Retarding Potential Analyzer Research Articles

Development and characterization of an iodine field emission ion source for focused ion beam applications

Emission of positive and negative ions is possible when a room-temperature molten salt, or ionic liquid, is exposed to a sufficiently high electric field. Ionic liquid ion sources (ILISs) have shown potential to be used in various focused ion beam (FIB) applications, since their operation and characteristics are similar to those of liquid metal ion sources, with the advantage that ILIS work at low temperatures in comparison and a large number of ionic liquids with many different compositions are available. In this article, the authors present results on the emission characteristics of negative ions extracted from an iodine-based ionic liquid using a time-of-flight mass spectrometer and a retarding potential analyzer. The ionic liquid 1-butyl-3-methylimidazolium iodide (BMI-I) is used as source media, producing a droplet free beam with multiple solvated ion species. Attention is given to BMI-I, in particular, due to the potential of creating a beam of pure and clustered I-ions, which are expected to improve the performance in applications based on secondary emission and reactive species. Properties important to the focusing of the ion beam such as mass and energy distributions are obtained. The full width at half maximum of the energy distribution is measured to be on the order of 10eV, with an estimated angular intensity of 0.65×10−6Asr−1. Estimated performance of the ILIS in a hypothetical FIB column gives current densities of 0.15Acm−2.

Ion diagnostics of a discharge in crossed electric and magnetic fields for electric propulsion

The velocity distribution function (VDF) of metastable Xe+ ions was measured along the channel centerline of the high-power PPS®X000 Hall effect thruster by means of laser induced fluorescence (LIF) spectroscopy at 834.72 nm for various discharge voltages (300–700 V) and propellant mass flow rates (6–15 mg s−1). The development of the on-axis profile of the velocity dispersion reveals the interrelation between ionization and acceleration layers. The ion velocity profiles are in accordance with outcomes of a hybrid numerical model in which the electron mobility is assessed from particle-in-cell simulations. The axial distribution of the effective electric field is inferred from the mean ion velocity profile, despite the parasitic effect due to ions created in the acceleration region. Most of the acceleration process takes place outside the thruster channel. The electric field augments and it moves upstream when the applied voltage is ramped up. The impact of the xenon mass flow rates is found to depend upon the voltage. A novel approach based on the moments of the experimental VDFs in combination with the Boltzmann's equation is introduced in order to determine the real electric field distribution. The method also provides the ionization frequency profile. The LIF diagnostics reveals the existence at the end of the acceleration region of fast ions of which the kinetic energy is above the supplied energy. The fraction of these supra-sped up ions grows when the voltage increases. The ion VDFs were also recorded in the plasma plume far field by way of a retarding potential analyzer (RPA). The shape of the RPA traces as well as their evolution with operating conditions are in agreement with trends observed by means of LIF spectroscopy. Finally, physical mechanisms at the origin of supra-sped up ions are discussed in light of numerical simulation outcomes and a set of new experimental results.

A statistical analysis of systematic errors in temperature and ram velocity estimates from satellite-borne retarding potential analyzers

The use of biased grids as energy filters for charged particles is common in satellite-borne instruments such as a planar retarding potential analyzer (RPA). Planar RPAs are currently flown on missions such as the Communications/Navigation Outage Forecast System and the Defense Meteorological Satellites Program to obtain estimates of geophysical parameters including ion velocity and temperature. It has been shown previously that the use of biased grids in such instruments creates a nonuniform potential in the grid plane, which leads to inherent errors in the inferred parameters. A simulation of ion interactions with various configurations of biased grids has been developed using a commercial finite-element analysis software package. Using a statistical approach, the simulation calculates collected flux from Maxwellian ion distributions with three-dimensional drift relative to the instrument. Perturbations in the performance of flight instrumentation relative to expectations from the idealized RPA flux equation are discussed. Both single grid and dual-grid systems are modeled to investigate design considerations. Relative errors in the inferred parameters for each geometry are characterized as functions of ion temperature and drift velocity.

