Grid Aperture Research Articles

Characteristic and Mechanism Analysis of Aperture Barrel Erosion at Edge of Decelerator Grid of Three-Grid Ion Thrusters

To study the characteristics and mechanism of ion thruster decelerator grid aperture erosion at the edge, immersed finite element particle-in-cell (IFE-PIC) method combined with the sputtering erosion model was used to simulate the distribution of beam ions between grids. The characteristics and mechanism of ion thruster decelerator grid aperture barrel erosion at the edge were obtained. The results show that the direct impact of beam ions leads to polygonal erosion morphology. The reason for the uneven erosion depth in the circumferential direction is that the influence of the upstream sheath of the surrounding aperture on the edge aperture is too large, resulting in uneven distribution of beam ions. Hence, to avoid this kind of direct ion etching, the polygonal aperture is suggested at the edge of ion optics.

Optical emission from a ‘beam mode’ transparent cathode glow discharge

We investigate optical emission from a low-pressure ‘beam mode’ inertial electrostatic confinement glow discharge, created using cylindrical grid electrodes, having cathode inside a grounded, concentric anode. Spectral line distributions were measured in helium from this and a hollow cathode mode, with four lines emitted from singlet and triplet states with n = 3 and L = P and D compared against simple models for impact excitation by electrons and by ions and neutrals, suggesting much beam mode emission is caused by fast neutrals, and a significant component also caused by electrons. Beam mode emission occurs in a pattern of radial lines, that pass through aligned electrode apertures and extend across the chamber. Shadows cast by the electrode wires indicate electrically-neutral, beam-like sources of excitation, that cross the electrode assembly from bright radial lines in the opposing inter-electrode space, and calculated potential distributions show lensing surfaces around the anode grid apertures, focussing for inward-drifting ions. These indicate the emission pattern is caused by radially-convergent ion beams, and co-linear beams of fast neutrals produced in these. Emission was also observed with various alterations made to the electrode arrangement: with the anode removed, no beams were observed; with the cathode apertures rotated out of phase with those of the anode, bright radial beams between the electrodes followed the distribution of anode apertures; similar beams appeared with the grid cathode replaced by a solid version. In these experiments, progressive obstruction of pathways through the cathode caused increasing reduction in the beam pattern of emission beyond the anode radius, and in discharge perveance at similar pressure and voltage. Beam-like emission was also observed for a parallel-planar configuration, in which a cathode grid was held between two identical anode grids. The beam mode is shown to be a convergent, anode-focussed ion beam discharge, distinct to the star mode described elsewhere.

Numerical and theoretical modeling of the sheath upstream of ion optics: sheath structure transition and its effect on the beam divergence

The grasp of the sheath characteristics is fundamental to evaluate the extraction capabilities of ion optics and accommodate the wide application of ion sources. A one-dimensional theoretical model is developed to investigate the sheath upstream of ion optics as well as the matching relation between the ion optics and plasma, by simplifying and decomposing the Poisson’s equation at the outer surface and centerline of the grid aperture. The one-dimensional model is validated by 2D3V hybrid simulations which are also applied to visualize the sheath structure and ion beam divergence. With the increase of plasma density, it is found that the upstream sheath will transform gradually into a sheath near the plate electrode at first and then enter the screen aperture with a sheath edge approximately paralleling to the meniscus. Accordingly, the structure of the upstream sheath can be classified into four kinds which correspond to different beam divergence. The structure transition of the upstream sheath reflects the interaction between the extraction field and plasma, and the ion optics is considered to work at the matching point when the plasma is relatively balanced with the extraction field. Around the matching point, a small beam divergence angle can be achieved without the occurrence of over-perveance. Then a matching model is proposed based on the characteristics of the potential distribution at the matching point. It is verified to be effective of the model for quickly analyzing the ion beam divergence characteristics and determining an ideal operating range of the ion optics.

Electromagnetic-Polarization-Selective Composites with Quasi-1D Van der Waals Fillers: Nanoscale Material Functionality That Mimics Macroscopic Systems.

