Design Study of Damped Accelerating Cavity Based on the TM020-Mode and HOM Couplers for the KEK Light Source Project
Design Study of Damped Accelerating Cavity Based on the TM020-Mode and HOM Couplers for the KEK Light Source Project
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KEK光源项目基于TM020模式和HOM耦合器的阻尼加速腔设计研究
DOI：
10.18429/jacow-ipac2017-thpik036
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发表时间：
2017
期刊：
影响因子：
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通讯作者：
N. Yamamoto
中科院分区：
文献类型：
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作者：
Takeshi Takahashi;S. Sakanaka;N. Yamamoto
我们研究了 1.5 GHz 三次谐波射频系统的常导阻尼加速腔的设计。为了减少束隙导致的射频电压瞬态变化，我们参考Ego等人的开创性工作，选择TM020模式进行加速[1, 2]。为了抑制有害的寄生模式，我们在 TM020 模式电场较弱的位置放置了杆型高阶模式 (HOM) 耦合器。电磁仿真表明，除 TM120 和 TM021 模式外，大多数主要寄生模式均具有良好的阻尼效果。如果我们采取一些措施来有效抑制这些模式，那么该腔对于高次谐波腔来说是有希望的。简介 在基于多弯消色差晶格的 3-GeV KEK 光源 (KEK-LS) 项目 [3, 4] 中，谐振频率为 1.5 GHz 的三次谐波腔将用于减轻光束内散射和 Touschek 效应 [5]。对于这些谐振腔，我们将使用 TM020 谐振模式进行梁加速。由于TM020模式具有低Rsh/Q和高空载Q值，因此对于减少束隙引起的射频电压波动非常有利。根据 KEK-LS 的典型参数，正常导电 (NC) 腔体预计会有约 4.5% p-p 的小波动 [5]。通过应用额外的射频电压主动补偿[5]，预计束团延长可与超导腔获得的束团延长相当。然后，NC 谐波腔由于其坚固性和低构建/运营成本而成为 KEK-LS 的有吸引力的解决方案。 Ego 等人首先提出了一种使用 TM020 模式的新型阻尼腔方案。 [1, 2]。在他们的设计中，腔体沿着 TM020 模式的磁节点配备了两个圆周槽。除 TM020 模式外，寄生模式均使用安装在这些槽中的吸波材料进行阻尼。在该方案中，期望获得优异的寄生模式阻尼，同时在存储环的直线部分中占据较小的空间。因此，该腔作为新一代同步加速器光源的射频腔非常有吸引力。我们在本文中提出了一种替代腔体设计，该设计基于相同的 TM020 模式，但采用杆型 HOM 耦合器来阻尼寄生模式。 HOM 耦合器 [6, 7] 已被证明在许多腔体中都是可靠的。我们将这些 HOM 耦合器放置在 TM020 模式的电场大约为零的位置，以便它们几乎不与 TM020 模式耦合。在该方案中，HOM耦合器所需的开口可以被隔离并且更小，这有利于腔体机械结构的加固。腔体设计 配置 图 1 显示了我们设计的腔体的横截面。在安装 HOM 耦合器之前，初步确定腔体的内半径 (R2)，以使 TM020 模式的谐振频率符合 1.5 GHz 的频率。 TM020模式的主要参数如表1所示。对于阻尼寄生模式，四个HOM耦合器对称地安装在腔体的侧壁上。每个 HOM 耦合器由一根拉杆天线和一根同轴传输线组成。这些HOM耦合器位于TM020模式电场非常弱的位置，这确保它们几乎不与TM020模式耦合。 HOM 耦合器的尺寸暂定如图 1 所示。为了最小化每个腔体的占用长度，这些 HOM 耦合器应设计为纵向紧凑。表 1：TM020 模式参数 谐振频率 (fres) 1499.75 MHz 空载 Q (Q0) 37,400 (34,400**) Rsh/Q * 77.2 射频电压/腔 (Vc) 156 kV 耗散功率/腔 (Pc) 9.2 kW * 由 Rsh=(Vc) 定义2/件。 ** 带 HOM 耦合器 (L1=35 mm)。图 1：谐波腔的横截面（上半部分）。 TM020模式的电场用红色箭头示意性地示出。单位：毫米。寄生模式 表 2 显示了主要寄生的谐振频率、纵向 Rsh/Q 和横向 Rt/Q ____________________________________________ † takeshi.takahashi@kek.jp THPIK036 IPAC2017 会议记录，丹麦哥本哈根 ISBN 978-3-95450-182-3 4172 Co py rig ht © 20 17 CC-B Y3。 0 和各自作者 07 加速器技术 T06 室温 RF ic 模式，针对轴对称腔进行计算。为了避免耦合群不稳定性 (CBI)，这些模式的负载 Q (QL) 应得到良好阻尼。我们根据这个要求估计了目标QL，并将其显示在表2的第五列中。在这个估计中，我们要求CBI的增长率（这是用基于刚性束模型的简单公式估计的）应该小于辐射阻尼率；使用 KEK-LS 的参数，同时假设环中有五个谐振腔。请注意，这些目标 QL 将来应进行修改，以便始终包含三次谐波射频对光束不稳定性的影响。表 2：主要寄生模式的特性，以及四个 HOM 耦合器的目标负载 Q (QL) 和估计外部 Q (Qex)（L1=35 mm） 模式 f (GHz) Rsh/Q () Rt/Q (/m) QL * Qex TM010 0.6524 168.1 45 32 TM110 1.0370 1026.4 32 15 TE111 1.6543 9.2 3500 不适用TM011 1.7149 6.2 460 < 55 TM111 1.8913 148.3 220 < 20 TM120 1.8969 554.2 59 380 TE121 2.1327 10.8 3000 不适用TM021 2.1865 34.6 65 33100 TM030 2.3517 12.8 160 < 57 TM121 2.4675 632.0 52 < 53 TM130 2.7479 20.3 1600 不适用TE131 2.7903 4.0 8100 不适用TM031 2.8464 39.3 44 < 39 * 目标值。 ** 估计的。 n.a.：尚未估计。电磁仿真 我们使用电磁仿真代码 ANSYS HFSS 优化了腔体。模型腔如图 2 所示。我们将 HOM 耦合器的四个出口指定为端口 3-5，其中假设这些端口以 50 Ω 的特性阻抗终止。我们还将临时探针连接到仿真模型的波束端口，并将端口 1 和 2 分配给它们的出口。对于 TM020 模式，这些试验性探针的外部 Q (Qex) 约为 1.4×109。我们暂定HOM耦合器拉杆天线的长度（图1中的L1）为20毫米。然后，我们优化了 HOM 耦合器的位置（图 1 中的 R1），以尽量减少它们与 TM020 模式的耦合。图 3 显示了计算得出的 TM020 模式外部 Q 作为 R1 的函数。我们暂时选择 R1=76.1 mm，这使得外部 Q 值最大化。请注意，HOM 耦合器应准确定位，例如在 ±0.2 mm 以内，以获得 Qex > 10。 图 2：谐波腔的仿真模型。图 3：计算得出的 TM020 模式外部 Q（使用四个耦合器）与 HOM 耦合器位置 (R1) 的函数关系。杆长：L1=20mm。图 4：主要寄生模式的估计外部 Q（使用四个 HOM 耦合器）与杆长度的关系
