Tuning of 2.998 GHz S-band hybrid buncher for injector upgrade of LINAC II at DESY
Tuning of 2.998 GHz S-band hybrid buncher for injector upgrade of LINAC II at DESY
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DOI：
10.1016/j.nima.2014.05.043
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发表时间：
2014-10
影响因子：
1.4
通讯作者：
Y. Nie;C. Liebig;M. Hüning;M. Schmitz
中科院分区：
文献类型：
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作者：
Y. Nie;C. Liebig;M. Hüning;M. Schmitz
DESY上对直线加速器II的注入器升级旨在提高其可靠性，并减轻在数百MeV的相对较高能量下由于电子损失而引起的组件的辐射激活。因此，一个2.998 GHz的混合聚束器已经被开发出来，并将安装在现有的2.998 GHz预聚束器和直线加速器II之间。它包括一个用于快速电子加速的1单元驻波(SW)部分和一个用于进一步聚束和加速的13单元行波(TW)部分。本文重点介绍了其射频调谐过程。该调谐策略结合了复杂电场的非共振拉珠测量和计算局部反射系数的线性模型。证明了局部反射系数的虚部直观地表示了场的分布，据此可以对结构进行单元间的调谐。在调谐过程中，特别注意了西南段的场强增强，以确保其捕获束流的能力。同时分析了结构固有频率和目标频率两种不同频率下的场幅相、全局和局部反射系数，以避免过度调谐。调试结果令人满意。对于目标频率，TW段的场不平坦度从±9%降低到±4%，而西南段的场显著增强。同时，在TW剖面中，相邻单元之间的相位推进与标称值120°的偏差已从±5°减小到±2°。通过ASTRA模拟，验证了从束流动力学性能来看，结构的剩余失谐量是可以接受的。
The injector upgrade of LINAC II at DESY aims to improve its reliability and mitigate the radiological activation of components due to electron loss at relatively high energy of hundreds of MeV. Therefore, a 2.998 GHz hybrid buncher has been developed and will be installed in between an existing 2.998 GHz pre-buncher and LINAC II. It comprises a 1-cell standing-wave (SW) section for rapid electron acceleration and a 13-cells traveling-wave (TW) section for further beam bunching and acceleration. This paper focuses on its radio-frequency tuning procedure. The tuning strategy combines a non-resonant bead-pull measurement of complex electric field and a linear model for local reflection coefficient calculation. It is demonstrated that imaginary part of the local reflection coefficient represents the field distribution straightforwardly, based on which the structure can be tuned from cell to cell. During tuning, special attention has been paid to the field enhancement in the SW section to ensure its beam-capturing capability. Field amplitude and phase, global and local reflection coefficients have been analyzed for two different frequencies simultaneously, i.e. the intrinsic frequency of the structure and the target frequency, to avoid over-tuning. The tuning result is satisfying. For the target frequency, field unflatness of the TW section has been reduced from ±9% to ±4%, and field in the SW section has been enhanced significantly. Meanwhile, in the TW section, the deviation of phase advances between adjacent cells from the nominal value 120° has been reduced from the range ±5° to ±2°. By using ASTRA simulation, it has been verified that the residual detuning of the structure is acceptable in view of the beam dynamics performance.