Energy Spectra Of Ions Research Articles

Plasma Pressures in the Heliosheath From Cassini ENA and Voyager 2 Measurements: Validation by the Voyager 2 Heliopause Crossing

AbstractWe report “ground truth,” 28‐ to 3,500‐keV in situ ion and 5.2‐ to 55‐keV remotely sensed ENA measurements from Voyager 2/Low Energy Charged Particle detector and Cassini/Ion and Neutral Camera, respectively, that assess the components of the ion pressure in the heliosheath. In this process, we predict an interstellar neutral hydrogen density of ∼0.12 cm−3 and an interstellar magnetic field strength of ∼0.5‐nT upstream of the heliopause in the direction of V2, that is, consistent with the measured magnetic field and neutral density measurements at Voyager 1 from August 2012, when the spacecraft entered interstellar space, to date. Further, this analysis results in an estimated heliopause crossing by V2 of ∼119 AU, as observed, suggesting that the parameters deduced from the pressure analysis are valid. The shape of the >5.2‐keV ion energy spectra play a critical role toward determining the pressure balance and acceleration mechanisms inside the heliosheath.

Jumping the anode layer in the zone of the E × B discharge

New features and peculiarities of anomalous self-sustained Hall E × B discharge have been discovered. The presence of a minimum density of ions n upon the magnetic field induction B is confirmed for a wide range of discharge parameters. Likewise, the operation mode for a maximum density at the optimal values of the radial and longitudinal components of magnetic induction was discovered. Both effects are accompanied by the changes in the position of the combustion zone E × B of the discharge in the anode-cathode interval and the transformation of the distribution function of the ion energy. The threshold nature of the processes in the plasma E × B of the discharge can be manifested on the curves n = f(B) as an abrupt 3 to 4 times decrease in density upon the increase in B by not more than 10%. An abrupt increase in the density of ions (up to 16 times), occurring with a slight increase in the density of neutrals (∼1.2 times), is recorded when there are jumps of the anode layer from the anode region to the cathode one and vice versa. A thin structure of ion energy spectra explained by the isomagnetic potential jumps, which generate ion density jumps in a narrow energy range, was found. In the search for the reasons for the restructuring of the discharge and generation of isomagnetic jumps, the possible effect of nonlocal gradient-drift and electron-cyclotron drift instabilities on electron mobility and ion acceleration is briefly discussed.

Particle Acceleration in Kinetic Simulations of Nonrelativistic Magnetic Reconnection with Different Ion–Electron Mass Ratios

Abstract By means of fully kinetic particle-in-cell simulations, we study whether the proton-to-electron mass ratio m i /m e influences the energy spectrum and underlying acceleration mechanism during magnetic reconnection. While kinetic simulations are essential for studying particle acceleration during magnetic reconnection, a reduced m i /m e is often used to alleviate the demanding computing resources, which leads to artificial scale separation between electron and proton scales. Recent kinetic simulations with high mass ratios have suggested new regimes of reconnection, as electron pressure anisotropy develops in the exhaust region and supports extended current layers. In this work, we study whether different m i /m e changes the particle acceleration processes by performing a series of simulations with different mass ratio (m i /m e = 25–400) and guide field strength in a low-β plasma. We find that mass ratio does not strongly influence reconnection rate, magnetic energy conversion, ion internal energy gain, plasma energization processes, ion energy spectra, and the acceleration mechanisms for high-energy ions. Simulations with different mass ratios are different in electron acceleration processes, including electron internal energy gain, electron energy spectrum, and the acceleration efficiencies for high-energy electrons. We find that high-energy electron acceleration becomes less efficient when the mass ratio gets larger because the Fermi-like mechanism associated with particle curvature drift becomes less efficient. These results indicate that when particle curvature drift dominates high-energy particle acceleration, the further the particle kinetic scales are from the magnetic field curvature scales (∼d i ), the weaker the acceleration will be.

Dissociative ionization of the H2O molecule bombarded by swift singly charged and slow highly charged ions

Fragment ion energy spectra of the water molecule have been measured in conventional crossed‐beam experiments by the impact of 46 keV/u energy, singly charged ions (SCIs) and 4.3 keV/u energy, highly charged ions (HCIs). Double differential cross sections have been determined and a comparative analysis has been performed. We found that the fragmentation spectra for SCIs and HCIs are very similar, indicating that both collisions lead to the same fragmentation channels. This suggests that the Coulomb explosion of the water molecule is dominantly determined by the charge state of the transient molecular ions, and it is almost independent from the primary ionization mechanism. Differences were observed not only between the SCI and HCI impact‐induced fragmentation cross sections, but between those obtained by the 60 keV N6+ and 70 keV O7+ projectiles. The differences were attributed to the selectivity of the electron capture process for HCIs. Multiple target ionization cross sections have been deduced from the fragment ion spectra. We found contributions of up to fivefold ionization for SCIs and up to sixfold ionization for HCIs.