We report on the preparation of flexible polymer composite films with aligned metallic fillers composed of atomic chain bundles of quasi-one-dimensional (1D) van der Waals material, tantalum triselenide (TaSe3). The material functionality, embedded at the nanoscale level, is achieved by mimicking the design of an electromagnetic aperture grid antenna. The processed composites employ chemically exfoliated TaSe3 nanowires as the grid building blocks incorporated within the thin film. Filler alignment is achieved using the "blade coating" method. Measurements conducted in the X-band frequency range demonstrate that the electromagnetic transmission through such films can be varied significantly by changing the relative orientations of the quasi-1D fillers and the polarization of the electromagnetic wave. We argue that such polarization-sensitive polymer films with unique quasi-1D metallic fillers are applicable to advanced electromagnetic interference shielding in future communication systems.

3D-printing of a complete modular ion mobility spectrometer

Three-dimensional printing (3D-printing) is an asset for rapid prototyping. Therefore, this technology is applied in amateur, fundamental as well as in applied research communities. Moreover, the fused deposition modeling even allows a cheap and fast 3D-printing of functional filaments such as conductive or ferro-magnetic materials. Further applicable materials enable the production of electronic circuits, electrodes or even complete analytical sensors.Here, a 3D-printed stand-alone drift tube ion mobility spectrometer (IMS) completely manufactured in one process is presented. The ionization chamber, the Bradbury-Nielsen gate, the partly conductive drift tube, and the detector including an aperture grid and a Faraday plate were printed by dual extrusion 3D-printing. Therefore, non-conductive polylactic acid (PLA) was utilized for the housing and conductive PLA was used for the electrodes, the ion gate and the detector. Due to a magnetic quick lock system and a modular design, each part of this innovative ion mobility spectrometer can be exchanged in an instant. All parts were validated individually and as a whole by comparing them to an ion mobility spectrometer produced by computerized numerical control machining consisting of polytetrafluoroethylene (PTFE).To investigate the performance, parameters regarding the resolution, resolving power, signal-to-noise ratio, running-in period and long-term stability were used involving a set of isomeric volatile organic compounds (VOCs). Notably, every single 3D-printed component as well as the completely 3D-printed IMS could keep up with the traditionally produced ones. Furthermore, the signal intensity was comparable to the signal intensity of the PTFE-IMS in exemplary optimization steps. The resolution, resolving power and signal-to-noise ratio could be improved by the variation of the Bradbury-Nielsen gate design, thus demonstrating the potential of the 3D-printing for design optimization and rapid prototyping of analytical instrumentation.

The 6,500-H Life Test Results of 30 cm Diameter Ion Thruster

The life test of the 30 cm diameter ion thruster developed by the Lanzhou institute of physics began in Apr. 2018 and ended in Jan. 2020, lasting 6,500 h. This paper introduces the results of the 6,500-h life test of the 30 cm diameter ion thruster completed on the ground, including the ground facility for life test, the variations of the thrusters working performance, the times of the breakdown and power restart, and the erosion of the key components. The results show that the performance parameters, such as thrust, specific impulse and efficiency, do not change obviously during the test and magnetic field of the discharge chamber has no change. With the increase of test time, breakdown times increased significantly, whereas the power restart time decreased correspondingly. The diameter of the cathode orifice decreases gradually and there is a blockage risk of orifice. However, the diameter of the keeper orifice increases and presents an inverted cone erosion pattern. The diameter of the decelerator grid aperture expanded from 1.6 to 1.8 mm from 0 to 1,000 h and slightly enlarged after that. The aperture diameter of the accelerator grid presented linear enlargement but the pits-and-grooves erosion is obvious.

E × B electron drift current across the aperture of an ion source surrounded by a cusped magnetic field profile

In negative ion sources, a cusped magnetic field is generated by magnets placed around each aperture of the extraction grid in order to limit the co-extracted electron current. In spite of this suppression magnetic field, the co-extracted electron current is large, on the same order as the negative ion current extracted from the plasma. In this paper, we study the mechanisms of electron extraction from the plasma through a cusped aperture in a simplified situation, in the absence of negative ions, with the help of a three-dimensional Particle-In-Cell Monte Carlo Collisions model. The calculation results show that the electron current extracted from the plasma is small for an infinite slit aperture with a suppressed (cusped) magnetic field and significantly increases in the case of finite slit or circular grid apertures. We find that the E×B electron drift plays an important role in the extraction of electrons through a finite slit grid aperture and that current driven micro instabilities are present in the aperture region. This work is relevant to negative ion sources and micro-ECR neutralizers designed for space propulsion.

