Field Gradient Research Articles

Variational model-based reconstruction techniques for multi-patch data in Magnetic Particle Imaging

Magnetic Particle Imaging is an emerging imaging modality through which it is possible to detect tracers containing superparamagnetic nanoparticles. The exposure of the particles to dynamic magnetic fields generates a non-linear response that is used to locate the particles and produce an image of their distribution. The bounding box that can be covered by a single scan curve depends on the strength of the gradients of the magnetic fields applied, which is limited due to the risk of causing peripheral nerve stimulation (PNS) in the patients. To address this issue, multiple scans are performed. The scan data must be merged together to produce reconstructions of larger regions of interest. In this paper we propose a mathematical framework which can deal with rather general multi-patching scenarios including rigid transformations of the field of view (FoV), the specimen and of the scanner. We show the flexibility of this framework in a variety of different scanning scenarios. Moreover, we describe an iterative reconstruction algorithm that yields a reconstruction of the target distribution by minimizing a convex functional which includes positivity constraints and sparsity enforcing priors. We show its convergence to a minimizer and perform numerical experiments on simulated data.

The Xe isotope nuclear resonance frequency shift of NMR angular velocity sensors due to transverse gradient magnetic fields

The nuclear magnetic resonance (NMR) angular velocity sensor detects angular velocity by measuring the shift in the nuclear spin precession frequency, which is of fundamental interest. Recent studies have revealed the parameter dependency of nuclei frequency shifts induced by non-uniformly distributed polarized pump beams, temperature, and magnetic field properties. In this study, we investigated the effect of the linear transverse gradient magnetic field on the frequency ratio shift of the Xe isotope nuclear resonance frequency in the NMR sensors. A theoretical analysis method was proposed based on the Fermi contact interaction in atomic polarization and the spin-diffusion relaxation of 129Xe and 131Xe nuclei. The frequency ratio shift of the Xe isotope under different x-axis gradient magnetic fields was measured experimentally. Furthermore, we eliminated the equivalent residual magnetic field through a feedback system and compensated for the original gradient magnetic field in the system, which contributed to accurately revealing the frequency shift induced by the magnetic field gradient. The results indicate that the frequency ratio shift of the Xe isotope is proportional to the strength of the second-order linear transverse gradient magnetic field. This study provides a reference for the analysis and evaluation of the presence of the gradient magnetic field in the NMR angular velocity sensor.

Elastin-Like Peptide as a Model for Disordered Proteins: Diffusion Behaviour in Self-Crowding Conditions.

Despite the current high interest, there is limited information on diffusion data for intrinsically disordered proteins (IDPs). This study investigates the effect of crowding on the diffusion behaviour of an elastin-like peptide (ELP), by combined pulse field gradient (PFG) and static field gradient (SFG) NMR techniques. We interpret our findings in terms of highly dynamic chain assemblies with weak interactions, resulting in ELP diffusion that is primarily governed by the viscous flow of the solvent. The diffusion behaviour of the peptide appears to resemble that of globular proteins rather than flexible linear polymers over a wide concentration range.

A research program to measure the lifetime of spin polarized fuel

The use of spin polarized fuel could increase the deuterium-tritium (D-T) fusion cross section by a factor of 1.5 and, owing to alpha heating, increase the fusion power by an even larger factor. Issues associated with the use of polarized fuel in a reactor are identified. Theoretically, nuclei remain polarized in a hot fusion plasma. The similarity between the Lorentz force law and the Bloch equations suggests polarization can be preserved despite the rich electromagnetic spectrum present in a magnetic fusion device. The most important depolarization mechanisms can be tested in existing devices. The use of polarized deuterium and 3He in an experiment avoids the complexities of handling tritium, while encompassing the same nuclear reaction spin-physics, making it a useful proxy to study issues associated with full D-T implementation. 3He fuel with 65% polarization can be prepared by permeating optically-pumped 3He into a shell pellet. Dynamically polarized 7Li-D pellets can achieve 70% vector polarization for the deuterium. Cryogenically-frozen pellets can be injected into fusion facilities by special injectors that minimize depolarizing field gradients. Alternatively, polarized nuclei could be injected as a neutral beam. Once injected, the lifetime of the polarized fuel is monitored through measurements of escaping charged fusion products. Multiple experimental scenarios to measure the polarization lifetime in the DIII-D tokamak and other magnetic-confinement facilities are discussed, followed by outstanding issues that warrant further study.