Isotope effects on the ionization of hydrogen molecular ions in strong laser fields

Isotope effects on the ionization of hydrogen molecular ions in strong laser fields are studied by numerically simulating the time-dependent Schr\odinger equation. Though the isotopes of hydrogen molecular ions have identical Born-Oppenheimer potential curves, the distinct ionization scenarios are observed for single-photon, multiphoton, and tunneling ionization. For multiphoton and tunneling ionization, the ionization rate of ${{\mathrm{H}}_{2}}^{+}$ is larger than that of ${{\mathrm{D}}_{2}}^{+}$ and ${{\mathrm{T}}_{2}}^{+}$. The ratio of the intensity-dependent tunneling ionization probabilities of ${{\mathrm{H}}_{2}}^{+}$ (${{\mathrm{D}}_{2}}^{+}$) and ${{\mathrm{T}}_{2}}^{+}$ can be qualitatively explained by the adiabatic tunneling theories if a few-femtosecond ultrashort pulse is used. For tens of femtosecond laser pulses, the nuclear movement is unavoidable and the time-dependent Schr\odinger equation simulation is necessary in order to calculate the ionization probability accurately. For single-photon ionization, the electron-nuclei joint energy spectra are distinct though the total ionization probabilities are similar. The elastic constant of potential curves can be retrieved by comparing the single-photon-induced electron-nuclei joint energy spectra of different isotopic molecular ions, which offers a potential technique to image molecules.

Pressure dependence of singly and doubly charged ion formation in a HiPIMS discharge

Generation of singly charged Ar+ and Ti+, doubly charged Ar2+ and Ti2+, and of Ar2+ and Ti2+ dimer ions in a high power impulse magnetron sputtering (HiPIMS) discharge with a Ti cathode was investigated. Energy-resolved mass spectrometry was employed. The argon gas pressures varied between 0.5 and 2.0 Pa. Energy spectra of monomer ions are composed of low- and high-energy components. The energetic position of the high-energy component is approximately twice as large for doubly charged ions compared to singly charged ions. Intensities of Ar2+ and Ti2+ dimer ions are considerably smaller during HiPIMS compared to dc magnetron sputtering.

The impact of surface oxidation on energy spectra of keV ions scattered from transition metals

Studying the initial stages of surface oxidation is of great relevance to understand how oxygen alters the physical and chemical properties at the interface of the host material to the environment and is therefore, crucial for improvement in manifold technological applications. We investigated the influence of surface oxygen on ion spectra recorded for keV noble gas ions backscattered from metal surfaces in low energy ion scattering (LEIS). Initially pure Zn and Ta surfaces, chosen for their well-characterized properties in ion-neutralization in LEIS, have been oxidized and ion spectra for pure and oxidized surfaces have been compared. Oxygen on the surface significantly influences shape and intensity of the backscattered ion spectrum at all energies: for both metal systems, the surface scattered ion yield of the metal is drastically decreasing under oxygen presence. The observed decrease, however, cannot be explained by the reduction in the surface areal density of the metal constituents exclusively. At least for Zn an additional significant change in charge exchange behavior is necessary to explain the observations. In contrast to the generally observed decrease in the yield of ions scattered from the outermost surface, the change in shape and intensity of the reionization background are found to show opposing trends and different energy dependencies for Zn and Ta.

A method to convert spectra from slab microdosimeters in therapeutic ion-beams to the spectra referring to microdosimeters of different shapes and material

Experimental studies of microdosimetry in therapeutic ion beams have been performed using several detectors. The differences among the microdosimeters lie on the shapes, the site sizes, and the material. The present study proposes a method to extract from the heterogeneous information collected with the different microdosimeters a univocal specification of the radiation quality of the radiation field. Historically the specification of the radiation quality is provided either in terms of linear energy transfer (LET) or in terms of lineal energy, y. The first part of this study focuses on identifying the correlation between the distributions of LET and the lineal energy spectra as well as the correspondence between their mean values calculated from the frequency and dose distributions. The evaluation is inspired by the method of LET analysis described by Kellerer (1972 Phys. Med. Biol. 17 232–40) with adaptation to the peculiarities of ion-beam therapy where the pristine irradiation is unidirectional and made of a single species of essentially mono-energetic ions. The second objective of this study is to interpret the spectra collected by a slab and perform the necessary conversion to estimate what the spectrum would be if it was collected by a detector different in shape, material, or size. An example is provided using as starting point the simulated lineal energy spectrum of carbon ions impinging a slab detector of graphite and applying the method to convert it to the spectra that would be obtained in the same radiation field with spherical, cylindrical, and slab detectors made of water.

