Grid Gap Research Articles

Beam Optics Analysis for the 120-keV Multiaperture Accelerator of Neutral Beam Injector

Neutral beam injection is an important auxiliary heating method for magnetic confinement nuclear fusion. To further optimize the steady-state operation of the high-temperature plasma in the Experimental Advanced Superconducting Tokamak (EAST), a higher beam energy is required for the high-power, long-pulse neutral beam injection system. The most critical technology to achieve this goal is the upgrade of the ion source accelerator. This paper analyzes the beam optics of the multiaperture accelerator for the extraction of a deuterium ion beam of 120 keV/50 A with a divergence angle of less than 1 deg root-mean-square. The effects of the number of grids, shapes of plasma grids (PGs), grid gaps, voltage ratio, and plasma parameters on beam optics were assessed by numerical simulation. The results allowed optimization of the PG shape; they also indicated that the tetrode configuration can reduce the beam divergence angle effectively while the triode configuration can extract a higher current. These conclusions provide guiding significance for the selection and parameter design of the EAST neutral beam injector ion positive source with a 120-keV accelerator.

Geant4 Simulation of Scatter Radiation Removal: Comparison and Validation of Anti-scatter Grid and Air Gap for X-ray Mammography

Abstract: X-ray mammography modality provides excellent low-contrast resolution images with low scatter radiation, making it the gold standard in diagnosing breast cancer. Anti-scatter grid and air gap techniques are typically used to further minimize the scatter radiation and improve image quality. Thus, Geant4 simulation was used to investigate the effectiveness of these techniques in removing scatter radiation in X-ray mammography. The effectiveness of an anti-scatter grid was evaluated using the Bucky factor, where it linearly increased with increasing the anti-scatter grid ratio. It was found that increasing the grid frequency affects the Bucky factor depending on the design of the grid ratio. This research proved that designing an anti-scatter grid with high grid frequency (80 lp/mm), low grid ratio (2:1), and proper orientation minimized common anti-scatter grid artifacts. The effectiveness of the air gap technique was also evaluated using the air gap dose factor. It increased non-linearly with increasing magnification. This research validated that using smaller pixel sizes and small focal spot sizes improved spatial resolution with magnification. Our simulation validated that the anti-scatter grid and air gap were effective techniques in removing scatter radiation. By comparing these techniques, the anti-scatter grid was more effective in removing scatter radiation at the expense of increasing the radiation absorbed dose with the exception of 2.0 magnification. It’s recommended to be extremely cautious when using 2.0 magnification or a grid ratio higher or equal to 8:1. These parameters may cause the radiation absorbed dose to be increased by several folds. Keywords: Geant4, Gate, X-ray mammography, Scatter removal, Anti-scatter grid, Air gap.

Characteristics of the pressure profile in the accelerator on the RF negative ion source at ASIPP

Neutral beam injection (NBI) systems based on negative hydrogen ion sources—rather than the positive ion sources that have typically been used to date—will be used in the future magnetically confined nuclear fusion experiments to heat the plasma. The collisions between the fast negative ions and neutral background gas result in a significant number of high-energy positive ions being produced in the acceleration area, and for the high-power long-pulse operation of NBI systems, this acceleration of positive ions back to the ion source creates heat load and material sputtering on the source backplate. This difficulty cannot be ignored, with the neutral gas density in the acceleration region having a significant impact on the flux density of the backstreaming positive ions. In the work reported here, the pressure gradient in the acceleration region was estimated using an ionization gauge and a straightforward 1D computation, and it was found that once gas traveled through the acceleration region, the pressure dropped by nearly one order of magnitude, with the largest pressure drop occurring at the plasma grid. The computation also revealed that the pressure drop in the grid gaps was substantially smaller than that in the grid apertures.

Variable grid gap for a wider operational space of NBI-heating at ASDEX upgrade

A novel extraction system with variable grid gap has been developed for the Neutral Beam Injection (NBI) at ASDEX Upgrade to be able to adjust beam energy and power independently.The ASDEX Upgrade NBI is composed of two beamlines (one operating at 60 kV, one at 93 kV) with four ion sources each, delivering a total heating power up to 20 MW. To achieve low beam divergence which reduces transmission losses in the duct, the system has to be operated, like any NBI, at the perveance optimum Πopt which is proportional to Iext/Uext3/2, (Iext = extracted current, Uext = extraction voltage). The extracted beam power Pext = Iext Uext = Πopt Uext5/2 has therefore a strong dependence on the extraction voltage. If beam energies lower than the nominal values are required, the extracted current has to be reduced accordingly, leading to a drastic reduction of the heating power.The optimum perveance Πopt is specific for each extraction system and defined by its geometry, approximately by the ratio of the grid aperture diameter to the gap between the first and the second grid (extraction gap).We have developed an extraction system with a “variable gap“ between those grids which can be changed in situ and remotely between shots. Varying this gap modifies Πopt which allows to set beam energy and beam current independently in a wide range while maintaining minimum divergence. This extends the operational space of NBI significantly and adds experimental versatility for ASDEX Upgrade.A conventional extraction grid system was modified while maintaining interchangeability with the existing systems and keeping the impact on beam optics minimal. The paper discusses the consequences of such variable gap for beam optics and describes the technical boundary conditions, the final design and prototype tests.

