Ion Current Density Distribution Research Articles

Investigation of the directional Faraday cup and improvement of the comparison between direct and indirect thrust measurements of a magnetic nozzle ECR thruster

The thrust of an electric propulsion device estimated from electrostatic probe measurements may be different from direct thrust measurements. In order to reduce this discrepancy for electron cyclotron resonance thrusters (ECRT), a new diagnostic is presented: the directional Faraday cup (FCDi). Thanks to a peculiar design, it assesses the angular distribution of ion current density in the plume of plasma thrusters. First, a theoretical model of the angular selectivity of the FCDi is introduced. It is validated with both simulations and experiments. Guidelines are given to use the FCDi in the plume of ECRT. Second, the observed discrepancies between direct thrust measurements made on a thrust stand and indirect ones are reduced when using the FCDi instead of a planar Faraday probe with a guard ring (FPGR). Relative errors come down to 10% with the FCDi, which are compared to 20%–30% observed with the FPGR. An analysis demonstrates that it is not due to the effect of local ion trajectories but comes from the magnitude of the ion current measured. A large sheath in front of the negatively biased FPGR seems to be the cause of this phenomenon. The grounded opening of the FCDi reduces this phenomenon and improves the ion flux measurement accuracy. This new probe, with angular selectivity characteristics, allows for the comparison of the ion flux ejection direction with the magnetic field line and contributes to a better description of ion population dynamic in the magnetic nozzle of the thruster.

Simultaneously measured direct and indirect thrust of a FEEP thruster using novel thrust balance and beam diagnostics

For the characterisation of an electric propulsion system, the determination of the thrust has a crucial role. The thrust of an Indium FEEP Multiemitter (IFM) Nano Thruster laboratory model is measured directly with a thrust balance and indirectly via beam diagnostics. Both measurements are carried out simultaneously to enable mutual verification with high accuracy. The novel mN-torsion thrust balance by FOTEC resolves six different thrust magnitudes, ranging from 10μN to 1N. It is based on the so-called force-feedback method using a voice coil actuator to determine the thrust, which leads to an accuracy of better than 2%. The indirect thrust measurements were performed with a high-precision beam diagnostics system by FOTEC. A semi-circular diagnostics arm equipped with 23 digital Faraday cups is used to measure the ion current density distribution of the entire beam. The thrust results of both systems show an agreement with a deviation less than 5% for thruster operation points ranging from 50 to 450μN. This proves that the performance of a FEEP thruster can be characterised very precisely by indirect trust measurements.

Quantifying Uncertainty in Predictions of Spacecraft Erosion Induced by a Hall Thruster

The impact of sputtering yield model uncertainty on the predicted erosion of a meshed reflector wire exposed to a Hall effect thruster plume is investigated. Quantifying this uncertainty is critical for making informed assessments of reflector lifetime. The erosion is modeled by assuming known ion current density and energy distribution at the location of the wire. The wire surface is then discretized, and wear is evaluated incrementally over time. The confidence in model predictions is quantified where the major source of uncertainty is assumed to stem from the material sputtering yield. The model is run to simulate reflector erosion after 10 h of exposure to a 3 kW class Hall thruster operating at 300 V. The simulated results then are compared to experimental measurements from a dedicated wear test performed on a series of mesh coupon samples. The experimental results are shown to fall largely within the 95% credible intervals from model predictions, though the uncertainty in the maximum predicted erosion is found to be up to 190% of the maximum predicted erosion. These results are discussed in the context of predicting lifetime of reflectors on orbit and the need for margin in this component design.

Deposition of tungsten disilicide films by DC magnetron sputtering at ultra-low operating pressure

We describe our work on tungsten disilicide film deposition by planar magnetron sputtering at low operating gas pressure (down to 0.08 Pa). The sputter target was a tungsten disilicide composite with diameter 125 mm, and the DC magnetron current 0.1–1 A. We have explored the dependence of film homogeneity over the 100 mm diameter substrate on substrate temperature and distance from the magnetron, and the spatial distribution of ion current density and the effect of operating pressure on the roughness and resistivity of the films.