Measurement of plasma channels in the Venus wake

Plasma channels have been suggested to account for the observation of ionospheric holes in the Venus nightside ionosphere. They are produced by the erosion of the solar wind on the magnetic polar regions of the Venus ionosphere, and are related to a sharp plasma transition that extends along the flanks of the wake and that is located downstream from the bow shock. The sharp plasma (intermediate) transition is a feature that has been detected with instruments onboard all the main spacecraft that have probed the Venus wake. From the early flyby transit of the Mariner 5 spacecraft to the later orbit crossings of the Venera 10, the Pioneer Venus and more recently the Venus Express there has been accumulated evidence of the presence of that transition by the flanks of the Venus wake. We report here measurements made with the ASPERA-4 instrument of the Venus Express that were conducted in orbit crossings by the midnight plane over a 4 year period (between 2006 and 2009), and that show different positions of the intermediate transition in the Venus wake at locations closer to the sun-Venus axis near solar minimum conditions by 2009. Measurements conducted in this latter time period also indicate that the intermediate transition by the midnight plane derives from the plasma channels and that is replaced by a broad velocity layer in orbits that probed farther away from that plane. It is argued that changes in the position of the intermediate transition with respect to the plasma channels are related to processes produced by the transport of solar wind momentum to the Venus polar ionosphere. The energy spectra of planetary ions measured in VEX orbits that probed by the midnight plane show momentum transport from the solar wind protons. This result is contrary to the conclusions reported by Collinson et al. (2014) in the sense that there is ¨no difference in the peak energy of the ionospheric outflow¨ when measured inside and outside an ionospheric hole.

Shaping of ion energy spectrum due to ionization in ion acceleration driven by an ultra-short pulse laser

In the interaction between an ultra-short laser pulse and an ultra-thin film target, field ionization dominates the plasma formation. The charge and density distribution of the plasma are determined by the laser pulse and the target, and play a crucial role on the following ion acceleration process. In this paper, we study the role of ionization in laser ion acceleration with particle-in-cell simulations. The ion energy spectra in the simulations including ionization are significantly shaped comparing with those from the simulations using a pre-ionized plasma. The shaping of the ion energy spectra is robust in a wide parameter range, in term of laser temporal profile, intensity, duration and target material. This work also shows the possibility to generate quasi-monoenergetic ion beams resulted by ionization. The benchmark simulation well reproduces the narrow bandwidth ion energy spectra, the evolution of the particle number with charge state, and the leaps on the ion energy, those observed in the previous experiments. This study reveals the necessity to include ionization in modeling laser ion acceleration, and provides an approach to understand the shaping of ion energy spectra in the experiments.

Simulation of cathode surface sputtering by ions and fast atoms in Townsend discharge in argon-mercury mixture with temperature-dependent composition

The mixture of argon and mercury vapor is used as the background gas in different types of gas discharge illuminating lamps. The aim of this work was development of a model, describing transport of electrons, ions and fast atoms in the one-dimensional low-current gas discharge in argon-mercury mixture, and determination of the dependence of their contributions to the cathode sputtering, limiting the device service time, on the temperature.For simulation of motion of electrons we used the Monte Carlo method of statistical modeling, whereas the ion and metastable excited atom motion, in order to reduce the calculation time, we described on the basis of their macroscopic transport equations, which allowed to obtain their flow densities at the cathode surface. Then, using the Monte Carlo method, we found the energy spectra of ions and fast atoms, generated in collisions of ions with mixture atoms, at the cathode surface and also the effective coefficients of the cathode sputtering by each type of particles.Calculations showed that the flow densities of argon ions and fast argon atoms, produced in collisions of argon ions with slow argon atoms, do not depend on the temperature, while the flow densities of mercury ions and fast argon atoms generated by them grow rapidly with the temperature due to an increase of mercury content in the mixture.There are represented results of modeling of the energy spectra of ions and fast atoms at the cathode surface. They demonstrate that at low mercury content in the mixture of the order of 10–3 the energies of mercury ions exceed that of the other types of particles, so that the cathode is sputtered mainly by mercury ions, and their contribution to sputtering is reduced at a mixture temperature decrease.