A study of the influence of different grid structures on plasma characteristics in the discharge chamber of an ion thruster

The grid structure has significant effects on the discharge characteristics of an ion thruster. The discharge performances of a 30 cm diameter ion thruster with flat, convex and concave grids are studied. The analysis results show that the discharge chamber with a convex grid has a larger ‘magnetic-field free area’ than the others, and the parallelism of the magnetic-field isopotential lines and anode is generally the same in the three models. Plasma densities of the three structures at the grid outlet are in the range of 3.1 × 1016–6.9 × 1017 m−3. Along the thruster axis direction, the electron temperature in the chamber with the convex and concave grids is in the range of 3.3–3.5 eV, while that with a flat grid is lower, in the range of 3.1–3.5 eV. In addition, the convex and the concave grids have better uniform distribution of electron temperature. Moreover, the collision frequency ratios show that the axial degree of ionization of the three models is the highest, and the flat grid has the highest discharge efficiency, followed by the convex grid and the concave grid is the least efficient. The test and simulation results of the 30 cm diameter ion thruster with the convex grid show that the measurement and calculation results are 3.67 A and 3.44 A, respectively, and the error above mainly comes from the ignorance of the doubly charged ions and parameter settings in the model. The comparison error between the simulation and measurement of beam current density is mainly caused by the actual thermal deformation of the grids during the discharge process, which leads to the change in electric potential distribution and variation of the focusing characteristics of the grids. Upon consideration of discharge performance and the thermal grid gap variation, it can be concluded that the flat and concave grids are more suitable for small-diameter ion thrusters, while the convex grid is a more reasonable choice for the higher-power and larger-diameter thrusters.

Design and performance test of discharge chamber and grid for multi-mode ion thrusters

In view of the application requirements of electric propulsion system for China’s asteroid deep space exploration mission, the overall scheme is designed, an ion thruster prototype model is established by using a four-ring-cusp field discharge chamber, 30-cm beam current extraction diameter three-grid ion optics system. Reasonableness and compatibility of discharge chamber and grid design are verified experimentally and theoretically . The test results are shown below. The ion thruster can operate steadily over an input power envelope of 277–3120 W, thrust increases linearly from 9.9 to 117.2 mN, specific impulse rises from 1269 to 3492 s, the beam divergence angle drops from 30.7° to 26.8° and stabilizes above a certain power value, the thrust vector angle is less than 1.5° and beam flatness parameter is greater than 0.75 at different operating points. The maximum percentage reduction in grid gap aberration is 90% with the strain relief molybdenum mounting ring thermal design. This research provides a reference for multi-mode ion thruster design and in-orbit engineering applications.

Electric supplementary heating technology of municipal heating for city heat exchange station

Municipal heating in northern areas generally suffers from heat sources, heat grid gaps, and high dependence on fossil fuels. It is difficult to guarantee the heating effect of incremental buildings at the end of the building, and relying on traditional capacity expansion methods has caused a waste of social resources to a certain extent. A “no capacity increase” municipal heating electric supplementary heating technology for urban heat exchange stations is proposed. First, the design concept of the electric supplementary heating system is explained. Then, the investment calculation model of the electric supplementary heating system is established. Finally, an office building in Shijingshan, Beijing is used as a research example to perform simulation calculations.

Accelerator Grid Life Modeling of T6 Ion Thruster for BepiColombo

The T6 ion thruster is providing in-space propulsion for the BepiColombo mission, currently en route to Mercury. Simulations of the accelerator grid erosion during flight operation were carried out using the CEX2D and CEX3D codes in order to verify that the grid lifetime will be sufficient to impart the 5.75 total impulse per thruster required for the mission. Before calculating in-space erosion rates, the models were benchmarked against data from long-duration wear tests. Two-dimensional simulations tracked the time-dependent grid geometry and calculated changes in the electron backstreaming limit as the grids eroded. Sensitivity studies assessed the impact of uncertainty in the operating screen-to-accelerator grid gap on . Three-dimensional simulations employed a particle-in-cell model for the charge exchange ions and were able to accurately reproduce the nonaxisymmetric pits and grooves erosion pattern observed on the downstream face of the accelerator grid following ground testing. Charge exchange ion space charge effects were found to broaden the pit erosion features in flight, further reducing the peak erosion rate relative to that observed on the ground. The simulations predicted that T6 will have sufficient margin against both electron backstreaming and accelerator grid wearthrough over the duration of the mission.

