Charge State Evolution Research Articles

Temporal evolution of ion energy distribution functions and ion charge states of Cr and Cr-Al pulsed arc plasmas

To study the temporal evolution of ion energy distribution functions, charge-state-resolved ion energy distribution functions of pulsed arc plasmas from Cr and Cr-Al cathodes were recorded with high time resolution by using direct data acquisition from a combined energy and mass analyzer. The authors find increases in intensities of singly charged ions, which is evidence that charge exchange reactions took place in both Cr and Cr-Al systems. In Cr-Al plasmas, the distributions of high-charge-state ions exhibit high energy tails 50 μs after discharge ignition, but no such tails were observed at 500 μs. The energy ratios of ions of different charge states at the beginning of the pulse, when less neutral atoms were in the space in front of the cathode, suggest that ions are accelerated by an electric field. The situation is not so clear after 50 μs due to particle collisions. The initial mean ion charge state of Cr was about the same in Cr and in Cr-Al plasmas, but it decreased more rapidly in Cr-Al plasmas compared to the decay in Cr plasma. The faster decay of the mean ion charge state and ion energy caused by the addition of Al into a pure Cr cathode suggests that the mean ion charge state is determined not only by ionization processes at the cathode spot but also by inelastic collision between different elements.

Quasi-equilibrium in charge-state evolution for carbon, sulfur, and tungsten ions after passing through carbon foils

SynopsisCharge-state evolution of 2.0 MeV/u carbon, sulfur, and 1.0 MeV/u tungsten ions after carbon-foil penetration has been studied experimentally in the foil thickness range covering both pre- and after the equilibrium. It was observed for the first time that mean charge-states for Sq+ (q = 6-14) initial ions evolved to coincide in 12.3 at a target thickness of 6.9 μg/cm2, showing a “quasi-equilibrium,” and further evolved simultaneously to a real equilibrium value of 12.68, whereas those for Sq+ (q = 15, 16) initial ions evolved directly to the real equilibrium, without through the “quasi-equilibrium,” until the target thickness was increased to 100 μg/cm2. Such kind of shell-effect was observed also for C-ions, and could be qualitatively reproduced by some simulations.

A STEADY-STATE PICTURE OF SOLAR WIND ACCELERATION AND CHARGE STATE COMPOSITION DERIVED FROM A GLOBAL WAVE-DRIVEN MHD MODEL

The higher charge states found in slow ($<$400km s$^{-1}$) solar wind streams compared to fast streams have supported the hypothesis that the slow wind originates in closed coronal loops, and released intermittently through reconnection. Here we examine whether a highly ionized slow wind can also form along steady and open magnetic field lines. We model the steady-state solar atmosphere using AWSoM, a global magnetohydrodynamic model driven by Alfv{\'e}n waves, and apply an ionization code to calculate the charge state evolution along modeled open field lines. This constitutes the first charge states calculation covering all latitudes in a realistic magnetic field. The ratios $O^{+7}/O^{+6}$ and $C^{+6}/C^{+5}$ are compared to in-situ Ulysses observations, and are found to be higher in the slow wind, as observed; however, they are under-predicted in both wind types. The modeled ion fractions of S, Si, and Fe are used to calculate line-of-sight intensities, which are compared to EIS observations above a coronal hole. The agreement is partial, and suggests that all ionization rates are under-predicted. Assuming the presence of suprathermal electrons improved the agreement with both EIS and Ulysses observations; importantly, the trend of higher ionization in the slow wind was maintained. The results suggest there can be a sub-class of slow wind that is steady and highly ionized. Further analysis shows it originates from coronal hole boundaries (CHB), where the modeled electron density and temperature are higher than inside the hole, leading to faster ionization. This property of CHBs is global, and observationally supported by EUV tomography.

