Grid Aperture Research Articles

High-Resolution Drift Tube Ion Mobility Spectrometer with Ultra-Fast Polarity Switching.

Besides safety and security applications, ion mobility spectrometry (IMS) is increasingly used in other fields such as medicine, environmental monitoring and food quality analysis. However, some applications require gas chromatographic separation before analysis by IMS. Furthermore, different compounds in the sample may form positive or negative ions during ionization and therefore simultaneous detection of both ion polarities is highly beneficial to avoid two chromatographic runs of the same sample. This can be achieved by ultra-fast polarity switching of a single drift tube IMS, allowing for quasi-simultaneous detection of both ion polarities. By using a ramped aperture voltage during the switching process, we overcome the issue of excessive displacement currents at the detector during polarity switching, which usually lead to overdriving the output signal of the high-gain transimpedance amplifier. Furthermore, mechanical aperture grid oscillations caused by polarity switching were also reduced through the ramped aperture voltage. This enables a polarity switching time of only 7 ms at a drift voltage of 8 kV and a drift length of 103 mm, leading to a high resolving power of RP = 117. Requiring 50 ms to acquire a pair of positive and negative spectrum, the IMS achieves an acquisition rate of 20 Hz. It reaches limits of detection of 20 pptv for dimethyl methylphosphonate and 40 pptv for methyl salicylate. For demonstration, different hop varieties were investigated and could be clearly differentiated by considering both, the positive and negative spectra.

CIGALE: an innovative gas neutralizer based high efficiency neutral beam injector concept for future fusion reactors

This paper outlines the main features of a new high efficiency (η > 62%) high power (∼18 MW D0) neutral beam (NB) concept based on pragmatic solutions suitable with the reactor requirements. The injector is modular (several beamlines in parallel) with independent ion sources referenced to the ground potential and gas neutralizers held at +1 MV. This topology leads to numerous simplifications; it overcomes the main issues of conventional NB systems, such as the complex 1 MV electrical setup, the difficult ion source remote maintenance, the high caesium consumption. The other key parameter is the gas neutralization concept which minimizes the amount of gas by operating at a low gas target and low neutralizer duct conductance. The implementation of an energy recovery system for the residual 1 MeV D− is essential to attain a high wall-plug efficiency. These specific features require thin laminar D− beams provided by a pre-acceleration up to 100 keV in slotted grid apertures to form thin blade-like beamlets, followed by the post-acceleration to 1 MeV by merging the beamlets in a single beam in five gaps (+200 kV per gap). All these specific aspects minimize the beams losses and thermal loads along the beamline and enhance the injector reliability and availability.

Exploring the possibility of adding DGGS support to the S-100 Universal Hydrographic Data Model

reference systems. A DGGS is a spatial reference system consisting of multiple layers of grids with increasing resolution which partition the globe into polygon shaped cells of equal area. Individual cells can be easily identified and basic Geographic Information System (GIS) operations can be performed on data stored within the cells. Advantages of using DGGS for spatial data include the ability to integrate data layers of different data types in a statistically uniform way, the possibility to avoid distortions caused from projection, and the capability to scale analysis across multiple resolution levels. The benefits of using DGGS for hydrographic data, specifically Electronic Navigational Charts (ENC) are examined. The addition of DGGS to S-100 would provide a uniform gridding system for ENC cells, and would eliminate distortions in the Polar Regions caused when projecting rectangular cells based on latitude and longitude. The Open Geospatial Consortium (OGC) led a pilot project for which one of the participants developed a server that offers S-100 marine protected area data organized according to DGGS. Because of this success and the availability of open-source software for the implementation of DGGS, the BSH (German Federal Maritime and Hydrographic Agency) investigated the feasibility and possible benefits of using Discrete Global Grid Systems for S-100 data. The outcome was successful. However, it is recognized that DGGS as a standard is not yet advanced enough and the available software tools are inadequate for wide implementation. More investigation needs to be done, and additionally the best grid shape, orientation and aperture for a uniform grid for hydrographic data need to be determined. Overall, DGGS should be considered for future versions of S-100, the new universal hydrographic data model, because the benefits for ENCs and hydrographic data are clear and in the future, it is expected that DGGS will revolutionize GIS.