Characterization of an RF excited broad beam ion source operating with inert gases

Ion current density distributions, mass spectra, and ion energy distribution functions (IEDFs) for the use of Ar and mixtures of Ar/He and Ar/Ne in an RF broad beam ion source have been measured as a function of the extracted ion beam current and the applied RF-power. For the measurements, an array of Faraday probes and an energy-selective mass spectrometer have been used. Regardless of the gases or gas mixtures used, a broadening of the ion current density profile is measured at constant grid voltages with increasing RF-power. The IEDFs of all species are also dependent on the applied RF-power and show a bimodal shape with an increasing energy shift for a higher RF-power. The data show a significant contribution of charge exchange processes after the ions left the ion beam source. Besides ions of the process gas, additional species of ions originating from the erosion processes of the ion beam source, can be found in the mass spectrum. The measurements show that the operating conditions of the ion source have a significant influence on the beam properties and thus can also affect the results of ultraprecise ion beam-assisted processes.

IFM Nano Thruster performance studied by experiments and numerical simulations

Field emission electric propulsion thrusters are characterised by their low thrust range, which makes them ideal for the precise control of spacecraft. Decisive for precise control are the properties of the thruster ion beam, which includes the beam divergence angle and the thrust vector. The analysis of these properties is also necessary in order to be able to estimate the interactions of the beam with components of the spacecraft. Due to such interactions, solar panels or electrical instruments on board the spacecraft could be damaged by sputtering effects.The spatial ion current density and energy distribution of a test crown emitter beam, with different specifications compared to the IFM Nano thruster, were examined experimentally with a diagnostic system, including Faraday cups and a retarding potential analyser. In addition to the analysis of the beam profile of an emitting crown, a single emitting needle was analysed. Based on these experimental analyses, an ion trajectory simulation model was developed to determine the theoretical ion current density distribution. This model includes the properties of a liquid metal ion source, where the ion trajectories start from their point of origin, the so-called Taylor cone jet cap. The benchmark of the model shows that the thrust vector and divergence angle correspond to the experimental results and shows the identical calculations for different thruster parameters, like emission current and electrode voltage. The simulation allows for the optimisation of existing and novel thruster geometries in terms of performance, reliability and longevity.

Experimental and numerical investigation on the asymmetry of the current density extracted through a plasma meniscus in negative ion accelerator

In multi-beamlet negative ion accelerators for neutral beam injectors, the transverse magnetic field necessary for suppressing the co-extracted electrons induces a deflection of the negative ion beamlets that must be corrected. For the design, particle-tracing simulation codes are used to compute ion trajectories and optical properties of the beamlets in the acceleration stage. In these codes, uniform boundary conditions are normally assumed for the ion current density distribution at the surface (called meniscus), where negative ions are extracted from the plasma and form beamlets, which are accelerated across the apertures of the accelerator grids. Recently, experimental campaigns dedicated to the accurate measurement of the beamlet deflection in the acceleration phase revealed higher deflection than foreseen by simulations. In this work, we demonstrate that an agreement with the experimental data can be obtained by incorporating in the numerical simulations a non-radially symmetric distribution of the ion current density extracted across the meniscus surface. In the first part, the asymmetry of the ion current density at the meniscus is studied in an empirical way, by analysing and fitting the experimental results obtained with different operating parameters. In the second part, we show that the ion current asymmetry estimated by this procedure is well consistent with the flow pattern of H− ions calculated in the meniscus zone using a detailed particle in cell (PIC) model of the ion source in the presence of transverse magnetic field.

Deposition of Cu-films by a planar magnetron sputtering system at ultra-low operating pressure

The effect of operating pressure in the ultra-low range (down to 7 × 10−2 Pa) on film deposition by planar magnetron sputtering using a copper target has been studied. The magnetron discharge power was 500 W (target diameter 125 mm) in DC mode. The distance from the target to the deposition substrate was varied from 25 to 55 cm. The axial distribution of plasma ion current density and ion mass-to-charge ratio were measured under the same conditions by a moveable Langmuir probe and a modified quadrupole mass spectrometer, respectively. The influence of working gas pressure on film parameters such as deposition rate and roughness was investigated.