Multinuclear PFGSTE NMR description of 39K, 23Na, 7Li, and 1H specific activation energies governing diffusion in alkali nitrite solutions

While pulsed field gradient stimulated echo nuclear magnetic resonance (PFGSTE NMR) spectroscopy has found widespread use in the quantification of self-diffusivity for many NMR-active nuclei, extending this technique to uncommon nuclei with unfavorable NMR properties remains an active area of research. Potassium-39 (39K) is an archetypical NMR nucleus exhibiting an unfavorable gyromagnetic ratio combined with a very low Larmor frequency. Despite these unfavorable properties, this work demonstrates that 39K PFGSTE NMR experiments are possible in aqueous solutions of concentrated potassium nitrite. Analysis of the results indicates that 39K NMR diffusometry is feasible when the nuclei exhibit spin–lattice and spin–spin relaxation coefficients on the order of 60–100 ms and 50–100 ms, respectively. The diffusivity of 39K followed Arrhenius behavior, and comparative 23Na, 7Li, and 1H PFGSTE NMR studies of equimolal sodium nitrite and lithium nitrite solutions led to correlations between the enthalpy of hydration with the activation energy governing self-diffusion of the cations and also of water. Realizing the feasibility of 39K PFGSTE NMR spectroscopy has a widespread impact across energy sciences because potassium is a common alkali element in energy storage materials and other applications.

Gradient magnetic field measurement using first-order gradiometers equipped with 50 mm diameter pickup coils made out of a 2G high-Tc superconducting tape

Two types of first-order gradiometers equipped with 50 mm diameter pickup coils were made out of an REBaCuO High-Tc superconducting (HTS) flexible tape. The HTS tapes were formed into the HTS gradiometers by ``cut-and-wind'' method without cable jointing.The HTS gradiometers were examined at 77 K for magnetic sensitivity to a gradient field applied by an external coil. The axial-type HTS gradiometer eliminated the magnetic field output of the device induced by a uniform field, and enhanced the magnetic field output induced by a gradient field on the axis of the pickup coil. The planar-type HTS gradiometer made it possible to detect a 20 mm diameter steel cylinder placed beside the pickup coil in a uniform field of 100 µT by detecting the spatial field gradient in the vicinity of a ferromagnetic object.

Self-phoretic oscillatory motion in a harmonic trap.

We consider the motion of a harmonically trapped overdamped particle, which is submitted to a self-phoretic force, that is proportional to the gradient of a diffusive field for which the particle itself is the source. In agreement with existing results for free particles or particles in a bounded domain, we find that the system exhibits a transition between an immobile phase, where the particle stays at the center of the trap, and an oscillatory state. We perform an exact analysis giving access to the bifurcation threshold, as well as the frequency of oscillations and their amplitude near the threshold. Our analysis also characterizes the shape of two-dimensional oscillations that take place along a circle or a straight line. Our results are confirmed by numerical simulations.