Calculation on 2 cm ECRIT Thrust Regulation Based on 3D PIC Numerical Simulation Method

Drag-free control is the key technology for space gravitational wave detection, which will be completed by miniature thruster. 2 cm Electron Cyclotron Resonance Ion Thruster (ECRIT) may be used for the drag-free control. Therefore, the 3D PIC method is used to estimate the thrust regulation of 2 cm ECRIT through simulating the property of ion extracting from the double-grid system. Through analyzing the thrust performance and grid focusing property under different grid apertures, the better grid structure is selected for the subsequent calculation. On this basis, the influence of grid voltage and the ion density on the performance of the thruster is analyzed. The result shows that reducing grid aperture can reduce the thrust, but it also affects the focusing property of the grid. Changing the ion density can adjust the thrust in a wide range. At given grid structure and ion density, there is a suitable grid accelerating voltage range to ensure the best ion focus characteristics. Considering the thrust performance and focusing characteristics, the grid structure with screen and accelerating grid aperture in 0.6 mm and 0.34 mm is selected. When the ion density is 1×1 017, 0.7×1 017, 0.4×1 017 and 0.2×1 017 m-3, the thrust can range from 5.05 to 141.44 μN through adjusting the grid accelerating voltage. This study can provide information for analyzing the possibility of the thruster application.

Lifetime Considerations for Electrospray Thrusters

Ionic liquid electrospray thrusters are capable of producing microNewton precision thrust at a high thrust–power ratio but have yet to demonstrate lifetimes that are suitable for most missions. Accumulation of propellant on the extractor and accelerator grids is thought to be the most significant life-limiting mechanism. In this study, we developed a life model to examine the effects of design features, operating conditions, and emission properties on the porous accelerator grid saturation time of a thruster operating in droplet emission mode. Characterizing a range of geometries and operating conditions revealed that modifying grid aperture radius and grid spacing by 3–7% can significantly improve thruster lifetime by 200–400%, though a need for explicit mass flux measurement was highlighted. Tolerance analysis showed that misalignment can result in 20–50% lifetime reduction. In addition, examining the impact of electron backstreaming showed that increasing aperture radius produces a significant increase in backstreaming current compared to changing grid spacing. A study of accelerator grid bias voltages revealed that applying a reasonably strong accelerator grid potential (in the order of a kV) can minimize backstreaming current to negligible levels for a range of geometries.

Influence of screen gird aperture diameter in outer region on performance of dual-mode ion thruster

To eliminate the erosion in the apertures at the edge of the decelerate grid of a dual-mode ion thruster three-grid optical system, a new method of reducing the diameter of the apertures in the outer region of the screen grid is proposed. In order to investigate the method of influencing the discharge loss and the uniformity of the beam current density and also reducing the aperture erosion region and erosion rate of the decelerate grid, two kinds of three-grid optical systems are designed and fabricated with the same material and physical parameters except the diameter of the apertures in the outer region of the screen grid. The first kind is that the diameter of the aperture in the outer region of the screen grid is equal to that in middle region of the screen grid, it is designated as the same aperture diameter grid optical system. The second kind of three-grid optical system is that the diameter of screen apertures whose center distance from the center of grid is larger than 0.95 times the beam radius, is reduced by 26% and therefore the physical transparency of optical system is reduced by 8.8%, it is designated as the small aperture diameter grid optical system. The comparison between the same aperture diameter grid optical system and small aperture diameter grid optical system is performed by assembling them into a 30-cm-diameter dual-mode ion thruster, and the ion thruster performance test, beam flatness test and 600-h-endurance test are conducted in the two typical operation modes, namely the low thrust-high specific impulse mode and large thrust-high power mode. A comparison of small aperture diameter grid optical system with the same aperture diameter grid optical system shows that the discharge loss of the 30-cm dual-mode ion thruster in the low thrust-high specific impulse mode and large thrust-high power mode are reduced by 10% and 21% respectively, the beam flatness is decreased by 3% and 10% respectively, the number of rows of the apertures which are sputtered by beam ions at the edge of the decelerate grid is reduced from 5 to 1. In addition, the erosion rate is significantly reduced and the erosion phenomenon disappears after 900-h-long-duration operation. These results signify that reducing the screen grid aperture diameter in the outer region is an effective method to eliminate aperture erosion at the edge of the decelerate grid of a dual-mode ion thruster three-grid optical system and that this method will not reduce the efficiency of the thruster, but cause the beam current uniformity to worsen.

