Retarding Grid Research Articles

Magnetic Nozzle and RPA Simulations vs. Experiments for a Helicon Plasma Thruster Plume

The experimental characterization of electrodeless plasma thrusters with a magnetic nozzle is fundamental in the process of increasing their maturity to reach the industrialization level. Moreover, it offers the unique opportunity of validating existing numerical models for the expansion of a magnetized plasma plume, and for the synthetic simulation of diagnostics measurements, like those of a retarding potential analyzer, which provides essential information regarding the ion beam energy distribution function. Simulations to experiments comparison ultimately enables a better understanding of the physical processes behind the observed experimental curves. In this work, input experimental data of a Helicon plasma plume is used to simulate both a magnetic nozzle expansion in the divergent field region, and the corresponding measurements of a retarding potential analyzer, through dedicated small-scale simulations of this diagnostics tool. Magnetic nozzle simulation and experimental results agree well in terms of the angular distribution of the ion current at 40 cm distance from the source, and also in the prediction of the energies of the two main peaks of the ion energy distribution function: a first one at 45 eV due to source ions, and a second one, at 15–20 eV, due to ions from charge-exchange and ionization collisions in the plume. Finally, the small-scale simulation of the retarding potential analyzer permits to assess the parasitic effects caused by the ion current collected by the different analyzer grids. The inclusion of the retarding and electron suppression grids currents in the overall I-V characteristic is shown to correct almost entirely these effects on the obtained ion velocity distribution.

Spherical micro-hole grid for high-resolution retarding field analyzer.

A wide-acceptance-angle spherical grid composed of numerous micro cylindrical holes was developed to be used for the retarding grid of a display-type retarding field analyzer (RFA) and to enhance the energy resolution (E/ΔE). Each cylindrical hole with a diameter of 50 µm and a depth of 80 µm is directed to the spherical center. The inner radius of the spherical grid is 40 mm. The holed area corresponds to an acceptance angle of ±52°. The E/ΔE of an RFA equipped with the developed holed grid was estimated to be 2000 from a measured Au 4f photoemission spectrum. A clear photoelectron hologram was observed in the Mo 4p core-level region of MoS2, indicating that the RFA with the holed grid is effective for photoelectron holography.

The gridded retarding ion drift sensor for the petitSat cubeSat mission.

The Gridded Retarding Ion Drift Sensor (GRIDS) is a small sensor that will fly on the 6 U petitSat CubeSat. It is designed to measure the three-dimensional plasma drift velocity vector in the Earth's ionosphere. The GRIDS also supplies information about the ion temperature, ion density, and the ratio of light to heavy ions present in the ionospheric plasma. It utilizes well-proven techniques that have been successfully validated by similar instruments on larger satellite missions while meeting CubeSat-compatible requirements for low mass, size, and power consumption. GRIDS performs the functions of a Retarding Potential Analyzer (RPA) and an Ion Drift Meter (IDM) by combining the features of both types of instruments in a single package. The sensor alternates RPA and IDM measurements to produce the full set of measurement parameters listed above. On the petitSat mission, GRIDS will help identify and characterize a phenomenon known as plasma blobs (or enhancements).

Energetic ions during plasma-enhanced atomic layer deposition and their role in tailoring material properties