Heat sources for Variscan high‐temperature–low‐pressure metamorphism: Petrochronological constraints from the Trois Seigneurs massif, French Pyrenees

AbstractHigh‐temperature–low‐pressure metamorphism is commonly associated with intermediate to felsic magmatism in continental orogenic belts. The heat budgets and transfer mechanisms responsible for such elevated temperatures and partial melting of the upper crust are uncertain. The Trois Seigneurs massif, French Pyrenees, preserves a structurally continuous record of Variscan high‐temperature–low‐pressure metamorphism through a sequence of upper‐to‐mid‐crustal Paleozoic metasedimentary rocks. Conventional thermobarometry and phase equilibria calculations show that metamorphic conditions span ~2.5 kbar, 575°C to suprasolidus conditions of ~6 kbar, 700°C. Peak temperatures depend strongly on depth: temperature gradients of 50–60°C/km are present through the uppermost 12 km of the section; deeper portions (12–20 km) define restricted temperature conditions of ~650–700°C. The lowest‐grade metamorphic rocks preserve the largest spread in monazite 206Pb*/238U dates, from c. 325–285 Ma, while the spread in dates is restricted to c. 305–290 Ma in the highest‐grade rocks. Within this spread, each sample yields a well‐defined population of monazite 206Pb*/238U dates with peaks at c. 305 Ma in the andalusite schists, 295 Ma in the sillimanite schists, and 300 Ma in the migmatite sample. Monazite trace‐element compositions capture a systematic change with decreasing date and increasing metamorphic grade, including a more negative Eu‐anomaly and decreasing Sr concentrations, consistent with co‐crystallizing feldspar; increasing HREE and Y contents, consistent with xenotime breakdown; and decreasing Th/U, reflecting increasing U content during breakdown of inherited zircon. Zircon rims from a granite unit that formed via partial melting of the Paleozoic sedimentary package yields a 206Pb/238U‐207Pb/235U concordia age of 304.1 ± 3.73 Ma. These rims have trace‐element compositions reflecting cogenetic apatite and zircon growth during granite formation. Zircon from a calc‐alkaline granodiorite intrusion preserves a 40 Ma record of melt‐related activity in the lower crust that preceded the regional thermal climax. We interpret these petrochronological data to show that the Trois Seigneurs field gradient including andalusite schist and biotite granite samples represents a genuine geotherm through Variscan orogenic crust during the regional thermal climax at 305 Ma. When combined with constraints from other Pyrenean massifs, the form of the geotherm is consistent with a thermal scenario in which heat is advected to the upper crust by intermediate‐composition magmas generated in the lower crust. A simple thermal model for this process indicates that anatexis in the upper crust may plausibly occur within 10 Ma of the initiation of the lower‐crustal melting. Such a thermal scenario, however, requires focusing of melt through a fertile lower crust and an elevated Moho heat flux. We suggest that this process may have controlled the attainment of high‐temperature–low‐pressure metamorphic conditions along the Variscan belt and may currently be operating in zones of post‐orogenic continental extension.

Dating prograde metamorphism: U–Pb geochronology of allanite and REE-rich epidote in the Eastern Alps

We use U–Pb dating of allanite and REE-rich epidote in three polymetamorphosed units from the Eastern Alps to constrain the timing of prograde metamorphism. All three units (Ennstal, Wölz and Rappold Complex) record several metamorphic cycles (Variscan, Permian and Eoalpine) and presently define an Eoalpine (Cretaceous) metamorphic field gradient from lower greenschist to amphibolite facies. For U–Pb data, a method is introduced to test the magnitude of 230Th disequilibrium and potentially approximate the Th/U ratio of the reservoir out of which allanite and REE-rich epidote grew. We also show that the modelled stability of epidote-group minerals in the REE-free MnNCKFMASH and MnNCKFMASHTO systems and REE-bearing systems is nearly identical. By combining the stability fields of (clino-)zoisite and epidote modelled in REE-free systems with known geothermal gradients for the region, REE-rich epidote growth is constrained to 200–450 °C and 0.2–0.8 GPa during prograde metamorphism. In the Rappold Complex, allanite cores yield a Variscan age of ca. 327 Ma. In the Ennstal and Wölz Complex, allanite growth during the Permian event occurred at ca. 279–286 Ma. Importantly, recrystallized allanite laths and REE-rich epidote overgrowths in samples from all three units yield prograde Eoalpine ages of ca. 100 Ma, even though these units subsequently reached different peak conditions, most likely at different times. This suggests that all units were buried roughly at the same time during the onset of Eoalpine continental subduction. This interpretation leaves room for the model proposing that diachronous peak metamorphic conditions reported for the field gradient may be related to the inertia of thermal equilibration rather than tectonic processes.

