The excitation and evolution of magnetohydrodynamic density waves are considered in a differentially rotating thin gaseous disk embedded with both an azimuthal magnetic field and a vertical magnetic field perpendicula...The excitation and evolution of magnetohydrodynamic density waves are considered in a differentially rotating thin gaseous disk embedded with both an azimuthal magnetic field and a vertical magnetic field perpendicular to the disk. Our results shows that the effect of isopedic vertical magnetic fields tend to stabilize the fast MHD density waves while slow MHD density waves are not affected by such vertical magnetic fields for either rigid or differential rotation.展开更多
The purpose of this study was to present a lock-in-amplifier model for analyzing the behavior of signal harmonics in magnetic particle imaging (MPI) and some simulation results based on this model. In the lock-in-ampl...The purpose of this study was to present a lock-in-amplifier model for analyzing the behavior of signal harmonics in magnetic particle imaging (MPI) and some simulation results based on this model. In the lock-in-amplifier model, the signal induced by magnetic nanoparticles (MNPs) in a receiving coil was multiplied with a reference signal, and was then fed through a low-pass filter to extract the DC component of the signal (output signal). The MPI signal was defined as the mean of the absolute value of the output signal. The magnetization and particle size distribution of MNPs were assumed to obey the Langevin theory of paramagnetism and a log-normal distribution, respectively, and the strength of the selection magnetic field (SMF) in MPI was assumed to be given by the product of the gradient strength of the SMF and the distance from the field-free region (x). In addition, Gaussian noise was added to the signal induced by MNPs using normally-distributed random numbers. The relationships between the MPI signal and x were calculated for the odd- and even-numbered harmonics and were investigated for various time constants of the low-pass filter used in the lock-in amplifier and particle sizes and their distributions of MNPs. We found that the behavior of the MPI signal largely depended on the time constant of the low-pass filter and the particle size of MNPs. This lock-in-amplifier model will be useful for better understanding, optimizing, and developing MPI, and for designing MNPs appropriate for MPI.展开更多
The magnetic fluid is used as working medium in pilot stage of electro-hydraulic servoamplifier. Utilizing the magnetization viscous character of the fluid, a new type of electro-hydraulicservo amplifier without any m...The magnetic fluid is used as working medium in pilot stage of electro-hydraulic servoamplifier. Utilizing the magnetization viscous character of the fluid, a new type of electro-hydraulicservo amplifier without any moving parts in its pilot stage has been developed. This research provides a way for improving the frequency property and reliability of electro-hydraulic servoelements.展开更多
On line measurement of heavy dc current has been studied for many years, and devices based on magnetic modulator, magnetic amplifier, optic fiber have been developed, but harsh environments made all of them are insuff...On line measurement of heavy dc current has been studied for many years, and devices based on magnetic modulator, magnetic amplifier, optic fiber have been developed, but harsh environments made all of them are insufficient for on line measurement. A new sensing theory using magnetic modulator and magnetic amplifier is discussed. The open loop characteristic of the double detector has only one operating point corresponding to zero ampere turns. The double detector comparator possesses not only high precision, low drift, high linearity and good antimagnetic property from magnetic modulator comparators, but also the perfect stability and reliability from magnetic amplifiers. A double dc comparator with a ratio of 5 000A/1A has been constructed for testing and performance evaluation. The performance and test results show: The error of the double detector dc comparator measured with stable dc currents between 10% and 100% rated current is less than 5×10 -5 . The double detector dc comparator has successfully achieved rapid and accurate dc step response at 10 000A which is twice over the rated current, which shows the fire new detector has perfect stability and reliability.展开更多
A cryogenic low noise amplifier （LNA） using Agilent high electron mobility transistor （HEMT） for 380 MHzto 480 MHz is designed and fabricated, and the excellent cryogenic performance in superconducting receiver fr...A cryogenic low noise amplifier （LNA） using Agilent high electron mobility transistor （HEMT） for 380 MHzto 480 MHz is designed and fabricated, and the excellent cryogenic performance in superconducting receiver front-end for communication system is achieved. A special input impedance matching topology is implemented to provide low noise figure （NF） and good input matching in this cryogenic LNA design. The measurement results show that the NF is within 0.25 dB from the minimum NF of a single transistor, the power gain is above 20 dB, the flatness is within 1 dB, and the maximum input return loss is lower than -20 dB in bandwidth.展开更多
The magamp (magnetic amplifier) is widely used in power supplies due to its low cost, simplicity and other advan-tages. This paper discusses a novel application of the magamp in switching power supplies, where the mag...The magamp (magnetic amplifier) is widely used in power supplies due to its low cost, simplicity and other advan-tages. This paper discusses a novel application of the magamp in switching power supplies, where the magamp is used to regulate pulse width modulation (PWM) instead of power signal in the main circuit. This method extends the application of the magamp in power supplies, and makes it possible to further regulate control signal when PWMs have been generated. Based on this applica-tion, a new current-sharing (CS) scheme using the magamp is proposed, which uses a modified inner loop CS structure. In this scheme PWMs are generated by one main controller, and CS is achieved by regulating PWMs using a magamp in each module. Compared with traditional application of the magamp, the new CS scheme can be used in most topologies and only requires magamps of low power capacity. Then a test circuit of parallel power supply is developed, in which CS is achieved by a PWM regulator with the magamp. The proposed scheme is also used to upgrade an electroplate power to make it capable of paralleling supplies. Experimental results show that the proposed scheme has good CS performance.展开更多
文摘The excitation and evolution of magnetohydrodynamic density waves are considered in a differentially rotating thin gaseous disk embedded with both an azimuthal magnetic field and a vertical magnetic field perpendicular to the disk. Our results shows that the effect of isopedic vertical magnetic fields tend to stabilize the fast MHD density waves while slow MHD density waves are not affected by such vertical magnetic fields for either rigid or differential rotation.
