Electrostatic Transport Research Articles

Recent advancements with the H− injector performance for the Spallation Neutron Source operation and upgrade

The Spallation Neutron Source (SNS) located at the Oak Ridge National Laboratory is an accelerator-based, pulsed neutron facility utilized for a broad range of scientific research and industrial applications. In the past 5 years, the facility has reliably been operated at its baseline design beam power of 1.4 MW, and it is currently undertaking an upgrade to double its power to 2.8 MW. A 65-keV H− injector, which comprises an rf-driven H− ion source and an electrostatic low energy beam transport, delivers the required high-current, time-structured H− beam to the accelerator. The H− injector can reliably provide 50-60 mA beam current at 6% duty-factor (1 ms, 60 Hz) for a 3-4 month service cycle. To ensure sufficient operational margins for the SNS routine operations and ongoing upgrade requirements, the injector system has been continuously improved on a R&D test stand. This paper presents the operational status and recent advancements with the injector system performance including the enhanced ion source beam output capability up to ∼110 mA, the improved ion source plasma ignition reliability using a 27.12-MHz RF system, and the upgraded LEBT beam chopper system etc.

Discovery of a Dust Sorting Process on Boulders Near the Reiner Gamma Swirl on the Moon

AbstractIn a database of lunar fractured boulders (Rüsch & Bickel, 2023, https://doi.org/10.3847/psj/acd1ef), we found boulders with reflectance features dissimilar to previously known morphologies. We performed a photo‐geologic investigation and determined that the features correspond to a dust mantling on top of boulders with a unique photometric behavior. We next performed a photometric model inversion on the dust mantling using Bayesian inference sampling. Modeling indicates that the dust photometric anomaly is most likely due to a reduced opposition effect, whereas the single scattering albedo is not significantly different from that of the nearby background regolith. This implies a different structure of the dust mantling relative to the normal regolith. We identified and discussed several potential processes to explain the development of such soil. None of these mechanisms can entirely explain the multitude of observational constraints unless evoking anomalous boulder properties. Further study of these boulders can shed light on the workings of a natural dust sorting process potentially involving dust dynamics, a magnetic field, and electrostatic dust transport. The presence of these boulders appears to be limited to the Reiner K crater near the Reiner Gamma magnetic and photometric anomaly. This close spatial relationship further highlights that poorly understood processes occur in this specific region of the Moon.

Design and experiments of a compact electrostatic low energy beam transport with a double Einzel-lens for a transportable neutron source.

A transportable, compact, accelerator-based neutron source is under development at Xian Jiaotong University. An electrostatic low energy beam transport (LEBT) structure with a double Einzel-lens setup was adopted due to its short length and low power consumption. It can transport a pulsed proton beam to the radio frequency quadrupole with a required beam current of 15 mA and an energy of 30keV. We performed detailed structure optimization and beam tracking to achieve beam matching and small emittance growth. In addition, the fast chopper, beam steering, and diagnostic devices are integrated into the LEBT. The fabrication and assembly of the proton injector have been completed, and beam commissioning was carried out to measure the beam current and Twiss parameters. The design strategy, beam simulation, and experimental results are presented and discussed in this paper.

Global gyrokinetic simulations of electrostatic microturbulent transport in LHD stellarator with boron impurity

Global gyrokinetic simulations of electrostatic microturbulent transport for discharge # 166256 of the Large Helical Device stellarator in the presence of boron impurity show the co-existence of the ion temperature gradient (ITG) turbulence and trapped electron mode (TEM) turbulence before and during boron powder injection. ITG turbulence dominates in the core, whereas TEM dominates near the edge, consistent with the experimental observations. Linear TEM frequency increases from ∼80 kHz to ∼100 kHz during boron injection, and ITG frequency decreases from ∼20 kHz to ∼13 kHz, consistent with the experiments. The poloidal wave number spectrum is broad for both ITG (0–0.5 mm−1) and TEM (0–0.25 mm−1). The nonlinear simulations with boron impurity show a reduction in the heat conductivity compared to the case without boron. The comparison of the nonlinear transport before and during boron injection shows that the ion heat transport is substantially reduced in the region where the TEM is dominant. However, the average electron heat transport throughout the radial domain and the average ion heat transport in the region where the ITG is dominant are similar. The simulations with boron show the effective heat conductivity values qualitatively agree with the estimate obtained from the experiment.

