Impact of cool-down conditions at T c on the superconducting rf cavity quality factor
Impact of cool-down conditions at T c on the superconducting rf cavity quality factor
复制标题
T c 冷却条件对超导射频腔品质因数的影响
DOI：
10.1103/physrevstab.16.102002
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发表时间：
2013
影响因子：
--
通讯作者：
J.Knobloch
中科院分区：
文献类型：
--
作者：
J.M.Vogt;O.Kugeler;J.Knobloch
从能量回收直线加速器到加速器驱动系统(ADS)，许多下一代高梯度加速器的应用都依赖于连续波(CW)操作，超导射频(SRF)系统是使其成为可能的技术。然而，尽管SRF腔消耗的功率很小，但它们必须由液氦冷却，对于许多CW加速器来说，低温装置的复杂性以及投资和运营成本可能被证明是令人望而却步的。我们研究了通过提高Nb腔的剩余电阻()来降低动态损耗的方法。材料处理和磁屏蔽都会产生影响。此外，我们还发现，当优化了Nb超导相变的降温条件时，可以显著降低Nb的超导相变温度。我们认为，冷却条件不仅影响外部磁通被捕获到腔内的程度，而且还会产生热电电流，进而产生可以捕获的额外磁通。因此，我们研究了一个模拟氦储罐中的SRF腔的简单模型Nb-钛系统中通量的产生以及通量捕获和释放的动力学。我们确实发现，在超导转变过程中，沿系统的温度梯度可以产生热电电流和磁通，随后可以被俘获。这些效应可以解释观察到的空腔随降温条件的变化。
Many next-generation, high-gradient accelerator applications, from energy-recovery linacs to accelerator-driven systems (ADS) rely on continuous wave (CW) operation for which superconducting radio-frequency (SRF) systems are the enabling technology. However, while SRF cavities dissipate little power, they must be cooled by liquid helium and for many CW accelerators the complexity as well as the investment and operating costs of the cryoplant can prove to be prohibitive. We investigated ways to reduce the dynamic losses by improving the residual resistance () of niobium cavities. Both the material treatment and the magnetic shielding are known to have an impact. In addition, we found thatcan be reduced significantly when the cool-down conditions during the superconducting phase transition of the niobium are optimized. We believe that not only do the cool-down conditions impact the level to which external magnetic flux is trapped in the cavity but also that thermoelectric currents are generated which in turn create additional flux that can be trapped. Therefore, we investigated the generation of flux and the dynamics of flux trapping and release in a simple model niobium-titanium system that mimics an SRF cavity in its helium tank. We indeed found that thermal gradients along the system during the superconducting transition can generate a thermoelectric current and magnetic flux, which subsequently can be trapped. These effects may explain the observed variation of the cavity’swith cool-down conditions.