Review of Scientific Instruments最新文献

Long-Time Coherent Integration (LTCI) utilizes digital integration to combine multiple coherent cycles, thereby improving the signal-to-noise ratio (SNR). Our previous work introduced single-bit LTCI, an approach optimized for FPGA implementation, but faced challenges of output saturation at high SNR levels and inherent limitations in SNR gain (SNRG), which are insufficient for certain applications. This paper presents a threshold tracking method that improves the performance of single-bit LTCI in high-SNR scenarios. In addition, a sampling rate enhancement technique and a Kalman filtering method are introduced to further enhance the SNR of the processed signals. An FPGA-based prototype was developed to validate these methods. The results demonstrate that the threshold tracking method extends the measurable input SNR range to 30. Under the specified conditions, the sampling rate enhancement technique yields a 30% improvement in SNR over the original method, while the Kalman filter reduces noise levels to 60% of their original values.

We present an active alignment and stabilization control system for laser setups based on a thin-disk regenerative amplifier. This method eliminates power and pointing instability during the warm-up period and improves long-term stability throughout the entire operation. The alignment method is based on a four-mirror control system consisting of two motorized mirrors placed within the regenerative amplifier cavity, two additional motorized mirrors external to the amplifier cavity, and four camera detectors. The implemented stabilization system achieves significant performance improvements, increasing the power stability from 1.87% to 0.79% RMS and the peak-to-peak stability from 7.43% to 3.88%. Furthermore, the system significantly enhances beam positional stability, achieving up to a sixfold improvement in certain sensor measurements. The advantage of this method is the removal of long-term pointing instability by adding a second controlled motorized mirror to the cavity, in addition to using only one cavity end mirror for optimizing the overlap with the pump spot on the cavity medium. To achieve higher precision in pointing, the nonlinear hysteresis effect of the piezoelectric actuator is mitigated. Although the power and pointing stability of the cavity are secured, pointing instability at the input of the compressor occurs. This issue is resolved by two additional controlled motorized mirrors external to the cavity.

In this paper, two models for simulating the shot noise and electronic noise performances of resonant photodetectors designed for homodyne measurements are presented. One is based on a combination of a buffer and a low-noise amplifier, and the other is based on an operational amplifier. Through the comparisons between the numerical simulation results and the experimentally obtained data, excellent agreements are achieved, which show that the models provide a highly efficient guide for the development of a high signal-to-noise ratio (SNR) resonant photodetector. Furthermore, we demonstrate a high SNR resonant photodetector for homodyne measurements at the 147 MHz optical sideband, achieving a 20.8 dB SNR of the shot noise to the electronic noise with a 2 mW optical signal input, utilizing a combination of a buffer and a low-noise amplifier. Concurrently, we have obtained another resonant photodetector at the 1.14 GHz optical sideband, which exhibits a 13 dB SNR based on an operational amplifier.

A method to determine electron temperature within a plasma by the spectral analysis of atomic tungsten emission has been explored. The technique was applied to a post-discharge region immediately following a high voltage nanosecond pulsed discharge in air with tungsten electrodes. Atomic tungsten lines are readily observed in the weak emission spectrum within the post-discharge region for many microseconds. Intensity ratios were measured at various times after the pulsed discharge for a select pair of neutral tungsten emission lines at 400.88 and 401.52 nm, where the upper electronic levels of each transition are at 3.46 and 5.52 eV respectively. This significant difference in upper state energy causes their line intensity ratio to vary as the electron temperature changes. In addition to the emission spectra, the absolute electron temperature could be accurately measured in our lab using laser Thomson scattering to calibrate the new tungsten emission line intensity ratio method. An analysis is presented that calculates electron temperature from these tungsten emission data assuming a Maxwellian electron energy distribution contributing to direct electron impact excitation to the upper states of each transition. The results included the derivation of a calibration factor between the two experimental methods representing a previously unreported ratio of Einstein A coefficients for the 400.88-401.52 nm transitions. This derivation provides a method for future measurement of absolute electron temperature by the 400.88-401.52 nm tungsten line intensity ratio without the need for laser Thomson scattering calibration.

Ignition of the lubricating fluid in a mechanical system is a highly undesirable and unsafe condition that can arise from the elevated temperatures and pressures to which the lubricant is subjected. It is therefore important to understand the fundamental chemistry behind its ignition to predict and prevent this condition. Lubricating oils, particularly those with a mineral oil base, are very complex mixtures of thousands of hydrocarbons. Additionally, these oils have very low vapor pressures and high viscosities. These physical characteristics present considerable barriers to examining and understanding lubricant ignition chemistry. Therefore, a novel experimental design was devised to create and introduce a lubricant aerosol into a shock-tube facility in a reliable yet relatively simple manner. In this way, the lubricant can be quasi-homogeneously introduced into the shock tube where it will be vaporized by the incident shock wave, and combustion can be observed behind the reflected shock wave. To characterize the technique and anchor it with previously established methods, n-hexadecane was chosen to be tested both with the endwall injection and the well-established, heated shock tube techniques. This comparison showed good agreement, proving the ability of the simple technique to produce reliable ignition delay time (IDT) results. From here, Jet-A was also tested with the current injection technique and compared to a previous generation of the technique to highlight the advantages of the present method. Then, IDT results for mineral oil were collected to establish a baseline IDT set to which off-the-shelf lubricants and additional mixtures can be compared. Finally, IDTs for the off-the-shelf, mineral-based lubricant Mobil DTE 732 were obtained and compared to the baseline as well as the n-hexadecane results. All experiments were conducted near atmospheric pressure and for temperatures between 1084 and 1530 K. An analysis of the system estimated the effective stoichiometry to be around ϕ = 1.15. Although no kinetics mechanisms exist for lubrication oils, preliminary model predictions made by modern chemical kinetics mechanisms for an alkane with 16 carbon atoms were then compared to the results to elucidate some of the chemistry this new method will allow the community to probe. This paper establishes the new method as a viable way to study and compare the ignition behavior of lubricating oils and other very low-vapor-pressure fuels in a shock tube.