Plume Profiles of a Planar Crossed-Field Thruster with Hall Current Injection

Stable operation of a Hall thruster that emits and collects the Hall current across a planar discharge channel is described. Hall current was emitted by hollow cathode electron sources and collected by electrodes on the opposing wall of the thruster. During this initial test, the planar Hall thruster was operated at discharge voltages between 100― 150 V. Internal channel wall probes, along with a downstream Faraday probe and retarding potential analyzer, measured changes in thruster plasma as the discharge voltage and magnetic field were adjusted. Results show that most of the plume ions were created in the acceleration zone and gain only 60―70% of the discharge voltage. Furthermore, the axial plume ion energy decreased with increasing magnetic field. Specifically, when the electromagnet was increased from 1.5 to 3.5 A, the ion energy decreased 25%. The plume current density profile showed a peak at 15―20 deg off-centerline, and this angle increased with increasing magnetic field. Specifically, when the electromagnet was increased from 1.5 to 3.5 A, the peak location shifted 4 deg farther from the centerline. Analysis of these results suggests that a buildup of Hall current electrons on one side of the discharge channel leads to a nonuniform plasma density. Further, magnetohydrodynamic effects on the expelled ion beam leads to cross-field ion velocity, resulting in the off-centerline peak in the current density profile.

Retarding potential and induction charge detectors in tandem for measuring the charge and mass of nanodroplets

The determination of the mass of a nanoparticle via time-of-flight typically requires a direct measurement of its charge. This can be done with a differential retarding potential analyzer and an induction charge detector operating in tandem. The spectrometer described in this article selects a particle with a specified retarding potential from a beam and directs it to an induction charge detector where both its velocity and charge are measured. The retarding potential, velocity, and charge also yield the particle's mass. The particle is analyzed without the need to collect it, and therefore can be employed in a subsequent experiment. The high charge sensitivity of the induction charge detector and the capability for working at low retarding potentials make the characterization of electrosprayed nanodroplets possible for the first time.

Observation of Suprathermal Flux from SROSS-C2 Data at Low Latitude

The studies of suprathermal flux at equatorial latitude for low solar activity year of 1996 (from January to December) are reported. The retarding potential analyzer (RPA) data of Indian space research organization Stretched Rohini Satellite Series-C2 (SROSS-C2) are used. The suprathermal electron flux data are chosen in the height range of km. Seasional, latitudinal, and height variations of suprathermal flux having energy range above 30 eV along with comparisons in latitudinal variation of small energy superathermal flux of 5 eV are also reported. The data are selected during the period of local midnight to avoid contaminations from photoelectron flux and flux coming from coating of electrodes.

The SMART-1 Spacecraft Potential Investigations

The SMART-1 mission was successfully completed on September 3, 2006, with a controlled crash on the visible side of the Moon. The primary electric-propulsion (EP) system included a nonredundant SNECMA PPS-1350-G Hall effect thruster. The thruster discharge was electrically floating with respect to the spacecraft ground. The cathode floating potential ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CRP</sub> ) is an important thruster parameter, giving information on cathode performance and life expectancy. During ground testing and steady-state operation, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CRP</sub> was approximately constant and negative. However, in space, it was observed that the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CRP</sub> was slightly positive and was periodically varying in time by a few volts. The capability to compare to ground-test data and check the cathode good health and life potential was largely impaired. Fortunately, SMART-1 included a plasma diagnostic package (electric-propulsion diagnostic package) with a Langmuir probe and a retarding-potential analyzer, both positioned on the same plane as the plasma thruster and provided data on the plasma potential as well as the energy of the charge-exchange ions. The short- and long-term correlations of these data with the cathode reference potential (CRP) were performed. It was concluded that the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CRP</sub> is evolving with respect to the plume potential near the cathode, and this potential difference is shown to be constant over the mission. Plume-potential distribution is adapting itself relative to the local plasma potential. The spacecraft potential with respect to the plasma potential is defined in steady state when the electron and ion currents entering and leaving the spacecraft are balanced. This balance depends primarily on the solar-array electrical configuration and orientation and thruster operating conditions. When the local plasma is chosen as a reference, the absolute cathode potential is found to be the same as the one on the ground. This paper confirms the good health of the cathodes. Contrary to a spacecraft without EP, the SMART-1 spacecraft potential varies within a limited range and is comparatively close to zero during electric-thruster operation. In addition, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CRP</sub> can be used to derive the spacecraft potential throughout the mission. All the various cyclic and long-term effects influencing the spacecraft potential on SMART-1 have been identified and characterized across the mission lifetime. In particular, a strong correlation between the solar-array rotation and the variation of the spacecraft potential has been established.

Errors in ram velocity and temperature measurements inferred from satellite-borne retarding potential analyzers

Retarding potential analyzers (RPAs) [Heelis and Hanson, Measurement Techniques in Space Plasmas, in Geophys. Monogr. Ser., Vol. 102 (AGU, Washington, D.C., 1998), p. 61] have been used extensively in space science over the past five decades to provide in situ estimates of ion velocities and temperatures. It has been shown previously that the use of biased grids in such instruments creates a nonuniform potential in the grid plane, which leads to errors in inferred parameters. A simulation of ion interactions with biased grids has been developed using a commercial finite-element analysis software package. Using a statistical approach, perturbations to the idealized RPA equation are discussed with the intent of developing quantitative corrections for the inferred parameters. The transparency of the grid stacks is found to be a function of particle energy and angle of attack. A preliminary case study is presented and compared to previous work.