Partial Acquisition of Spectral Projections Accelerates Four-dimensional Spectral-spatial EPR Imaging for Mouse Tumor Models: A Feasibility Study.

Our study aimed to accelerate the acquisition of four-dimensional (4D) spectral-spatial electron paramagnetic resonance (EPR) imaging for mouse tumor models. This advancement in EPR imaging should reduce the acquisition time of spectroscopic mapping while reducing quality degradation for mouse tumor models. EPR spectra under magnetic field gradients, called spectral projections, were partially measured. Additional spectral projections were later computationally synthesized from the measured spectral projections. Four-dimensional spectral-spatial images were reconstructed from the post-processed spectral projections using the algebraic reconstruction technique (ART) and assessed in terms of their image qualities. We applied this approach to a sample solution and a mouse Hs766T xenograft model of human-derived pancreatic ductal adenocarcinoma cells to demonstrate the feasibility of our concept. The nitroxyl radical imaging agent 2H,15N-DCP was exogenously infused into the mouse xenograft model. The computation code of 4D spectral-spatial imaging was tested with numerically generated spectral projections. In the linewidth mapping of the sample solution, we achieved a relative standard uncertainty (standard deviation/| mean |) of 0.76 μT/45.38 μT = 0.017 on the peak-to-peak first-derivative EPR linewidth. The qualities of the linewidth maps and the effect of computational synthesis of spectral projections were examined. Finally, we obtained the three-dimensional linewidth map of 2H,15N-DCP in a Hs766T tumor-bearing leg in vivo. We achieved a 46.7% reduction in the acquisition time of 4D spectral-spatial EPR imaging without significantly degrading the image quality. A combination of ART and partial acquisition in three-dimensional raster magnetic field gradient settings in orthogonal coordinates is a novel approach. Our approach to 4D spectral-spatial EPR imaging can be applied to any subject, especially for samples with less variation in one direction.

Impact of variable viscosity, thermal conductivity, and Soret–Dufour effects on MHD radiative heat transfer in thin reactive liquid films past an unsteady permeable expandable sheet

AbstractSignificance of magnetohydrodynamic effect on a viscous (temperature‐dependent) and chemically reactive thin fluid film flow past an unsteady permeable stretchable plate with Soret–Dufour effects, nonlinear thermal radiative, and suction under the action of a convective type of boundary condition is analyzed. The problem consists of nonlinear governing basic equations that are highly nonlinear due to the existence of nonlinear thermal radiative terms in the energy equation. Analytical solutions are challenging to achieve for such types of problems, so a numerical scheme adopts the numerical solution. Computed solutions indicate that decreasing the Dufour number (and simultaneously increasing the Soret number) enhances heat flux, whereas the reverse trend is estimated for the concentration gradient field. The influence of magnetization indicates a decrement in the thin liquid film velocity distribution, whereas an increment is observed in temperature and solutal gradient profiles. Further, an enhancement in thermoradiative values focuses on decreasing the heat flux profiles, whereas a decreasing trend is determined in the solutal gradient by incrementing the Schmidt number. The variations of the velocity field, temperature, and concentration gradients are shown for the unsteady parameter lying in the range [0.8, 1.4]. Similarly, the range of different parameters utilized are [0.0, 1.0], [0.0, 2.0], [0.8, 1.5], [0.5, 2.0], [0.0, 1.0], [0.2, 3.0], [1.0, 2.0], [0.0, 3.0], [0.4, 1.0], and [0.4, 1.0]. The novelty of the present study lies in its analysis of complex fluid dynamics phenomena and their implications for various industrial processes and engineering applications, including coating processes, heat exchangers, microfluidics, and biomedical engineering. The insights gained from the study can contribute to developing more efficient and innovative research in these areas. Further, we have compared the present results with those available in the literature under some special cases and found them to be in excellent agreement.

