Design and simulation of the coupler of single resonant cavity bunch length monitor
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摘要: 单谐振腔束团长度监测器利用谐振腔内的两个本征模式测量ps量级的电子束团长度,它的关键是如何将两个不同频率的模式互不干扰地耦合提取出来。为解决这个问题,基于低通和带通滤波器的理论,提出了同轴滤波耦合结构和膜片加载波导滤波结构,借助CST微波工作室对滤波器进行建模并仿真得到其S参数。为测试耦合器的应用效果,设计了一套带有该耦合结构的单谐振腔束团长度监测器探头,根据国家同步辐射实验室基于可调谐红外激光的能源化学研究大型实验装置FELiChEM的束流特点,在CST内对所设计的探头进行束流模拟仿真。仿真结果表明,该耦合器可以实现对特定模式的耦合,并有效降低其它模式的干扰,采用同轴滤波耦合结构和膜片加载波导滤波结构的谐振腔监测器可以实现对FELiChEM装置束团长度的高精度测量,测量误差小于2 %。Abstract: The single-cavity bunch length monitor uses two eigenmodes in the resonant cavity to measure the bunch length in the order of picoseconds. The key is how to couple and extract two modes of different frequencies without interfering with each other. To solve this problem, based on the theory of low-pass and band-pass filters, a coaxial filter coupling structure and a diaphragm-loaded waveguide filter structure are proposed. The filters are modeled and simulated in CST Microwave Studio to obtain S parameters. In order to test the effect of the coupler, a bunch length monitor with the coupling structure is designed. According to the beam characteristics of the National Synchrotron Radiation Laboratory based on the tunable infrared laser energy chemistry research large-scale experimental device (FELiChEM), a beam simulation is performed on the designed monitor in CST. The simulation results show that the coupler can realize the coupling of specific modes and effectively reduce the interference of other modes. The resonant cavity monitor adopting the coaxial filter and the diaphragm-loaded waveguide filter can achieve high-precision measurement of the bunch length of the FELiChEM, and the measurement error is less than 2%.
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Key words:
- resonant cavity /
- bunch length /
- filters /
- CST /
- beam /
- free electron laser
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随着科学技术的发展,核技术具有零碳排放、能源独立、安全等诸多优势,在人类社会中的地位越来越重要。然而,核辐射事故却为核技术发展迅速蒙上了一层阴影。1986年,苏联切尔诺贝利核电站发生了迄今为止人类历史上最严重的核辐射事故[1]。2011年,日本东北海岸发生了里氏9.0级的强烈地震和海啸,造成了福岛第一核电站的1~3号机组反应堆熔毁[2]。由于反应堆内部高温和高辐射等极端环境,人类无法直接进入进行勘察和处置工作,因此在福岛事故中使用了多种类型和功能的机器人。光纤激光器具有高功率、高光束质量,光束可以远距离柔性传输等优点,可以用于无人区开展激光切割救援等工作[3]。比如Shin等人研究了用10 kW光纤激光器拆除核设施的150 mm厚的厚钢板和大型管道的切割性能[4]。当然,光纤激光器在辐射环境中也会受到影响[5],高能射线会导致增益光纤产生色心等各类缺陷,这些缺陷引起的额外光吸收增加了传输损耗,降低了光纤激光器性能。
课题组基于光纤激光器存在的自漂白效应,利用60CO辐照源探索不同辐照剂量率下的光纤激光器暗化与自漂白的平衡关系。实验先采用低功率光纤振荡器进行不同辐照剂量率下激光器输出功率演化和去辐照后自漂白研究。使用的光纤激光振荡器实验结构如图1所示,谐振腔由常规商业掺镱光纤(YDF)、高反射光纤光栅(HR-FBG)、低反射光纤光栅(OC-FBG)构成,中心波长为976 nm的泵浦源(LDs)通过前向(2+1)×1泵浦信号合束器(FPSC)注入到谐振腔中,激光经过包层光滤除器(CLS)后由光纤端帽(QBH)扩束输出。
首先,利用较高辐照剂量率研究在去辐照后的自漂白效应,结果如图2(a)所示。图2(a)的(I)为未辐照阶段,持续时间为680 s,由于水冷机周期性制冷使得功率计温度周期变化导致测试激光功率也存在周期变化,激光器功率起伏为1.44%;需要注意的是,这个是主要功率测量误差导致,并不是激光器本身功率起伏。图2(a)中(II)为辐照阶段,在总辐照时间298 s内,辐照总剂量为14 900 rad,激光器输出功率从150 W下降至105 W。图2(a)的(III)为去辐照后的自漂白阶段,在光纤激光器的泵浦光子与热效应的共同作用下,激光器输出功率从118 W恢复di至145 W,与初始功率相差仅5 W,表明自漂白效应可以较为有效地恢复由于辐照导致的激光功率下降。
然后,为了探索不同剂量率的自漂白与在线辐照相互作用是否可以达到平衡,开展了不同剂量率的对比研究,结果如图2(b)所示。图2(b)中,总辐照剂量为2 400 rad,红色、蓝色曲线分别对应辐照剂量率为50 rad/s、1 rad/s时激光器归一化输出功率演化情况;在辐照剂量率为50 rad/s时,激光输出功率下降了3%;在辐照剂量率1 rad/s时,功率起伏1.22%,考虑到这里的周期性起伏主要由于水冷机周期性制冷导致,可以认为在低辐照剂量率下,光纤激光器自漂白导致的功率提升与辐照导致的功率下降基本达到平衡。
进一步地,基于图2(b)的实验结果,我们验证了1 kW级光纤激光器中自漂白与辐照平衡的实验现象。在辐照剂量率为0.1 rad/s时,激光器输出激光功率曲线演化如图2(c)所示。从实测功率曲线来看,在总辐照剂量为190 rad的整个辐照过程中,光纤激光器的输出功率都稳定在1 050 W以上,即使考虑前述由于水冷机导致的功率变化,激光器的功率起伏在1.79%以内。如果不考虑水冷机周期性制冷影响,激光器的功率起伏在0.66%以内。
实验首次验证了在一定辐照剂量率下,光纤激光器自漂白效应导致的激光功率提升可以平衡辐照效应导致的功率下降,为相关场景应用的光纤激光器设计提供了有效支撑。后续,我们将继续深入相关研究,探索不同类别、不同结构激光器辐照与自漂白平衡的机理、阈值和可能的应用。
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表 1 FELiChEM的束流参数
Table 1. Electron beam parameters of FELiChEM
energy/MeV energy spread/keV bunch charge/nC bunch length/ps micro-pulse repetition rate/GHz 25−60 <240 1.0 2−5 0.476 表 2 带通滤波器参数
Table 2. Parameters of band pass filter
d01/mm d12/mm a/mm b/mm l0/mm l1/mm l2/mm t/mm 10.54463 6.26978 28.499 12.624 15 21.19 22.47 1.2 表 3 束团长度仿真结果
Table 3. Simulation results of bunch length
bunch length/ps VTM010/10−7 VTM030/10−7 VTM010/VTM030 measured bunch length/ps relative error/% 2 2.175 0.966 2.251 2.03 1.50 3 2.174 0.959 2.266 2.99 0.33 5 2.168 0.937 2.313 4.91 1.80 10 2.147 0.840 2.557 9.93 0.70 15 2.116 0.700 3.024 14.94 0.40 20 2.074 0.542 3.828 19.97 0.15 -
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