Positive Ion Emission Research Articles

Efficiency Mechanisms in Porous-Media Electrospray Thrusters

The overall propulsion efficiency for ion-mode electrospray thrusters has been predicted to be as high as 90%; however, experimental measurements currently fall far short of these predictions. Further complicating this is that for passively fed electrospray thrusters, the mass flow rate, which is required to obtain the propulsion efficiency and specific impulse, is not directly controlled or measured, and so this parameter is typically estimated by assuming all mass loss is due to the emitted ion current. Presented here is a detailed investigation into the efficiencies associated with a porous-media-based electrospray thruster operated in the purely ionic regime using the ionic-liquid propellant 1-ethyl-3-methylimidazolium tetrafluoroborate in both positive and negative ion emission modes. Measurements of performance metrics that affect thruster efficiency are discussed, including the transmission, angular, polydispersive, energy, and mass utilization efficiencies, in order to determine their impact on overall efficiency. The overall propulsion efficiency and specific impulse are also calculated using a variety of methods to better investigate how the assumptions made to estimate the mass flow rate affect these parameters. These results suggest that the efficiency of these devices may primarily be limited by the presence of additional mass loss mechanisms other than ion emission occurring during thruster operation.

Fabrication and characterization of a fully conventionally machined, high-performance porous-media electrospray thruster

While electrospray devices have been used in a variety of applications for decades, they have recently seen a surge in research within the field of electric propulsion. These research efforts have helped significantly improve the understanding of electrospray thruster operation and optimization, however they have primarily been focused on capillary-based, droplet emitting devices due to the more readily available manufacturing techniques. In contrast, ion emitting, porous-media-based electrospray devices are less developed both theoretically and experimentally. Presented here are fabrication methods and thruster characterization results for an entirely conventionally machined, high performance porous-media electrospray thruster. The goal of this work was to explore the performance capabilities of an ion-mode electrospray thruster which could be fabricated and tested rapidly using techniques readily available to virtually any institution, with the hope of enabling more academic and industrial development of this technology. The thruster described here consisted of 576 emitters conventionally machined out of porous borosilicate glass and is able to maintain stable operation up to ± 700 µA of emitted ion current. The overall thruster design is described, and detailed fabrication steps are presented for this device. Additionally, performance characteristics are discussed for both positive and negative ion emission, including I–V curves and direct thrust measurements, as well as measurements of the emitted ion angular, 2D spatial, mass, and energy distributions. Examples of the performance of this device compared to other devices found in the literature are also discussed.

Low-temperature chemistry induced by cosmic rays: positive and negative ion desorption from nitrile-bearing astrophysical ice analogues

ABSTRACT In cold core of dark molecular clouds, where the UV radiation from external sources is strongly attenuated, cosmic rays can induce chemical reactions on the surface of ice-covered grains promoting the ejection of the processed material to the gas phase. We report the positive and negative secondary ion emission from pure CH3CN, C2H3CN, and i-C3H7CN ices due to the bombardment of heavy ions (252Cf fission fragments), simulating the incidence of cosmic rays on to icy surfaces. The secondary ions emitted from each sample were analysed by time-of-flight mass spectrometry, using Plasma Desorption Mass Spectrometry technique. Several ionic species were identified, indicating strong fragmentation on the frozen surface. Proton-transfer processes are suggested to play a role for positive ion desorption, as evidenced by the protonated RCNH+ parent molecules and (RCN)nH+ ionic clusters. The high electron affinity of the cyano radical seems to contribute to the strong emission of CN−, as well as anions attributed to the CHmCN− fragment and (RCN)nCN− cluster series. Sputtering and desorption of ion clusters (positive and negative) induced by heavy ion bombardment are suggested to constitute a route by which new neutral or ionized molecular species may be delivered to the gas phase where thermal desorption is negligible.

