Research On Realization And Application Of ?T-level Magnetic Fields Of High Homogeneity Inside Magnetically Shielded Rooms

Magnetic shields screen geomagnetic fields and all types of man-made magnetic interference,reducing the magnetic field to the n T level.In recent years,physicists around the world have been conducting precise quantum measurements based on using magnetic shields to look for Beyond-the-Standard-Model physics.This type of experiment requires the use of coils to reproduce a?T-level magnetic field inside magnetic shields,and places stringent requirements on the magnetic field homogeneity.The objective of this thesis is to realize a highly homogeneous magnetic field environment with a gradient of less than1 p T/cm,and apply it to electric dipole moment(EDM)measurements.A homogeneous n T-level background magnetic field is a prerequisite for achieving a?T-level field environment of high homogeneity.Existing studies regarding magnetic shields have focused on the attenuation of the magnetic field amplitude,but little attention was paid to the analysis of the internal field gradient.This thesis first analyzes the magnetic field distribution of a magnetically shielded spherical shell under a uniform background field,and then extends the model to a non-uniform background field,revealing that the shielding effect increases with the increasing order of the excitation magnetic field.Furthermore,the damage of apertures to the magnetic field environment inside magnetic shields is considered,and the relationship between the shielding factor and the size of apertures is analytically deduced,revealing that apertures produce internal field gradients even under a uniform background field.However,this effect can be reduced by a proper arrangement of apertures.The internal magnetic field at the ideal magnetic equilibration state are calculated by finite element simulation for magnetically shielded rooms(MSR),and verified experimentally with the MSR prototype of Harbin institute of technology and the upgraded Berlin magnetically shielded room(BMSR-2.1),located at Physikalisch-Technische Bundesanstalt in Germany.A n T-level field environment with a gradient of1 p T/cm is realised,as a basis for reproducing uniform?T-level magnetic fields.A uniform magnetic field of?T magnitude is generated by a coil set inside MSRs.Since the coil size is limited by the dimension of MSRs and the ferromagnetic material causes distortion of magnetic fields,the parameters of such shield-coupled coils must be specifically optimized in order to achieve high homogeneity.An improved mirror method model considering finite-thickness material is established for high-permeability plates,which improves the accuracy of field calculation compared to the classical mirror method.Based on this model,an approximate formula for calculating the generated magnetic field inside a cylindrical shield is proposed,and its systematic error as well as application range are analyzed.A semi-analytical model of the generated magnetic field inside MSRs is established,which can efficiently realize the optimization of coil spacings.The optimized model enables the coil to be as close to the ferromagnetic material as possible,according to which a built-in uniform coil structure is proposed with the advantages of high uniformity,low vibration and no space occupation.The optimized coil is installed in the innermost layer of the BMSR-2.1,and the magnetic field distribution is measured by superconducting quantum interferometer devices(SQUIDs),showing that the magnetic field gradient is better than 1 p T/cm in the central region of 10 cm at a background field of 2.3?T.Magnetic sensor calibration is essential for accurate magnetic sensing and guar-antees the realization of?T-level magnetic fields with high uniformity.Conventional high-precision magnetometer calibration methods require an excellent geomagnetic envi-ronment and are often performed in remote locations.This thesis proposes a convenient in-situ calibration technique for magnetic sensors using?T-level magnetic fields of high uniformity.Using Monte-Carlo simulations,three calibration methods are compared,showing that the scalar method can achieve the highest accuracy in practical applications,and the homogenous?T-level field environment can improve the calibration accuracy of the offset.The created homogenous magnetic field inside BMSR-2.1 is used to calibrate a three-axis fluxgate,which significantly reduces the measurement error.In addition,for multi-channel SQUID devices that cannot be calibrated using the scalar method,a calibra-tion method using an accurate coil array is proposed and applied to a 304-channel SQUID system,supporting the quantum precision measurements performed inside BMSR-2.1.The key application of homogenous?T-level magnetic fields is the EDM measure-ment.This thesis discusses the method of measuring permanent¹²⁹Xe EDM using a homogenous?T-level field environment.An international team has conducted a¹²⁹Xe atomic EDM measurement inside BMSR-2.This thesis highlights the influence of mag-netic field uniformity on the measurement results.The new upper limit of the¹²⁹Xe electric dipole moment is obtained using the pattern combination analysis method.Fur-thermore,a novel analysis method based on global phase fitting is proposed with a distinct advantage on statistical sensitivity,and an application of this method to obtained data further improves the measurement result and yields the lowest upper limit of¹²⁹Xe EDM d_Xe<8.3×10^-28e cm,improving the world record by a factor of 8.0.This thesis comprehensively analyses the internal field gradient of MSRs and the shield-coupled coil model,and proposes a built-in B₀coil structure with winding's posi-tions being adjustable,and realizes a?T-level magnetic field environment with gradients as low as 1 p T/cm.These presented methods can be applied to various magnetic shields and support the construction of a first-class multilayered MSR of Harbin Institute of Tech-nology,which is a national major scientific and technological infrastructure for China.The magnetic sensor calibration using the homogenous?T-level magnetic field environ-ment provides a novel solution,and the quantum physics measurements based on this environment open a new window to look for Beyond-the-Standard-Model physics.