Control of H_{2} Dissociative Ionization in the Nonlinear Regime Using Vacuum Ultraviolet Free-Electron Laser Pulses.

The role of the nuclear degrees of freedom in nonlinear two-photon single ionization of H_{2} molecules interacting with short and intense vacuum ultraviolet pulses is investigated, both experimentally and theoretically, by selecting single resonant vibronic intermediate neutral states. This high selectivity relies on the narrow bandwidth and tunability of the pulses generated at the FERMI free-electron laser. A sustained enhancement of dissociative ionization, which even exceeds nondissociative ionization, is observed and controlled as one selects progressively higher vibronic states. With the help of abinitio calculations for increasing pulse durations, the photoelectron and ion energy spectra obtained with velocity map imaging allow us to identify new photoionization pathways. With pulses of the order of 100fs, the experiment probes a timescale that lies between that of ultrafast dynamical processes and that of steady state excitations.

Micro-spectrometer for fusion plasma boundary measurements.

In situ probes are being developed to make direct, spatially resolved measurements of the ion energy spectra in the edge of tokamak plasmas while being easily replaced and requiring minimal resources. The ion spectrometers will consist of a combined collimator and energy analyzer fabricated from silicon and mated to a detector to yield a form factor of approximately 2.0 cm × 1.5 cm × 0.2 cm. Results of fabrication and testing of the combined collimator and energy analyzer element are presented.

Energy Spectrum and Charge Composition of Ions of Laser-Produced Gd and Al Plasmas

The energy spectrum and the distribution of the total charge over the ion energy are studied for plasma produced by partially coherent radiation of the neodymium glass laser with a power density of ~1013 W/cm2 in the focusing spot incident on continuous gadolinium (Gd) and aluminum(Al) plates. The spectra of plasma ions for gadolinium and aluminumtargets are presented at laser radiation energies of 2, 7, 20 J and 1 J, respectively. For the gadolinium target, an increase in the peak amplitudes in ion spectra with increasing influencing radiation energy and manifestations of higher-energy ions are observed. The difference in the ion emission for gadolinium and aluminum targets is also recorded at close radiation energies of 2 and 1 J, respectively.

Concomitant Double Ion and Electron Populations in the Earth's Magnetopause Boundary Layers From Double Reconnection With Lobe and Closed Field Lines

AbstractWhile double ion populations, with a cold population originating from the solar wind and a hotter one from the magnetosphere, are frequently observed in the low‐latitude boundary layers at the Earth's magnetopause, similar double electron populations are observed less often. A preliminary study of magnetopause crossings characteristics was used to determine the typical locations and energy spectra of ion and electron populations near the magnetopause. Then, we set up an automated detection algorithm for identifying regions with concomitant double populations in both ion and electron energy spectra. The statistical study was carried out on 7 years of Time History of Events and Macroscale Interactions during Substorms particle data, to determine the interplanetary magnetic field conditions in the upstream solar wind for these double populations. The results suggest that such concomitant ion and electron double population boundary layers form preferentially in the subsolar region and under northward interplanetary magnetic field but with a significant BY component. We interpret this finding as the result of reconnection of the same magnetosheath field line in both hemispheres with at least one end reconnecting in its hemisphere at lower latitude with a closed magnetospheric field line that already contains a hot electron source.

Ion energy spectra directly measured in the interaction volume of intense laser pulses with clustered plasma

The use of gas cluster media as a target for an intense femtosecond laser pulses is considered to be uniquely convenient approach for the development of a compact versatile pulsed source of ionizing radiation. Also, one may consider cluster media as a nanolab to investigate fundamental issues of intense optical fields interaction with sub-wavelength scale structures. However, conventional diagnostic methods fail to register highly charged ion states from a cluster plasma because of strong recombination in the ambient gas. In the paper we introduce high-resolution X-ray spectroscopy method allowing to study energy spectra of highly charged ions created in the area of most intense laser radiation. The emission of CO2 clusters were analyzed in experiments with 60 fs 780 nm laser pulses of 1018 W/cm2 intensity. Theory and according X-ray spectra modeling allows to reveal the energy spectra and yield of highly charged oxygen ions. It was found that while the laser of fundamental frequency creates commonly expected monotonic ion energy spectrum, frequency doubled laser radiation initiates energy spectra featuring of distinctive quasi-monoenergetic peaks. The later would provide definite advantage in further development of laser-plasma based compact ion accelerators.