CHARGE STATE EVOLUTION IN THE SOLAR WIND. III. MODEL COMPARISON WITH OBSERVATIONS

We test three theoretical models of the fast solar wind with a set of remote sensing observations and in-situ measurements taken during the minimum of solar cycle 23. First, the model electron density and temperature are compared to SOHO/SUMER spectroscopic measurements. Second, the model electron density, temperature, and wind speed are used to predict the charge state evolution of the wind plasma from the source regions to the freeze-in point. Frozen-in charge states are compared with Ulysses/SWICS measurements at 1 AU, while charge states close to the Sun are combined with the CHIANTI spectral code to calculate the intensities of selected spectral lines, to be compared with SOHO/SUMER observations in the north polar coronal hole. We find that none of the theoretical models are able to completely reproduce all observations; namely, all of them underestimate the charge state distribution of the solar wind everywhere, although the levels of disagreement vary from model to model. We discuss possible causes of the disagreement, namely, uncertainties in the calculation of the charge state evolution and of line intensities, in the atomic data, and in the assumptions on the wind plasma conditions. Last, we discuss the scenario where the wind is accelerated from a region located in the solar corona rather than in the chromosphere as assumed in the three theoretical models, and find that a wind originating from the corona is in much closer agreement with observations.

Charge state distribution of 16O from the 4He(12C,16O)γ reaction of astrophysical interest studied both experimentally and theoretically

In astrophysics, 4He(12C,16O)γ reaction places an important role. At Kyushu University Tandem accelerator Laboratory (KUTL), the measurement of 4He(12C,16O)γ cross section is in progress in the energy range of astrophysical nuclear reaction. Since the charge state of product 16O ions after passing through the gas target is spread and only one charge state can be measured at terminal detector, it is necessary to know the charge state distribution of 16O ions passing through the He gas target precisely. Here, we report the charge state distribution of the 16O recoils both experimentally and theoretically. Experimentally, we measured the equilibrium charge state distribution of 16O ions in the windowless helium gas target with the beam energy of primary 16O ions at 7.2, 4.5, and 3.45MeV at KUTL. The measured results showed a Gaussian distribution for the charge state fraction. Theoretically, we proposed a framework for the charge state distribution study. Charge state distribution was computed by solving a set of differential equations including a series of charge exchange cross sections. For the ionization cross section, plane-wave Born approximation was applied and modified by taking target atomic screening as a function of momentum transfer into account. For the capture cross section, continuum distorted wave approximation was applied and the influence of the gas target density was taken into account in the process of electron capture. Using above charge exchange cross sections, the charge state evolution was simulated. According to the equilibrium distribution, we compared the theoretical calculation to the experimental data. After taking into account the density effects in the charge exchange process, the theoretical charge state distributions shows a good agreement with the experimental data. Both experimental and theoretical results are useful to understand the charge fraction of recoil oxygen created via 4He(12C,16O)γ reaction, especially in the energy range of astrophysical nuclear reaction.

Charge stripping ofU238ion beam by helium gas stripper

Development of a nondestructive, efficient electric-charge-stripping method is a key requirement for next-generation high-intensity heavy-ion accelerators such as the RIKEN Radioactive-Isotope Beam Factory. A charge stripper employing a low-$Z$ gas is an important candidate applicable to high-intensity uranium beams for replacing carbon-foil strippers. In this study, a high-beam-transmission charge-stripping system employing helium gas for $^{238}\mathrm{U}$ beams injected at $10.8\text{ }\text{ }\mathrm{MeV}/\mathrm{u}$ was developed and demonstrated for the first time. The charge-state evolution measured using helium in a thickness range of $0.24--1.83\text{ }\text{ }\mathrm{mg}/{\mathrm{cm}}^{2}$ is compared with theoretical predictions. Energy attenuation and energy spread due to the helium stripper are also investigated.