Influence of Grid Aperture Ratio on Electron Transmittance and Electron Beam Spot Size in Field Emission Processes of Carbon Nanotubes

Field emission is an important work mode for electron sources, and carbon nanotubes (CNTs) have been extensively studied for their good emission properties. It is well known that the parameters of the grid deeply influence the field emission performance of CNTs, a relationship that requires further elucidation. Therefore, in this study, the relationship between the grid aperture ratio and electron transmittance was studied through simulations and experiments. This study’s results indicate that the electron transmittance improved as the grid aperture ratio increased. Meanwhile, electron beam spot simulations and imaging experiments indicate that an increased grid aperture ratio will expand the cathode electron divergence, leading to a larger electron beam spot size. These results demonstrate that there is a trade-off in maintaining the grid aperture ratio between high electron transmittance and relatively small electron beam spot size, and the optimum grid aperture ratio is between 75% and 85%. These results will provide a reference for the design and optimization of X-ray tubes and other electron sources.

Multi-hole grid thruster plume simulation model based on single grid aperture analytical model

This paper presents a plume divergence angle prediction model for ion thruster. Firstly, an analytical model using Gaussian distribution for single grid hole plume is proposed. The parameters of this single hole model are obtained from the particle simulation results of ion optics which utilizes the immersed finite element Particle-in-cell Monte Carlo collision (IFE-PIC-MCC) method. Then, by the extension of the single hole model, a three-dimensional plume model for the whole ion optics is proposed. This plume model can be used to predict the divergence angle of ion thruster plume. Finally, a numerical example with the simulation of a 217-hole ion optics is provided to demonstrate the features of the proposed method.

Langmuir-probe measurement through the plasma grid aperture of hydrogen negative ion source

The extraction physics of negative-ion sources is still unknown because of the complication of the plasma-beam boundary (meniscus), which includes electronegative plasma, the magnetic field, and a surface production of negative ions. This paper shows results from Langmuir probe measurements inside of the extraction aperture (EA) and the criteria for locating the meniscus. The criteria are defined by the appearance and disappearance of Maxwellian feature of plasma. The result gives direct information on the location and shape of the plasma-beam boundary.

Differential sputtering yield measurement of ion grid materials by novel arch array system

In this experimental study the differential sputtering yield and total sputtering yield of ion thruster grid materials (Mo and C/C composite) under low-energy ion bombardment has been investigated. A novel combinable aluminum arch array is developed to determine the spatial distributions of sputtered atoms in different azimuthal angles. The quartz crystal microbalance (QCM) method is used to validate the typical distribution profile of sputtered atoms at the 180° azimuthal angle. The distribution characteristics of sputtered atoms as well as the specific regions of maximum differential sputtering yield under different incident energies, incident angles and ion species (Xe, Kr, Ar) have been discussed. The sputtering resistance performance of C/C composite materials have been compared with Mo metal by measuring the total sputtering yield. The spatial distributions of C/C differential sputtering yield are found highly anisotropic even under normal incidence, a multi-region discontinuous distribution characteristic of maximum differential sputtering yield have been observed in C/C composite material. The experimental results about the spatial distribution of sputtered atoms are of great importance for establishing the sputter-redeposition models of grid apertures to accurately predict the evolution of grid morphology and lifetime of ion thruster.

A study of grid failure mode drivers and methods for accelerated life testing of a 30 cm diameter ion thruster

In view of the high cost caused by the 1:1 lifetime verification test of ion thrusters, the lifetime acceleration test should be considered. This work uses the PIC-MCC (Particle-in-Cell Monte-Carlo Collision) method to analyze the five failure factors that lead to the failure of the accelerator grid of a 30 cm diameter ion thruster under the working mode of 5 kW. Meanwhile, the acceleration stress levels corresponding to different failure factors are obtained. The results show that background pressure has the highest stress level on the grid’s erosion. The accelerator grid aperture’s mass sputtering rate under the rated vacuum degree (1 × 10−4 Pa) of 5 kW work mode is 8.78 times that of the baseline vacuum degree (1 × 10−6 Pa), and the mass sputtering rate under worse vacuum degree (5 × 10−3 Pa) is 5.08 times that of 1 × 10−4 Pa. Under the influence of the other four failure factors, namely, the voltage of the accelerator grid, upstream plasma density, the screen grid voltage and mass utilization efficiency, the mass sputtering rates of the accelerator grid hole are 2.32, 2.67, 1.98 and 2.51 times those of the accelerator grid hole under baseline condition, respectively. The ion sputtering results of two 30 cm diameter ion thrusters (both installed with new grids assembly) after working for 1000 h show that the mass sputtering rate of the accelerator grid hole under vacuum conditions of 5 × 10−3 Pa is 4.54 times that under the condition of 1 × 10−4 Pa, and the comparison error between simulation results and test results of acceleration stress is about 10%. In the subsequent ion thruster lifetime verification, the working vacuum degree can be adjusted according to the acceleration stress level of background pressure, so as to shorten the test time and reduce the test cost.

