Review of Scientific Instruments最新文献

A method is proposed for estimating the power spectral density (PSD) of time series that uses median smoothing in the frequency domain. The "Median method" for PSD estimation rejects deterministic noise peaks in the PSD while preserving stochastic signals and noise sources. For a PSD averaging factor M, deterministic noise sources are suppressed by a factor of ∼M in power when applying the Median method. In addition, the Median method leads to a reduction of spectral leakage by a factor of ∼M relative to traditional methods. An increase of up to 44% in the standard deviation in the PSD estimate from the Median method is the trade-off for these advantages. In the context of a stochastically driven simple harmonic oscillator, the estimation of its parameters (stiffness, Q factor, and resonance frequency) using the Median method is much more robust against the presence of deterministic noise peaks and more accurate than linear PSD estimation methods.

The National Ignition Facility (NIF) has 48 Real-Time Nuclear Activation Detectors distributed around the target chamber capable of measuring deuterium-triton reaction neutron yields with high precision. In this work, we extend this functionality to deuterium-deuterium (DD) reaction neutrons using a nuclear reaction that occurs in the detector's scintillator material. The corresponding decay of the activated material has a very short half-life of 5 s, which necessitates rapid data collection immediately following an experiment. In this regime, dead time can be very high (>50%) adding significant uncertainty to the measurement. To combat this, we have developed a dead time model that can self-consistently describe the measured data. Initial results show reasonable agreement (within 20%) with DD neutron yields from neutron time-of-flight spectrometers.

High-frequency technological low-temperature plasmas play a key role in various industrial processes of high societal relevance, such as semiconductor manufacturing and gas conversion. Due to their complexity, the fundamentals of their operation are typically not understood and process development is done empirically. The continuous increase in process requirements with respect to precision and reproducibility, however, necessitates knowledge-based approaches toward process development and monitoring. Diagnostic techniques used for this should be non-invasive, have short measuring times, and have low equipment costs. A valuable tool to understand plasma processes is to measure the spatio-temporally resolved dynamics of energetic electrons with phase resolved optical emission spectroscopy (PROES), as these electrons generate the plasma through ionization and reactive radicals through dissociation of the neutral gas. However, PROES is typically performed based on expensive intensified charge-coupled device (ICCD) cameras, is slow, and requires large windows for optical access to the plasma, which do not exist in commercial reactors. To overcome these limitations, we present a modified version of this diagnostic, Fiber PROES, which is based on an optical fiber in combination with a photo-multiplier tube operated in a photon-counting mode. Compared to classical PROES, only a small fiber access port is required, which is typically available in commercial plasma reactors, the costs are strongly reduced, and the measurement speed is increased. We demonstrate that Fiber PROES yields similar results compared to classical ICCD-camera-based PROES by comparing measurements taken in geometrically symmetric capacitively coupled radio frequency plasma based on both PROES variants.

Fourier-transform infrared spectroscopy (FTIR) is a powerful analytical method not only for the chemical identification of solid, liquid, and gas species but also for the quantification of their concentration. However, the chemical quantification capability of FTIR is significantly hindered when the analyte is surrounded by a strong IR absorbing medium, such as liquid solutions. To overcome this limit, here we develop an IR fiber microprobe that can be inserted into a liquid medium and obtain full FTIR spectra at points of interest. To benchmark this endoscopic FTIR method, we insert the microprobe into bulk water covering a ZnSe substrate and measure the IR transmittance of water as a function of the probe-substrate distance. The obtained vibrational modes, overall transmittance vs z profiles, quantitative absorption coefficients, and micro z-section IR transmittance spectra are all consistent with the standard IR absorption properties of water. The results pave the way for endoscopic chemical profiling inside bulk liquid solutions, promising for applications in many biological, chemical, and electrochemical systems.

This paper presents the design of a parallel compliant pure rotational micropositioning stage driven by two voice coil motors aimed at achieving high precision and high responsiveness in pure rotation micropositioning. The stage consists of two leaf-spring based driving arms and a pair of symmetric isosceles trapezoidal mechanisms. The parallel structure of the stage can effectively enhance the system's stiffness and response speed. By analyzing the kinematic and dynamic properties of the stage, the design parameters of the stage are determined to meet positioning requirements across various application scenarios. Finite element analysis is then conducted to validate the accuracy of the analytical model. To validate the proposed design, a stage prototype was constructed and subjected to experimental testing. The testing results demonstrate that the stage can achieve a maximum rotation angle of 41.6 mrad. Furthermore, a feedback controller was also designed to validate the system's stability and tracking accuracy. The trajectory tracking experimental results show that the stage exhibits excellent positioning accuracy and dynamic response to both sine and step signal inputs, confirming its potential in the micropositioning field. Finally, the drift of the rotational center of the stage was observed and evaluated under a microscope, validating the good performance of the rotation accuracy of the stage.