Study on influencing factors of ion current density measurement in corona discharge of HVDC transmission lines

The ground level ion current density produced by corona discharge in high voltage direct current (HVDC) transmission lines can reflect the operation status of the lines, but the distorted electric field at the edge of the Wilson plate seriously affects the measurement results of ion current density. In order to measure the ground level ion current density accurately and directly, a new reduced scale wire-plate experimental device in which the Wilson plate is flush with the grounding plate is designed. The influence of protective annulus width and the height of the Wilson plate from the grounding plate on ground level ion current density are studied. In addition, the differences between the micro-current galvanometer method and the sampling resistance method in the measurement of ion current density are compared. Finally, the ground level ion current density distributions of unipolar and bipolar HVDC transmission lines are measured. The results show that the edge effect of the Wilson plate can be neglected when the width of protective annulus is less than 2 mm, and the ion current density is nonlinear with the height of the Wilson plate from the grounding plate. Moreover, the internal resistance of the digital voltmeter seriously affects the measurement results and it is necessary to correct the results. Finally, at the same applied voltage, the ground level ion current density in the negative conductor region is higher than that in the positive conductor region.

Procedure of Predicting the Lifetime of the Accelerating Electrode of an Ion-Extraction System of a Gas-Discharge Ion Source

The state of studies on determination of the lifetime of ion thrusters (IT), including their methodology, is analyzed. The erosion of the accelerating electrode (AE) of an ion-extraction system caused by charge-exchange ions is a critical process responsible for the IT lifetime. It is shown that the quickest and most accurate information on the nature of AE erosion and rate at the initial test stage can be obtained by a method based on the use of multilayer coatings, consisting of several pairs of optical contrast materials, preliminarily applied to the AE surface. The method allows the use of simple photographic devices for recording the erosion pattern and is especially efficient in the case of a highly nonuniform nature of erosion. The calculation procedure is presented for the parameters of diagnostic multilayer coatings, including the optimal layer thickness and total coating thickness, which allow data to be obtained on the surface erosion profile and rate, and on the charge-exchange ion current density distribution with the highest resolution during the given test time.

Effect of the mesh emission electrode shape on the distribution of the plasma density generated in the working chamber

The research of the influence of the shape of emission mesh electrode on the distribution of the plasma density generated in the hollow cathode of low-pressure glow discharge was implemented. Radial and azimuthal distributions of the ion current density to the probe were measured for six shapes of the mesh emission electrode. For each shape of the emission electrode the non-uniformity factor of the ion current density distribution was calculated. These studies allow to optimize the electrode system to generate a high uniformity plasma (±25%).

The facility effects of a Hall effect thruster's relative exhaust direction in ground tests

This research focuses on understanding how the relative pumping orientation and backpressure of a vacuum facility affects the measured performance and plasma plume parameters of a Hall-effect thruster (HET). A HET was operated in two opposite directions: plume ejection towards the vacuum pump and plume ejection away from the pump. The HET plume for each orientation was individually characterized at three vacuum facility operating pressures to study the effect of the facility pumping orientation and backpressure on the measured plume parameters and thruster performance. The ion current density, divergence angle, beam current, and ion energy distribution were measured using a Faraday probe and a retarding potential analyzer (RPA). The variation in the direction of the plume with respect to the pumping orientation leads to a clear difference in the measured performance of the HET when operating at an elevated backpressure. These are the first experiments that take the effect of the pumping orientation of a vacuum facility with respect to the thruster's exhaust direction into consideration. The results here will aid future research in selecting an adequate thruster exhaust direction when performing different Hall thruster ground tests such as plume diagnostics, lifetime tests, and erosion tests.

Ion current density distribution in a pulsed non-self-sustained glow discharge with a large hollow cathode

The paper reports on azimuthal and radial distributions of the ion saturation current density measured with a single cylindrical and a plane probe in the plasma of a low-pressure (≈1 Pa) pulsed non-self-sustained glow discharge with a large (0.21 m3) hollow cathode at discharge currents higher than 100 A. It is shown that increasing the discharge operating voltage and decreasing the operating pressure in the range 0.4–1 Pa improves the uniformity of the ion current density distribution.