Numerical investigation of the electrostatic dust transport around a lunar lander

Since the Apollo era, lunar dust has been one of the major problems for lunar surface explorations, since these very abrasive and adhesive grains are harmful for both the instruments and the human beings. Normally, the lunar surface can get charged with an electric field of several V/m, which is not strong enough to launch the dust grains from the surface. However, the electric field may be enhanced when the solar wind and the sunlight are shadowed by an obstacle. Such a shadowing effect can be caused by either a natural topographic feature or a man-made object. Here we study the shadowing effect of a lunar lander with particle-in-cell simulations. It is found that the shadowed region is negatively charged, with a surface potential as low as −30 V and an electric field as large as 20 V/m. A local dust cloud is caused by the electrostatic dust transport around the lander, with a number density of about 104 m−3 at 1 m altitude and up to 105 m−3 near the surface. The surface potential is determined by the temperature of solar wind, but a higher temperature can bring a weaker electric field and a smaller dust cloud, due to the enhanced Debye length. The electrostatic dust transport favors a larger shadowed area, caused by either a larger solar zenith angle or a higher lander height. In addition, the dust transport favors a conductive lander surface, where the maximum dust density can reach 106 m−3. Our results are helpful for the future lunar explorations and have general implications in studying the dust environment of other airless bodies.

Aromatic self-assembled monolayers with pentafluoro-λ6-sulfanyl (−SF5) termination: Molecular organization and charge transport properties

A series of molecules with oligophenylene backbone, thiolate anchoring group, and pentafluoro-λ6-sulfanyl (−SF5) tail group was synthesized and used as precursors to form self-assembled monolayers (SAMs) on Au(111) substrates. The resulting SAMs feature dense molecular packing, upright molecular orientation, and chemically homogeneous SAM-ambient interface, comprised entirely of the −SF5 moieties. These SAMs exhibit exceptional wetting and electrostatic properties, showing advancing water contact angles up to 103° and work function values up to 5.96 eV—probably the highest values reported for any aromatic monolayers on gold. They also feature a comparably low value of the tunnelling decay coefficient (0.38 ± 0.07 Å−1), typical of oligophenylene backbone, which is not affected by the introduction of the −SF5 group. The latter also hardly affects the current densities at a specific bias compared to analogous monolayers with other electronegative tail groups. The superior electrostatic and good charge transport properties of the designed, SF5-terminated SAMs make them potentially useful for interface engineering in organic electronics and photovoltaics.

Electrostatic Transport Characteristics of Olivine Particles under Electron irradiation in Vacuum

Electrostatic Transport Characteristics of Olivine Particles under Electron irradiation in Vacuum

Experiments on the Electrostatic Transport of Charged Anorthite Particles under Electron Beam Irradiation

To reveal the effect of secondary electron emission on the charging properties of a surface covered by micron-sized insulating dust particles and the migration characteristics of these particles, for the first time we used a laser Doppler method to measure the diameters and velocities of micron-sized anorthite particles under electron beam irradiation with an incident energy of 350 eV. Here, anorthite particles are being treated as a proxy for lunar regolith. We experimentally confirm that the vertical transport of anorthite particles is always dominant, although horizontal transport occurs. In our experiments, some anorthite particles were observed to have large vertical velocities up to 9.74 m s−1 at the measurement point. The upper boundary of the vertical velocities V z of these high-speed anorthite particles are well constrained by its diameter D, that is, linearly depends on D −2. These velocity–diameter data provide strong constraints on the dust charging and transportation mechanisms. The shared charge model could not explain the observed velocity–diameter data. Both the isolated charge model and patched charge model appear to require a large dust charging potential of −350 to −78 V to reproduce the observed data. The microstructures of the dusty surface may play an important role in producing this charging potential and in understanding the pulse migration phenomenon observed in our experiment. The presented results and analysis in this paper are helpful for understanding the dust charging and electrostatic transport mechanisms in airless celestial bodies such as the Moon and asteroids in various plasma conditions.

H- ion source operational performance and latest development at the Spallation Neutron Source

The U.S. Spallation Neutron Source (SNS) at Oak Ridge National Laboratory is the world’s most powerful accelerator-driven pulsed neutron source. An H- injector feeds the accelerator with high current (>50 mA), time structured (1ms, 60 Hz) H- beam. The injector consists of an RF-driven, Cs-enhanced H- ion source and an electrostatic low energy beam transport section. In the recent three run cycles, an H- ion source operated ∼4 months for each run with excellent availability near 99.9%. The presently ongoing run is also on track to finish another ∼4 months run with similar high availability. Lately, we have tested a solid-state RF system for the ion source to replace the existing vacuum-tube type RF amplifier for the sake of improving the ease of operation and maintenance. Progress has also been made on the continued development of an external antenna RF H- ion source with a focus on improvement to its plasma ignition scheme.

