Development of Superconducting RF Double Spoke Cavity at IHEP
Development of Superconducting RF Double Spoke Cavity at IHEP
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DOI：
10.18429/jacow-srf2019-thp089
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发表时间：
2019-08
期刊：
影响因子：
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通讯作者：
Quan Zhou;F. He;W. Pan
中科院分区：
文献类型：
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作者：
Quan Zhou;F. He;W. Pan
中国散裂中子源（CSNS）旨在产生散裂中子。计划对CSNS进行升级，通过在漂移管直线加速器（DTL）之后插入一个超导射频直线加速器（SRF linac）来提高束流功率。高能物理研究所（IHEP）正在为CSNS的SRF直线加速器研发一个工作在β0为0.5的325 MHz双辐条腔。该腔的形状经过优化，在使Ep/Ea最小化的同时，保持Bp/Ep处于合理的较低水平。同时，进行了机械设计以检查应力、洛伦兹力失谐和微音效应，并使压力敏感度最小化。提出了一种新的射频耦合方案以避免电子直接撞击陶瓷窗。在腔的制造和后处理之后，在2K垂直测试下，该腔在Eacc = 13.8 MV/m和Q0 = 1.72×10¹⁰时达到Bp为120 mT。
引言
CSNS升级包括一个SRF质子直线加速器。该SRF直线加速器将把40 mA的质子束峰值加速到303 MeV，并且在SRF直线加速器的中β段采用双辐条腔。辐条腔由工作在TEM模式的半波谐振器（HWR）演变而来。与HWR相比，辐条腔中可以采用多间隙结构，这节省了纵向空间并提高了有效梯度。与椭圆腔相比，辐条结构具有更高的分流阻抗，同时，由于其高的单元间耦合，它在机械上更稳定并且呈现出稳定的场分布[1]。因此双辐条腔是中β应用的首选。
电磁设计
电磁设计包括几何形状优化、耦合端口设计。
几何形状优化
对腔的几何形状进行优化以在运行期间实现最大加速梯度（Eacc）。超导腔性能的一个限制是在表面电场高的地方发生场致发射（FE）；另一个限制是在表面磁场高的地方发生猝灭。因此，峰值表面场与梯度的比值，即Ep/Eacc和Bp/Eacc，是几何形状优化的主要品质因数。根据先前加速器驱动次临界系统（ADS）项目的经验，SRF腔在Bp低于90mT时很少猝灭；然而，FE可能在Ep低至35 MV/m时发生，并且在束流运行一段时间后观察到FE起始的退化。因此主要的优化目标是降低Ep/Eacc，同时使Bp/Ep保持在2.57 mT/(MV/m)以下。Ep/Eacc对辐条的中心部分和端盖锥形形状，即Tk、Sgl、Sal和Sbl最为敏感，如图1所示。辐条的底部对Bp/Eacc有更大的影响，并且为了进一步降低Ep/Eacc，它被偏向于跑道形状[2]。优化后的主要几何参数如图1所示。最终腔长Cl为729 mm，腔直径Cd为560 mm。
图1：优化参数。H场分布 E场分布
图2：双辐条腔的电磁场分布
经过优化，Ep/Eacc达到3.4，Bp/Eacc为8.7 mT/(MV/m)；如果腔在Ep为35 MV/m的情况下运行，那么Bp为90mT，并且梯度可以达到10.3 MV/m，这高于7.3 MV/m的项目目标。腔的表面场分布如图2所示，详细设计参数列于表1。
表1：双辐条腔的主要参数
描述 结果
频率（MHz） 325
R - 孔径（mm） 50
Epk/Eacc 3.4
Bpk/Eacc（mT/(MV/m)） 8.67
G*R/Q（Ω²） 5.08×10⁴
df/dP（Hz/mbar） 3.35
LFD因子（Hz/(MV/m)²） -10.9
调谐敏感度（kHz/mm） 93
腔刚度（kHz/kN） 16.78
（最后一个“od T”似乎是不完整的内容）
The China Spallation Neutron Source (CSNS) is designed to produce spallation neutrons. CSNS upgrade is planned to increase beam power by inserting a SRF linac after drift tube linac (DTL). IHEP is developing a 325 MHz double spoke cavity at β0 of 0.5 for the CSNS SRF linac. The cavity shape was optimized to minimize Ep/Ea while keeping Bp/Ep reasonably low. Meanwhile, mechanical design was applied to check stress, Lorentz force detuning and microphonic effects, and to minimize pressure sensitivity. A new RF coupling scheme was proposed to avoid electrons hitting directly on ceramic window. After fabrication and post processing of cavity, the cavity reached Bp of 120 mT at Eacc = 13.8 MV/m and Q0 = 1.72E10 under vertical test at 2K. INTRODUCTION The CSNS upgrade consists of a SRF proton linac. The SRF linac will accelerate peak proton beam of 40 mA up to 303 MeV, and double spoke cavity is adopted for the SRF linac medium β section. Spoke cavity evolves from half-wave resonator (HWR) operating in TEM mode. Compared with HWR, Multi-gap structure is possible in spoke cavity, which saves longitudinal space and increase the real-estate gradient. Compared with elliptical cavity, the spoke structure has higher shunt impedance, meanwhile, it is mechanically more stable and exhibit a stable field profile due to the high cell-to-cell coupling [1]. Thus double spoke cavity is a preferred candidate for medium β application. ELECTROMAGNETIC DESIGN Electromagnetic design includes geometry optimization, coupling port design. Geometry Optimization The cavity geometry is optimized to achieve maximum accelerating gradient (Eacc) during operation. One limit to the performance of a superconducting cavity is field emission (FE) at where surface electric field is high; another limit is quenching at where surface magnetic field is high. So the peak surface field to gradient ratio, i.e. Ep/Eacc and Bp/Eacc, are the major figure of merits for geometry optimization. Based on the experience of previous ADS project, SRF cavities seldom quench below Bp of 90mT; though, FE may occur at Ep as low as 35 MV/m, and the degradation of FE onset is observed over some time of beam operation. So the main optimization target is to reduce Ep/Eacc, while the Bp/Ep is kept below 2.57 mT/(MV/m). The Ep/Eacc is most sensitive to the central part of the spoke and the end-cover cone shape, i.e. Tk, Sgl, Sal, and Sbl, as shown Fig. 1. The base of the spoke has more influence on Bp/Eacc, and it is biased to a racetrack shape in order to further reduce Ep/Eacc [2]. The major geometry parameters optimized are shown in Fig. 1. The final cavity length Cl is 729 mm, and the cavity diameter Cd is 560 mm. Figure 1: Parameters for optimization. H-field distribution E-field distribution Figure 2: Electromagnetic field distribution of double spoke cavity. After optimization, Ep/Eacc achieved 3.4, and the Bp/Eacc is 8.7 mT/(MV/m); in case the cavity is operated with Ep of 35 MV/m, then the Bp is 90mT, and the gradient can reach 10.3 MV/m, which is higher than the project target of 7.3 MV/m. The surface field profile of the cavity is shown in Fig. 2, while detailed design parameters are listed in Table 1. Table 1: The Major Parameters of Double Spoke Cavity Description Results Frequency (MHz) 325 R-aperture (mm) 50 Epk/Eacc 3.4 Bpk/Eacc (mT/(MV/m) 8.67 G*R/Q (Ω2) 5.08e4 df/dP (Hz/mbar) 3.35 LFD factor (Hz/(MV/m)2) -10.9 Tuning sensitivity (kHz/mm) 93 Cavity rigidity (kHz/kN) 16.78 od T