Metal ion filtering of vacuum arc ion source through an inclined-aperture extraction grid

The paper reports a novel method of increasing the fraction of H+ ions produced by vacuum arc ion sources with metal hydride cathodes, which applies the ionic selectivity of an inclined-aperture extraction grid to separate and filter heavy metal ions. A simple theoretical model is built to describe the filtering mechanism. Since H+ ions and Tin+ (n = 1 ∼ 3) ions produced by vacuum arc discharge have a great difference in the ratio of charge state and kinetic energy, H+ ions are easy to pass through the inclined-aperture grid, while most of the Ti ions are blocked and absorbed by the grid wall. Using a 2D particle-in-cell simulation, the ionic selectivity of an inclined-aperture extraction grid is demonstrated. The numerical simulation results show that after ion filtering through the extraction grid, the fraction of H+ ions is increased from 39% to more than 80%. The increased amplitude of H+ ion fraction depends on the thickness of the grid and the angle of grid apertures. In addition, an extraction grid with different aperture angles for the extraction of divergent vacuum arc plasma is proposed.

Influence of the Decelerator Grid on the Optical Performance of the Ion Thruster

In order to reduce the erosion of the ion thruster accelerator grid, which is caused by charge-exchange (CEX) ions, the 2-grid optical system is added to a decelerator grid to block the reflux CEX ions. The previous experiment and simulation results have proven that the decelerator grid can effectively reduce the Pit and Groove erosion. However, the influence of the decelerator grid on the optical performance has not yet been studied well. In this paper, a three-dimensional Immersed Finite Element Method-Particle in Cell-Monte Carlo Collision (IFE-PIC-MCC) algorithm was adopted to investigate the effect of the decelerator grid on the optical performance under crossover and normal circumstances. Results show that the decelerator grid has no effect on the focusing state and the distribution of beam ions. It also has little effect on the CEX ions from the upstream and extraction (center) regions. However, it has great influence on the downstream CEX ions. When the upstream plasma number density is small, the decelerator grid will cause most of the downstream reflux CEX ions to impinge on the accelerator grid aperture barrel, resulting in the significant increase of the Barrel erosion of the accelerator grid. With the increase of the upstream plasma number density, the downstream reflux CEX ions tend to impact the downstream surface of the decelerator grid, which means the decelerator grid begins to block the downstream backflow of CEX ions.

System design for a "checkerboard" imager.

In order to solve the problem of uneven spatial frequency sampling and lack of zero frequency sampling in the segmented planar imaging detector for electro-optical reconnaissance (SPIDER), we propose a "checkerboard" imaging system design using a square grid aperture arrangement, show its aperture matching method, and demonstrate that it can achieve the uniform sampling of all integer multiples of fundamental frequency in the highest frequency range with zero frequency in the direction of two orthogonal arrays. Through computer simulation, it is verified that the imaging effect of the "checkerboard" imaging system is superior to that of the SPIDER imaging system and the traditional single aperture imaging system at the same system diameter.

Coupling of a High-Resolution Ambient Pressure Drift Tube Ion Mobility Spectrometer to a Commercial Time-of-flight Mass Spectrometer.