Electrostatic powder attraction for the development of a novel recoating system for metal powder bed-based additive manufacturing

The field of electrostatics and electrophotography offers great potential for applying powder layers for powder bed-based additive manufacturing processes, such as powder bed fusion of metals using a laser beam (PBF-LB/M). The authors introduce a highly flexible powder layer forming process using the electrophotographic principle. The focus is on full-area powder attraction by electrostatic mass transport of a typical gas atomized PBF-LB/M steel alloy (20MnCr5). A simulation model was created to predict the formation of the electric field. This model was used to determine parameter preselection for experimental validation. After successful full-area powder attraction, a selective powder attraction approach was investigated.

Molecular Dynamics of Electrostatic Channeling on the Surface of Malate Dehydrogenase-Citrate Synthase

Multi-enzyme catalytic cascade reactions exist in many metabolic pathways in nature; the benefits of these reactions include high efficiency and reduced time and labor costs [1].The design and application of efficient cascade reactions in biofuel cells, sensors, and chemical conversion requires an understanding of intermediate channeling between adjacent cascade active sites. Electrostatic channeling, for example, utilizes electrostatic force to control the direct transfer of charged intermediates [2]. Here, we study the electrostatic channeling of an oxaloacetate (OAA) intermediate between malate dehydrogenase (MDH) and citrate synthase (CS) active sites within the tricarboxylic acid (TCA) cycle.The MDH-CS supramolecular assembly includes a positively charged surface between neighboring active sites that guides the movement of the negatively charged oxaloacetate, thus improving the transfer efficiency [3].To study the interaction between OAA and the charged enzyme surface and identify OAA’s transfer pathways, we simulated this system using molecular dynamics (MD). Specifically, since both MDH and CS have two identical subunits and thus two active sites, we ran 50 MD simulations for 10 ns near each of the four active sites, resulting in 200 parallel runs of simulations. Then we analyzed the resulting MD dataset with a Markov State Model (MSM) [4].We identified multiple energy basins in MSM, including four active site basins, a desorption basin, and an intermediate basin. MSM successfully predicted the the transition rates between those energy basins and identified the dominant transport pathways of OAA from MDH to CS. The visualization demonstrates that the dominant transport pathway crosses Arg65, providing further support for the experimental observation that Arg65 plays an critical role in the electrostatic transport process of OAA [3].This study improves the field’s understanding of natural substrate channeling mechanisms and can help with the design of artificial cascade reactions. References K. C. Nicolaou, D. J. Edmonds, and P. G. Bulger, Angew. Chemie - Int. Ed., 45, 7134–7186 (2006). doi:10.1002/anie.200601872. I. Wheeldon, S. D. Minteer, S. Banta, S. Calabrese Barton, P. Atanassov, and M. Sigman, Nat. Chem (2016). doi:10.1038/NCHEM.2459 B. Bulutoglu, K. E. Garcia, F. Wu, S. D. Minteer, and S. Banta, ACS Chem. Biol., 11, 2847–2853 (2016). doi:10.1021/acschembio.6b00523F. Noé, C. Schütte, E. Vanden-Eijnden, L. Reich, and T. R. Weikl, Proc. Natl. Acad. Sci. U. S. A., 106, 19011–19016 (2009). doi:10.1073/pnas.0905466106 Figure 1

Electrostatic Dust Transport Effects on Shaping the Surface Properties of the Moon and Airless Bodies across the Solar System

Electrostatic Dust Transport Effects on Shaping the Surface Properties of the Moon and Airless Bodies across the Solar System

Influence of the DLC Passivation Conductivity on the Performance of Silicon High-Power Diodes Over an Extended Temperature Range

The diamond-like carbon (DLC) is important for passivation of junction termination in high power devices due to its excellent electrical, mechanical, and thermal properties. While the role of conductivity and polarization of the DLC layer on the blocking capability of a p-n junction has been explained recently, the thermal behavior still needs to be addressed. For this purpose, the diode leakage current was measured on large area power diodes with negative bevel coated by the DLC in a typical industrial range between 300 and 413 K. An unusual deviation from the expected Arrhenius law was experimentally observed. A predictive TCAD model, which incorporates the effect of the DLC layer, has been developed to study the impact of the DLC layer parameters on diode thermal performance. Both the electrostatic features and charge transport mechanisms through and along the DLC/Silicon interface have been modeled over a wide range of temperatures. Different DLC/Silicon doping combinations have been analyzed to explain the main effects determining the temperature dependence of diode leakage current and breakdown voltage. A complete validation of the TCAD approach has been achieved.