CHARGE STATE EVOLUTION IN THE SOLAR WIND. RADIATIVE LOSSES IN FAST SOLAR WIND PLASMAS

We study the effects of departures from equilibrium on the radiative losses of the accelerating fast, coronal hole-associated solar wind plasma. We calculate the evolution of the ionic charge states in the solar wind with the Michigan Ionization Code and use them to determine the radiative losses along the wind trajectory. We use the velocity, electron temperature, and electron density predicted by Cranmer et al. as a benchmark case even though our approach and conclusions are more broadly valid. We compare non-equilibrium radiative losses to values calculated assuming ionization equilibrium at the local temperature, and we find that differences are smaller than 20% in the corona but reach a factor of three in the upper chromosphere and transition region. Non-equilibrium radiative losses are systematically larger than the equilibrium values, so that non-equilibrium wind plasma radiates more efficiently in the transition region. Comparing the magnitude of the dominant energy terms in the Cranmer et al. model, we find that wind-induced departures from equilibrium are of the same magnitude as the differences between radiative losses and conduction in the energy equation. We investigate which ions are most responsible for such effects, finding that carbon and oxygen are the main source of departures from equilibrium. We conclude that non-equilibrium effects on the wind energy equation are significant and recommend that they are included in theoretical models of the solar wind, at least for carbon and oxygen.

The evolution of ion charge states in cathodic vacuum arc plasmas: a review

Cathodic vacuum arc plasmas are known to contain multiply charged ions. Twenty years after ‘Pressure ionization: its role in metal vapour vacuum arc plasmas and ion sources’ appeared in volume 1 of Plasma Sources Science and Technology, this is a great opportunity to re-visit the issue of pressure ionization, a non-ideal plasma effect, and put it in perspective to the many other factors that influence observable charge state distributions, such as the role of the cathode material, the path in the density–temperature phase diagram, the ‘noise’ in vacuum arc plasma as described by a fractal model approach, the effects of external magnetic fields and charge exchange collisions with neutrals. A much more complex image of the vacuum arc plasma emerges, putting decades of experimentation and modeling in perspective.

Li+-ion neutralization on metal surfaces and thin films

Li${}^{+}$ ions with energies ranging from 0.3 to 2 keV are scattered from Au(110) and Pd(100) surfaces and from ultrathin Ag film grown on Au(111) in order to study electron transfer phenomena. We find that neutralization occurs quite efficiently and find an anomalous ion energy dependence of the neutral fraction for Au(110) and Pd(100) surfaces previously noted for Au(111). The dependence of the neutral fraction on the azimuthal angle of the Au(110) and Pd(100) surfaces is reported. In the case of Ag monolayer on Au(111), results are similar to the case of the Ag(111) surface. To understand the anomalous ion energy dependence, we present a theoretical study using density functional theory (DFT) and a linearized rate equation approach, which allows us to follow the Li charge state evolution for the (111) surfaces of Ag, Au, and Cu, and for the Ag-covered Au(111) surface.

Energy loss of argon in a laser-generated carbon plasma

The experimental data presented in this paper address the energy loss determination for argon at 4 MeV/u projectile energy in laser-generated carbon plasma covering a huge parameter range in density and temperature. Furthermore, a consistent theoretical description of the projectile charge state evolution via a Monte Carlo code is combined with an improved version of the CasP code that allows us to calculate the contributions to the stopping power of bound and free electrons for each projectile charge state. This approach gets rid of any effective charge description of the stopping power. Comparison of experimental data and theoretical results allows us to judge the influence of different plasma parameters.

X-Ray Scattering Measurements of Radiative Heating and Cooling Dynamics

Spectrally and time-resolved x-ray scattering is used to extract the temperature and charge state evolution in a near solid density carbon foam driven by a supersonic soft x-ray heat wave. The measurements show a rapid heating of the foam material (approximately 200 eV/ns) followed by a similarly fast decline in the electron temperature as the foam cools. The results are compared to an analytic power balance model and to results from radiation-hydrodynamics simulations. Finally, the combination of charge state and temperature extracted from this known density isochorically heated plasma is used to distinguish between dense plasma ionization balance models.

Development of a time-of-flight detector for the Rare-RI Ring project at RIKEN

We have tested the timing properties of several thin plastic scintillators (10–500μm) designed for the Rare-RI Ring project at the RI beam factory. The experiments were performed at the heavy-ion synchrotron facility, HIMAC (Heavy Ion Medical Accelerator in Chiba), using a Xe beam with an energy of 200MeV/nucleon. The scintillation light was read out by fast-timing photomultipliers through standard fishtail-shaped light guides. The resultant timing resolutions reached a few tens of picoseconds, which satisfied our requirement for the project. We also measured charge-state distributions for Xe42+ and Fe20+ ions penetrating carbon foils of 10–500μg/cm2 thickness. The results were compared with a theoretical model calculation for charge-state evolution.

