## 1 Introduction  
Ultra-low-field nuclear magnetic resonance (ULF NMR) is one of the hot research areas in the applications of superconducting quantum interference device (SQUID) [1; 2; 3]. Compared with the measurement field (\(B_{m}\)) strength in the conventional high-field apparatuses (HF-MRs) [4], the measurement field strength in ULF NMR is several orders of magnitude lower, which brings some advantages including inhomogeneous line broadening comparable to natural line widths or narrower, absence of distortion artifact with metal and chemical shift due to the strong main field and ease of \(B_{m}\) tuning over many octaves. As well as lower cost and decrease in volume, it is possible to obtain unique images with enhanced longitudinal relaxation time contrast [5]. The significantly lower measurement magnetic field in comparison with 1.5 T hospital MRI or 9 T lab MRI causes the reduction of signal-to-noise ratio, thus normally ULF NMR introduces strong pre-polarizing field (\(B_{p}\)) and works in magnetically shielded environments [6]. A pulsed \(B_{p}\) whose amplitude is 2-3 orders-of-magnitude stronger is critical to SNR enhancement since the signal amplitude is proportional to \(B_{p}\) strength [7; 8; 9; 10]. However, pulsed operation of the \(B_{p}\) coil may magnetize and induce large eddy currents around shielding structures such as a magnetically or conductive shielded room. The transient magnetic field generated by the eddy currents may cause severe image distortions and signal loss, especially when the large pre-polarizing coil are employed for in vivo imaging [7]. A method for the assessment of magnetic properties of ferromagnetic materials at low magnetic field values was presented by [PERSON], [PERSON] et al. [11] and they designed a magnetically shielded room which can be effective against low-frequency external magnetic field disturbances. [PERSON] et al. [12] calculated and simulated the shielding effectiveness of the shielded room with different thicknesses of aluminum sheets to increase the SNR and improve image quality. [PERSON] et al. [13] according to the relative position of the \(B_{p}\) coil and the shielded room, described two physical models of the conductive shielded room to simulate the transient fields at the sample position: the \(B_{p}\) coil plane parallel to the ceiling and floor of the shielded room and the coil plane perpendicular to the walls. In [14], superconducting connectivity technique was utilized to connect input coil and pickup coil in order to obtain the best SNR of the fetal magnetocardiography signals in a thin magnetically shielded room. Suppression of the residual magnetic field (\(B_{r}\)) makes it possible to increase the upper value of \(B_{p}\) and the magnetization intensity of the object to be measured. In [15], an effective method to design self-shielded polarizing coils for ULF NMR by connecting shielding coils and the polarizing coil in series with reversed currents was presented and the magnetic fields caused by the eddy currents were largely reduced. In their works the residual magnetic field enters steady state at about 300 ms while in our works the residual magnetic field enters steady state at about 110 ms. In [16], cancellation coils (CC) with discrete current loops connected in serial to the \(B_{p}\) coil was implemented using the method of inverse problem and it effectively neutralizes the magnetic field produced by the \(B_{p}\) coil on the SR walls. [PERSON] et al. [17] introduced a dynamical cancellation method in which a precisely designed current waveform was applied to a separate coil after turning off the polarizing pulse. Complicated calculation is needed to figure out the cancellation current density distribution in [16] or the precisely designed cancellation current waveform in [17], while it was avoided in this study. By changing the number of turns of the CC which is connected in series with the \(B_{p}\) coil in the designed structure in this study, the residual magnetic field was suppressed effectively.