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.

Modeling grid erosion in the NEXT ion thruster using the CEX2D and CEX3D codes

NASA’s Evolutionary Xenon Thruster (NEXT) is a candidate for future deep space missions that offers high efficiency and specific impulse over a large power throttling range. One of the key life-limiting components is the ion accelerator system, which is subject to sputter erosion by low energy discharge plasma ions incident on the upstream screen grid and higher energy charge exchange ions that impact the downstream accelerator grid. The grid erosion codes CEX2D and CEX3D were validated with data from tests of NEXT as well as the NSTAR ion thruster and then used to assess the time to failure in space due to screen grid erosion and electron backstreaming caused by accelerator grid aperture erosion. Screen grid erosion was found to be important only at the lowest throttle levels, and was conservatively estimated to lead to failure after processing over 900 kg of xenon. The first failure mode at high power levels was found to be electron backstreaming due to accelerator grid hole wall erosion, which would occur after processing over 700 kg of propellant.

Ion thruster accelerator grid erosion mechanism under extreme conditions of cylindrical erosion and chamfer erosion

In this paper, the abnormal experimental phenomenon on barrel erosion under extreme working conditions in the ultra-long life experiment (>10000 h) of ion thruster ion optics is studied by the Immersed-Finite-Element Particle-In-Cell Monte-Carlo-Collision (IFE-PIC-MCC) method and the grid erosion evaluation model. The transport process of beam ions and Charge Exchange (CEX) ions in the grid system, and the characteristics and mechanisms of the aperture barrel erosion under extreme erosion conditions (i.e. the cylindrical erosion and chamfer erosion) were systematically studied. Thanks to the advantage of the IFE method for dealing with complex boundaries in structured mesh, the aperture barrel erosion morphology of the accelerator grid is reconstructed accurately based on the experimental results. The results show that, with the evolution of working conditions, the mechanism of the aperture barrel erosion changes significantly, which relies heavily on the accelerator grid morphology. The change of the accelerator grid aperture barrel morphology has a significant effect on the behavior of CEX ions, and only affects the local electric field distribution, but has no effect on the upstream plasma sheath. As the erosion progresses, the erosion position moves downstream along the grid aperture axis direction, and the erosion range becomes narrower. Regardless of the erosion phase, the erosion rate of the CEX ions located downstream of the decelerator grid is the largest. The erosion rate is related to the mean incident energy and angle, and their variation is closely related to the position and trajectory of CEX ions.

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.

Simulation and Optimization of CNTs Cold Cathode Emission Grid Structure

Carbon nanotubes (CNTs) show significant advantages in the development of cold cathode X-ray tubes due to their excellent field emission performance; however, there are still some problems, such as short lifetime and the low emission current of large-area CNTs. In this paper, a front-grid carbon nanotube array model was established, and the electric field intensity near the tip of the CNTs' electric field enhancement factor was analytically calculated. A simulation model of a CNT three-dimensional field emission electron gun was established by using computer simulation technology (CST). The effects of grid wire diameter, grid aperture shape, and the distribution of grid projection on the cathode surface on the cathode current, anode current, and electron transmission efficiency were analyzed. The aperture ratio was used to evaluate the grid performance, and the simulation results show that the ideal aperture ratio should be between 65% and 85%. A grid structure combining a coarse grid and a fine grid was designed, which can make the electric field intensity around the grid evenly distributed, and effectively increased the cathode emission current by 24.2% compared with the structure without the fine grid. The effect of grid aperture ratio on the electron transmission efficiency was tested. The simulation results and optimized structure can provide a reference for the grid design of cold cathode emission X-ray tubes.

Initial Results From the SPIDER Beamlet Current Diagnostic

Source for the production of ions of deuterium extracted from a radio frequency plasma (SPIDER) is the full-scale prototype for the ITER heating neutral beam (HNB) source. Beam operation in SPIDER, with the eventual aim of achieving 100-keV, 46-A hydrogen negative ion beam for 1 h, has mainly been performed in volume operation without cesium. Recently, with the introduction of cesium, SPIDER has moved to the surface operational regime, with a noted increase in beam current. Electrical and calorimetric measurements of the beam current have previously shown discrepancies between the two, possibly due to background gas interactions in the accelerator and upstream of the STRIKE calorimeter. Taking advantage of the reduced number of open apertures due to the plasma grid (PG) mask, a series of fluxgate current transducers have been installed in SPIDER to measure the current of individual beamlets. These beamlet current monitors (BCMs), positioned downstream of the grounded grid (GG) around individual grid apertures, have provided direct measurement of the current of up to five individual beamlets during the recent SPIDER campaigns with and without cesium. This contribution describes the installation of the BCM dc sensors, analysis of the dc signal, and initial characterization of the beamlet direct current component in volume and surface operational regimes.