Ion mobility spectrometry provides information about molecular structures of ions. Hence, high resolving power allows separation of isomers which is of major interest in several applications. In this work, we couple our high-resolution ion mobility spectrometer (IMS) with a resolving power of Rp = 100 to a time-of-flight mass spectrometer (TOF-MS). Besides, the benefit of an increased resolving power such an IMS-MS also helps analyzing and understanding the ionization processes in IMS. Usually, the coupling between IMS and TOF-MS is realized by synchronizing data acquisition of the IMS and MS resulting in two-dimensional data containing ion mobility and mass spectra. However, due to peak widths of less than 100μs in our high-resolution IMS, this technique is not practicable due to significant peak broadening during the ion transfer into the MS and an insufficient data acquisition rate of the MS. Thus, a novel but simple interface between the IMS and MS has been designed which minimizes ion losses, allows recording of ion mobility at full IMS resolving power, and enables a shuttered transmission of ions into the MS. The interface is realized by replacing the Faraday plate used in IMS by a Faraday grid that is shielded by two additional aperture grids. For demonstration, positive product ions of benzene, toluene, and m-xylene in air are investigated. The IMS is equipped with a radioactive 3H source. Besides the well-known product ions M+ and M·NO+, a dimer ion is also observed for benzene and toluene, consisting of two molecules and three further hydrogen atoms. Graphical Abstract ᅟ.

Modeling of negative ion extraction from a magnetized plasma source: Derivation of scaling laws and description of the origins of aberrations in the ion beam

We model the extraction of negative ions from a high brightness high power magnetized negative ion source. The model is a Particle-In-Cell (PIC) algorithm with Monte-Carlo Collisions. The negative ions are generated only on the plasma grid surface (which separates the plasma from the electrostatic accelerator downstream). The scope of this work is to derive scaling laws for the negative ion beam properties versus the extraction voltage (potential of the first grid of the accelerator) and plasma density and investigate the origins of aberrations on the ion beam. We show that a given value of the negative ion beam perveance correlates rather well with the beam profile on the extraction grid independent of the simulated plasma density. Furthermore, the extracted beam current may be scaled to any value of the plasma density. The scaling factor must be derived numerically but the overall gain of computational cost compared to performing a PIC simulation at the real plasma density is significant. Aberrations appear for a meniscus curvature radius of the order of the radius of the grid aperture. These aberrations cannot be cancelled out by switching to a chamfered grid aperture (as in the case of positive ions).

Experimental study on pull-out response of geogrid embedded in sand

Geosynthetics have been widely employed to reinforce the soils over the last two decades. Pullout resistance and interface friction resistance of the geogrids are crucial design parameters for some geotechnical applications. In this study, the effects of pull-out resistance of geogrid material embedded in sand soil was examined by using simple shear test set-up with a little modification. Thus, a series of experiments carried on sand soil with different relative densities and gradations. In the tests, poor graded sand was reinforced with geogrid. The reinforcement material has 0.85 mm thickness and 1 cm grid aperture. It can be inferenced from experiments that behavior of pull-out resistance had resemblance to classical load–displacement behavior of soil. Classically when displacement increases pull-out resistance increases until a certain displacement but after a certain pull-out resistance when displacement increases pull-out resistance decreases. The tests showed that, the increase in relative density leaded to increase in pull-out load. Due to the interlocking of soil grains with geogrid gaps, interface friction between soil and geogrid was also observed greater than internal friction of soil.