Plasma-enhanced atomic layer deposition (PEALD) has obtained a prominent position in the synthesis of nanoscale films with precise growth control. Apart from the well-established contribution of highly reactive neutral radicals towards film growth in PEALD, the ions generated by the plasma can also play a significant role. In this work, we report on the measurements of ion energy and flux characteristics on grounded and biased substrates during plasma exposure to investigate their role in tailoring material properties. Insights from such measurements are essential toward understanding how a given PEALD process at different operating conditions can be influenced by energetic ions. Ion flux-energy distribution functions (IFEDFs) of reactive plasmas typically used for PEALD (O2, H2, N2) were measured in a commercial 200 mm remote inductively coupled plasma ALD system equipped with RF substrate biasing. IFEDFs were obtained using a gridded retarding field energy analyzer and the effect of varying ICP power, pressure and bias conditions on the ion energy and flux characteristics of the three reactive plasmas were investigated. The properties of three material examples—TiOx, HfNx and SiNx—deposited using these plasmas were investigated on the basis of the energy and flux parameters derived from IFEDFs. Material properties were analyzed in terms of the total ion energy dose delivered to a growing film in every ALD cycle, which is a product of the mean ion energy, total ion flux and plasma exposure time. The properties responded differently to the ion energy dose depending on whether it was controlled with RF substrate biasing where ion energy was enhanced, or without any biasing where plasma exposure time was increased. This indicated that material properties were influenced by whether or not ion energies exceeded energy barriers related to physical atom displacement or activation of ion-induced chemical reactions during PEALD. Furthermore, once ion energies were enhanced beyond these threshold barriers with RF substrate biasing, material properties became a function of both the enhanced ion energy and the duration for which the ion energy was enhanced during plasma exposure. These results have led to a better insight into the relation between energetic ions and the ensuing material properties, e.g. by providing energy maps of material properties in terms of the ion energy dose during PEALD. It serves to demonstrate how the measurement and control of ion energy and flux characteristics during PEALD can provide a platform for synthesizing nanoscale films with the desired material properties.

The Mochi LabJet Experiment for Measurements of Canonical Helicity Injection in a Laboratory Astrophysical Jet

Abstract The Mochi device is a new pulsed power plasma experiment designed to produce long, collimated, stable, magnetized plasma jets when set up in the LabJet configuration. The LabJet configuration aims to simulate an astrophysical jet in the laboratory by mimicking an accretion disk threaded by a poloidal magnetic field with concentric planar electrodes in front of a solenoidal coil. The unique setup consists of three electrodes, each with azimuthally symmetric gas slits. Two of the electrodes are biased independently with respect to the third electrode to control the radial electric field profile across the poloidal bias magnetic field. This design approximates a shear azimuthal rotation profile in an accretion disk. The azimuthally symmetric gas slits provide a continuously symmetric mass source at the footpoint of the plasma jet, so any azimuthal rotation of the plasma jet is not hindered by a discrete number of gas holes. The initial set of diagnostics consists of current Rogowski coils, voltage probes, magnetic field probe arrays, an interferometer and ion Doppler spectroscopy, supplemented by a fast ion gauge and a retarding grid energy analyzer. The measured parameters of the first plasmas are ∼1022 m−3, ∼0.4 T, and 5–25 eV, with velocities of ∼20–80 km s−1. The combination of a controllable electric field profile, a flared poloidal magnetic field, and azimuthally symmetric mass sources in the experiment successfully produces short-lived (∼10 μs, ≳5 Alfvén times) collimated magnetic jets with a ∼10:1 aspect ratio and long-lived (∼100 μs, ≳40 Alfvén times) flow-stabilized, collimated, magnetic jets with a ∼30:1 aspect ratio.

Wide-angle display-type retarding field analyzer with high energy and angular resolutions.

Deployments of spherical grids to obtain high energy and angular resolutions for retarding field analyzers (RFAs) having acceptance angles as large as or larger than ±45° were explored under the condition of using commercially available microchannel plates with effective diameters of approximately 100 mm. As a result of electron trajectory simulations, a deployment of three spherical grids with significantly different grid separations instead of conventional equidistant separations showed an energy resolving power (E/ΔE) of 3200 and an angular resolution of 0.6°. The mesh number of the wire mesh retarding grid used for the simulation was 250. An RFA constructed with the simulated design experimentally showed an E/ΔE of 1100 and an angular resolution of 1°. Using the RFA and synchrotron radiation of 900 eV, photoelectron diffraction (PED) measurements were performed for single-crystal graphite. A clear C 1s PED pattern was observed even when the differential energy of the RFA was set at 0.5 eV. Further improvement of the energy resolution was theoretically examined under the assumption of utilizing a retarding grid fabricated by making a large number of radially directed cylindrical holes through a partial spherical shell instead of using a wire mesh retarding grid. An E/ΔE of 14 500 was predicted for a hole design with a diameter of 60 μm and a depth of 100 μm. A retarding grid with this hole design and a holed area corresponding to an acceptance angle of ±7° was fabricated. An RFA constructed with this retarding grid experimentally showed an E/ΔE of 1800. Possible reasons for the experimental E/ΔE lower than the theoretical values are discussed.