Compact retarding potential analyzers enabled by glass-ceramic vat polymerization for CubeSat and laboratory plasma diagnostics

We report the design, fabrication, and characterization of novel, digitally manufactured, compact retarding potential analyzers (RPAs), i.e., multi-electrode instruments that can be used as in-orbit mass spectrometers and as on-ground/in-orbit ion energy analyzers. Unlike most RPAs reported in the literature, our devices enforce active aperture alignment across the grid stack, maximizing ion transmission. The RPA electrode housing (the most critical component of the RPA, as it precisely positions the electrodes to maximize grid stack transmission while avoiding space charge effects and providing electrical insulation) is additively manufactured in Vitrolite® (a printable glass-ceramic) via vat polymerization, resulting in non-porous, high-temperature compatible, and high-vacuum compatible hardware. Metrology of as-printed (green) Vitrolite Ⓡ parts shows an in-plane, per axis manufacturing accuracy of 60 µm (i.e., the in-plane length of the voxels) and an out-of-plane manufacturing accuracy of 100 µm (i.e., the height of the slices). Vitrolite Ⓡ 3D-printed objects annealed at 900 °C show a ~5 % shrinkage, a slightly larger maximum tensile stress, a similar Young’s modulus, and a larger compression yield stress compared to green samples. Also, the assembly misalignment between the grids and a 3D-printed housing annealed at 900 °C is a twofold larger compared to using a green housing. Annealing 3D-printed Vitrolite Ⓡ at temperatures above 900 °C causes significant distortion of the printed part due to material reflow, which is incompatible with a precision engineering application. Four different RPA designs were synthesized to probe the ionosphere (design with floating grid alignment at the aperture level) and laboratory plasmas (designs with floating grid aperture alignment at the cluster level). Simulations conducted in this study show that the RPA design that enforces floating grid aperture alignment at the aperture level attains the best performance, although RPA designs that implement floating grid aperture alignment at the cluster level are more resilient to grid misalignment. Experimental characterization of the RPAs using an ion source and a helicon plasma source is consistent with expected performance and the literature. Plasmas with a Debye length as small as 50 µm were successfully characterized using the reported sensors, matching the performance of state-of-the-art RPAs manufactured via semiconductor microfabrication. • Study reports the first plasma sensors enabled by glass-ceramic 3D printing. • Four sensor designs for probing ionospheric and laboratory plasmas were developed. • Sensors successfully characterized plasmas with a Debye length as small as 50 µm. • Device performance on par with that of state-of-the-art, Si microfabricated sensors.

Wide-range characteristics of beam perveance and saddle point potential of LIPS-200 ion thruster

Both the long-life and multi-mode versions of LIPS-200 ion thruster are under investigation in LIP (Lanzhou Institute of Physics). To confirm the feasible ranges of the beam current and accel (abbreviation for accelaration) grid potential to apply to the thruster, the wide-range beam perveance (the state of beam focus) and saddle point potential (the lowest potential along beamlet centerline) characteristics of LIPS-200 are studied with a test-verified PIC-MCC (Particle in Cell-Monte Carlo Collisions) model. These characteristics are investigated with both the initial and the eroded states of the accel grid aperture diameter. The results show that the feasible ranges of these parameters with respect to perveance/crossover (overfocused) limit extend as the operating time accumulates, while the feasible range of accel grid potential narrows due to a reduced EBSF (electron backstreaming failure) margin. The feasible ranges determined by the initial condition are: (i) the beam current up to 0.981 A, and (ii) the accel grid potential up to −85 V. A 23% enlargement of the aperture diameter would bring up to 48 V of EBSF margin loss.

Development of a high dynamic range retarding potential analyzer for electric propulsion plume diagnosis.

