A low-temperature superconducting quantum interference device(low-Tc SQUID)can improve the depth of exploration.However,a low-Tc SQUID may lose its lock owing to oscillations in the current or the occurrence of spikes...A low-temperature superconducting quantum interference device(low-Tc SQUID)can improve the depth of exploration.However,a low-Tc SQUID may lose its lock owing to oscillations in the current or the occurrence of spikes when the transmitter is switched off.If a low-Tc SQUID loses its lock,it becomes impossible for the low-Tc SQUID TEM system to function normally and stably for a long period of time.This hinders the practical use of the system.In field experiments,the transmitting current is accurately measured,the voltage overshoot and current spike data are recorded,and the gradient of the primary magnetic field at the center of the transmitting loop is calculated.After analyzing the results of field experiments,it was found that when the gradient of the primary magnetic field far exceeds the slew rate of a low-Tc SQUID,the low-Tc SQUID loses its lock.Based on the mechanisms of the transmitting oscillation,an RC serial and multi-parallel capacity snubber circuit used to suppress such oscillation is proposed.The results of simulation and field experiments show that,when using a 100 m×100 m transmitting loop,the gradient of the primary magnetic field is suppressed from 101.4 to 2.4 mT/s with a transmitting current of 40 A,and from 29.6 to 1.4 mT/s with a transmitting current of 20 A.Therefore,it can be concluded that the gradient of the primary magnetic field is below the slew rate of a low-Tc SQUID after adopting the proposed RC serial and multi-parallel capacity snubber circuit.In conclusion,the technique proposed in this paper solves the problem of a lost lock of a low-Tc SQUID,ensuring that the low-Tc SQUID TEM system functions stably for a long period of time,and providing technical assurance for ground TEM exploration at an additional depth.展开更多
In order to detect extremely weak magnetic signals,superconducting quantum interference device(SQUID)gradiometers are widely used to suppress environmental noise.A hardware SQUID gradiometer consists of a niobium grad...In order to detect extremely weak magnetic signals,superconducting quantum interference device(SQUID)gradiometers are widely used to suppress environmental noise.A hardware SQUID gradiometer consists of a niobium gradio-antenna and an SQUID,which are coupled via an input coil.Here gradiometer imbalance may greatly reduce its noise suppression performance.The gradiometer balance depends on the geometrical forms of the antenna wound by niobium wire.We describe a simple method based on Faraday's law for the pre-calibration of the gradiometer balance at room temperature,before the gradiometer is set up.The pre-calibrating results are compared with the measured balance of an SQUID gradiometer system.This method may be used for sifting hardware gradiometers for multi-channel systems.展开更多
The voltage biased(SQUID)bootstrap circuit(SBC)was recently introduced as an effective means to reduce the preamplifier noise contribution.We analyze the tolerances of the SBC noise suppression performance to spreads ...The voltage biased(SQUID)bootstrap circuit(SBC)was recently introduced as an effective means to reduce the preamplifier noise contribution.We analyze the tolerances of the SBC noise suppression performance to spreads in SQUID and SBC circuit parameters.It is found that the tolerance to spread mainly caused by the integrated circuit fabrication process could be extended by a one-time adjustable current feedback.A helium-cooled niobium SQUID with a loop inductance of 350 pH is employed to experimentally verify the analysis.From this work,design criteria for fully integrated SBC devices with a high yield can be derived.展开更多
Using a second-order helium-cooled superconducting quantum interference device gradiometer as the detector,ultra-low-field nuclear magnetic resonance(ULF-NMR)signals of protons are recorded in an urban environment wit...Using a second-order helium-cooled superconducting quantum interference device gradiometer as the detector,ultra-low-field nuclear magnetic resonance(ULF-NMR)signals of protons are recorded in an urban environment without magnetic shielding.The homogeneity and stability of the measurement field are investigated.NMR signals of protons are studied at night and during working hours.The Larmor frequency variation caused by the fluctuation of the external magnetic field during daytime reaches around 5 Hz when performing multiple measurements for about 10 min,which seriously affects the results of averaging.In order to improve the performance of the averaged data,we suggest the use of a data processor,i.e.the so-called time-domain frequency correction(TFC).For a 50-times averaged signal spectrum,the signal-to-noise ratio is enhanced from 30 to 120 when applying TFC while preserving the NMR spectrum linewidth.The TFC is also applied successfully to the measurement data of the hetero-nuclear J-coupling in 2,2,2-trifluoroethanol.展开更多