Modeling of shear failure in fractured reservoirs with a porous matrix

A finite volume-based numerical modeling framework using a hierarchical fracture representation (HFR) has been developed to compute flow-induced shear failure. To accurately capture the mechanics near fracture manifolds, discontinuous basis functions are employed which ensure continuity of the displacement gradient across fractures. With these special basis functions, traction and compressive forces on the fracture segment can be calculated without any additional constraints, which is extremely useful for estimating the irreversible displacement along the fracture (slip) based on a constitutive friction law. The method is further extended to include slip-dependent hydraulic aperture change and grid convergent results are obtained. Further, the change in hydraulic aperture is modeled using an asymptotic representation which respects the experimentally observed behavior of pore volume dilation due to shear slip. The model allows the initial rapid increase in hydraulic aperture due to shear slip and asymptotically approaches a finite value after repeated shearing of a fracture segment. This aperture increase is the only feedback for mechanics into the fluid flow for a linear elastic mechanics problem. The same model is also extended to include poroelastic relations between flow and mechanics solver. The grid convergence result in the case of poroelastic flow-mechanics coupling for flow-induced shear failure is also obtained. This proves the robustness of the numerical and analytical modeling of fracture and friction in the extended finite volume method (XFVM) set-up. Finally, a grid convergent result for seismic moment magnitude for single fracture and fracture network with random initial hydraulic and friction properties is also obtained. The b-value, which represents the slope of seismic moment occurrence frequency decay vs seismic moment magnitude, which is approximately constant in a semi-logarithmic plot, is estimated. The numerical method leads to converged b-values for both single fracture and fracture network simulations, as grid and time resolutions are increased. For the resulting linear system, a sequential approach is used, that is, first, the flow and then the mechanics problems are solved. The new modeling framework is very useful to predict seismicity, permeability, and flow evolution in geological reservoirs. This is demonstrated with numerical simulations of enhancing a geothermal system.

A study of the geogrid–subballast interface via experimental evaluation and discrete element modelling

This paper presents a study of the interface of geogrid reinforced subballast through a series of large-scale direct shear tests and discrete element modelling. Direct shear tests were carried out for subballast with and without geogrid inclusions under varying normal stresses of $$\sigma _n =6.7$$ to $$45\hbox { kPa}$$ . Numerical modelling with three-dimensional discrete element method (DEM) was used to study the shear behaviour of the interface of subballast reinforced by geogrids. In this study, groups of 25–50 spherical balls are clumped together in appropriate sizes to simulate angular subballast grains, while the geogrid is modelled by bonding small spheres together to form the desired grid geometry and apertures. The calculated results of the shear stress ratio versus shear strain show a good agreement with the experimental data, indicating that the DEM model can capture the interface behaviour of subballast reinforced by geogrids. A micromechanical analysis has also been carried out to examine how the contact force distributions and fabric anisotropy evolve during shearing. This study shows that the shear strength of the interface is governed by the geogrid characteristics (i.e. their geometry and opening apertures). Of the three types of geogrid tested, triaxial geogrid (triangular apertures) exhibits higher interface shear strength than the biaxial geogrids; and this is believed due to multi-directional load distribution of the triaxial geogrid.

Response of H− ions to extraction field in a negative hydrogen ion source

In order to investigated the dynamics of H− ions and understand the extraction process inside filament-arc-driven plasmas in a Cs-seeded negative ion source, diagnostic experiments using a directional Langmuir probe combined with photodetachment measurement have been conducted. Two-dimensional flow pattern of H− ion flow has been obtained as well as the profile of H− ion density. The result shows that H− ions come from the direction of plasma grid surface, flow to the plasma in the extraction region. During beam extraction, H− ion flow turns to the beam direction. The stagnation point of H− ion flow is at ∼20mm apart from the plasma grid. Around the stagnation point, the maximum decrement of H− density caused by beam extraction appears. The results indicate that H− ions are extracted from the plasma volume. The extraction process occurs in a region near the plasma grid aperture with boundary of ∼20mm apart from the plasma gird.

Assessment of geogrids in gravel roads under cyclic loading

Performance of geogrid in gravel roads subjected to repeated loads was investigated through laboratory testing in the laboratory of faculty of engineering at Shoubra and finite element analysis. Twenty two laboratory model tests under cyclic loading were conducted on road sections consisting of base course layer with and without geogrid reinforcement overlaying weak subgrade. Parameters investigated included base layer thickness, grid aperture size, geogrid tensile strength, number of geogrid layers, and geogrid location. The experimental results indicated that the inclusion of one geogrid sheet placed at the base of course layer reduced the vertical deformation by about 18 to 64% depending on the base course layer thickness. The vertical deformation depth increased rapidly during the initial load cycles; thereafter the rate of settlement is reduced as the number of loading cycles increased. The most effective location of geogrid was found to be in the top quarter of the base course layer. When the results of the laboratory tests were compared with the analytical solution using finite element program ABAQUS, the FE results were in good agreement with the experimental test results.