用于ACME电偶极矩测量的、位于μ金属屏蔽层内的紧凑型主动屏蔽磁场线圈
Compact Actively-Shielded Magnetic Field Coil within Mu-Metal Shields for ACME Electric Dipole Moment Measurements
AI总结：
本文设计了一套含两层主动屏蔽线圈与三层μ金属屏蔽的紧凑型磁场系统，为ACME III eEDM测量提供稳定均匀磁场，使磁场相关系统不确定性较ACME II减小约40倍。
AI中文摘要：
本文设计、构建并验证了一套主动屏蔽线圈与μ金属屏蔽层组成的系统，可提供ACME III电子电偶极矩（eEDM）测量所需的稳定且空间均匀的磁场。该系统包含两层载流线圈，被三层铁磁屏蔽层包裹，在用于探测ThO分子进动的1m×4.2cm×4.2cm内部体积内，磁场变化小于1nT（10μG）。可拆解的屏蔽层由矩形μ金属板制成，其中最大的一块质量为19kg、面积为2.18m×0.75m，可由两人轻松搬运，刚好适配大型退火炉。该组件设计便于低应力安装与操作，以抑制μ金属磁性能变化，同时为线圈内的设备提供模块化访问，便于维护与升级。近静态外部环境场在屏蔽的进动体积内被削弱达10^5倍。在ACME用于压制系统不确定性的磁场反转过程中，“主动屏蔽”线圈可大幅抵消其外部杂散场，最小化μ金属的磁化。尽管屏蔽层仅位于线圈外侧10cm处，但无需在每次磁场反转后对屏蔽层进行消磁；例如，当每30秒反转一次磁场时，非反转残余场可在长达17小时内保持低于1nT。与前代ACME II装置相比，测得的性能表明，尽管ACME III的进动体积是ACME II的5倍，且使用3层磁屏蔽而非5层，但其与磁场相关的系统不确定性预计将减小40倍。
英文摘要：
A system of actively-shielded coils and mu-metal shields is devised, constructed and shown to provide the stable and spatially uniform magnetic field needed for the ACME III electron electric dipole moment (eEDM) measurement. Two layers of current-carrying coils, enclosed within three layers of ferromagnetic shields, produce a field that varies by less than 1 nT (10 $\uG$) within the 1 m $\times$ 4.2 cm $\times$ 4.2 cm interior volume in which a beam of ThO molecules are probed as they precess. The demountable shields are constructed from rectangular mu-metal plates. The largest, with a mass of 19 kg and an area of 2.18 m $\times$ 0.75 m, is easily carried by two people and just fits within a large available annealing oven. The assembly design facilitates low-stress mounting and handling to suppress changes in the magnetic properties of the mu metal, and also provides modular access to apparatus within the coils for maintenance and upgrades. The nearly static external ambient field is reduced within the shielded precession volume by up to a factor of $10^5$. During the magnetic field reversals that ACME uses to suppress systematic uncertainties, the ``actively-shielded'' coil largely cancels out its external fringing field to minimize the magnetization of the mu metal. Even though the shields are only 10 cm outside the coils, shield degaussing after every magnetic field reversal is not required. The non-reversing residual field stays below 1 nT for up to 17 hours when the field is reversed every 30 seconds, for example. The measured performance, compared to the previous generation ACME II apparatus, suggests that the magnetic-field-related systematic uncertainties for ACME III will be smaller by an estimated factor of 40 despite a five times longer precession volume and the use of three magnetic shielding layers rather than five.
