Transmitting oscillation suppression of low-Tc SQUID TEM system based on RC serial and multi-parallel capacity snubber circuit

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.

Snubber Circuit for Marine Controlled-Source Electromagnetic Transmitter

A high-power marine controlled-source electromagnetic transmitter(HP-MCSET)transmits a highfrequency conversion current on the sea floor.Some problems exist when the direct-current to alternating-current(DC-AC)launch bridge(LB)is used in the marine controlled-source electromagnetic transmitter(MCSET).There is a high voltage peak in the LB when the insulated gate bipolar transistor(IGBT)is turned on and off.In some cases,the voltage stress of the IGBT can be exceeded,which may cause IGBT damage.Because the rise of the current steepness is relatively low and the output voltage has a voltage peak in the LB,a snubber circuit is added to the IGBT to suppress the voltage peak to improve the output current and voltage waveform.The suppression of the voltage peaks is analyzed and compared for several groups of snubber circuits.To meet the performance requirements of the MCSET,the optimal snubber circuit is selected to effectively suppress the voltage peaks at an output current of 1 kA.This method is verified by using a 70 kW MCSET and the experimental waveforms are provided.The simulation of the inductance obstruction load in seawater is necessary to determine the conditions for actual marine environment experiments.