Zoltan Sternovsky

Trapped ion effect on shielding, current flow, and charging of a small object in a plasma

The problem of electrostatic shielding around a small spherical collector immersed in nonflowing plasma, and the related problem of electron and ion flow to the collector, date to the origins of plasma physics. Calculations have typically neglected collisions, on the grounds that the mean free path is long compared to the Debye length. However, it has long been suspected that negative-energy trapped ions, created by occasional collisions, could be important. This paper presents self-consistent analytic calculations of the density and distribution function of trapped and untrapped ions, the potential profile, the ion and electron current to the collector, and the floating potential and charge of the collector. Under typical conditions for dust grains immersed in a discharge plasma, trapped ions are found to dominate the shielding near the grain, substantially increase the ion current to the grain, and suppress the floating potential and grain charge, even when the mean free path is much greater than the Debye length.

Contact charging of lunar and Martian dust simulants

micron dust grain is typically more than 10 5 elementary charges and varies linearly with dust size. The measured contact charge of a dust particle increases with repeated agitation of the surface. The average contact charge also varies linearly with the work function of the contacting surface. The contact charging with oxidized metal surfaces is found to be independent of the metal’s work function. The effective work functions of the planetary analogs are determined by extrapolation to be 5.8 eVand 5.6 eV for the lunar and Martian dust simulants, respectively. INDEX TERMS: 3914 Mineral Physics: Electrical properties; 3947 Mineral Physics: Surfaces and interfaces; 5470 Planetology: Solid Surface Planets: Surface materials and properties; 6225 Planetology: Solar System Objects: Mars; 6250 Planetology: Solar System Objects: Moon (1221); KEYWORDS: Dust, contact charging, lunar dust, Martian dust, work function

A permanent, asymmetric dust cloud around the Moon

Interplanetary dust particles hit the surfaces of airless bodies in the Solar System, generating charged and neutral gas clouds, as well as secondary ejecta dust particles. Gravitationally bound ejecta clouds that form dust exospheres were recognized by in situ dust instruments around the icy moons of Jupiter and Saturn, but have hitherto not been observed near bodies with refractory regolith surfaces. High-altitude Apollo 15 and 17 observations of a ‘horizon glow’ indicated a putative population of high-density small dust particles near the lunar terminators, although later orbital observations yielded upper limits on the abundance of such particles that were a factor of about 104 lower than that necessary to produce the Apollo results. Here we report observations of a permanent, asymmetric dust cloud around the Moon, caused by impacts of high-speed cometary dust particles on eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths striking the Moon at lower speeds. The density of the lunar ejecta cloud increases during the annual meteor showers, especially the Geminids, because the lunar surface is exposed to the same stream of interplanetary dust particles. We expect all airless planetary objects to be immersed in similar tenuous clouds of dust.

Micrometeoroid impact charge yield for common spacecraft materials

The impact ionization charge yield is experimentally measured from four common materials used in space and specifically on the two STEREO spacecraft (germanium-coated black Kapton, beryllium copper, multilayer insulation, and solar cells). Cosmic dust particle impacts on spacecraft have been detected by electric field and plasma and radio wave instruments. The accurate interpretation of these signals is complicated by many factors, including the details of the spacecraft antenna system, the local spacecraft plasma environment, and our understanding of the physics of the impact process. The most basic quantity, the amount of charge liberated upon impact, is generally considered poorly constrained and is suspected to depend on the target material. Here we show that for common materials used on spacecraft this variability is small for impacts around 10 km/s, and the impact charge yield can be approximated by 80 fC for a 1 pg projectile. At higher speeds (∼50 km/s), variation of up to a factor of 5 is observed. The measured yields in the 10–50 km/s range are compared to measurements and predictions from the literature and are found to be lower than predicted by at least a factor of 12 at 10 km/s and at least a factor of 1.7 at 50 km/s. Impact charge is also found to depend on angle of incidence; the data suggest a maximum at 45°.

3 MV hypervelocity dust accelerator at the Colorado Center for Lunar Dust and Atmospheric Studies

A hypervelocity dust accelerator for studying micrometeorite impacts has been constructed at the Colorado Center for Lunar Dust and Atmospheric Studies (CCLDAS) at the University of Colorado. Based on the Max-Planck-Institüt für Kernphysik (MPI-K) accelerator, this accelerator is capable of emitting single particles of a specific mass and velocity selected by the user. The accelerator consists of a 3 MV Pelletron generator with a dust source, four image charge pickup detectors, and two interchangeable target chambers: a large high-vacuum test bed and an ultra-high vacuum impact study chamber. The large test bed is a 1.2 m diameter, 1.5 m long cylindrical vacuum chamber capable of pressures as low as 10(-7) torr while the ultra-high vacuum chamber is a 0.75 m diameter, 1.1 m long chamber capable of pressures as low as 10(-10) torr. Using iron dust of up to 2 microns in diameter, final velocities have been measured up to 52 km/s. The spread of the dust particles and the effect of electrostatic focusing have been measured using a long exposure CCD and a quartz target. Furthermore, a new technique of particle selection is being developed using real time digital filtering techniques. Signals are digitized and then cross-correlated with a shaped filter, resulting in a suppressed noise floor. Improvements over the MPI-K design, which include a higher operating voltage and digital filtering for detection, increase the available parameter space of dust emitted by the accelerator. The CCLDAS dust facility is a user facility open to the scientific community to assist with instrument calibrations and experiments.

