保证粒子在加速过程中始终处于加速时区。由于跑道型结构分布并不对称,可能造成感应加速电场的不均匀性,我们利用OPERA-3D模拟感应电场的分布,通过模拟我们发现设计模型的不同半径轨道上电场差异很小,完全可以满足加速的需要。通过对以上两个关键技术的研究分析,我们认为实现氦离子FFAG加速器的感应加速方式是可行的,其相关的概念设计也可以借鉴予未来中国高功率FFAG加速器的发展。更多还原

【Abstract】 From 1990s, Fixed Field Alternate Gradient (FFAG) accelerator has been significantly developed for its advantages of a larger dynamic aperture and a higher output beam power comparing with conventional synchrotrons or cyclotrons. This type of accelerator can be used as the driver of the Accelerator Driven System (ADS) and the Spallation Neutron Source (SNS), the collider of the neutrino factory, or be used in the tumor treatment, etc. A small He ion FFAG accelerator has been designed at National Synchrotron Radiation Laboratory in University of Science and Technology of China to study the generating mechanism of Helium (He) in the iron caused by the neutron radiation and the corresponding embitterment problem induced by He, with highest He ion energy of 36 MeV. One superperiod of this accelerator is composed of three combined function bending magnets, with a defocusing, focusing and defocusing sequence. The revolutionary period of the He ion in this accelerator varies from hundreds of ns to several us. By considering the low beam energy and the low revolution frequency characteristics of this accelerator, we proposed an acceleration cavity based on the induction-acceleration principle. Comparing with conventional Metal Alloy broad-band cavity, the average accelerating beam current using this type of accelerating cavity can be higher.First, we studied the fast solid-state modulator used for the induction accelerating cavity. The large variation of revolutionary period of particles in the FFAG accelerator requires a cavity with the capability of varying the frequencies simultaneously in the range of the revolutionary frequency. The conventional linear type modulator cannot fulfill this requirement. In this dissertation, we choose the semiconductor switching device based technology which can produce a continuous wave in order of MHz frequency to build to modulator. MOSFET, a commonly used power switching element is used in our research for its advantage of fast switching speed (about 10 ns). After analyzing the circuit, we built a modulator prototype with 5 induction-adders as well as quick synchronized clocking and driving circuits. A testing result of a 3 kV pulsed voltage, a 118 A pulsed current and a 100 ns pulse length have been obtained, and the impulse rise time is short enough for the accelerating the He ion in the FFAG accelerator. Except for in the accelerator area, the solid-state technology developed in this dissertation can also be used in the radar and medical processing areas. In the second part, we designed a racetrack shape cavity used for the FFAG accelerator. Typically, an RF cavity is operated in a fixed frequency, while for the cavity used in the FFAG accelerator, a cavity with rapid frequency tuning capability is required to accelerate the particles in thousands of turns. The RF cavity with a low quality factor can gain a tunable frequency, but its strong wake field and easily breaked up electric field restrict the use of this type of cavity in FFAG. A conceptual design of induction cavity for the FFAG accelerator is given in this part of dissertation. To match the vacuum chamber, a racetrack cross section shape is used. We calculated distributed parameters of this cavity, as well as the accelerating voltage waveform using an equivalent transformer model and a transmission line model. This cavity is designed to have a 5kV accelerating voltage,200 ns pulse length, and the power assumption of 180kW, and to accelerate a beam with a 10 mA average current. The pulse form is adjusted to have a slightly rising tail to minimize the phase delay caused by the unflatten waveform. Due to the asymmetry of the racetrack cavity, the particles with difference transverse coordinate may not be accelerated by a same voltage. Thus we studied the 3D inducted electric field using the code OPERA-3D, and found that the accelerating voltages at different horizontal location are within the accelerator physics design tolerance.The technologies developed in this dissertation demonstrated that it is possible to accelerating charged particles in the FFAG accelerator using the induction type cavity.更多还原