Europa’s ice-related atmosphere: The sputter contribution

Europa, Jupiter’s innermost icy satellite, is embedded well within Jupiter’s magnetospheric plasma, an intense flux of ions and electrons that approximately co-rotate with Jupiter. The plasma can be thought of as consisting of two populations: The cold, thermal plasma containing charged particles with energies ranging from 1 eV to 1 keV, and the hot, energetic plasma containing charged particles with energies ranging from 10 keV to 100 MeV. When the charged particles interact with Europa’s surface, they not only chemically and physically alter the icy surface, but also liberate material from the surface through a process called sputtering, which in turn forms a tenuous atmosphere.In this paper we calculate the sputter contribution to the atmosphere by modeling the formation of Europa’s ice-sputtered atmosphere ab initio. We consider the species H, H2, O, OH, H2O, O2, HO2, H2O2, and O3, all of which are related to the water–ice surface. Whereas the ice sputter yields of H2O, H2, and O2 have been well established, the ice sputter yields (and the resulting density profiles) of H, O, OH, HO2 and O3 are small and largely unknown. As model input we use available plasma ion and electron energy spectra as well as available water-ice sputter yields. Based on first principles, i.e., without applying any scaling to observed data, we calculate atmospheric densities ab initio.Our results match available observational data and previously published modeling efforts well. Europa’s exosphere is dominated by thermally accommodated O2 close to the surface (below a few 100 km), and the much lighter H2 molecules at higher altitudes. The water-ice related species that stick to the surface (freeze out) are liberated by cold and hot plasma sputtering in about equal amounts. In addition, in the case of H2, O2, and H2O2, electrons contribute almost as significantly to the sputter yield as ions do.

Multiprobe characterization of plasma flows for space propulsion

Plasma engines for space propulsion generate plasma jets (also denominated plasma plumes) having supersonic ion groups with typical speeds in the order of tens of kilometers per second, which lies between electron and ion thermal speeds. Studies of the stationary plasma expansion process using a four-grid retarding field energy analyzer (RFEA), an emissive probe (EP) and a Langmuir probe (LP), all mounted on a three dimensionally (3D) displaced multiprobe structure are discussed. Specifically, the determination of plasma beam properties from the RFEA current-voltage (IV) characteristic curves is presented. The experimental results show the ion energy spectra to be essentially unchanged over 300 mm along the plasma-jet expansion axis of symmetry. The measured ion velocity distribution function (IVDF) results from the superposition of different ion groups and has two dominant populations: A low-energy group constituted of ions from the background plasma is produced by the interaction of the plasma jet with the walls of the vacuum chamber. The fast-ion population is composed of ions from the plasma beam moving at supersonic speeds with respect to the low-energy ions. The decreasing spatial profiles of the plasma-jet current density are compared with those of the low-energy ion group, which are not uniform along the axis of symmetry because of the small contributions from other ion populations with intermediate speeds.

Origins of plateau formation in ion energy spectra under target normal sheath acceleration

Target normal sheath acceleration (TNSA) is a method employed in laser–matter interaction experiments to accelerate light ions (usually protons). Laser setups with durations of a few 10 fs and relatively low intensity contrasts observe plateau regions in their ion energy spectra when shooting on thin foil targets with thicknesses of the order of 10 μm. In this paper, we identify a mechanism which explains this phenomenon using one-dimensional particle-in-cell simulations. Fast electrons generated from the laser interaction recirculate back and forth through the target, giving rise to time-oscillating charge and current densities at the target backside. Periodic decreases in the electron density lead to transient disruptions of the TNSA sheath field: peaks in the ion spectra form as a result, which are then spread in energy from a modified potential driven by further electron recirculation. The ratio between the laser pulse duration and the recirculation period (dependent on the target thickness, including the portion of the pre-plasma which is denser than the critical density) determines if a plateau forms in the energy spectra.

LEIS analysis of the W surface during water vapor adsorption

The low energy ion spectroscopy (LEIS) experimental results of the steam adsorption by W samples are presented. It is shown that use of the ion scattering spectroscopy has allowed us to measure the thickness of adsorbate water layer on the tungsten surface with good accuracy. Tungsten at room temperature is completely covered by a water monolayer after 20 min (the partial steam pressure p≃2×10−5 Pa) of exposure. The water film thickness can be obtained by analyzing energy spectra of hydrogen ions scattering on surface. To transfer the energy scale (in energy spectra) into the depth scale with 10−15% accuracy one can use the approximation formula confirmed by simulation. In these experiments, the water film thickness on W does not exceed 40−45 Å.