Laboratory measurements of initial launch velocities of electrostatically lofted dust on airless planetary bodies

Electrostatic dust lofting may play an important role in surface evolution of airless bodies as indicated from a number of unexplained observations, including the lunar horizon glow and dust ponding on asteroids, for example. The initial launch velocity of a charged dust particle determines its range of motion, hence it is a critical quantity to assess the dynamics of electrostatic dust transport. Here we report on laboratory measurements of the launch velocity of electrostatically lofted dust and its relationship with the dust size. For irregularly-shaped dust, similar-sized particles show a large velocity dispersion, indicating a large variation in the inter-particle cohesive force. The maximum launch velocity is shown to decrease with the increase in the dust radius and it is on the order of ~0.7 m/s for 15 μm radius particles. Derived from the patched charge model (Wang et al., 2016a. Geophys. Res. Lett. 43, 6103–6110), the theoretical expectation of an inverse velocity-size relationship of lofted dust is shown to be consistent with the maximum launch velocity measurements. The dust shape is found to have a significant effect on the lofting process, showing that irregularly-shaped particles are lofted with much larger velocities than spherical particles. The initial launch angle relative to the surface shows a broad distribution.

Gyrokinetic understanding of the edge pedestal transport driven by resonant magnetic perturbations in a realistic divertor geometry

Self-consistent simulations of neoclassical and electrostatic turbulent transport in a DIII-D H-mode edge plasma under resonant magnetic perturbations (RMPs) have been performed using the global total-f gyrokinetic particle-in-cell code x-point gyrokinetic code (XGC), in order to study density pump-out and electron heat confinement. The RMP field is imported from the extended magneto-hydrodynamics code M3D-C1, taking into account the linear two-fluid plasma response. With both neoclassical and turbulence physics considered together, the XGC simulation reproduces two key features of experimentally observed edge transport under RMPs: increased radial particle transport in the pedestal region that is sufficient to account for the experimental pump-out rate and suppression of the electron heat flux in the steepest part of the edge pedestal. In the simulation, the density fluctuation amplitude of modes moving in the electron diamagnetic direction increases due to interaction with RMPs in the pedestal shoulder and outward, while the electron temperature fluctuation amplitude decreases.

Charge characteristics (surface charge vs. zeta potential) of membrane surfaces to assess the salt rejection behavior of nanofiltration membranes

The charge characteristics (i.e., surface charge and zeta potential) of two different nanofiltration (NF) membranes were characterized by the potentiometric titration and electrophoresis methods under various experimental conditions, i.e., pH, ionic strength, ionic species, and organic fouled membranes to clarify the salt rejection mechanism of the NF membranes. The both charge characteristics of the NF membranes clearly represented the variation in the salt rejection behavior at different ionic strengths and for electrostatically fouled membranes. However, the surface charge of the NF membranes was independent of the ionic species and with the neutrally fouled membrane, whereas the zeta potential changed substantially with changes in the ionic species and the neutrally fouled membrane. Therefore, the variation in salt rejection behavior for different ionic species and with a neutrally fouled membrane was better described by the zeta potential values, which reflected the ion transport properties (i.e., ion valency and diffusivity) affecting the electrophoretic and electroosmotic flows at the shear plane of the membranes. These results indicate the electrophoresis method is considered as a promising tool for predicting the electrostatic transport of various ions at the shear plane of the membrane surfaces, intimately associated with the salt rejection behavior in the NF membrane processes.

Electrostatic dust transport on the terminator of atmosphereless bodies

Small dust particles may levitate over the surface of atmosphereless bodies, but to study electrostatic dust transport during a complete rotational period it is necessary to evaluate the surface potential at the terminator zone. We simulate the photo-electron layer and the dust charging process according to previously used models for dust levitation, but we propose a modification of the models to add the effects produced by surface roughness for low solar elevation angles. The correction for surface roughness on the model showed that a positive value for the surface potential and the electric field appear at the terminator. We found that submicron particles could be in stable levitation for small gravity fields. Our results also showed that the finest regolith grains can be lost into space due to the electrostatic repulsion.