Simulation-Based Quantification of Alkali-Metal Evaporation Rate and Systematic Errors From Current–Voltage Characteristics of Langmuir–Taylor Detectors

This article presents an analysis for deducing the total emission rate of cesium from Langmuir–Taylor detectors embedded on the cesium ovens for a large negative-ion (NI) source. The angular distribution of the Cs emission is simulated to obtain the flux intercepted by the detector and its intensity along its length. The space-charge-limited emission of surface-ionized Cs positive ions is also studied with particle-in-cell methods, to include the geometric features of the Cs emission nozzles. Combining the two methods reproduces with good agreement the full current–voltage characteristics measured by the detector. The proposed approach can be effectively applied to obtain the total emission rate, as it was verified in the absence of impurities. The measurement uncertainties in terms of shape and alignment of the detector filament are investigated and their influence is quantified. Measuring and controlling the Cs emission rate is essential for the success and repeatability of cesium operations in the prototype NI beam source for the ITER neutral beam.

Influence of Collector Plasma on the Performance of an L-Band MAC-MILO

The influence of collector plasma has been investigated for an L-band magnetically insulated line oscillator (MILO) with metal array cathode (MAC), which showed obvious performance degradation after 3000 shots in repetitive mode. Positive ion emission model is applied to simulate plasma generation from the collector surface. Different types of positive ions have been applied to simulate different collector materials. The simulation results reveal that the collector plasma has a significant influence on the output characteristics of the original MAC-MILO. The ladder cathode structure is introduced to reduce plasma formation rate on the collector surface. The simulation and experimental results show that the optimized structure reduces the influence of collector plasma successfully.

Migration of silver on the nanoporous glasses surface under the action of an electric field.

Experimentally, the formation of micro- and nano-labyrinths in grained silver film on the surface of nanoporous glass was shown. The movement of silver ions from the bulk of the nanoporous glass towards the glass surface gradually occurs under direct current application. A similar effect was observed under direct electron irradiation of grained silver film. The proposed mechanism assumes that observed effects are caused by the formation of "hot spots" with a strong electric field, by ionic field emission of positive silver ions from the small charged nanoparticles and by trapping these ions by large nanoparticles.

The CHIANTI atomic database

The freely available CHIANTI atomic database was first released in 1996 and has had a huge impact on the analysis and modeling of emissions from astrophysical plasmas. It contains data and software for modeling optically thin atom and positive ion emission from low density (≲1013 cm−3) plasmas from x-ray to infrared wavelengths. A key feature is that the data are assessed and regularly updated, with version 8 released in 2015. Atomic data for modeling the emissivities of 246 ions and neutrals are contained in CHIANTI, together with data for deriving the ionization fractions of all elements up to zinc. The different types of atomic data are summarized here and their formats discussed. Statistics on the impact of CHIANTI to the astrophysical community are given and examples of the diverse range of applications are presented.

Positive and negative ion emission from microdroplets by MeV energy ions

SynopsisWe have developed a new experimental setup that allowed us to study collision interactions between fast ion beams and liquid droplets under a vacuum condition. Droplets of water and ethanol are irradiated with 0.4-1.5 MeV H+ and 2.0 MeV C2+ ions. The droplet diameter is estimated from energy loss measurements of projectile ions penetrating through droplets. Time-of-flight mass spectra of positive and negative secondary ions exhibit a series of cluster ions generated via protonation and deprotonation.

Molecular cluster emission in sputtering of amino acids by argon gas-cluster ions

Ten pure amino acid specimens (alanine, arginine, asparagine, glutamine, glutamic acid, isoleucine, leucine, phenylalanine, threonine, and tyrosine) and three binary mixtures (phenylalanine/tyrosine, phenylalanine/arginine, and arginine/tyrosine) were bombarded by 10keV argon gas-cluster ions Ar1000+ and the sputter-induced emission of positive secondary ions was investigated by orthogonal time-of-flight secondary ion mass spectrometry (TOF-SIMS). A prolific flux of protonated molecular cluster ions (Mn+H)+ was observed, with up to n=22 for alanine. In most cases, the cluster yields Yn were found to follow an overall exponential decay with cluster size n, Yn∝exp(−βn), although a non-monotonous decrease was found for some clusters. The parameter β varies between ∼0.2 and ∼0.65 for the different amino acids; it seems to scale inversely with the respective sputtering yields of the materials. Generally, the yields of dimers relative to monomers are surprisingly high, amounting to 20–170% whereas those of trimers and tetramers are in the range of 10–90%. In amino acid samples with an (unintentionally) enhanced Na content, Na-cationized cluster (Mn+Na)+ were found in addition to (Mn+H)+. For both molecular cluster species the yields exhibit a very similar dependence on the cluster size n. From the binary mixtures, the emission of both homo- and heteromolecular clusters was observed; their size distributions seem to indicate that the two components were not homogeneously distributed within the samples.