Studying the Influence of some Parameters of Opening and Cleaning Machines on Cleaning Efficiency (Dept.T)

The aim of this work is studying the influence of some parameters of blowroom opening and cleaning machines on cleaning efficiency. Erect of grid gap width on waste and good fibre elimination as well as cleaning efficiency is studied for opening and cleaning machines and for blowroom line. Factorial analysis were carried out to determine if the variation in cleaning efficiency is significant or by accident. For bale opener the variables were cotton type (trash content), grid gap width and spiked lattice and evener roller selling with interaction 2 x 3 x 5. Also for cleaner RN factorial analysis were carried out for grid gap width and seed roller speed with interaction 4 x 3. Results showed that trash content and grid gap width as well as the other variables have a significant effect on cleaning efficiency. Every elimination of foreign matter is accompanied by a simultaneous elimination of good fibres. For opening and cleaning machines, as grid gap width increased trash and good fibres eliminated increased, but the rate of good fibres climination is uglier than the rate of trash climination. For blowroom line increasing grid gap width of opening and cleaning machines from narrow to open Ievel raised the cleaning efficiency from 43% to 56% and good fibres climinated increased by 30%.

Grid Gap Variation of Ion Thruster During Startup in Orbit

The grids play a central role in ion thrusters and considerably affect both the performance and lifetime. In this article, the primary goal is to study the variation of the grid gap during the startup process of an ion thruster in orbit. Therefore, we established a detailed model of ion thrusters using ANSYS. We have carried out several experiments to valid both thermal and mechanical performances of the model. The results reveal that the temperature of the ion thruster in orbit periodically varies. The grid gap between the screen grid and the accelerator grid decreases at first and then increases during the startup process, in which the minimum depends on the initial temperature and the thermal control. Higher temperature and more thermal control will raise the minimum gap. To launch a 30-cm-diameter ion thruster from the lowest temperature point and ensure a safe gap, 50 °C thermal control would do a great help.

Evaluation of mobile 3D light detection and ranging based canopy mapping system for tree fruit crops

In this study, 3D light detection and ranging (LiDAR) and an inertial measurement unit (IMU) were integrated on a ground vehicle for mapping tree fruit crops. A custom interface was developed in robot operating system for synchronous communication with hardware modules and for continuous field data collection. Point cloud data processing methods were developed for reconstruction and estimation of canopy parameters including height, voxel grid and convex hull methods-based volume and gap fractions in the canopy. The system was tested in apple tree and grapevine canopies. Overall, the mobile 3D LiDAR mapping system provided realistic representation of the canopy compared to the manual methods. For apple trees, the manual canopy volume measurements were strongly correlated to volume derived from the mobile 3D LiDAR mapping system (Convex hull: r = 0.81, Voxel grid: r = 0.51). The voxel grid method adequately considered gaps in the canopy during volume estimation and performed better than the convex hull method. The system was also evaluated for estimating canopy growth in grapevines with different rates of subsurface irrigation treatments. Deficit irrigation treatments did not show any significant effect on the canopy growth due to a high moisture content in the soil, resulting from high winter snowpack prior to that particular season. Nonetheless, the 3D LiDAR mapping system was able to aid in visualization of the temporal changes in canopy growth during the growing season. Change in vine canopy volume for the treatments followed a similar trend to the area ratio estimated from normalized differential vegetation index images derived from small unmanned aerial system based multispectral imagery. Overall, the 3D LiDAR based canopy mapping system and pertinent data mining algorithms can be the useful tool to the growers in rapid assessment of perennial fruit crop canopies for real-time management decision making.

Dynamic characteristics of beam perveance under thermal deformation for a three-grid ion thruster

In this study, the dynamic characteristics of beam perveance under thermal deformation in a three-grid ion thruster are analyzed and discussed by a combination of theory and experiment. Beam extraction in the apertures of the grids is simulated by the particle-in-cell–Monte Carlo collision method. The influences of thermal deformation on ion density thresholds in underperveance and overperveance conditions are analyzed and calculated. Finally, a verification experiment is performed. The simulated and experimental results show the gradually increasing underperveance threshold but decreasing overperveance threshold with the grid gap widening. Unlike under normal perveance in the cold state, the extracted beam exhibits a local anomalous perveance under thermal deformation with overperveance or underperveance occuring in the middle or edge area, respectively. The simulation and experimental results agree well with a maximum relative error of less than 4.1%. This work offers a reference for uniformity of optimization of beam density for ion thrusters.