Evolution of charge and spin state of transition metals in the LaMn1−xCoxO3 perovskite series

Charge and spin state evolution of the transition metals in the LaMn1−xCoxO3 perovskite series was studied by means of high resolution x-ray absorption and emission spectroscopy. The gradual charge transfer from Mn to Co sites was observed upon increasing Co doping. The high spin of the Mn3+∕4+ and Co2+ ions is preserved in the entire solid solution. Co3+ ions reveal low spin state at low doping, while the gradual increase of the average Co3+ spin state is observed at higher cobalt concentrations. The results plausibly explain the observed magnetization anomalies.

Charge and state population in dilute plasmas from beam-foil spectroscopy

Beam-foil spectroscopy has a long history for determining charge state evolution, radiative decay lifetimes, spectral widths, and satellite structure. Many theoretical results predict a range of populations and state dependencies, but high resolution or a very large bandpass is normally required to test or investigate these. The current investigation has tested some key assumptions and models as applied to this relativistic regime, particularly for few-electron iron resonances and satellites. We conclude that there are two specific production mechanisms, one inside the target to low-n levels and one dominated by capture mechanisms at the exit of the foil, with particularly significant Rydberg cascades. Several systematics in high-precision measurements of these systems are discussed and quantified for the first time. The 2s–1s and 4f–2p satellites are explicitly investigated, and a dominant systematic is uncovered that is due to the variable location of spectral emission, downstream of the beam-foil target. PACS Nos.: 32.70.Jz, 34.10.+x, 52.20.Hv, 34.60.+z

Simulation of charge breeding for trapped ions

The well established program SUK (Sukzessive Ionisierung) for the evaluation and graphical presentation of charge state evolution has been improved by including RR (radiative recombination) and CX (charge exchange). It has options for continuous or pulsed feed of single charged ions, ion heating by Coulomb collisions of electrons limited by loss, residual gas pressure and plotting controls. SUK recently has been renamed to CBSIM (charge breeding simulation) and is run from a GUI (graphical user interface). Future developments are intended to make the program universal for any charge breeding simulation, including ECRs.

Gas-solid difference in charge-changing cross sections for bare and H-like nickel ions at200MeV∕u

It is well known that the density of the target can have a crucial impact on charge-changing collisions of partially ionized heavy ions. However, the basic understanding of this experimental observation is hampered by the difficulty in knowing the charge-state evolution of projectiles inside solids. Therefore, the present experiments with $200\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}∕\mathrm{u}$ bare and H-like nickel ions were performed to study charge-changing cross sections in different monatomic and compound gases and solids. The experimental results clearly demonstrate that the electron-loss cross sections in solids increase by about 40% compared to gases. The results support the Bohr-Lindhard model which predicts this gas-solid difference originating from enhanced ionization of excited ions. The experimental results are compared with recent theoretical estimates.

Distribution of exit silicon ions over excited states after penetrating through carbon foils at 2.65, 4.3 and 6.0 MeV/u

New experimental data on the charge-state ( q) evolution fractions F q ( D) and equilibrium charge-state fractions F q ∞ are presented when 2.65 MeV/u C 2+, O 3+, Ne 4+ and Si 5+ projectile ions passed through carbon foils with thickness of D = 10–200 μg/cm 2. The equilibrium fractions F q ∞ for Si + C collisions (i.e. for silicon ions colliding with carbon foils) are compared with the data at 4.3 and 6.0 MeV/u previously reported, and the observed significant difference is explained by different energy dependencies of the electron-capture and projectile-ionization cross sections. Detailed information about nl-state distribution functions N q ( nl) of exit silicon ions is obtained for these three energies by solving linear balance equations for N q ( nl) values where n and l are the principal and orbital quantum numbers, respectively, and ∑ nl N q ( nl ) = F q ∞ . The collision cross sections and radiative decay rates as the coefficients of these balance equations are calculated for each ion-beam energy. Based upon the present method, it is found that the fractions of excited Si ions after the carbon foil are about 57%, 33% and 15% at energies of 2.65, 4.3 and 6.0 MeV/u, respectively, clearly indicating the significant reduction of the excited-ion component with the collision energy increasing. The calculated equilibrium charge-state fractions F q ∞ for Si ions are in good agreement with available experimental data.