Design of an energy analyzer for low energy 1+ charged ion beams at RISP Project

Accurate measurement of the energy spread with a compact device has been accomplished by developing a new prototype of a retarding field energy analyzer (RFEA). The device is capable of handling all kinds of low energy mono-charged ion beams at RISP Project. Numerical simulations have been performed in order to study and evaluate dependency of RFEA energy resolution to beam emittance, beam energy, beam energy spread, space charge effects, and most significantly the optics system. Simulation results have shown that the use of developed optics system with particular voltage applied on the focusing cylinder and suitable positioning of the retarding grid may lead to high efficiency energy resolutions not exceeding 0.31 eV for beam energy going up to 20 keV . Small errors on the measured energy spread are obtained despite the presence of degradations stemming from various energy resolution dependencies. Typical mono-charged ion beams delivered from ion trap devices, ISOL beam-line and stable ion sources have been studied. The error on the optimum measured energy spread for beams delivered from ion trap devices is 5 ∼ 10% owing to beam emittances. For a larger beam size, delivered from an ISOL beam-line, a larger error occurs on the measured energy spread at around 23.5%. The energy spread of ion beams delivered from stable ion sources with good optical quality relative to the ISOL sources can be measured with an error around 21.7%. This prototype enables accurate measurement of the average beam energy with an error around 1.5 eV per 20 keV beam energy.

The New Methodology and Chemical Contrast Observation by Use of the Energy-Selective Back-Scattered Electron Detector.

We report on a robust method for chemical element-sensitive imaging by scanning electron microscopy (SEM). The commercial Auriga FE-SEM microscope (Carl Zeiss, Oberkochen, Germany), equipped with an energy-selective grid detector (EsB) as a part of the experimental setup, was applied for generation of chemical contrast at low accelerating voltages, which is gentle for sensitive samples. The EsB-grid detector, conceptually adapted by us as an energy retarding field analyzer (RFA), was used to detect the two-dimensional (2D) energy spectrum for the first time. The electron energy spectrum measured by sweeping the retarding grid potential revealed thresholds corresponding to electronic transitions in the specimen, followed by 2D-derivation treatment applied just at the observed thresholds. This allowed chemical mapping by SEM. In this report the 273 eV Auger transition in carbon deposited onto the Si(100) sample was chosen as a source for chemical contrast in the SEM image. In addition to Auger electrons, we expect analogous energy-selective contrast enhancement for inelastically scattered electrons, for example, in plasmonic contrast and elastically scattered electrons, for example in phase contrast, our method, proved for carbon, is expected to apply to a broader list of elements as a general capability of chemical mapping, at several-fold better lateral resolution when compared with energy dispersive spectroscopy (EDS).

Analysis of the Anomalous Phenomenon in the Retarding Potential Analyzer Measurements

An anomalous phenomenon was observed in the retarding potential analyzer (RPA) measurements of the energy of the ion beam from an 8 cm argon ion source. The current-voltage (I–V) curve, which should theoretically descend, went up as the ion retarding potential was increased. Various explanations, such as the Townsend discharge theory and secondary electron emission etc. were proposed but denied by the theory application condition or the experiment results. An angle of about −10° was found between the axes of the ion beam and the RPA according to the contours of the ion beam density and direction. The particle simulation and experiment of the sum of the collector and wall current were conducted at different incident ion angles. The trends of the I–V curve in simulation results conformed with the experimental results in most cases. The ion trajectories were simulated at different retarding potentials with an incident angle of −10°. According to these results, the reason for the anomalous phenomenon is that when there is a specific angle between the axes of the ion beam and the RPA, more ions are repelled from the vicinity of the ion retarding grid to avoid striking on the grid as the ion retarding potential increases. These redundant ions reach the plate and thus lead to the formation of an ascending I–V curve.