文摘The purpose of this study was to present a lock-in-amplifier model for analyzing the behavior of signal harmonics in magnetic particle imaging (MPI) and some simulation results based on this model. In the lock-in-amplifier model, the signal induced by magnetic nanoparticles (MNPs) in a receiving coil was multiplied with a reference signal, and was then fed through a low-pass filter to extract the DC component of the signal (output signal). The MPI signal was defined as the mean of the absolute value of the output signal. The magnetization and particle size distribution of MNPs were assumed to obey the Langevin theory of paramagnetism and a log-normal distribution, respectively, and the strength of the selection magnetic field (SMF) in MPI was assumed to be given by the product of the gradient strength of the SMF and the distance from the field-free region (x). In addition, Gaussian noise was added to the signal induced by MNPs using normally-distributed random numbers. The relationships between the MPI signal and x were calculated for the odd- and even-numbered harmonics and were investigated for various time constants of the low-pass filter used in the lock-in amplifier and particle sizes and their distributions of MNPs. We found that the behavior of the MPI signal largely depended on the time constant of the low-pass filter and the particle size of MNPs. This lock-in-amplifier model will be useful for better understanding, optimizing, and developing MPI, and for designing MNPs appropriate for MPI.
文摘The magnetic fluid is used as working medium in pilot stage of electro-hydraulic servoamplifier. Utilizing the magnetization viscous character of the fluid, a new type of electro-hydraulicservo amplifier without any moving parts in its pilot stage has been developed. This research provides a way for improving the frequency property and reliability of electro-hydraulic servoelements.
文摘On line measurement of heavy dc current has been studied for many years, and devices based on magnetic modulator, magnetic amplifier, optic fiber have been developed, but harsh environments made all of them are insufficient for on line measurement. A new sensing theory using magnetic modulator and magnetic amplifier is discussed. The open loop characteristic of the double detector has only one operating point corresponding to zero ampere turns. The double detector comparator possesses not only high precision, low drift, high linearity and good antimagnetic property from magnetic modulator comparators, but also the perfect stability and reliability from magnetic amplifiers. A double dc comparator with a ratio of 5 000A/1A has been constructed for testing and performance evaluation. The performance and test results show: The error of the double detector dc comparator measured with stable dc currents between 10% and 100% rated current is less than 5×10 -5 . The double detector dc comparator has successfully achieved rapid and accurate dc step response at 10 000A which is twice over the rated current, which shows the fire new detector has perfect stability and reliability.
基金This work was supported by the National Nature Science Foundation of China under Grant No. 60471001.
文摘A cryogenic low noise amplifier （LNA） using Agilent high electron mobility transistor （HEMT） for 380 MHzto 480 MHz is designed and fabricated, and the excellent cryogenic performance in superconducting receiver front-end for communication system is achieved. A special input impedance matching topology is implemented to provide low noise figure （NF） and good input matching in this cryogenic LNA design. The measurement results show that the NF is within 0.25 dB from the minimum NF of a single transistor, the power gain is above 20 dB, the flatness is within 1 dB, and the maximum input return loss is lower than -20 dB in bandwidth.
基金Project (No. 50677063) supported by the National Natural Science Foundation of China
文摘The magamp (magnetic amplifier) is widely used in power supplies due to its low cost, simplicity and other advan-tages. This paper discusses a novel application of the magamp in switching power supplies, where the magamp is used to regulate pulse width modulation (PWM) instead of power signal in the main circuit. This method extends the application of the magamp in power supplies, and makes it possible to further regulate control signal when PWMs have been generated. Based on this applica-tion, a new current-sharing (CS) scheme using the magamp is proposed, which uses a modified inner loop CS structure. In this scheme PWMs are generated by one main controller, and CS is achieved by regulating PWMs using a magamp in each module. Compared with traditional application of the magamp, the new CS scheme can be used in most topologies and only requires magamps of low power capacity. Then a test circuit of parallel power supply is developed, in which CS is achieved by a PWM regulator with the magamp. The proposed scheme is also used to upgrade an electroplate power to make it capable of paralleling supplies. Experimental results show that the proposed scheme has good CS performance.