Evaluation of ion current density distribution on an extraction electrode of a radio frequency ion thruster

The radial distributions of ion current density on an ion extracting electrode of a radio frequency (RF) ion thruster (RIT) with an inductive plasma source were obtained using probe diagnostics of the RF xenon plasma. Measurements were carried out using a plane wall probe simulator and the VGPS-12 Probe System of Plasma Sensors Co. At xenon flow rate q = 2 sccm plasma pressure was 2 · 10−3 Torr, incident RF generator power varied in the range Pg = 50−250 W with RF power absorbed by plasma up to Pp = 220 W. Ion current densities were determined using semi- and double-logarithmic probe characteristics by linear extrapolations of their ion branches to probe floating potentials. The same parameters were also measured in undisturbed plasma by a classic cylindrical probe. They exceeded plane probe data by more than two times, showing the effectiveness of plasma sheath reproduction of the RIT ion extracting electrode by the plane wall probe simulator. Slight non-uniformity of the resulting plasma distributions and simplified RIT model design showed that the studied device with flat antenna coil and ferrite core could be considered as a promising prospect for RITs of new generation.

Experimental measurements of the hollow cathode DC glow discharge parameters in Ar and He plasmas

In this article, we focus on some of the fundamental parameters of SS316L hollow cathode glow discharge. Four SS316L samples are placed at different locations on the cathode surface and the current passed through them is measured in Ar and He glow discharge plasmas. The wall current densities of Ar and He are in the range of 8–25 µA cm−2 and 8–35 µA cm−2, respectively. Results also show that with decreasing working pressure, the ion flux and current density distribution on the wall surface becomes more uniform. The ion flux of the Ar and He is in the range of 1013 to 1014 . Total energy losses of Ar and He are measured at the pressure range of 1.4–5.5 × 10−2 torr and 3.2–7.1 × 10−1, respectively. In both Ar and He, total energy losses decreased with increasing pressure. The secondary electron emission coefficients of Ar and He, which are evaluated for the hallow cathode configuration, are about 0.42 and 0.26, respectively, and are higher in comparison with the plate cathode configuration.

Ablation plasma transport using multicusp magnetic field for laser ion source

We propose a plasma guiding method using multicusp magnetic field to transport the ablation plasma keeping the density for developing laser ion sources. To investigate the effect of guiding using the magnetic field on the ablation plasma, we demonstrated the transport of the laser ablation plasma in the multicusp magnetic field. The magnetic field was formed with eight permanent magnets and arranged to limit the plasma expansion in the radial direction. We investigated the variation of the plasma ion current density and charge distribution during transport in the magnetic field. The results indicate that the plasma is confined in the radial direction during the transport in the multicusp magnetic field.

Preliminary results concerning the simulation of beam profiles from extracted ion current distributions for mini-STRIKE.

The Radio Frequency (RF) negative hydrogen ion source prototype has been chosen for the ITER neutral beam injectors due to its optimal performances and easier maintenance demonstrated at Max-Planck-Institut für Plasmaphysik, Garching in hydrogen and deuterium. One of the key information to better understand the operating behavior of the RF ion sources is the extracted negative ion current density distribution. This distribution-influenced by several factors like source geometry, particle drifts inside the source, cesium distribution, and layout of cesium ovens-is not straightforward to be evaluated. The main outcome of the present contribution is the development of a minimization method to estimate the extracted current distribution using the footprint of the beam recorded with mini-STRIKE (Short-Time Retractable Instrumented Kalorimeter). To accomplish this, a series of four computational models have been set up, where the output of a model is the input of the following one. These models compute the optics of the ion beam, evaluate the distribution of the heat deposited on the mini-STRIKE diagnostic calorimeter, and finally give an estimate of the temperature distribution on the back of mini-STRIKE. Several iterations with different extracted current profiles are necessary to give an estimate of the profile most compatible with the experimental data. A first test of the application of the method to the BAvarian Test Machine for Negative ions beam is given.

