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超导加速腔的优点在于高的功率传输效率、低的腔体功率损耗。所以要求提取耦合器具有低的耦合度,避免过度负载以保证超导腔具有较高的品质因数。超导腔的功率传输系统包括馈入功率的主耦合器、超导谐振腔和功率提取耦合器。具体的示意图如下图6所示。
图 6 二端口腔体模型示意图[9]
根据超导腔二端口模型[9]有如下推导:提取耦合器与超导腔体的耦合度为
$$ \beta_{\rm e} =\frac{{P}_{\mathrm{e}}}{{P}_{0}} = \frac{{Q}_{0}}{{Q}_{\mathrm{e}}} \text{,}\; $$ 其中:
$ {P}_{0}$ 、$ {P}_{\mathrm{e}} $ 分别为为腔体自身腔壁损耗功率和代表谐振腔通过端口对外泄露的功率;$ {Q}_{\mathrm{e}} $ 为外部品质因数;Q0为腔体固有品质因数,同时也是描述谐振腔射频特性好坏的重要参数:$$ {Q}_{0} = \frac{\omega U}{{P}_{0}} \text{,}\quad {Q}_{\mathrm{e}} = \frac{\omega U}{{P}_{\mathrm{e}}} \text{。} $$ U是腔中电磁场的总储能,
$ P/\omega $ 为一个周期内腔体中的损耗能量。谐振腔与外电路的耦合使腔内的损耗增大。
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摘要: 我国自主研发的超导加速器CAFe是ADS加速器的前端示范装置,其对加速器的运行稳定性有非常高的要求。在CAFe的运行中存在场致发射效应干扰超导腔提取信号的问题,是造成超导腔运行故障最频繁的问题之一,这严重降低了加速器的稳定性。这一问题是由于超导腔内的场致发射电子在相关微波传输器件上引起了放电现象,干扰了信号的传输并造成低电平控制系统对超导腔的错误控制,最终导致超导腔失谐和加速器停机故障。本文通过优化提取天线的结构,避免场致发射电子进入提取耦合器,从而解决传输信号异常的问题。优化后的天线在线运行效果良好,解决了超导腔这种提取信号异常的问题,提高了加速器运行的稳定性。Abstract: The China ADS Front-end demonstration facility(CAFe) is the prototype accelerator for the China Accelerator Driven Subcritical system(ADS), and exhibits extremely high requirement on operation stability. During the operation of CAFe, the field emission effect interferes with the pickup signal from the superconducting cavity, which is one of the most common causes for beam trips and other operation faults. Such interference seriously harmed the accelerator operation stability. The origin of the problem comes from the fact that field emission electrons cause discharge on the microwave transmission device, which interferes with the signal transmission and causes the low-level radio frequency system to control the superconducting cavity incorrectly, and finally leads to the superconducting cavity failure and accelerator shutdown. In this paper, the structure of the pick-up antenna was optimized to avoid the field emission electrons from entering the pick-up coupler, so as to solve the problem of abnormal signal. The optimized antenna showed desirable results during the online operation tests, the problem of abnormal signal extraction was solved, and the stability of accelerator operation was improved.
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Key words:
- superconducting accelerator /
- stable operation /
- field emission effect /
- flashover /
- coupler
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图 6 二端口腔体模型示意图[9]
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