A Human-Scale Clinically Ready Electromagnetic Navigation System for Magnetically Responsive Biomaterials and Medical Devices.

Magnetic navigation systems are used to precisely manipulate magnetically responsive materials enabling the realization of new minimally invasive procedures using magnetic medical devices. Their widespread applicability has been constrained by high infrastructure demands and costs. The study reports on a portable electromagnetic navigation system, the Navion, which is capable of generating a large magnetic field over a large workspace. The system is easy to install in hospital operating rooms and transportable through health care facilities, aiding in the widespread adoption of magnetically responsive medical devices. First, the design and implementation approach for the system are introduced and its performance is characterized. Next, in vitro navigation of different microrobot structures is demonstrated using magnetic field gradients and rotating magnetic fields. Spherical permanent magnets, electroplated cylindrical microrobots, microparticle swarms, and magnetic composite bacteria-inspired helical structures are investigated. The navigation of magnetic catheters is also demonstrated in two challenging endovascular tasks: 1) an angiography procedure and 2) deep navigation within the circle of Willis. Catheter navigation is demonstrated in a porcine model in vivo to perform an angiography under magnetic guidance.

The impact of magnesium content on lithium-magnesium alloy electrode performance with argyrodite solid electrolyte

Solid-state lithium-based batteries offer higher energy density than their Li-ion counterparts. Yet they are limited in terms of negative electrode discharge performance and require high stack pressure during operation. To circumvent these issues, we propose the use of lithium-rich magnesium alloys as suitable negative electrodes in combination with Li6PS5Cl solid-state electrolyte. We synthesise and characterise lithium-rich magnesium alloys, quantifying the changes in mechanical properties, transport, and surface chemistry that impact electrochemical performance. Increases in hardness, stiffness, adhesion, and resistance to creep are quantified by nanoindentation as a function of magnesium content. A decrease in diffusivity is quantified with 6Li pulsed field gradient nuclear magnetic resonance, and only a small increase in interfacial impedance due to the presence of magnesium is identified by electrochemical impedance spectroscopy which is correlated with x-ray photoelectron spectroscopy. The addition of magnesium aids contact retention on discharge, but this must be balanced against a decrease in lithium diffusivity. We demonstrate via electrochemical testing of symmetric cells at 2.5 MPa and 30∘C that 1% magnesium content in the alloy increases the stripping capacity compared to both pure lithium and higher magnesium content alloys by balancing these effects.

Microstructural and mechanistic insights into the Tension–Compression asymmetry of rapidly solidified Fe–Cr alloys: A phase field and strain gradient plasticity study

Rapid solidification in Additively Manufactured (AM) metallic materials results in the development of significant microscale internal stresses, which are attributed to the printing induced dislocation substructures. The resulting backstress due to the Geometrically Necessary Dislocations (GNDs) is responsible for the observed Tension–Compression (TC) asymmetry. We propose a combined Phase Field (PF)-Strain Gradient J2 Plasticity (SGP) framework to investigate the TC asymmetry in such microstructures. The proposed PF model is an extension of Kobayashi’s dendritic growth framework, modified to account for the orientation-based anisotropy and multi-grain interaction effects. The SGP model has consideration for anisotropic temperature-dependent elasticity, dislocation strengthening, solid solution strengthening, along with GND-induced directional backstress. This model is employed to predict the solute segregation, dislocation substructure and backstress development during solidification and the post-solidification anisotropic mechanical properties in terms of the TC asymmetry of rapidly solidified Fe–Cr alloys. It is observed that higher thermal gradients (and hence, cooling rates) lead to higher magnitudes of solute segregation, GND density, and backstress. This also correlates with a corresponding increase in the predicted TC asymmetry. The results presented in this study point to the microstructural factors, such as dislocation substructure and solute segregation, and mechanistic factors, such as backstress, which may contribute to the development of TC asymmetry in rapidly solidified microstructures.