2D fluid simulation of capacitively coupled plasma with cylindrical electrode for roll-to-roll processing

RF glow discharges are widely used in deposition, etching and surface treatments for semiconductor materials. Especially, roll-to-roll processing is necessary for flexible display or thin film solar cell modules. For a roll-to-roll thin film deposition processing, it is necessary to use cylindrical grounded electrode. The effects of electrode shapes on the plasma properties of capacitively coupled rf argon discharge at 13.56 MHz are investigated using self-consistent two-dimensional fluid simulation. The simulation model includes blocking capacitor and external circuit effects. Comprehensive analysis has been made in terms of time-varying current–voltage characteristic, spatial distributions of plasma density and ion current density on the substrate. The effects of applied voltage (Vf=200∼800[V]) and gas pressure (p=0.5 ∼ 2 Torr) on the plasma properties are also investigated. We found that flat electrode has a little bit higher impedance than that of cylindrical ground electrode. However the difference between the two configurations is less than 5% therefore we may conclude that the shape of grounded electrode does not significantly affect current–voltage characteristics of the discharge system. For roll-to-roll reactor version, displacement current come from dielectric layer between electrodes reaches more than 75% of total displacement current. Large displacement current results in degradation of power factor and matching circuit efficiency. For the cylindrical electrode, increasing electrode gap length along lateral direction makes that the location of peak electron density moves toward center of the reactor. The effect of the cylindrical electrode on the plasma spatial structure is reflected in the ion current density at substrate (dielectric layer) on grounded electrode. At low voltage and low power condition, about 90% of input power is consumed for electron heating. The efficiency decreases with increasing power and decreasing pressure as a consequence of increasing power to drive ion current. Electron heating efficiency decreased with pressure. The increase of pressure mainly reduces the diffusive component of the loss rate for both charged and neutral species.

Influence of Cathode Shape on Vacuum Arc Thruster Performance and Operation

Vacuum arc thrusters (VATs) are a potentially useful technology for use as microthrusters, given their simplicity, low mass, and low-power requirements. However, further development is still needed to improve and mature their performance and operation. In an effort to improve thruster characteristics, we investigate the use of conically shaped convergent cathode surface profiles in a low-power coaxial VAT design. Experimental measurements of ion current density distribution, thrust, and erosion rate were taken for a range of cathode profiles and materials. Results show that modifying the profile of the cathode face can affect the plasma jet's plume distribution and even improve thrust production in certain cases. One likely cause of the thrust improvement is speculated to be increased charge exchange collisions resulting in a lower average ion charge state of the ejecting plasma.

Modelling of tokamak glow discharge cleaning I: physical principles

Glow discharge cleaning (GDC) is a common technique for the conditioning of tokamak vessel walls in order to improve plasma performance and will be one of the primary conditioning techniques in ITER. The GDC discharge is a dc low-temperature plasma discharge, operated in the absence of the toroidal magnetic field, between one or more anodes inserted into the vessel, and the entire vessel wall serving as a cathode. This paper presents a self-consistent 2D model of the GDC discharge with the aim of improving fundamental understanding and predicting the wall ion current density distribution in ITER. The model combines a standard fluid model of the quasineutral plasma bulk with non-standard fluid equations for the fast electrons accelerated by the cathode sheath, based on transport coefficients and rate coefficients deduced from a Monte Carlo simulation. Examples of model results are shown in order to illustrate the general principles of the GDC discharge and the influence of the model input parameters. An important insight gained from this work is that the GDC discharge operates basically as a hollow-cathode discharge: the plasma is sustained mainly by ionization by secondary electrons emitted from the cathode, accelerated ballistically through a thin cathode sheath, penetrating the plasma as a fast electron beam, and trapped by the cathode fall surrounding the plasma on all sides. The electric field distribution inside the plasma, which determines the ion flux distribution on the vessel walls, is controlled by low-energy plasma bulk electrons. The relatively small surface area of the anode leads to the formation of an anode glow affecting the plasma uniformity. Comparisons with experimental data and predictions for ITER are presented in a companion paper.