一种可用于极化~3He实验的新型磁场系统

A new magnetic field system for ~3He polarization

原子核自旋极化的~3He气体已被深入研究并广泛用于各种科学实验.在过去的极化~3He实验中,为了减小磁场梯度对纵向弛豫时间的影响,通常会建造大尺寸的亥姆霍兹线圈来提供所需均匀度的主磁场环境.本文通过计算得到了新的六正方形线圈系统,可以为极化~3He实验提供小型高均匀性的磁场装置.其中线圈系√统内部超过30%的区域磁场梯度满足(|▽B_x|~2+|▽B_y|~2)/B_0 <10^(-4)cm^(-1),这一均匀区域比例超过了现在所有用于极化~3He实验的线圈装置.对于其他需要大均匀区域磁场环境的研究实验,新的六线圈系统也具有很好的应用价值.

The nuclear spin-polarized ^3He gas has been in depth studied and widely used in various scientific experiments.The polarized ^3He gas can be used as a polarized neutron target to study the reaction of neutrons with charged particles or photon beams. On the other hand, spin polarized ^3He gas is a good probe for detecting the new interactions in the supernormal model, and has many other applications as follows： the spin-dependent interaction can be studied quantitatively by measuring the NMR frequency shift but the spin-dependent interaction can also be studied by measuring the relaxation time of polarized ^3He gas; the polarized ^3He gas can be applied to magnetometers and magnetic resonance imaging（MRI）; the highly polarized ^3He gas can be used as a neutron spin filter for neutron polarization and polarization analysis because of the high correlation between the absorption cross section of the neutron in polarized ^3He nucleus and the spin orientation. At present, the three major domestic sources of neutron, CMRR, CARR, and CSNS, are used to study the neutron polarization and polarization analysis techniques based on spin polarized ^3He gas. The longitudinal（or spin-lattice） relaxation time（i.e., T1） of ^3He is a key parameter that limits the polarizability of ^3He gas. In order to reduce the effect of magnetic field gradient on the longitudinal relaxation time of polarized ^3He gas, large-sized Helmholtz coils are usually constructed to provide the main magnetic field where the uniformity in the magnetic field central region reaches 10^-4cm^-1. To obtain enough magnetic field uniformity, some magnetic field systems even exceed 1.5 m in size. However, it is expected to have a small magnetic field configuration from the view of practicality and convenience. For the common size（〈0.1 m × 0.1 m × 0.1 m） of ^3He cells, Merritt coil and Saddle coil can effectively reduce the size of the magnetic field apparatus. However, for electron scattering experiments of ^3He cells, the chamber length can be 40 cm. The system length exceeds 1 m even by using the Merritt coil. In this work, a new six-coil system for ^3He polarization is obtained. Within the coils, the magnetic field gradient satisfies the requirement that √｜▽Bx｜^2＋｜▽By｜^2/B0 10^-4cm^-1 in more than 30% area, which is better than all the existing coils used in polarized ^3He experiments and can be applied to the future ^3He instruments. For other experiments that require magnetic field to have a large uniform area, the new six-coil system is also a good option.