A new high dynamic range retarding potential analyzer design is proposed to measure the ion energy distribution in electric propulsion plasma plumes. The main difference between the proposed probe and previous designs is an ion-optical system with a high negative accelerating potential. Applying the principle of acceleration and focusing of the ion flow provides the correct operation of the probe in a dense plasma in which the Debye length can be ten times smaller than the diameter of the grid apertures. As a result, the new probe has high transparency with an extended operating range of measured plasma densities. Another advantage of the probe is its accurately known and time-stable transparency for ions, so the probe can measure the absolute values of the ion current density like the Faraday probe. The analyzer geometry, which provides measurement in the range of ion current densities up to 100 A/m2 at ion energies up to 2000eV with an error of no more than 2%, was determined using numerical simulation. We present the measurement results of the plasma plume of a Hall effect thruster with a nominal discharge power of 2.3 kW.

Ultra-Fast Polarity Switching, Non-Radioactive Drift Tube for the Miniaturization of Drift-Time Ion Mobility Spectrometer.

Drift-time ion mobility spectrometer (DT-IMS) is a promising technology for gas detection and analysis in the form of miniaturized instrument. Analytes may exist in the form of positively or negatively charged ions according to their chemical composition and ionization condition, and therefore require both polarity of electric field for the detection. In this work the polarity switching of a drift-time ion mobility spectrometer based on a direct current (DC) corona discharge ionization source was investigated, with novel solutions for both the control of ion shutter and the stabilization of aperture grid. The drift field is established by employing a switchable high voltage power supply and a serial of voltage regulator diode, with optocouplers to drive the ion shutter when the polarity is switched. The potential of aperture grid is stabilized during the polarity switching by the use of four diodes to avoid unnecessary charging cycle of the aperture grid capacitor. Based on the proposed techniques, the developed DT-IMS with 50 mm drift path is able to switch its polarity in 10 ms and acquire mobility spectrum after 10 ms of stabilization. Coupled with a thermal desorption sampler, limit of detection (LoD) of 0.1 ng was achieved for ketamine and TNT. Extra benefits include single calibration substance for both polarities and largely simplified pneumatic design, together with the reduction of second drift tube and its accessories. This work paved the way towards further miniaturization of DT-IMS without compromise of performance.

Negative ion density in the ion source SPIDER in Cs free conditions

The SPIDER experiment, operated at the Neutral Beam Test Facility of Consorzio RFX, Padua, hosts the prototype of the H−/D− ion source for the ITER neutral beam injectors. The maximization of the ion current extracted from the source and the minimization of the amount of co-extracted electrons are among the most relevant targets to accomplish. The Cavity Ring-Down Spectroscopy diagnostic measures the negative ion density in the source close to the plasma grid (the plasma-facing grid of the ion acceleration system), so to identify the source operational parameters that maximize the amount of negative ions which can be extracted. In this study SPIDER was operated in hydrogen and deuterium in Cs-free conditions, therefore negative ions were mostly produced by reactions in the plasma volume. This work shows how the magnetic filter field and the bias currents, present in SPIDER to limit the amount of co-extracted electrons, affect the density of negative ions available for extraction. The results indicate that the magnetic filter field in front of the acceleration system should be set between about 1.6 mT, condition that maximizes the density of available negative ions, and about 3.2 mT, condition that minimizes the ratio of electron current to ion current. The negative ion density also resulted to be maximized when the plasma grid and its surrounding bias plate was positively biased against the source body with a total current in the range 0–100 A. The paper shows also how much, in Cs-free conditions, the electric fields in the acceleration system can affect the density of negative ions in the source, close to the plasma grid apertures.

Structural characteristics of the upstream sheath of the ion optics and its application in evaluating the beam performance of an ion thruster

To evaluate the extraction capabilities of ion optics and promote the generation of highly collimated ion beams for propulsion, the properties of the upstream sheath of the ion optics and how those properties relate to the beam divergence are investigated numerically and theoretically. The characteristics of the beam divergence at different grid parameters are studied from the behaviors of the impingement current and divergence angle obtained by simulations. Additionally, the simulations indicate the existence of an optimal structure for the upstream sheath of the ion optics, one that corresponds to a moderate focusing effect and a relatively small divergence angle. The plasma densities at the dividing points of different sheath structures are then derived with the matching model of the ion optics and the Child–Langmuir law, coupled with semi-empirical approaches based on the simulation results. According to the theoretical analyses, the range of existence of the most-desirable sheath structure depends on the strength of the penetration of the extraction field, the voltage between the grid apertures, and the distance between the upstream surfaces of the grids. Also, sensitivity analyses are performed with the numerical partial derivatives of the models to investigate how the grid parameters affect the sheath structures. The plasma densities at the dividing points generally vary synchronously with the changes of grid parameters, but the ranges of variations are different. Consequently, the desirable sheath structure and operating conditions of the ion optics can be achieved by correctly adjusting the grid parameters.