Influence of Quadrupolar Molecular Transitions within Plasmonic Cavities.

Optical nanocavities have revolutionized the manipulation of radiative properties of molecular and semiconductor emitters. Here, we investigate the amplified photoluminescence arising from exciting a dark transition of β-carotene molecules embedded within plasmonic nanocavities. Integrating a molecular monolayer into nanoparticle-on-mirror nanostructures unveils enhancements surpassing 4 orders of magnitude in the initially light-forbidden excitation. Such pronounced enhancements transcend conventional dipolar mechanisms, underscoring the presence of alternative enhancement pathways. Notably, Fourier-plane scattering spectroscopy shows that the photoluminescence excitation resonance aligns with a higher-order plasmonic cavity mode, which supports strong field gradients. Combining quantum chemistry calculations with electromagnetic simulations reveals an important interplay between the Franck-Condon quadrupole and Herzberg-Teller dipole contributions in governing the absorption characteristics of this dark transition. In contrast to free space, the quadrupole moment plays a significant role in photoluminescence enhancement within nanoparticle-on-mirror cavities. These findings provide an approach to access optically inactive transitions, promising advancements in spectroscopy and sensing applications.

Identification of structural defects and their impact on the magnetic memory, static and cyclic strength of VNS9-SH thin sheet trip-steel

Identification of microstructure defects, which are stress concentrators (SC) during the operation of mechanical engineering products, is an important scientific and practical task relevant for manufacturing enterprises. This problem becomes especially urgent and difficult for critical helicopter parts made of sheet TRIP steel VNS9-Sh (23Kh15N5AM3-Sh) and operating under cyclic loads due to the complex microstructure of the steel and small thickness of strips and sheets. To assess the impact of structural defects on the cyclic strength of products made of the steel under study, the specimens were preliminary sorted proceeding from the results of their testing using the method of metal magnetic memory (MMM) and metallographic studies. The MMM method is a structure-sensitive procedure which provides information about the presence of structural defects that arise during the manufacture. Magnetic anomalies in the form of sharp local changes in the intrinsic stray magnetic field (SSMF) (H) and its gradient |ΔH| along the length of the controlled section Δx, were identified on the surface of sheets cut from five different batches. A conventional classification of the identified anomalies was made according to the magnitude of the magnetic field gradient. The specimens of two types were cut in zones of magnetic anomalies and outside them: type 1 — for cyclic tests and type 2 — for metallographic studies. The geometric parameters and field gradient values of magnetic anomalies on specimens of type 1 and type 2 were the same. Metallographic studies in zones of maximum magnetic field gradient on type 2 specimens revealed defects in the form of a strip at the boundary of different structures, which is a structural stress concentrator (SSC) and a source of the inhomogeneity and changes in the magnetic properties. Type 1 specimens with similar magnetic anomalies and Type 1 specimens cut from sheets outside zones of magnetic anomalies were then selected for cyclic testing. Comparative tests for cyclic strength of the specimens with and without specified SSC were carried out. It is shown that the presence of SSC zones in the specimens reduces the number of cycles to failure during cyclic tests by 1 – 2 orders of magnitude compared to the specimens free of SSC. Based on cyclic tensile tests of specimens, a limiting value of the magnetic field gradient was determined that corresponds to the acceptable level of stress concentration on structural defects. This value is recommended for use as a rejection criterion when examining a new tape by the MMM method.

Interior Exciton Extraction by Spatial-Controlled Iodine Doping in BiOBr Photocatalysts.