Charging of dust particles on surfaces

Experimental investigations have been made of the charge on dust particles resting upon a metal surface in vacuum. The surface is agitated so that the particles drop through a small hole and a Faraday cup beneath measures the charge on each particle. The surfaces are metals (Hf, Zr, V, W, Co, Ni, Pt, and stainless steel) and the dust grains are both metallic conductors (Zn, V, and stainless steel) and insulators (silica and alumina) in the size range of 50–200 μm. The contact charge is consistent with a model based upon the grain capacitance and the effective contact potential between the grain and surface. An electric field above the surface induces an additional charge on metallic grains consistent with Gauss’s law. The induced charge on insulating grains increases with repeated contact. UV irradiation may increase or decrease the charge depending upon the relative importance of photoemission and photoconductivity.

Large area mass analyzer instrument for the chemical analysis of interstellar dust particles

A new instrument to analyze the chemical composition of dust particles in situ in space has been developed. The large target area ( approximately 0.2 m(2)) makes this instrument well suited for detecting a statistically significant number of interstellar dust grains or other dust particles with a low flux. The device is a reflectron-type time-of-flight mass spectrometer that uses only flat electrodes for the generation of the parabolic potential. The instrument analyzes the ions from the impact generated plasma due to hypervelocity dust impacts onto a solid target surface. The SIMION ion optics software package is used to investigate different potential field configurations and optimize the mass resolution and focusing of the ions. The cylindrically symmetric instrument operates with six ring electrodes and six annular electrodes biased to different potentials to create the potential distribution of the reflectron. The laboratory model of the instrument has been fabricated and tested. Hypervelocity dust impacts are simulated by laser ablation using a frequency doubled Nd:YAG laser with approximately 8 ns pulse length. The experimental data show typical mass resolution m/Deltam approximately 200.

Laboratory investigation of antenna signals from dust impacts on spacecraft

We describe laboratory experiments which reproduce characteristic signals observed on spacecraft, believed to be caused by dust impact. A simulated spacecraft, including an antenna system using a facsimile of the preamplifier electronics from the STEREO/WAVES instrument, was bombarded by 10 km/s submicron-sized dust at the University of Colorado Institute for Modeling Plasma, Atmospheres, and Cosmic Dust accelerator facility. Signal variation was investigated as a function of the DC potentials of both the spacecraft and the antennas. We observed (1) signals corresponding to modification of the spacecraft body potential, an important process believed to be responsible for the so-called “triple hit” antenna signals on STEREO, (2) a few-eV energy distribution for the electrons and ions released in the impact leading to (3) signals corresponding to direct recollection of a substantial fraction of the impact charge by the spacecraft antennas, even at modest antenna bias potentials. We also observe (4) an unexpected class of fast antenna signals, which do not appear to be caused by charge recollection by either the spacecraft or the antennas and may be induced by charge separation in the expanding plasma cloud. Similar signals are also commonly observed by the STEREO/WAVES instrument but have not previously been analyzed.

Mass spectrometry of hyper-velocity impacts of organic micrograins

The study of hyper-velocity impacts of micrometeoroids is important for the calibration of dust sensors in space applications. For this purpose, submicron-sized synthetic dust grains comprising either polystyrene or poly[bis(4-vinylthiophenyl)sulfide] were coated with an ultrathin overlayer of an electrically conductive organic polymer (either polypyrrole or polyaniline) and were accelerated to speeds between 3 and 35 km s(-1) using the Heidelberg Dust Accelerator facility. Time-of-flight mass spectrometry was applied to analyse the resulting ionic impact plasma using a newly developed Large Area Mass Analyser (LAMA). Depending on the projectile type and the impact speed, both aliphatic and aromatic molecular ions and cluster species were identified in the mass spectra with masses up to 400 u. Clusters resulting from the target material (silver) and mixed clusters of target and projectile species were also observed. Impact velocities of between 10 and 35 km s(-1) are suitable for a principal identification of organic materials in micrometeoroids, whereas impact speeds below approximately 10 km s(-1) allow for an even more detailed analysis. Molecular ions and fragments reflect components of the parent molecule, providing determination of even complex organic molecules embedded in a dust grain. In contrast to previous measurements with the Cosmic Dust Analyser instrument, the employed LAMA instrument has a seven times higher mass resolution--approximately 200--which allowed for a detailed analysis of the complex mass spectra. These fundamental studies are expected to enhance our understanding of cometary, interplanetary and interstellar dust grains, which travel at similar hyper-velocities and are known to contain both aliphatic and aromatic organic compounds.

Ion collection by cylindrical probes in weakly collisional plasmas: Theory and experiment

A theoretical approach has recently been described [Z. Sternovsky, S. Robertson, and M. Lampe, Phys. Plasmas 10, 300 (2003)] for including the effect of ion collisions in the orbit motion limited theory for cylindrical Langmuir probes. In plasmas with a single ion species, ion-neutral charge exchange collisions are dominant and their first order effect is to increase the magnitude of the collected ion current. Measurements in Ar and Ne gas discharges at plasma densities <109 cm−3 show that the theory is accurate only for probes with radii less than approximately half the Debye length. For larger probes, absorption of ions at the probe surface reduces the ion density locally causing the sheath to expand. This increases the volume from which the charge exchange ions are collected and further increases the ion current. Poor agreement between measurements and theory is also found, when the probe is placed close to the ionization source.