Study on temperature rising of stripping foil and stripped electron of China Spallation Neutron Source
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摘要: 负氢剥离注入是强流质子同步加速器累积束流的唯一可行性方案。目前中国散裂中子源（CSNS）采用负氢剥离方案为薄膜剥离注入。由负氢束流穿越剥离膜产生的能量沉积造成的膜片剧烈温升是影响剥离膜寿命和加速器稳定运行的关键问题。同时，剥离产生的高功率残余电子束会产生严重后果，包括：电子在膜中的电离作用造成膜温度升高；电子打在真空盒上造成真空盒热损伤；停留在真空管道中的电子可能被质子束流俘获，造成e-p不稳定性；产生的二次电子会引起严重的电子云效应。主要内容包括两部分：首先，利用有限元分析软件，考虑粒子通过剥离膜的平均穿越次数等参数，模拟剥离膜温升并对不同软件结果进行详细比较，得到剥离膜上的温度场分布，并对未来继续提高的束流功率做出膜表面温升的预测。其次，根据理论计算结果和蒙特卡罗程序Geant4模拟结果对剥离后电子分布进行分析，完善3D计算模型并综合考虑CSNS注入区的电磁场和束流条件，获得电子收集装置的合适位置，给出剥离电子收集方案。Abstract: Negative hydrogen stripping injection is the only feasible scheme for accumulating beam in high current proton synchrotrons. Currently, the China Spallation Neutron Source (CSNS) employs negative hydrogen stripping injection by using a stripping foil. The intense temperature rising of the foil caused by energy deposition from the negative hydrogen beam passing through the foil is a critical issue which affecting the foil's lifetime and the stable operation of the accelerator. Additionally, the residual high power electron beam generated during the stripping process may have severe consequences, including electron ionization within the foil causing further temperature increase, thermal damage to the vacuum box from electron impacts, e-p instability from electrons captured by the proton beam in the vacuum tube, and significant electron cloud effects from secondary electrons. This paper focuses on two main topics: first, comprehensive simulations of the foil’s temperature rise have been conducted using finite element analysis software, taking into account various parameters, including the average number of particle crossings. Simulation results under various software conditions are compared to obtain the temperature field distribution on the stripping foil and predict surface temperature increases for future higher beam power. Secondly, the electron distribution following the stripping process is analyzed based on theoretical calculations and Geant4 simulations. The 3D computational model is refined by considering the electromagnetic field and beam conditions in the CSNS injection area, and a scheme for capturing stripping electrons is proposed by determining the optimal position for the electron collection device.
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图 2 剥离后各产物占比与膜厚度的关系
Figure 2. Relationship between the proportion of each product and the thickness of the foil
图 3 不同注入能量下电子和质子在剥离膜上的沉积能谱
Figure 3. Energy deposition of electrons and protons on the stripping foil under different injection energy
图 4 开始注入束流后剥离膜上的温度变化
Figure 4. Temperature change of the stripping foil after injecting beam
图 5 膜片上束流中心位置在注入期间的温度变化
Figure 5. Temperature change of the center of the beam on the foil during injection
图 6 膜片上最高温度与循环质子束平均穿越次数的关系
Figure 6. Relationship between the maximum temperature of the foil and the average number of circulating proton beam crossings
图 11 不同注入时间下电子在真空盒表面的分布
Figure 11. Distribution of electrons on the vacuumbox at different injection time
表 1 反应微分截面[6]
Table 1. Reaction differential cross-section
energy/MeV ${\sigma _{ - 1,0}}/({10^{ - 18}}\;{\text{c}}{{\text{m}}^{\text{2}}})$ $ {\sigma _{ - 1,1}}/({10^{ - 18}}\;{\text{c}}{{\text{m}}^{\text{2}}}) $ ${\sigma _{0,1}}/({10^{ - 18}}\;{\text{c}}{{\text{m}}^{\text{2}}})$ 80 3.170 0.0560 1.240 300 1.216 0.0214 0.476 
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表 2 粒子沉积能量
Table 2. Deposition energy of particles
injection energy/MeV particle deposition energy of particle/(MeV·cm2·g−1) theoretical result NIST database Geant4 result 80 electron 6.364 6.497 6.873 proton 7.710 7.678 7.714 300 electron 2.730 2.745 2.733 proton 3.149 3.126 3.045
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表 3 剥离电子及其收集器参数
Table 3. Parameters of stripped electrons and collection instrument
angle
spread/radbeam
size/mmcentral particle
energy/keVpulse
width/μsmagnetic
strength/Tdiffuse beam
size/mmcollection
efficiency/%collection
power/W0.073 1.5×1.5 163.2588 500 0.018(±0.23%) 60×130 99.95 101.35
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