Extractor Grid Effects on Beam Characteristics of Dual-Stage Ion Thruster

High thrust density per thruster area is the most desired achievement in current ion thruster designs. Dual-stage ion thruster concept is introduced to improve the ion beam focusing capability of single-stage thrusters in order to maximize the achievable thrust density for a fixed thruster area. In this paper, a two-grid ion thruster is converted to a dual-stage one by adding an extractor grid. Then, improvement of ion beam divergence characteristic is numerically investigated through the parametric study of extractor grid's geometrical and operational variables by a hybrid-particle-in-cell method. The sensitivity analysis shows that the beam divergence angle is very sensitive to acceleration-to-extraction voltage difference ratio. The effect of acceleration-to-extraction grid gap ratio is also significant. The initial dual-stage model is modified based on this result. The minimum divergence angle for the modified model is about 3.38° resulting in 70% and 60% improvement against the minimum divergence angle of the two-grid single-stage and the initial dual-stage numerical models, respectively.

IEDF distortion and resolution considerations for RFEA operation at high voltages

Retarding field energy analyzers (RFEA) have been used extensively to measure the ion flux energy distribution function of plasmas. For consistency, the ion flux energy distribution function is referred to as the ion energy distribution function (IEDf) even though it more accurately represents the one-dimensional ion velocity distribution function. In the past, these devices have operated at voltages less than 1 kV. Higher operating voltages (>2 kV) are currently desired. For an RFEA to operate at these voltages, design changes are necessary that impact the energy resolution and cause space charge build-up. To investigate the effect the design changes have for a high voltage RFEA, electromagnetic simulations and particle-in-cell (PIC) simulations were used to analyze the electric field between the grids, the potential drop in the grid holes, and space charge build-up between the grids. Non-unique optimized dimensions for the RFEA increased the electric field uniformity. The optimization minimizes the electric field from distorting the IV curve or adversely affecting the energy resolution. It was found that a larger grid gap distance and smaller grid hole diameter decreases the potential drop in the grid holes improving energy resolution. IV curves from the PIC simulation were used to obtain space charge distorted IEDfs. The point at which space charge distorts the IV curve is dependent on the grid gap distance and incoming flux. Space charge build-up was found to only affect low energy ions which manifested by cutting off the low energy portion of the IEDf. To fix space charge distortions, the flux into the probe can be limited or it may be possible to account for the distortion when calculating the IEDf.

Feasibility Study on Numerical Life Qualification of Ion Thruster^|^apos;s Ion Optics Using the JIEDI Tool

The JIEDI (JAXA’s Ion Engine Development Initiative) tool has been developed to assess the ion acceleration grid erosion of an ion thruster. Validation of the JIEDI tool and sensitivity analysis of input parameters in the JIEDI tool were conducted in this paper. Comparison with the simulation results by the JIEDI tool and the endurance test results showed that the full lifetime test of μ10 prototype model was successfully reproduced by the JIEDI tool. Through the sensitivity analysis, sticking factor is most sensitive to estimate the accelerator grid erosion and electron backstreaming time. In the worst case of grid erosion, which is without re-deposition, the uncertainty in neutral mass flow rate through grid holes is important to estimate the accelerator grid erosion. 30% change in the neutral mass flow rate, which is caused by 6% change in propellant utilization efficiency, corresponds to about 20% change in the accelerator grid mass loss and in the increasing rate of minimum potential on the axis. In contrast to the accelerator grid erosion, the uncertainties in discharge voltage and grid gap, which affect the trajectory of mainstream ions, are important to estimate decelerator grid erosion. 40% change in discharge voltage corresponds to about 90% change in the decelerator grid mass loss.

Estimation of Ion Beam Trajectories for a Slot Aperture Accelerator of Long-Pulse Ion Source in Neutral Beam Injector

A neutral beam injection (NBI) system has been developed and is being tested for an Experimental Advanced Superconducting Tokamak (EAST) device. The NBI system needs to be employed for an auxiliary heating and current drive of EAST plasmas. The first long pulse ion source (LPIS-1) has been installed in the neutral beam test bed (NBTB) system, and the performance is being tested in the NBTB. The LPIS-1 consists of a magnetic bucket plasma generator with multipole cusp-fields and a set of tetrode accelerators with slit-type apertures (a transparency of 60%). The ion beam trajectories of the accelerator column are estimated for the LPIS-1, including an original structure, with the change of slit aperture distance, plasma grid shape, grid gap distance, and voltage ratio between a plasma grid and a gradient grid using the IGUN code. This kind of calculation for the ion beam trajectory may be useful for the estimation of beam extraction characteristics and the direction of accelerator upgrade or modification, prior to the experiments of ion beam extraction.