Emotion recognition based on electroencephalogram (EEG) has always been a research hotspot. However, due to significant individual variations in EEG signals, cross-subject emotion recognition based on EEG remains a challenging issue to address. In this article, we propose a dynamic domain-adaptive EEG emotion recognition method based on multi-source selection. The method considers each subject as a separate domain, filters suitable source domains from multiple subjects by assessing their resemblance, then further extracts the common and domain-specific features of the source and target domains, and then employs dynamic domain adaptation to mitigate inter-domain discrepancies. Global domain differences and local subdomain differences are also considered, and a dynamic factor is added so that the model training process first focuses on global distribution differences and gradually switches to local subdomain distributions. We conducted cross-subject and cross-session experiments on the SEED and SEED-IV datasets, respectively, and the cross-subject accuracies were 89.76% and 65.28%; the cross-session experiments were 91.63% and 67.83%. The experimental outcomes affirm the efficacy of the EEG emotion recognition approach put forward in this paper.

In this paper, a microwave thermal imaging system (MTIS) has been presented for debonding detection of radar absorbing materials (RAMs). First, an overview of the mechanism underlying microwave heating and the fundamental principle of defect detection within RAMs is presented. Then, a multifunctional MTIS capable of performing both microwave lock-in thermography (MLIT) and long-pulse microwave thermography (LPMT) has been introduced, specifically tailored for the in situ inspection of RAMs. In addition, in this system, the detection area for a single scan is 90 * 90 mm2, with the emission source operating at a frequency of 5.8 GHz and boasting a maximum output power of 20 W. Next, based on MTIS, the above-mentioned two thermography techniques are applied to detect defects in RAMs. In addition, thermal contrast (Tc) and signal-to-noise ratio are introduced for the analysis of imaging results. Finally, the results show that LPMT can be used for preliminary detection of debonding defects in RAMs, while MLIT can be further used for detailed detection of debonding defects in RAMs. In addition, the minimum detection time of this MTIS is 45 s, and the minimum detectable defect aperture is 3 mm.

In multi-dimensional nanopositioning and nanomeasuring devices, interference measurement is widely used. The three-dimensional (3D) target mirror serves as the spatial reference plane to achieve multidimensional interference measurements. However, the surface shape errors of the target mirror are superimposed on the geometric dimensions of the measured workpiece, which limits the system's overall measurement accuracy. This paper proposes a method for on-machine separation and compensation of the target mirror's surface shape errors based on the micro-nano-coordinate measuring machine (MNCMM) that employs interference measurement. This method provides the model for the separation and compensation of the surface shape errors. The MNCMM employs a home-made resonant probe and a reference flat crystal to achieve the separation experiment. Subsequently, an interpolation algorithm is used to compensate for the surface shape errors at any point in space according to the compensation model. By comparing the flatness measurement results of a standard flat crystal with a flatness of 50 nm before and after compensation, the flatness is reduced from 175 to 77 nm. It demonstrates the reliability of the method. This method can be widely applied to on-machine compensation for surface shape errors in multidimensional interference measurement systems.

Comprehensive Research Facility for Fusion Technology (CRAFT) is a technology development and validation platform for fusion technology in China. Neutral beam injection is one of the most important auxiliary heating and current drive methods in magnetically confined controlled fusion. Consequently, a negative ion based neutral beam injector (NNBI) testing facility with a beam energy of 400 keV is being developed in CRAFT. The core snubber, which provides an equivalent parallel resistance and inductance, will be used as the main surge suppression method for the CRAFT NNBI power supply system. In this paper, a core snubber for CRAFT NNBI based on Fe-based amorphous alloy is designed. The transmission line resistance, inductance, and capacitance of the high-voltage circuit have been considered during the design process. The snubber is made thin and long for even weight distribution while ensuring choke capacity. One of the snubber units was tested on the test bench. The test results indicate that the choke capacity of the snubber meets the requirement and can achieve significant suppression of peak ignition current and oscillation.

In this paper, we present the development of a nanosecond pulse generator utilizing semiconductor opening switches (SOS), designed to deliver high voltage and operate at a high repetitive frequency. The pulse generator comprises three main components: a primary charging unit, a magnetic pulse compression unit, and an SOS magnification unit. To ensure stable operation of the high-power charging unit at high repetitive frequencies, a rectifying resonant charging and energy recovery circuit are implemented, providing a 1 kV charging voltage at a 3 kHz repetition rate. The three-stage magnetic pulse compression is designed to reduce the pulse width from tens of microseconds to tens of nanoseconds, where self-demagnetization could be completed during repetitive frequency operation. To achieve an output voltage of 300 kV, multiple SOS switches are employed in a series. The developed pulse generator achieves a final output of 300 kV with a 3 kHz repetitive frequency under a load of 2 kΩ. Furthermore, the effects of multiple factors on the output performance are characterized by both simulation and measurement for a comprehensive analysis.