Ultrahigh Vacuum Cryostat System for Extended Low-Temperature Space Environment Testing

The range of temperature measurements have been significantly extended for an existing space environment simulation test chamber used in the study of electron emission, sample charging and discharge, electrostatic discharge and arcing, electron transport, and luminescence of spacecraft materials. This was accomplished by incorporating a new two-stage, closed-cycle helium cryostat which has an extended sample temperature range from to , with long-term controlled stability of . The system was designed to maintain compatibility with an existing ultrahigh vacuum chamber (base pressure ) that can simulate diverse space environments. These existing capabilities include controllable vacuum and ambient neutral gases conditions , electron fluxes (5-30-keV monoenergetic, focused, and pulsed sources over 10-4-1010 nA-cm-2), ion fluxes ( monoenergetic sources for inert and reactive gases with pulsing capabilities), and photon irradiation (numerous continuous and pulsed monochromated and broad band IR/VIS/UV [0.5-7 eV] sources). The new sample mount accommodates one to four samples of 1-2.5-cm diameter in a low-temperature carousel, which allows rapid sample exchange and controlled exposure of the individual samples. Custom hemispherical grid retarding field analyzer and Faraday cup detectors, custom high speed, high-sensitivity electronics, and charge neutralization capabilities used with , , and electrons/pulse pulsed-beam sources permit high-accuracy electron emission measurements of extreme insulators with minimal charging effects. In situ monitoring of surface voltage, arcing, and luminescence (250-5000 nm) have recently been added.

Second-harmonic generation from electron beam deposited SiO films

It is demonstrated that as-grown e-beam deposited SiOx thin films on fused silica substrates show a second-order nonlinear response that is dependent on film thickness. Using a Maker fringes method the effective nonlinear coefficient for a SiO thin film is estimated to be comparable to that of crystalline quartz. Variation of process parameters has been used to investigate the origin of the nonlinear response. The second-harmonic signal is very sensitive to annealing of the film and can be totally removed by annealing at a few hundred degrees. It is also demonstrated that a retarding grid that traps charged particles between the crucible and the sample reduces the nonlinear signal from a SiO thin film significantly. It is suggested that oriented dipoles arise during deposition due to a negatively charged film from oxygen ions, thus, resulting in a non-centrosymmetric film. Finally, using e-beam lithography, well-defined nonlinear 2D structures can be synthesized, thus opening the door to a new and practical way to create nonlinear structures for planar waveguide technology.

Retarding field energy analyser ion current calibration and transmission

Accurate measurement of ion current density and ion energy distributions (IEDs) is often critical for plasma processes in both industrial and research settings. Retarding field energy analysers (RFEAs) have been used to measure IEDs because they are considered accurate, relatively simple and cost effective. However, their usage for critical measurement of ion current density is less common due to difficulties in estimating the proportion of incident ion current reaching the current collector through the RFEA retarding grids. In this paper an RFEA has been calibrated to measure ion current density from an ion beam at pressures ranging from 0.5 to 50.0 mTorr. A unique method is presented where the currents generated at each of the retarding grids and the RFEA upper face are measured separately, allowing the reduction in ion current to be monitored and accounted for at each stage of ion transit to the collector. From these I–V measurements a physical model is described. Subsequently, a mathematical description is extracted which includes parameters to account for grid transmissions, upper face secondary electron emission and collisionality. Pressure-dependent calibration factors can be calculated from least mean square best fits of the collector current to the model allowing quantitative measurement of ion current density.