A new frequency-stepped Doppler backscattering (DBS) system has been integrated into a real-time steerable electron cyclotron heating launcher system to simultaneously probe local background turbulence (f < 10 MHz) and high-frequency (20-550 MHz) density fluctuations in the DIII-D tokamak. The launcher allows for 2D steering (horizontally and vertically) over wide angular ranges to optimize probe location and wavenumber response. The vertical steering can be optimized during a discharge in real time. The new DBS system employs a programmable frequency synthesizer with adjustable dwell time as a source to launch either O or X-mode polarized millimeter waves. This system can step in real-time over the entire E-band frequency range (60-90 GHz). This combination of capabilities allows for the diagnosis of the complex internal spatial structure of high power (>200 kW) helicon waves (476 MHz) injected from an external antenna during helicon current drive experiments in DIII-D. Broadband density fluctuations around the helicon frequency are observed during real-time scans of measurement location and wavenumber during these experiments. Analysis indicates that these broadband high-frequency fluctuations are a result of backscattering of the DBS millimeter-wave probe beam from plasma turbulence modulated by the helicon wave. It is observed that background turbulence is effectively locally "tagged" with the helicon wave electric field, forming images of the turbulent spectrum in the overall density fluctuation spectrum that appear as high-frequency sidebands of the turbulence. These observations of background turbulence and high-frequency fluctuations open up the possibility of monitoring local helicon wave amplitude by comparing the high-frequency signal amplitude to the simultaneously measured background turbulence. In combination with the real-time measurement location and wavenumber scanning capabilities (offered by real-time frequency-stepping and steering), this allows rapid determination of the spatial distribution of the helicon wave power during steady-state plasma operation. In the long term, such measurements may be used to validate predictive modeling (GENRAY [Smirnov and Harvey, Bull. Am. Phys. Soc. 40, 1837 (1995)] or AORSA [Lau et al., Nucl. Fusion 58 066004 (2018)]) of helicon current drive in DIII-D plasmas.

Transient grating spectroscopy (TGS) is a rapid and non-destructive technique for measuring thermal, acoustic, and elastic properties of solid materials with a multitude of uses across many areas of materials research. Current TGS systems require optics tables and cumbersome amounts of space for an entire setup, restricting TGS to being a lab-based method. This paper presents a new design for TGS systems that rotates the probe laser beams around the axis of the pump beam, allowing for an asymmetric probe, planar, optically 2D setup. This, in turn, allows the setup to be significantly simplified, which enables the setup presented in this paper to be roughly nine times smaller in volume than contemporary setups while being much easier to build, align, and operate. Part of the size reduction was enabled by a mono-homodyne system and the removal of the chopper. This system was benchmarked against an existing TGS system using a single-crystal tungsten sample. This showed that it can produce the same surface acoustic wave frequency data as the existing system. This design enables TGS to be more widely adopted for use in more varied and compact environments because of its smaller size and simplicity.

This work presents the hardware design and first results from a newly commissioned correlation reflectometer radiometer diagnostic that measures the cross-phase angle between electron density and temperature fluctuations in ASDEX Upgrade plasmas. This diagnostic employs cross correlations between signals measured by a tunable, continuous wave, perpendicular incidence, fluctuation reflectometer, and a 24-channel radiometer sharing the same line of sight. Novel measurements in the pedestal of a helium H-mode plasma with small edge localized modes show changes in the cross-phase angle between the electron density and temperature fluctuations from ∼90° to 120°, suggesting changes in the properties of the turbulence driving transport in the plasma edge.

In this work, a novel bipedal stepping piezoelectric actuator based on stick-slip principle is proposed. An integrated stator with symmetrically positioned dual driving feet has been developed, characterized by its element analysis. Key structural parameters were optimized, and a prototype was fabricated for a series of experiments. The experimental results demonstrate that the actuator attains a maximum output speed of 4550 μm/s at a voltage of 150 V and a frequency of 350 Hz. The stepping efficiency is measured at 0.886, with a maximum horizontal load capacity of 170 g. Furthermore, the actuator exhibits a displacement resolution of 180 nm, making it highly suitable for precision actuation and applications in fields such as biomedical engineering.

Ultra-high field magnetic resonance imaging (MRI) offers significant advantages in terms of signal-to-noise ratio and spatial resolution. In this study, we detail the development of a multi-channel home-built MRI console operating at 14 T. We propose a hybrid analog-digital framework that shifts high-frequency radio frequency transmission and reception issues to lower frequencies, utilizing software-defined radio technology to process these low-frequency signals. Digital pre-emphasis is used in gradient calculations to counteract the effects of eddy currents during gradient switching. Our console can transmit and receive at center frequencies up to 600 MHz. The pulse programmer module achieves a timing resolution of 20 ns, while the transmitter can independently generate waveforms with varying amplitude, frequency, phase, and envelope. The receiver's dual-stage gain control provides 63 dB of adjustable range, optimizing the magnetic resonance (MR) signal's dynamic range. After frequency conversion, the MR signals are digitized with 16-bit resolution and 100 MHz sampling rate. High-resolution water phantom images are acquired on the 14 T Bruker Ascend 600 nuclear magnetic resonance magnet, demonstrating its potential for clinical research and application.