Dissolution of a silver film in silicon glasses under electron bombardment

It is shown that the under the action of an electron beam with electron energy E > 10 keV and electron irradiation dose Q ≥ 20 mQ/cm2 on silicate glasses coated with a silver film 80–100 nm in thickness, the silver film is dissolved partially or completely. The main mechanisms inducing this effect are the formation of a negative space charge under the glass surface, field emission of positive silver ions from the field, and migration of silver ions to the space charge region.

Impact of laser wavelength on the emission of electrons and ions from thin gold films during femtosecond laser ablation

This experimental study supports computational evidence that the excitation of d-band electrons have a significant impact on the material response of gold to an ultrashort laser pulse. A voltage biased Langmuir probe was used to detect the electron and positive ion emission from a 20nm thick Au film at 343nm, 515nm and 1030nm laser wavelengths. The laser wavelengths used provided photon energies above and below the interband transition threshold (ITT) for d-band excitation in gold (1.9eV). Variations in optical properties of the Au film at different laser wavelengths were accounted for to ensure that comparisons were made at equal values of absorbed fluences. Data showed a significantly higher number of electrons emitted over a longer period when the energy of the incident photons exceeded the ITT. The electronic current generated at the probe indicated that the first electrons detected were primarily emitted via multiphoton photoemission. Electrons were also detected at later times when the photon energy exceeded the ITT and this was attributed to thermionic emission. The current generated from the positive ion emission was significantly lower than for electron emission, but indicated the formation of a plasma at a threshold fluence that increases with decreasing photon energy.

Prolific cluster emission in sputtering of phenylalanine by argon-cluster ion bombardment

Large Arn+ cluster ions (with n∼1500 Ar atoms per cluster) with a bombarding energy of 10keV were used to investigate the sputter-induced emission of positive secondary ions from a phenylalanine specimen by orthogonal time-of-flight SIMS. An abundant flux of phenylalanine cluster ions (Mn+H)+ with n≤12 was observed. The yield of dimers relative to monomers is found to amount to 50 − 60% whereas that of trimers and tetramers is roughly 10%. Tentatively, this prolific formation of these cluster species can then be ascribed to the concerted action of the large number of Ar atoms within their impact zone at the surface: these low-energy Ar species (with an average energy of only few eV) may effect the soft cleavage of the phenylalanine bonds in the solid and lead, eventually, to the intact emission of these phenylalanine moieties.

Peptide dissociation patterns in secondary ion mass spectrometry under large argon cluster ion bombardment

The analysis of organic and biological substances by secondary ion mass spectrometry (SIMS) has greatly benefited from the use of cluster ions as primary bombarding species. Thereby, depth profiling and three-dimensional (3D) imaging of such systems became feasible. Large Ar(n)(+) cluster ions may constitute a further improvement in this direction. To explore this option, large Ar(n)(+) cluster ions (with n ~1500 Ar atoms per cluster) were used to investigate the emission of positive secondary ions from two peptide specimens (angiotensin I and bradykinin) by orthogonal time-of-flight SIMS using bombarding energies 6, 10 and 14 keV. For both peptides, the protonated molecular ion is observed in the mass spectra. In addition, distinct fragmentation patterns were observed; these indicate that fragment ions under Ar cluster irradiation form primarily via cleavage of bonds along the peptide backbone whereas the rapture of side chains occurs much less frequently. These features appear to be similar to low-energy collision-induced dissociation pathways. Tentatively, these findings can then be ascribed to the concerted action of the large number of Ar atoms in the impact zone of cluster at the surface: these low-energy Ar species (with an average energy of few eV) may effect the cleavage of the peptide bonds and lead, eventually, to the emission of the fragment ions.