Ion kinetic energy measurements in two soft ion ejection methods from a quadrupole ion trap

The kinetic energy of ion beams upon ejection from a quadrupole ion trap was measured in view of coupling quadrupole ion traps with other mass analyzers. Measurements were performed using a retarding grid analyser for two modes of ion ejection from the quadrupole ion trap based on the decrease of the amplitude of the radiofrequency voltage on the ring electrode simultaneously combined with the application of: (i) a positive voltage on the ring electrode and (ii) a voltage between the end-cap electrodes. We show that ion kinetic energy at ejection depends on the DC ejection voltage and the amplitude of the RF voltage at the moment of ejection, which means that the kinetic energy of the ions that are ejected from the ion trap depends on the m/ z ratio. Ion focusing depends on the depth of the pseudo-potential well in the quadrupole ion trap.

Optimization of ion energy spread in inductively coupled plasma source designed for focused ion beam applications

The ion energy distribution of inductively coupled plasma ion source for focused ion beam application is measured using a four grid retarding field energy analyzer. Without using any Faraday shield, ion energy spread is found to be 50 eV or more. Moreover, the ion energy distribution is found to have double peaks showing that the power coupling to the plasma is not purely inductive, but a strong parasitic capacitive coupling is also present. By optimizing the various source parameters and Faraday shield, ion energy distribution having a single peak, well separated from zero energy and with ion energy spread of 4 eV is achieved. A novel plasma chamber, with proper Faraday shield is designed to ignite the plasma at low RF powers which otherwise would require 300–400 W of RF power. Optimization of various parameters of the ion source to achieve ions with very low energy spread and the experimental results are presented in this article.

Electron emission from tungsten induced by slow, fusion-relevant ions

Tungsten has recently been introduced as a new wall material for fusion, because it exhibits favourably low sputtering yields and a very low tritium retention as compared to the commonly used graphite wall and divertor tiles. We measure total electron emission yields due to impact of slow singly and multiply charged ions (deuterium, helium and carbon) on sputter-cleaned polycrystalline tungsten surfaces by using a current method in combination with a retarding grid. Results are presented in the eV to keV impact energy region as typical for fusion edge plasma conditions and discussed in terms of potential and kinetic electron emission.

Demonstration of electron clearing effect by means of a clearing electrode in high-intensity positron ring

In the beam pipe of high-intensity positron/proton storage rings, undesired electron clouds may be first produced by photoelectrons and the ionization of residual gases; then the clouds increase by the secondary electron emission. In this study, a strip-line clearing electrode has been developed to mitigate the electron-cloud effect in high-intensity positron/proton storage rings. The electrode is composed of a thin tungsten layer with a thickness of 0.1 mm formed on a thin alumina ceramic layer with a thickness of 0.2 mm. The narrow alumina gap between the electrode and the beam pipe decreases the beam impedance and also enhances the heat transfer from the electrode to the beam pipe. A test model has been installed in the KEK B-factory (KEKB) positron ring, along with an electron monitor with a retarding grid. The electron density in a field free region decreased by one order of magnitude was observed on the application of ±500 V to the electrode at a beam current of 1.6 A with 1585 bunches. The reduction in the electron density was more drastic in a vertical magnetic field of 0.77 T, that is, the electron density decreased by several orders by applying +500 V to the electrode at the same beam current. This experiment is the first experiment demonstrating the principle of the clearing electrode that is used to mitigate the electron-cloud effect in a positron ring.