Extracting interior photoinduced species to the surface before their recombination is of great importance in pursuing high-efficiency semiconductor-based photocatalysis. Traditional strategies toward charge-carrier extraction, mostly relying on the construction of an electric field gradient, would be invalid toward the neutral-exciton counterpart in low-dimensional systems. In this work, by taking bismuth oxybromide (BiOBr) as an example, we manipulate interior exciton extraction to the surface by implementing iodine doping at the edges of BiOBr plates. Spatial- and time-resolved spectroscopic analyses verified the accumulation of excitons and charge carriers at the edges of iodine-doped BiOBr (BiOBr-I) plates. This phenomenon could be associated with interior exciton extraction, driven by an energy-level gradient between interior and edge exciton states, and the following exciton dissociation processes. As such, BiOBr-I shows remarkable performance in photocatalytic C-H fluorination, mediated by both energy- and charge-transfer processes. This work uncovers the importance of spatial regulation of excitonic properties in low-dimensional semiconductor-based photocatalysis.

Tailoring data assimilation to discontinuous Galerkin models

AbstractIn recent years discontinuous Galerkin (DG) methods have received increased interest from the geophysical community. In these methods the solution in each grid cell is approximated as a linear combination of basis functions. Ensemble data assimilation (DA) aims to approximate the true state by combining model outputs with observations using error statistics estimated from an ensemble of model runs. Ensemble data assimilation in geophysical models faces several well‐documented issues. In this work we exploit the expansion of the solution in DG basis functions to address some of these issues. Specifically, it is investigated whether a DA–DG combination (a) mitigates the need for observation thinning, (b) reduces errors in the field's gradients, and (c) can be used to set up scale‐dependent localisation. Numerical experiments are carried out using stochastically generated ensembles of model states, with different noise properties, and with Legendre polynomials as basis functions. It is found that strong reduction in the analysis error is achieved by using DA–DG and that the benefit increases with increasing DG order. This is especially the case when small scales dominate the background error. The DA improvement in the first derivative is, on the other hand, marginal. We think this to be a counter‐effect of the power of DG to fit the observations closely, which can deteriorate the estimates of the derivatives. Applying optimal localisation to the different polynomial orders, thus exploiting their different spatial length, is beneficial: it results in a covariance matrix closer to the true covariance than the matrix obtained using traditional optimal localisation in state space.

Suppressing effect of shortening vapor cell stem on polarization-induced magnetic gradient relaxation of Xe in NMR co-magnetometers

The Rb polarization-induced magnetic field gradient is one of the crucial sources of Xe spin relaxation in NMR co-magnetometers. The additional volume of the atomic vapor cell stem can enhance the effective magnetic field gradient through the diffusion effect of the optically polarized-Rb atoms, which affects the spin relaxation properties of Xe and degrades the long-term stability of NMR co-magnetometers. Based on the discrete steps random walk diffusion model, the magnetic gradient relaxation properties of Xe diffusing in an asymmetric cell composed of a cubic part and a cylindrical tube (stem) are characterized. Simulation results indicate that a shorter stem can effectively prolong the nuclear spin relaxation time under a constant magnetic field gradient. To verify this conclusion, a cell stem truncating actuator is developed using a high-power CO2 laser to shorten the cell stem, and the magnetic field gradient coil is used to perform the gradient compensation to examine the impact of various stem lengths on the spin relaxation rate. Experiments indicate that under the influence of atomic diffusive motions, shortening the stem length can effectively suppress the polarization-induced magnetic gradient and increase the 129Xe transverse relaxation time from 4.5 s to 21 s. The study posts practical applications for vapor cells with a stem, which is expected to enhance the performance of atomic spin co-magnetometers.

Localization Method for Underwater Magnetic Sensors Based on the Optimization of Magnetic Field Gradients

Localization Method for Underwater Magnetic Sensors Based on the Optimization of Magnetic Field Gradients