Design formulae of unsymmetrical double-gap output cavities of klystrons

A new perspective is proposed that unsymmetrical double-gap output cavities of klystrons will have greater energy conversion efficiency of beam to microwave than the conventional symmetrical double-gap output cavities. The equivalent circuit method is used to get the relationships of the two gap electric fields in π-mode and 2π-mode unsymmetrical double-gap cavities in the case that the two gaps of the unsymmetrical double-gap cavities are approximately regarded as parallel grid gaps. The energy conversion efficiency in the process of an ideal bunching beam passing through the π-mode or 2π-mode unsymmetrical double-gap cavity is calculated by simulating the ideal cluster beam with a succession of periodically repeated electrons; then, the formulae of the optimal gap width, the optimal length of the drift tube between two gaps and the optimal gap voltage amplitude corresponding to the maximal conversion efficiency are obtained, which can be used to design π-mode and 2π-mode unsymmetrical double-gap output cavities of klystrons.

Electron Backstreaming Determination for Ion Thrusters

Electron backstreaming in ion thrusters is caused by the random flux of beam electrons past a potential barrier established by the accel grid. A technique that integrates this flux over the radial extent of the barrier reveals important aspects of electron backstreaming phenomena for individual beamlets, across the thruster beam, and throughout thruster life. For individual beamlets it was found that over 99% of the electron backstreaming occurs in a small annulus at the center of the beamlet that is less than 20% the area of the beamlet at the potential barrier established by the accel grid. For the thruster beam it was found that over 99% of the backstreaming current occurs inside of r = 6 cm for the over 28 cm diameter NSTAR grid. Initial validation against ELT data shows that the technique provides the correct behavior and magnitude of electron backstreaming limit, V(sub ebs). From the sensitivity analyses it is apparent that accel grid chamfering may be the dominant mechanism contributing to the sharp rise in the absolute value of V(sub ebs) observed in the ELT but does not explain the rise in ion transparency. Grid gap change also contributes to the absolute value of V(sub ebs) rise and large rises in ion transparency with thruster life for the center gridlet. Screen grid erosion contributes generally to rises in the absolute value of V(sub ebs) and ion transparency, but for the assumptions used herein, it appears to not have as much of an effect chamfering or grid gap change. Overall, it is apparent that accel grid chamfering, grid gap change, and screen grid erosion are important to the increase in electron backstreaming observed during the ELT.

The efficiency of a pitron

A technique is proposed for the calculation of the electron efficiency of a pitron, a self-oscillator that is based on a double-gap cavity and operates on the π-mode oscillations. For six generation zones, the parameters of grid gaps are optimized to ensure the maximum efficiency. Recommendations on the recalculation of the grid-gap parameters to the parameters of gridless gaps are formulated. It is shown that the total electron efficiency grows with the zone number from 14% for the zero zone to 65% for the fifth one. For practical application, it is recommended to employ the first and second zones with an electron efficiency of about 50% and 60%, respectively.

Shock wave interaction with porous plates

Previous detailed studies of the interaction of a shock wave with a perforated sheet considered the impact of a shock wave on a plate with regularly spaced slits giving area blockages of 60 and 67%, at various angles of incidence, and resulting in both regular and Mach reflection. The current work extends this study to a much wider variety of plate geometries. Blockage ratios of 20, 25, 33, 50, and 67 and inclinations of 45, 60, 75, and 90° to the shock wave were tested. Four different thicknesses of plate were tested at the same frontal blockage in order to assess the effects of gap guidance. Tests were conducted at two shock Mach numbers of 1.36 and 1.51 (inverse pressure ratios of 0.4 and 0.5). It is found that secondary reflected and transmitted waves appear due to the complex interactions within the grid gaps, and that the vortex pattern which is generated under the plate is also complex due to these interactions. The angle of the reflected shock, measured relative to the plate, decreases with plate blockage and the angle of inflow to the plate reduces with increasing blockage. By analysing the flow on the underside of the plate the pseudo-steady flow assumption is found to be a reasonable approximation. Both the pressure difference and the stagnation pressure loss across the plate are evaluated. It is found that over the range tested the plate thickness has a minimal effect.