Precise near-wall pH measurement in pressure-driven and electrically-driven flows using nanoscale laser-induced fluorescence imaging

The present study developed a near-wall pH measurement technique based on nanoscale laser-induced fluorescence to evaluate the spatio-temporal distributions of pH in the vicinity of a glass-solution interface. Fluorescein sodium salt, a pH-sensitive dye, was dissolved in working fluids and excited by an evanescent wave generated by total internal reflection of a laser beam. Near-wall pH distributions were determined based on a ratiometric calibration using a high-pH buffer solution (pH ⩾ 10) as a reference. The proposed technique was applied to both pressure-driven and electroosmotic flows to investigate the effects of interfacial potential and external electric field on proton concentration. The near-wall pH in pressure-driven flow was lower than the bulk pH by ~0.2 due to the local excess of protons attracted towards the negatively charged glass wall. This pH shift was especially noticeable in solutions with low ionic strength, which indicates that the near-wall pH is closely related to the thickness of the electric double layer. The pH shift in electroosmotic flow was smaller than that in pressure-driven flow by ~0.02 and decreased with electric field strength. In addition, a time-series measurement clearly visualized the temporal change in non-uniform pH distributions in electroosmotic flow at a spatial resolution of 5.2 × 5.2 µm. The measurement uncertainty was estimated to be 0.16 pH unit at 95% confidence level. These results demonstrated the feasibility of the present technique to evaluate spatio-temporal proton concentrations within the space 10–100 nm away from a solid–liquid interface. The proposed technique will therefore contribute to quantitative investigations into electrostatic interactions and ion transport in developing nanoscale electrochemical transport techniques and devices in, for example, biochemical analysis and water purification.

Numerical simulation of capture process of fine particles in electrostatic precipitators under consideration of electrohydrodynamics flow

In the present work, a numerical method was adopted to study the capture process of fine particles in two electrostatic precipitators (ESPs) with corrugated and parallel plates, respectively.11Electrostatic precipitators (ESPs) The simplified models were established to evaluate and predict the distribution of electrostatic field, gas dynamic, particle charging, and transport behavior. The electrohydrodynamic (EHD) flow with different applied voltage at various gas velocities was presented on the basis of two kinds of models, and the detailed influence on particle dynamic, including particle trajectory, migration velocity to collection plates, and particle capture had also been investigated thoroughly.22Electrohydrodynamic (EHD) Numerical results indicated that corrugated plate electrostatic precipitator (CP ESP),33Corrugated plate electrostatic precipitator (CP ESP) possessed greater electrostatic field characteristic than parallel plate electrostatic precipitator (PP ESP) and it also had a stronger capacity of resisting the influence of EHD flow, which was beneficial from the particular structure.44Parallel plate electrostatic precipitator (PP ESP) Besides, the EHD flow would make smaller particle present more apparent fluctuant trajectory near discharge wires when fine particles traveled in ESPs at a low gas velocity or with a high applied voltage, and that caused a more severe movement towards collection plates. Numerical results of the evolution of EHD flow showed that the faster elimination of ionic wind vortexes in CP ESP gave rise to higher collection efficiency. With the increment of gas velocity, the intensity of EHD flow exhibited a declining trend, and the difference of collection efficiency between two ESPs also decreased, which suggested that EHD flow had a significant effect on the <1 μm particle collection and the structure optimization could provide an available approach to evade the influence of EHD flow.

Impact of Bulk Trapping Phenomena on the Maximum Attainable Photovoltage of Semiconductor–Liquid Interfaces

In this work, we present a generic semiclassical approach for theoretically evaluating the impact of bulk trap states on the maximum attainable photovoltage in solar-assisted water-splitting reactions. Our method explicitly considers the charge contribution due to the capture and emission of conduction band electrons and valence band holes by trap states situated in the band gap, integrated within a self-consistent solution procedure for calculating electrostatic and charge transport properties. Utilizing the hematite photoanode as our model system, our approach reveals that bulk trap states may significantly degrade both the maximum attainable photovoltage and the concentration of mobile electrons (majority carriers) in the conduction band. These results suggest that bulk trap states can have two primary consequences in photoanodes. First, they may limit the degree to which a favorable cathodic shift in the photocurrent can be achieved (by lowering the maximum attainable photovoltage). Second, they may i...