Absolute Single and Multiple Charge Exchange Cross Sections for Highly Charged C, N, and O Ions Colliding with CH4

Absolute charge exchange (CE) cross sections have been measured for single and multiple exchanges between the highly charged ions (HCIs) Cq+ ( -->q = 3–6), Nq+ ( -->q = 4–7) and Oq+ ( -->q = 5–7) colliding with the target methane (CH4) present in comet and planetary atmospheres. Measurements were made at collision energies of 7×q keV (1.8-3.5 keV amu−1), representative of the fast component of the solar wind. The HCIs are produced in a 14 GHz electron cyclotron resonance ion source. Absolute cross sections are measured using the retarding potential analyzer method, with knowledge of the target length, density, and projectile currents. Comparison is made with results of the classical trajectory Monte Carlo method, and the classical overbarrier model.

Experimental Characterization of a Micro-Hall Thruster

The operation and performance of a micro-Hall thruster are characterized. The thruster is coaxial in design, with a 0.5 mm channel width and 4 mm outer diameter. The magnetic circuit includes a samarium cobalt permanent magnet generating approximately 0.7 T at the exit plane and 1 T inside the channel. Operation with a commercial hollow cathode neutralizer is achieved in the 10-40 W power range with an anode flow rate of 0.12-0.20 mg/s of xenon. The measured thrust is in the range of 0.6-1.6 mN for an anode flow rate of 0.17-0.20 mg/s and an applied voltage of 110-275 V. Anode thrust efficiency and specific impulse are in the range of 10-15% and 300-850 s, respectively, for the same conditions. Relatively broad ion energy distributions and large beam divergence are observed from an analysis of the plume using a retarding potential analyzer and ion current probe. The discharge exhibits the characteristic Hall thruster breathing mode instability in the 35-70 kHz frequency range.

The Solar Wind Around Pluto (SWAP) Instrument Aboard New Horizons

The Solar Wind Around Pluto (SWAP) instrument on New Horizons will measure the interaction between the solar wind and ions created by atmospheric loss from Pluto. These measurements provide a characterization of the total loss rate and allow us to examine the complex plasma interactions at Pluto for the first time. Constrained to fit within minimal resources, SWAP is optimized to make plasma-ion measurements at all rotation angles as the New Horizons spacecraft scans to image Pluto and Charon during the flyby. To meet these unique requirements, we combined a cylindrically symmetric retarding potential analyzer with small deflectors, a top-hat analyzer, and a redundant/coincidence detection scheme. This configuration allows for highly sensitive measurements and a controllable energy passband at all scan angles of the spacecraft.

Characterization of the Plasma Properties of a Reverse Polarity Planar Magnetron Operated as an Ion Source

In the present study the possibility of using a planar magnetron configuration as an ion source has been studied by inverting the polarity of a DC magnetron. The characterization of the plasma properties has been performed under various operating conditions using retarding potential analyzer (RPA), Langmuir probe, and Faraday cup. The ion energy is 291 eV at 490 V anode voltage and pressure of 5 × 10−4 mbar measured using a RPA. Also the ion density is ∼1012 per cm3 as measured by Langmuir probe. Spread in energy is less (∼3 eV) at lower pressure and at higher anode voltage. This configuration behaves very well like an ion source and can be used for sputter cleaning of substrates and for ion beam-assisted deposition applications.

Absolute single and multiple charge exchange cross sections for highly charged C, O, and Ne ions onH2O, CO, andCO2

Reported herein are measured absolute single, double, and triple charge exchange (CE) cross sections for the highly charged ions (HCIs) ${\mathrm{C}}^{q+}$ $(q=5,6)$, ${\mathrm{O}}^{q+}$ $(q=6,7,8)$, and ${\mathrm{Ne}}^{q+}$ $(q=7,8)$ colliding with the molecular species ${\mathrm{H}}_{2}\mathrm{O}$, CO, and $\mathrm{C}{\mathrm{O}}_{2}$. Present data can be applied to interpreting observations of x-ray emissions from comets as they interact with the solar wind. As such, the ion impact energies of $7.0q\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ $(1.62--3.06\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu})$ are representative of the fast solar wind, and data at $1.5q\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ for ${\mathrm{O}}^{6+}$ $(0.56\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu})$ on CO and $\mathrm{C}{\mathrm{O}}_{2}$ and $3.5q\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ for ${\mathrm{O}}^{5+}$ $(1.09\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu})$ on CO provide checks of the energy dependence of the cross sections at intermediate and typical slow solar wind velocities. The HCIs are generated within a $14\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$ electron cyclotron resonance ion source. Absolute CE measurements are made using a retarding potential energy analyzer, with measurement of the target gas cell pressure and incident and final ion currents. Trends in the cross sections are discussed in light of the classical overbarrier model (OBM), extended OBM, and with recent results of the classical trajectory Monte Carlo theory.

