## 4 Experimental Results  
An equipment representative of the configuration previously studied was manufactured, as illustrated in Figure 6, with a plastic holder supporting coil inside the SR. The current through the \(B_{p}\) coil, \(I_{p}\), during the measurement was set as shown in Figure 7. In the experiment, the current \(I_{p}\) was generated by control electronics and we used the magnetic sensor HMC2003 to monitor the the residual magnetic field. After the current \(I_{p}\) was completely switched off at 2 ms, the CC which was connected in series with \(B_{p}\) coil can effectively suppress the residual magnetic field in the SR.  
Figure 5: Curve fittings of the residual magnetic field in z direction with 70 turns CC. ‘+’ type of the red curve is experimental data after \(t_{s}\) = 2 ms; black curve \(B_{z\
eq 0z}\) is the fitting curve. Figure 6: Photo of the manufactured SR. A plastic holder was applied to support \(B_{p}\) coil and CC (front door of SR was temporarily removed). Considerable amounts of bolts were configured along the contact areas of the SR walls to better reduce the impedance of the eddy current. Figure 7: The current through the \(B_{p}\) coil during the test. The control electronics which can be seen in Figure 8 was used to generate the \(I_{p}\). The \(V_{DC}\) was provided by the adjustable source voltage and the voltage was adjusted according to the required turn-off time. By increasing the source voltage, we can turn off the \(I_{p}\) faster. The analog board was used to amplify the current feedback signal to realize the closed-loop control. The function of reset control circuit was used to reset the magnetic sensor HMC2003 and make it recover from the saturation state under the external magnetic field. Control logic of the electronics: the computer was connected to the DSP board through Ethernet and the DSP board outputs an interface to connect to the driver board. When it was needed to measure, the computer sends instructions to the DSP board (through Ethernet) and the DSP outputs PWM level on the port. At the same time, it detects the data of the current sensor, adjusts the PWM width to make the current \(I_{p}\) = 0.48 A, and lasts for a fixed time (\(>\) 2 s so that the eddy-current magnetic field produced by the rise of \(I_{p}\) had disappeared), and then sets the output to low level in 2 ms, completing one complete on/off cycle of the current \(I_{p}\). The measurement windows of most ULF NMR fall between 10 ms to 2 s, so effective reduction of \(B_{r}\) after 10 ms had a dominating impact on ULF NMR. By reducing the turnoff time of the current \(I_{p}\), we can advance the measurement time of ULF NMR. The measured \(B_{r}\) without and with CC are shown in Figure 9. At the beginning, the field of measured point keeps unchanged at around 35,000 nT until \(t\) = 1 ms. This was because the magnetoresistive sensor employed to measure the magnetic field remains saturated, and it was unable to reflect the actual strength of the magnetic field before \(t\) = 1 ms. Indeed, the magnetic field produced by the \(I_{p}\) = 0.48 A in the z direction was 1.79 mT, which exceeded the measuring range of the magnetoresistive sensor; the magnetic field produced by both \(I_{p}\) = 0.48 A and \(I_{c}\) = 0.48 A in the z direction was 1.68 mT, which also exceeded. \(B_{r}\) and its attenuation without and with CC at different times were shown in Tables 3 and 4, in which the values come from the magnetic field waveform in Figure 9. It can be seen that without CC, when \(I_{p}\) was completely switched off at 2 ms, the attenuation of modulus value of \(B_{r}\) at the measuring point in the SR was 98.42% while the attenuation was 99.89% with CC at the same time. The eddy-current magnetic field dominates \(B_{r}\) when \(I_{p}\) was completely switched off at 2 ms. At this moment, the \(B_{r}\) was 1851 nT with CC compared to 28,310 nT without CC, which was significantly suppressed. After \(t_{s}\) = 2 ms, only the eddy-current magnetic field was left in the SR.