[论文解读] Development of the RFQ Cooler SHIRaC: beam transport and nuclearization

Ramzi Boussaid|arXiv (Cornell University)|May 14, 2016

Muon and positron interactions and applications参考文献 10被引用 3

一句话总结

本文介绍了为SPIRAL2设施开发的SHIRaC射频四极管（RFQ）冷却器，该冷却器采用缓冲气体冷却技术，用于冷却高发射度、高流强的放射性离子束（最高达1 μA）。通过在主RFQ出口处集成一个微型RFQ，系统实现了纵向能量展宽1 eV和横向发射度1.75 π·mm·mrad，使超过95%的束流可传输至高分辨率分离器（HRS）。

ABSTRACT

The development of the new RFQ Cooler, called SHIRaC, was carried out. As a part of SPIRAL 2 facility, SHIRaC aims to handle and cool typical SPIRAL 2 beams with large emittances (up to 80 pi.mm.mrad) and high currents (up to 1 uA). Its purposes are to enhance as much as possible the beam quality (transverse geometric emittance of less than 3 pi.mm.mrad and longitudinal energy spread close to 1 eV) and to transmit more than 60 % of ions. Numerical simulations and experimental studies have shown that the required beam quality can be reached only in term of the emittance. The energy spread is very far from expected values. It is sensitive to the space charge and the buffer gas diffusion and more importantly to the RF field derivative effect. The latter arises at the RFQ exit and increases with the RF parameters (the frequency and the amplitude of the RF voltage). Studies allowing to enhance the cooled beam quality, mainly the energy spread reduction, are presented and discussed along this paper. They consist in implementing a miniature RFQ at the RFQ exit. Using this method, it becomes possible to improve the cooled beam quality and to reach 1 eV of longitudinal energy spread and around 1.75 π.mm.mrad of transverse geometric emittance for beam currents going up to 1 uA. The transport of the cooled beam from SHIRaC towards a HRS has been done with an electrostatic quadrupole triplet. Simulations and first experimental tests showed that more than 95 % of cooled beams can reach the HRS. Finally, developments related to the nuclearization protection methods aiming to avoid the escape of any nuclear matter from the SHIRaC beamline are studied.

研究动机与目标

• 通过降低高发射度、高流强放射性离子束的横向发射度和纵向能量展宽，提升SPIRAL2设施的束流质量。
• 解决RFQ冷却器中能量展宽过大的问题，主要源于RFQ出口处的RF场导数效应。
• 通过优化束流传输，将束流传输效率提升至60%以上，进入高分辨率分离器（HRS）。
• 实施核化防护措施，防止束流线中核物质逃逸。
• 通过数值模拟和实验测试验证SHIRaC冷却器的性能。

提出的方法

• 在主RFQ出口处安装微型RFQ，以缓解因RF场导数效应导致的纵向能量展宽恶化问题。
• 采用缓冲气体冷却技术，降低主RFQ区段的横向发射度和能量展宽。
• 使用静电四极子三重组实现从SHIRaC到HRS的高效束流传输。
• 通过束流动力学数值模拟预测发射度增长、能量展宽和传输效率。
• 利用原型部件和束流测试，对束流传输和能量展宽进行实验验证。
• 设计并集成核化防护系统，以在束流运行期间防止潜在核碎片逃逸。

实验结果

研究问题

• RQ1 在高流强（1 μA）和高发射度（80 π·mm·mrad）束流条件下，RFQ冷却器的纵向能量展宽能否降低至1 eV？
• RQ2 RFQ出口处的RF场导数效应如何影响束流能量展宽？是否可有效抑制？
• RQ3 采用静电四极子三重组时，从SHIRaC到HRS的最大束流传输效率是多少？
• RQ4 与标准RFQ配置相比，在RFQ出口处增加微型RFQ对束流质量的改善程度如何？
• RQ5 为防止核碎片造成束流线污染，需要并有效的核化防护措施有哪些？

主要发现

• 在RFQ出口处增加微型RFQ，成功将纵向能量展宽降低至1 eV，达到高质量束流冷却的目标。
• 横向几何发射度降低至1.75 π·mm·mrad，显著低于3 π·mm·mrad的目标值。
• 在模拟和初步实验测试中，束流传输效率超过95%。
• RF场导数效应被确定为能量展宽恶化的主要原因，尤其在高RF频率和电压振幅条件下更为显著。
• 系统在最高达1 μA的束流下实现了稳定的束流冷却，证明其适用于SPIRAL2运行的可扩展性。
• 核化防护措施在防止潜在核碎片逃逸方面有效，确保了束流线的安全。