Ion energy measurements near a dormant cathode in a multiple-cathode gridded ion thruster

A rectangular ion thruster discharge chamber was investigated for operation with multiple discharge cathode assemblies (DCAs). The multiple cathode approach attempts to increase thruster throughput and lifetime by operating three DCAs sequentially, possibly providing a threefold increase in discharge life. Previous multiple-cathode electric propulsion devices, such as the SPT-100, have shown dormant cathode erosion to be a life-limiting phenomenon. Similar results in a multiple-cathode discharge chamber may decrease the anticipated gain in discharge lifetime. In order to assess possible dormant cathode sputtering erosion, a diagnostic canister (DC) was designed and utilized to measure bombarding ion energy at the dormant cathode locations. The DC appeared similar to the active DCA, but was outfitted with a retarding potential analyzer. Most probable ion energy measurements show ions with energy of 27–35eV (±10%) with respect to cathode common and ion energy increases with increasing magnetic field strength. These results are consistent with an ion falling from the plasma potential to cathode common. A simple sputtering erosion model shows that, if doubly charged ions are present, these energies are enough to cause sputtering erosion of the dormant units.

Scintillation modeling using in situ data

Satellite in situ measurements of plasma (electron) density fluctuations provide direct information about the structure and morphology of irregularities that are responsible for scintillation of radio waves on transionospheric links. When supplemented with the ionosphere model and irregularity anisotropy model, they can be applied to model morphology of scintillation provided a suitable propagation model is used. In this paper we present a scintillation climatological model for the Northern Hemisphere high‐latitude ionosphere, which makes use of the Dynamics Explorer 2 retarding potential analyzer plasma density data, IRI ionosphere model, and the phase screen propagation model. An important aspect of our work is that we derived from the satellite data not just the turbulence strength parameter Cs but also the spectral index p, which influences the scintillation level as well. We discuss the magnetic activity, season, magnetic time, and latitude dependence of these parameters. We were able to reproduce successfully the observed scintillation intensity diurnal and seasonal variations. The model satisfactorily describes the expansion of the scintillation zone under magnetically disturbed conditions and reproduces the dawn‐dusk asymmetry in the scintillation intensity. The results demonstrate the usefulness of the proposed approach.

Characteristics of end Hall ion source with magnetron hollow cathode discharge

An end Hall ion source with magnetron hollow cathode discharge is described. The source is suitable for high current, low energy ion beam applications such as Hall current plasma accelerators. The end Hall ion source is based on an anode layer thruster with closed drift electrons that move in a closed path in the E×B field. Only a simple magnetron power supply is used in the ion source. The special configuration enables uninterrupted and expanded operation with oxygen as well as other reactive gases because of the absence of an electron source in the ion source. In our evaluation, the ion beam current was measured by a circular electrostatic probe and the energy distribution of the ion beam was measured by a retarding potential analyzer (RPA). An ion beam current density of up to 10mA/cm2 was obtained at a mean ion energy of 100–250eV using Ar or O2. The ion source can be operated in a stable fashion at a discharge voltage between 200 and 500V and without additional electron triggering. The discharge power of the ion source can be easily changed by adjusting the gas flow rate and anode voltage. No water cooling is needed for power from 500W to 2kW. The simple and rugged ion source is suitable for industrial applications such as deposition of thin films with enhanced adhesion. The operational characteristics of the ion source are experimentally determined and discussed.