Visible laser induced positive ion emissions from NaCl nanoparticles prepared by droplet rapid drying

A novel convenient way for the formation of sodium chloride (NaCl) nanoparticles on silicon wafer is proposed by using a droplet rapid drying method. The laser induced positive ion emissions from NaCl nanoparticles with and without water treatment is demonstrated by using a laser desorption/ionization time-of-flight mass spectrometer, with laser intensity well below the plasma formation threshold. It is found that the positive ion emissions from NaCl nanoparticles are obviously higher than that from microsize NaCl particles under soft 532nm laser irradiations, and water adsorption can efficiently enhance the ion emissions from NaCl nanoparticles. The initial kinetic energies of the emitted ions are estimated as 16–17eV. The synergy of the ultra-thermal effect in nanomaterials, the defect-mediated multiphoton processes, and the existence of intermediate states in NaCl-water interfaces are suggested as the mechanisms.

Neutral cesium deposition prior to SIMS depth profiling ‐ preliminary results on organic samples

In SIMS, the emission of negative secondary ions is strongly enhanced by the introduction of electropositive elements into the system. In our group, we have extensively studied SIMS analyses either performed with simultaneous cesium (Cs) deposition or with Cs deposition prior to the analysis. These studies were mainly performed on inorganic samples, focusing both on steady‐state and pre‐equilibrium conditions. The interest of Cs deposition prior to the SIMS analysis has been presented for inorganic as well as organic samples on the Cameca Sc‐Ultra and NanoSIMS50 instruments. The present work has been carried out on different organic samples including PTCDA and C60, which were covered with a layer of Cs prior to SIMS analysis. The diffusion of Cs into the sample as well as the change in emission of positive and negative secondary ions have been monitored by SIMS depth profiling by using Cs+ primary ions. Furthermore, the effect of air exposure on the obtained results has been investigated. The study on the same organic samples is completed by Cs deposition performed simultaneously to primary ion bombardment. Copyright © 2012 John Wiley & Sons, Ltd.

Sputtered ion emission under size‐selected Arn+ cluster ion bombardment

The emission of positive secondary ions from four amino acids (arginine, glycine, phenylalanine, and tyrosine) and from NaCl under irradiation with large Arn+ cluster ions was investigated by time‐of‐flight secondary ion mass spectrometry. Size‐selected 5.5‐keV Arn+ cluster ions with 300 ≤ n ≤ 2000 were used, and the flux of sputtered positive ions was monitored as function of cluster size n. For all cluster sizes, the protonated or the cationized molecular ions are observed in the mass spectra. In addition, the results show that with increasing cluster size, the number of fragment molecules strongly decreases in relation to the intact molecules, to the extent that the ratio of fragment to intact ions is 10% or less for the largest cluster ions. Generally, the ion yields Y+ were found to decrease for decreasing impact energy per cluster atom, E/n, and this attenuation was recorded down to values as low as E/n ~ 3 eV/atom. The general trend of Y+ versus E/n is similar for all targets and appears to follow a power‐law dependence, Y+ ∝ (E/n)x, with x ~ 4. This pronounced yield change could be ascribed to a reduction of the ionization probability which is envisaged to depend on the presence of free protons. Their number is expected to decrease concurrently with the decreasing amount of fragmentation for large cluster ions (i.e. for low energies/atom). Copyright © 2012 John Wiley & Sons, Ltd.

Strongly reduced fragmentation and soft emission processes in sputtered ion formation from amino acid films under large Arn+ (n ≤ 2200) cluster ion bombardment

The analysis of organic and biological substances by secondary-ion mass spectrometry (SIMS) has greatly benefited from the use of cluster ions as primary bombarding species. Thereby, depth profiling and three-dimensional (3D) imaging of such systems became feasible. Large Ar(n)(+) cluster ions may constitute a further improvement in this direction. To explore this option, size-selected Ar(n)(+) cluster ions with 300 ≤ n ≤ 2200 (bombarding energies 5.5 and 11 keV) were used to investigate the emission of positive secondary ions from four amino acid specimens (arginine, glycine, phenylalanine, and tyrosine) by time-of-flight SIMS. For all cluster sizes, the protonated molecule of the respective amino acid is observed in the mass spectra. With increasing cluster size the number of fragment ions decreases strongly in relation to the intact molecules, to the extent that the fraction of fragment ions amounts to less than 10% in some cases. Such 'soft' emission processes also lead the ejection of dimers and even multimers of the amino acid molecules. In the case of the phenylalanine, secondary ion species composed of up to at least seven phenylalanine moieties were observed. Tentatively, the ionization probability of the emitted molecules is envisaged to depend on the presence of free protons in the emission zone. Their number can be expected to decrease concurrently with the decreasing amount of fragmentation for large Ar(n)(+) cluster ions (i.e. for low energies per cluster atom).