Charge state evolution of 2 MeV/u sulfur ion passing through thin carbon foil

Charge state distribution and its evolution for 2.0MeV/u sulfur ions after passing through 0.9, 1.1, 1.5, 2.0, 3.0, 4.7, 6.9 and 10μg/cm2 carbon foils have been extensively studied following our previous paper [M. Imai, M. Sataka, K. Kawatsura, K. Takahiro, K. Komaki, H. Shibata, H. Sugai, K. Nishio, Nucl. Instr. and Meth. B 230 (2005) 63] to derive data for all the initial charge states between 6+ and 14+. Measured charge state distributions, their mean charge states and distribution widths do not flat off to establish equilibrium within the measured target thickness, and an overshooting feature of the distribution width for S6+ projectile is observed around 1μg/cm2 in the target thickness. Two kinds of calculations, one based on rate equations accounting only for single charge transfer and the other applying ETACHA code, show good agreements with the experimental evolutions of mean charge and distribution width.

Evaluation of excited nl-state distributions of fast exit ions after penetrating through solid foils. Part 2: Determination of the nl-state distribution fractions of exit ions

In Part 1 [T. Miyoshi, K. Noda, Y. Sato, H. Tawara, I.Yu. Tolstikhina, V.P. Shevelko, Nucl. Instr. and Meth. B, this issue] of our two reports, new experimental data on charge-state evolution Fq(D) and equilibrium fractions Fq∞ were presented when 4.3MeV/u projectile ions with the atomic number Z=6–26 pass through carbon. Also, in this paper, a ten-charge-state model for equilibrium fractions Fq∞ expressed in terms of the charge-changing cross sections was suggested.In the present work (Part 2), a new method is suggested to determine the nl-distributions Nq(nl) of exit ions with a charge q in the specific nl-states by solving the balance equations with all appropriate effective cross sections as coefficients. The Fq∞ and Nq(nl) values are normalized so that ∑qFq∞=1 and ∑nlNq(nl)=Fq∞. The efficiency of the present method suggested is illustrated by example of Ar ions colliding with a carbon foil at projectile energy of E=6.0MeV/u. It is found that the exit argon beam comprises about 12% of argon ions in the excited states. This method can be used for evaluation of the excited-ion components in the exit ion beam when the charge equilibrium has been reached.Besides, in this work, extending the method described in [Miyoshi et al., this issue], evaluation of nl-state distributions of exit ions at energies E=4–20MeV/u is performed for the case of Ar+C foil collisions, therefore, allowing to observe the nl-state distributions in a wide energy range.

Evaluation of excited nl-state distributions of fast exit ions after penetrating through solid foils. Part 1: Charge-state fractions for 4.3 MeV/u projectiles with atomic numbers Z = 6–26 passing through carbon foils

New experimental data on charge-state evolution fractions Fq(D) and equilibrium charge fractions Fq∞ are presented when 4.3MeV/u projectile ions with an atomic number Z=6–26 passed through carbon foils with thickness of D=10–350μg/cm2. These data are compared with the data at 6.0MeV/u previously reported, and the difference is explained in terms of the energy dependence of the charge-changing cross sections in solid carbon. Using the equations relating the equilibrium fractions Fq∞ with the collision cross sections describing interaction of projectiles with the target atoms, the nl state distributions of exit ions are predicted where n and l are the principal and orbital quantum numbers, respectively. A fitting procedure of getting information about these distributions is described. The influence and importance of the so-called target-density (gas–solid) effects on the collision cross sections for solid targets are also discussed. Using the fitting procedure described in this paper (Part 1), one can obtain information on nl state distributions, which survived after collisions of the projectiles with the target atoms.In the coming second part (Part 2) of our two reports, it will be shown how to estimate the absolute number of exit ions in these nl states.