We studied a design of a normal conducting, damped accelerating cavity for a 1.5-GHz third-harmonic rf system. To reduce the transient variation of rf voltage due to bunch gaps, we chose the TM020 mode for acceleration by referring to a pioneering work by Ego et al [1, 2]. To damp harmful parasitic modes, we placed rod-type higher-order-mode (HOM) couplers at the location where the electric fields of TM020 mode are weak. Electromagnetic simulations showed that most of the principal parasitic modes, except for TM120 and TM021 modes, damped well. If we incorporate some measures to effectively damp these modes, this cavity is promising for the higher harmonic cavity. INTRODUCTION In a proposed 3-GeV KEK Light Source (KEK-LS) project [3, 4] which is based on the multi-bend achromat lattice, the third-harmonic cavities having a resonant frequency of 1.5 GHz will be used for mitigating intrabeam scattering and Touschek effect [5]. For these harmonic cavities, we will use the TM020 resonant mode for beam acceleration. Because the TM020 mode has low Rsh/Q and high unloaded Q, it is very advantageous to reduce the fluctuations in rf voltage that are induced by bunch gaps. With typical parameters of the KEK-LS, a small fluctuation of ~4.5% p-p is expected with normal conducting (NC) cavities [5]. By applying additional active compensation of rf voltage [5], bunch lengthening which is comparable to that obtained with a superconducting cavity is expected. Then, the NC harmonic cavity can be an attractive solution for the KEK-LS due to their robustness and low construction/operational costs. A novel damped-cavity scheme using the TM020 mode was first proposed by Ego et al. [1, 2]. In their design, the cavity is equipped with two circumferential slots along the magnetic node of the TM020 mode. Parasitic modes, except for the TM020 mode, are damped using absorbing materials which are fit in these slots. In this scheme, excellent parasitic-mode damping is expected while occupying small space in the straight sections of storage rings. Therefore, this cavity is very attractive as rf cavities for the new-generation synchrotron light sources. We propose in this paper an alternative cavity design which is based on the same TM020 mode but with rodtype HOM couplers for damping parasitic modes. The HOM couplers [6, 7] have been proven reliable in many cavities. We place these HOM couplers where the electric fields of TM020 mode is approximately zero so that they hardly couple to the TM020 mode. In this scheme, openings needed for the HOM couplers can be isolated and smaller, which is advantageous for stiffening the mechanical structure of the cavity. DESIGN OF THE CAVITY Configuration Figure 1 shows a cross section of our designed cavity. An inner radius (R2) of the cavity was tentatively determined so that the resonant frequency of the TM020 mode fit a frequency of 1.5 GHz before attaching the HOM couplers. The principal parameters of the TM020 mode are shown in Table 1. For damping parasitic modes, four HOM couplers were attached symmetrically at a side wall of the cavity. Each HOM coupler consisted of a rod antenna and a coaxial transmission line. These HOM couplers locate where the electric fields of TM020 mode are very weak, which ensures that they hardly couple to the TM020 mode. The dimensions of the HOM coupler were tentatively chosen as shown in Fig. 1. To minimize the occupation length of each cavity, these HOM couplers should be designed to be longitudinally compact. Table 1: Parameters of the TM020 Mode Resonant frequency (fres) 