基金Project(XDB 0420200)supported by Strategy Priority Research Program(B)of China
文摘A low-temperature superconducting quantum interference device(low-Tc SQUID)can improve the depth of exploration.However,a low-Tc SQUID may lose its lock owing to oscillations in the current or the occurrence of spikes when the transmitter is switched off.If a low-Tc SQUID loses its lock,it becomes impossible for the low-Tc SQUID TEM system to function normally and stably for a long period of time.This hinders the practical use of the system.In field experiments,the transmitting current is accurately measured,the voltage overshoot and current spike data are recorded,and the gradient of the primary magnetic field at the center of the transmitting loop is calculated.After analyzing the results of field experiments,it was found that when the gradient of the primary magnetic field far exceeds the slew rate of a low-Tc SQUID,the low-Tc SQUID loses its lock.Based on the mechanisms of the transmitting oscillation,an RC serial and multi-parallel capacity snubber circuit used to suppress such oscillation is proposed.The results of simulation and field experiments show that,when using a 100 m×100 m transmitting loop,the gradient of the primary magnetic field is suppressed from 101.4 to 2.4 mT/s with a transmitting current of 40 A,and from 29.6 to 1.4 mT/s with a transmitting current of 20 A.Therefore,it can be concluded that the gradient of the primary magnetic field is below the slew rate of a low-Tc SQUID after adopting the proposed RC serial and multi-parallel capacity snubber circuit.In conclusion,the technique proposed in this paper solves the problem of a lost lock of a low-Tc SQUID,ensuring that the low-Tc SQUID TEM system functions stably for a long period of time,and providing technical assurance for ground TEM exploration at an additional depth.
基金Supported by the National High-Tech Research and Development Program of China under Grant No 2008AA02Z308and the Science and Technology Commission of Shanghai Municipality under Grant No 08JC1421800。
文摘In order to detect extremely weak magnetic signals,superconducting quantum interference device(SQUID)gradiometers are widely used to suppress environmental noise.A hardware SQUID gradiometer consists of a niobium gradio-antenna and an SQUID,which are coupled via an input coil.Here gradiometer imbalance may greatly reduce its noise suppression performance.The gradiometer balance depends on the geometrical forms of the antenna wound by niobium wire.We describe a simple method based on Faraday's law for the pre-calibration of the gradiometer balance at room temperature,before the gradiometer is set up.The pre-calibrating results are compared with the measured balance of an SQUID gradiometer system.This method may be used for sifting hardware gradiometers for multi-channel systems.
基金Supported by the Knowledge Innovation Program of the Chinese Academy of Sciences under Grant No KGCX2-YW-906 and KGCX2-EW-105the One Hundred Person Project of the Chinese Academy of Sciences.
文摘The voltage biased(SQUID)bootstrap circuit(SBC)was recently introduced as an effective means to reduce the preamplifier noise contribution.We analyze the tolerances of the SBC noise suppression performance to spreads in SQUID and SBC circuit parameters.It is found that the tolerance to spread mainly caused by the integrated circuit fabrication process could be extended by a one-time adjustable current feedback.A helium-cooled niobium SQUID with a loop inductance of 350 pH is employed to experimentally verify the analysis.From this work,design criteria for fully integrated SBC devices with a high yield can be derived.
基金Supported by the International Cooperation Key Project of Bureau of International Cooperation,CAS,under Grant No GJHZ1104Shanghai Natural Science foundation under Grant No 11ZR1444100.
文摘Using a second-order helium-cooled superconducting quantum interference device gradiometer as the detector,ultra-low-field nuclear magnetic resonance(ULF-NMR)signals of protons are recorded in an urban environment without magnetic shielding.The homogeneity and stability of the measurement field are investigated.NMR signals of protons are studied at night and during working hours.The Larmor frequency variation caused by the fluctuation of the external magnetic field during daytime reaches around 5 Hz when performing multiple measurements for about 10 min,which seriously affects the results of averaging.In order to improve the performance of the averaged data,we suggest the use of a data processor,i.e.the so-called time-domain frequency correction(TFC).For a 50-times averaged signal spectrum,the signal-to-noise ratio is enhanced from 30 to 120 when applying TFC while preserving the NMR spectrum linewidth.The TFC is also applied successfully to the measurement data of the hetero-nuclear J-coupling in 2,2,2-trifluoroethanol.