Comparison of gridded energy analyzer and laser induced fluorescence measurements of a two-component ion distribution

We present ion velocity distribution function (IVDF) measurements obtained with a five grid retarding field energy analyzer (RFEA) and IVDF measurements obtained with laser induced fluorescence (LIF) for an expanding helicon plasma. The ion population consists of a background population and an energetic ion beam. When the RFEA measurements are corrected for acceleration due to the electric potential difference across the plasma sheath, we find that the RFEA measurements indicate a smaller background to beam density ratio and a much larger parallel ion temperature than the LIF. The energy of the ion beam is the same in both measurements. These results suggest that ion heating occurs during the transit of the background ions through the sheath and that LIF cannot detect the fraction of the ion beam whose metastable population has been eliminated by collisions.

High resolution energy analyzer for broad ion beam characterization

Characterization of the ion energy distribution function (IEDF) of low energy high current density ion beams by conventional retarding field and deflection type energy analyzers is limited due to finite ion beam emittance and beam space charge spreading inside the analyzer. These deficiencies are, to a large extent, overcome with the recent development of the variable-focusing retarding field energy analyzer (RFEA), which has a cylindrical focusing electrode preceding the planar retarding grid. The principal concept of this analyzer is conversion of a divergent charged particle beam into a quasiparallel beam before analyzing it by the planar retarding field. This allows analysis of the beam particle total kinetic energy distribution with greatly improved energy resolution. Whereas this concept was first applied to analyze 5-10 keV pulsed electron beams, the present authors have adapted it to analyze the energy distribution of a low energy (<or=1 KeV) broad ion beam. In this paper we describe the RFEA design, which was modified from the original, mainly as required by the specifics of broad ion beam energy analysis, and the device experimental characterization and modeling results. Among the modifications, an orifice electrode placed in front of the RFEA provides better spatial resolution of the broad ion beam ion optics emission region and reduces the beam plasma density in the vicinity of analyzer entry. An electron repeller grid placed in front of the RFEA collector was found critical for suppressing secondary electrons, both those incoming to the collector and those released from its surface, and improved energy spectrum measurement repeatability and accuracy. The use of finer mesh single- and double-grid retarding structures reduces the retarding grid lens effect and improves the analyzer energy resolution and accuracy of the measured spectrum mean energy. However, additional analyzer component and configuration improvements did not further change the analyzed IEDF shape or mean energy value. This led us to conclude that the optimized analyzer construction provides an energy resolution considerably narrower than the investigated ion beam energy spectrum full width at half maximum, and the derived energy spectrum is an objective and accurate representation of the analyzed broad ion beam energy distribution characteristics. A quantitative study of the focusing voltage and retarding grid field effects based on the experimental data and modeling results have supported this conclusion.

Transmission of ∼10 keV electron beams through thin ceramic foils: Measurements and Monte Carlo simulations of electron energy distribution functions

Electron beams with particle energy of ~10 keV were sent through 300 nm thick ceramic (Si3N4 + SiO2) foils and the resulting electron energy distribution functions were recorded using a retarding grid technique. The results are compared with Monte Carlo simulations performed with two publicly available packages, Geant4 and Casino v2.42. It is demonstrated that Geant4, unlike Casino, provides electron energy distribution functions very similar to the experimental distributions. Both simulation packages provide a quite precise average energy of transmitted electrons: we demonstrate that the maximum uncertainty of the calculated values of the average energy is 6% for Geant4 and 8% for Casino, taking into account all systematic uncertainties and the discrepancies in the experimental and simulated data.

Determination of positron beam parameters by various diagnostic techniques

Various diagnostic techniques have been applied at the neutron-induced positron source Munich NEPOMUC in order to determine the positron beam parameters such as intensity, beam shape and energy distribution. The positron beam intensity is determined by the detection of the annihilation radiation of positrons, which annihilate in a movable target. The use of a micro-channel plate (MCP) detector with a CCD-camera allows a direct measurement of the positron beam shape and the lateral resolved intensity distribution. At NEPOMUC a movable MCP-assembly inside the evacuated beam line enables a quick examination of the beam shape during operation. A retarding grid was mounted inside the homogeneous magnetic guiding field in order to determine the distribution of the longitudinal positron momentum, and hence estimate the energy distribution of the positrons.