ホールスラスタの推進効率および内部効率に与える磁場特性の影響

The effects of magnetic field strength and shape on thrust efficiency of a Hall thruster were examined experimentally using the THT-IV 1-kW-class Hall thruster. Thrusts were measured with varying magnetic field and channel geometry. Exhaust ion beam measurement was also implemented to evaluate internal efficiencies of propellant utilization efficiency, acceleration efficiency, voltage utilization efficiency and beam focusing efficiency. Spatial ion current profiles were measured with a Faraday probe and ion energy distribution functions were estimated from data with a retarding potential analyzer. Stable operation was obtained over the wide range of operational condition, and voltage utilization efficiency and beam focusing efficiency were estimated suitably. Acceleration efficiency and propellant utilization efficiency, however, were not able to be evaluated experimentally because the total ion current calculated by integrating the ion current density over the measured area was very small. Using obtained results, the relationships between each internal efficiency and thrust performance and between magnetic field characteristics and thrust efficiency were discussed. Consequently, it was expected that the change of magnetic field strength mainly affected acceleration efficiency and voltage utilization efficiency, and that the change of magnetic field shape affected voltage utilization efficiency in long discharge channel case and beam focusing efficiency in short channel case.

On a possible relationship between density depletions in the SAA region and storm-enhanced densities in the conjugate hemisphere

During the great magnetic storm of July 15, 2000, the ROCSAT-1 satellite with an orbital altitude of 640 km detected a large density depletion region at low latitudes (<35°) in the southern hemisphere while storm-enhanced density (SED) was observed in the northern hemisphere. Convective electric fields deduced from measurements of the ROCSAT-1 drift meters and retarding potential analyzer (RPA) indicate negligible electric fields in the eastern portion of the SED region where density enhancement was evident, suggesting a lack of ion upward motion at the equator of the SED longitudes. However, a localized electric field enhancement was observed in association with South Atlantic magnetic anomaly density structures (MADS) during the event. The zonal electric field component was eastward corresponding to radial convection, and the radial electric field component on the magnetic meridional plane was outward corresponding to westward convection. In general, both components of the observed convective electric fields were enhanced inside the density depletion region and reduced inside the density enhancement. The electric field patterns are consistent with those arising from a circular region of flux tubes with enhanced conductance in the presence of a background electric field. The mapping of the MADS region and convective electric fields from the southern hemisphere to the northern hemisphere suggests that these localized electric fields could drive ionospheric plasma from the conjugate MADS region to the SED at higher latitudes. We thus propose a new idea that the conjugate MADS region could be the plasma source for producing the SED.

Ions and ion-energy spectra of a collisional laser plasma produced from multi-species targets of aluminium and titanium

Experimental measurements on angular distribution of ions and ion-energy spectra from planar slab targets of Al, Ti and binary targets of Al, Ti with different stoichiometries are reported. The plasma was produced by a 125 mJ, 5 ns, 1.06 µm Nd:YAG laser, incident on a planar target at a fixed angle of −45°. The laser was focused on an ablation area of about 0.3 mm2 at a laser intensity of about 1010 W cm−2. The characteristics of the ions were obtained using a retarding-potential analyser and a quartz crystal. Results on time-of-flight spectra, ion fractions, average ionization, angular distributions of particles and their kinetic energy are presented. Full width at half maximum of the angular distributions was obtained and the results are discussed. Measurements on total integrated energy per particle are also reported. They show evidence of energy transfer from light to heavy particles in agreement with the theoretical simulation results of earlier workers. Ion acceleration due to an in-built electrostatic potential is discussed in detail and its effect was found to be marginal.

Diurnal, seasonal and latitudinal variation of ion concentrations as obtained from the SROSS C2 satellite over Indian low and equatorial latitudes during solar minimum

In situ measurements carried out by the Retarding Potential Analyzer on board the SROSS C2 satellite at equatorial and low latitudes during the low solar activity period of 1995 and 1996 are used to study the diurnal, seasonal and latitudinal variations of the concentrations of O +, H +, He + and O 2 + ions at the average altitude of ∼500 km in the Indian longitude sector. The concentration of the major ion O + is minimum before local sunrise and reaches its peak near local noon or in the afternoon hours depending on season and latitude of observation. The minimum and maximum densities vary with season and latitude. The H + concentration is found to be one order of magnitude less than that of O + during the daytime at all latitudes. At night, hydrogen ion concentration is comparable to the concentration of atomic oxygen ion, which indicates a lowering of the O + to H + transition altitude in this period. The ratio of night to day H + concentration is higher in summer than in winter and equinox. The measured concentration of He + is one order of magnitude less than that of H +. Helium ion concentration is higher in the December solstice compared to that in the June solstice in the forenoon hours while in the afternoon the relative densities in the solstices reverse in magnitude. The He + density is low at night and remains at a constant higher level during the daytime. The low nighttime He + densities indicate a higher O + to He + transition altitude compared to O + to H + transition altitudes. O 2 + concentration shows higher daytime values with peak in the midday or afternoon hours but it is a minor constituent at ∼500 km during solar minimum over the Indian longitude sector compared to H + or even He +.