Characteristic Fragmentation of Polysiloxane Monolayer Films by Bombardment with Monatomic and Polyatomic Primary Ions in TOF-SIMS

This study reports the characteristic fragmentation patterns from two polysiloxane polymers that form ordered overlayer on silver substrates. Results are compared for the bombardment of various monatomic and polyatomic projectiles of Cs(+), C(60)(+) (10 keV), Bi(1)(+), and Bi(3)(+) (25 keV) in the high mass range time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra. Results are reported from sub-monolayer (solution cast) coverages of poly(dimethylsiloxane)s with the number average molecular weights (M(n)) of 2200 and 6140 Da, respectively, and Langmuir-Blodgett monolayers of poly(methylphenylsiloxane) with molecular weights (MW) from 600 and 1000 Da. For each film, Bi projectiles resulted in the emission of positive silver cluster ions from the substrate under the polymer overlayer and peaks corresponding to silver cluster ions with larger mass were observed by impact of polyatomic 25 keV Bi(3)(+) projectiles. In addition, depending on the change of energy of Bi (3) (+) , a different pattern of fragments was observed. With Cs(+) and C(60)(+) impact, however, the emission of silver cluster ions was not detected. In the case of C(60)(+) impact for PDMS-6140, peaks corresponding to silver-cationized intact oligomers were not observed. In this paper, these results are explained by the possible bombardment mechanism for each projectile, based on its mass, energy, and split trajectories of the component atoms under the polyatomic impact.

Positive ion emission from oxidized aluminum during ultraviolet excimer laser irradiation

We report quadrupole mass-selected time-of-flight measurements of positive ions from oxidized aluminum metal (and for comparison, single-crystal sapphire) during pulsed excimer laser irradiation at 193 and 248 nm. This work focuses on laser fluences well below onset of rapid etching or optical breakdown. By far the most intense emissions are due to Al+. On previously unexposed material, the ion kinetic energies are initially well above the photon energy, consistent with the ejection of Al+ sorbed at surface electron traps. During prolonged irradiation, the emission intensities and kinetic energies gradually fall. Emission from patches of oxide would account for previous reports of laser-induced Al+ emission from metallic aluminum surfaces cleaned by ion etching if patches of thin oxide were to survive the etching treatment.

SIMS analysis of hydrogen content in near surface layers of AISI 316L SS after electrolytic polishing under different conditions

Hydrogen concentration in AISI 316L stainless steel samples was monitored perpendicularly to the flat sample surface up to the depth of 0.8 μm, using secondary ion mass spectrometry (SIMS). Depth profile analysis was performed on the samples after electrolytic polishing under different conditions. Analyses were performed on SAJW-05 apparatus with quadrupole Balzers QMA-410 analyzer and Physical Electronics 06-350E ion gun. 5 keV Ar + primary ion beam of 100 μm diameter was scanned over 1 mm × 0.8 mm area. Positive (H +, C +, CH +, O +, Cr +, Fe +) and negative (H −, C −, CH −, O −, OH −, CrO −, FeO −) secondary ion emission was registered from central part of the scanned area (10% of area) during primary ion beam sputtering. Results of hydrogen depth profile analysis were compared for the samples after a conventional electrochemical polishing (EP), and magnetoelectropolishing (MEP). Both EP and MEP processes were carried out under the electrochemical conditions regarding also natural and forced convection. The steel samples taken of a hot-rolled sheet, as received (AR), and after abrasive polishing (MP), were used as a reference. Results show that the increased current density (up to 200 A/dm 2) and electrolyte stirring during electropolishing cause lowering of the hydrogen content in the samples, with the best result regarding hydrogen content decrease obtained on MEP200 sample. In fact, within the MEP process, characteristic with the self-contained electrolyte whirling, the contents of hydrogen are significantly decreased.