1499.75 MHz Unloaded Q (Q0) 37,400 (34,400**) Rsh/Q * 77.2 RF voltage/cavity (Vc) 156 kV Dissipated power/cavity (Pc) 9.2 kW * Defined by Rsh=(Vc) 2/Pc. ** With HOM couplers (L1=35 mm). Figure 1: Cross section (upper half) of the harmonic cavity. Electric fields of TM020 mode are schematically shown by red arrows. Unit: mm. Parasitic Modes Table 2 shows the resonant frequencies, the longitudinal Rsh/Q, and the transverse Rt/Q, of the principal parasit____________________________________________ † takeshi.takahashi@kek.jp THPIK036 Proceedings of IPAC2017, Copenhagen, Denmark ISBN 978-3-95450-182-3 4172 Co py rig ht © 20 17 CC -B Y3. 0 an d by th er es pe ct iv ea ut ho rs 07 Accelerator Technology T06 Room Temperature RF ic modes, which was calculated for an axially symmetric cavity. To avoid coupled-bunch instabilities (CBIs), the loaded Q’s (QL) of these modes should be damped well. We estimated the target QL’s from this requirement, and showed them in the fifth column of Table 2. In this estimation, we required that the growth rates of CBIs, which were estimated with simple formulae based on a rigid bunch model, should be less than the radiation damping rates; the parameters of the KEK-LS were used while assuming five harmonic cavities in the ring. Note that these target QL’s should be modified in future so that an effect of the third-harmonic rf on the beam instabilities is included consistently. Table 2: Properties of the Principal Parasitic Modes, as well as the Target Loaded Q (QL) and Estimated External Q (Qex) with Four HOM Couplers (at L1=35 mm) Mode f (GHz) Rsh/Q () Rt/Q (/m) QL * Qex TM010 0.6524 168.1 45 32 TM110 1.0370 1026.4 32 15 TE111 1.6543 9.2 3500 n.a. TM011 1.7149 6.2 460 < 55 TM111 1.8913 148.3 220 < 20 TM120 1.8969 554.2 59 380 TE121 2.1327 10.8 3000 n.a. TM021 2.1865 34.6 65 33100 TM030 2.3517 12.8 160 < 57 TM121 2.4675 632.0 52 < 53 TM130 2.7479 20.3 1600 n.a. TE131 2.7903 4.0 8100 n.a. TM031 2.8464 39.3 44 < 39 * Target value. ** Estimated. n.a.: not estimated yet. Electromagnetic Simulations We optimized the cavity using an electromagnetic simulation code, ANSYS HFSS. A modeled cavity is shown in Fig. 2. We assigned four exits of HOM couplers the ports 35, where these ports were assumed to be terminated by a characteristic impedance of 50 . We also attached tentative probes to the beam ports of the simulation model, and assigned the ports 1 and 2 to the exits of them. The external Q (Qex) of these tentative probes was ~1.4109 per each for the TM020 mode. We tentatively fixed the length (L1 in Fig. 1) of HOMcoupler’s rod antennas to be 20 mm. We then optimized the position (R1 in Fig. 1) of the HOM couplers so as to minimize the coupling of them to the TM020 mode. Figure 3 shows calculated external Q of TM020 mode as a function of R1. We tentatively chose R1=76.1 mm which maximized the external Q. Note that the HOM couplers should be positioned accurately, for example, within 0.2 mm to obtain Qex > 10. Figure 2: Simulation model of the harmonic cavity. Figure 3: Calculated external Q (with four couplers) of the TM020 mode as a function of the position (R1) of the HOM coupler. Rod length: L1= 20 mm. Figure 4: Estimated external Q (with four HOM couplers) of the principal parasitic modes vs. rod length of the
KEK同步辐射CDR
DOI：
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发表时间：
2017
期刊：
影响因子：
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作者：
Matsuura A;KInebuchi M
通讯作者：
KInebuchi M