AMR Magnetic Sensors in Flux Expulsion Studies

A. Netepenko
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Abstract

Magnetic flux expulsion properties of the superconducting material such as bulk niobium, widely used for the radio-frequency cavity fabrication, substantially affect the performance characteristics of the cavities. The quality factor of the SRF resonators can be significantly compromised due to the presence of the trapped flux vortices causing additional RF energy losses in the material. Large number of experiments have been carried out by different research groups to establish the correlation of the flux trapping in niobium cavities with the presence of impurities in niobium as well as various surface treatment methods. Majority of these experiments utilize commercially available cryogenic fluxgate magnetic sensors to measure the field before and after the niobium transition to the superconducting state to quantify the amount of flux trapped. One disadvantage of the typically used fluxgates is the size of the sensing volume. As an example, the Barting-ton F and G type cryogenic fluxgates have a sensing core length of about 30mm, which is comparable to the curvature radius of the cavity walls and hence the magnetic field lines curvature radius after the expulsion. Thus, the measured field value needs to be corrected to account for the sensor effective averaging over the sensing volume. In case of the sharper geometries, for instance if the flux expulsion to be measured on the edge of the rectangular niobium flat sheet with a thickness of ~5mm, the use of the fluxgate would be impractical.
AMR磁传感器排磁研究
大块铌等超导材料广泛应用于射频空腔的制造,其排磁特性对射频空腔的性能特性有很大的影响。由于被捕获的磁通涡流的存在,导致材料中额外的射频能量损失,SRF谐振器的质量因子可能会受到严重损害。不同的研究小组进行了大量的实验,以建立铌腔中通量捕获与铌中杂质的存在以及各种表面处理方法的相关性。这些实验大多利用市售的低温磁通门磁传感器来测量铌跃迁到超导状态前后的磁场,以量化被捕获的磁通量。通常使用的磁通门的一个缺点是感应体积的大小。例如,Barting-ton F型和G型低温磁通门的感应磁芯长度约为30mm,这与腔壁的曲率半径相当,因此与驱逐后的磁力线曲率半径相当。因此,需要对测量的场值进行校正,以考虑传感器对传感体积的有效平均。在较尖锐的几何形状的情况下,例如,如果要在厚度为~5mm的矩形铌平板的边缘测量通量,则使用磁通门将是不切实际的。
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