Highly Charged Ions Research Articles

Deceleration, precooling, and multi-pass stopping of highly charged ions in Be⁺ Coulomb crystals.

Preparing highly charged ions (HCIs) in a cold and strongly localized state is of particular interest for frequency metrology and tests of possible spatial and temporal variations of the fine structure constant. Our versatile preparation technique is based on the generic modular combination of a pulsed ion source with a cryogenic linear Paul trap. Both instruments are connected by a compact beamline with deceleration and precooling properties. We present its design and commissioning experiments regarding these two functionalities. A pulsed buncher tube allows for the deceleration and longitudinal phase-space compression of the ion pulses. External injection of slow HCIs, specifically Ar(13+), into the linear Paul trap and their subsequent retrapping in the absence of sympathetic cooling is demonstrated. The latter proved to be a necessary prerequisite for the multi-pass stopping of HCIs in continuously laser-cooled Be(+) Coulomb crystals.

Electromagnetic characteristics of a superconducting magnet for the 28 GHz ECR ion source according to the series resistance of the protection circuit

A linear accelerator, called RAON, is being developed as a part of the Rare Isotope Science Project (RISP) at the Institute for Basic Science (IBS). The linear accelerator utilizes an electron cyclotron resonance (ECR) ion source for providing intense highly-charged ion beams to the linear accelerator. The 28-GHz ECR ion source can extract heavy-ion beams from protons to uranium. The superconducting magnet system for the 28-GHz ECR ion source is composed of hexapole coils and four solenoid coils made with low-Tc superconducting wires of NbTi. An electromagnetic force acts on the superconducting magnets due to the magnetic field and flowing current in the case of not only the normal state but also the quench state. In the case of quench on hexapole coils, an unbalanced flowing current among the hexapole coils is generated and causes an unbalanced electromagnetic force. Coil motions and coil strains in the quench state are larger than those in the normal state due to the unbalanced electromagnetic force among hexapole coils. Therefore, an analysis of the electromagnetic characteristics of the superconducting magnet for the 28-GHz ECR ion source on series resistance of the protection circuit in the case of quench should be conducted. In this paper, an analysis of electromagnetic characteristics of Superconducting hexapole coils for the 28-GHz ECR ion source according to the series resistance of the protection circuit in the case of quench performed by using finite-elements-method (FEM) simulations is reported.

Preliminary simulation of beam extraction for the 28-GHz ECR ion source

The 28-GHz ECR (electron cyclotron resonance) ion source is under development to supply various beams from protons to uranium at the RISP (Rare Isotope Science Project). The superconducting magnet system for a 28-GHz ECR ion source consists of four solenoid coils and a saddle-type sextupole. A numerical simulation was accomplished to meet the design requirement of the ECR ion source by using the KOBRA3-INP code, which is the three dimensional ion optics code, to optimize the extraction system. The influence of the three-dimensional magnetic field and the space charge effect was considered to extract the highly-charged ion beam. In this paper, the design results for the extraction system were reported in detail.

Molecular-dynamics simulations of hillocks induced by highly-charged Arq+, Xeq+ ions impact on HOPG surface

The hillocks on highly oriented pyrolytic graphite (HOPG) surface induced by highly charged Arq+, Xeq+ ions are studied by using molecular-dynamics (MD) simulations. And a hybrid potential created by combining the ReaxFF potential with the repulsive ZBL potential is used to describe the interatomic interactions. The effects of incident highly charged ion (HCI)’s kinetic energy and the energy gain due to the interaction of HCI with its own image on the formation of the hillocks are considered in the present simulations. Our results show that both potential and kinetic energy of HCI may affect the hillock size. However, the potential energy of HCI increases dramatically with charge state, which is more important than kinetic energy in the formation of the hillock in extremely high charge states. And it is found that both the height and width of the hillock agree well with experimental data. In addition, the bond breaking and bond formation during the formation of the hillock are also investigated, and the results show that there are many σ bonds breaking and interlayer bonds formation in one layer or between two layers during this process. Furthermore, most of the interlayer bonds in HOPG surface induced by HCI impact are sp2 bond, although some interlayer sp3 bonds are also observed in the present work.

Favorable target positions for intense laser acceleration of electrons in hydrogen-like, highly-charged ions

Classical relativistic Monte Carlo simulations of petawatt laser acceleration of electrons bound initially in hydrogen-like, highly-charged ions show that both the angles and energies of the laser-accelerated electrons depend on the initial ion positions with respect to the laser focus. Electrons bound in ions located after the laser focus generally acquire higher (≈GeV) energies and are ejected at smaller angles with respect to the laser beam. Our simulations assume a tightly-focused linearly-polarized laser pulse with intensity approaching 1022 W/cm2. Up to fifth order corrections to the paraxial approximation of the laser field in the focal region are taken into account. In addition to the laser intensity, the Rayleigh length in the focal region is shown to play a significant role in maximizing the final energy of the accelerated electrons. Results are presented for both Ne9+ and Ar17+ target ions.

Can angle-resolved x-ray measurements help determine small level splittings in highly charged ions?

SynopsisThe angular distribution and photon-photon angular correlation have been studied for x-ray emissions from two-step radiative cascades that proceed via two or more overlapping intermediate resonances. From this study, we suggest that accurate angle-resolved measurements of the x-ray emissions may serve as a tool for determining small level splittings in excited highly-charged ions.

High-Performance Supercapacitor of Functionalized Carbon Fiber Paper with High Surface Ionic and Bulk Electronic Conductivity: Effect of Organic Functional Groups

Although carbon fiber paper (CFP) or nonwovens are widely used as a non-corrosive and conductive substrate or current collector in batteries and supercapacitors as well as a gas diffusion layer in proton exchange membrane fuel cells, the CFP cannot store charges due to its poor ionic conductivity and its hydrophobic surface. In this work, the chemically functionalized CFP (f-CFP) consisting of hydroxyl and carboxylic groups on its surface was produced by an oxidation reaction of CFP in a mixed concentrated acid solution of H2SO4:HNO3 (3:1 v/v) at 60°C for 1h. Other amide and amine groups modified CFP were also synthesized for comparison using a dehydration reaction of carboxylic modified CFP with ethylenediamine and n-butylamine. Interestingly, it was found that hydroxyl and carboxylic groups modified CFP behave as a pseudocapacitor electrode, which can store charges via the surface redox reaction in addition to electrochemical double layer capacitance. The aqueous-based supercapacitor of f-CFP has high areal, volumetric, and specific energy (49.0μW.h/cm2, 1960mW.h/L, and 5.2W.h/Kg) and power (3.0mW/cm2, 120W/L, and 326.2W/Kg) based on the total geometrical surface area and volume as well as the total weight of positive and negative electrodes. High charge capacity of the f-CFP stems from high ionic charge and pseudocapacitive behavior due to hydroxyl and carboxylic groups on its surface and high bulk electronic conductivity (2.03mS/cm) due to 1D carbon fiber paper. The acid treatment process here could be used to improve the charge storage capacity of other carbonaceous materials such as carbon nanotubes.

Analysis of extreme ultraviolet spectra from laser produced rhenium plasmas

Extreme ultraviolet spectra of highly-charged rhenium ions were observed in the 1–7 nm region using two Nd:YAG lasers with pulse lengths of 150 ps and 10 ns, respectively, operating at a number of laser power densities. The maximum focused peak power density was 2.6 × 1014 W cm−2 for the former and 5.5 × 1012 W cm−2 for the latter. The Cowan suite of atomic structure codes and unresolved transition array (UTA) approach were used to calculate and interpret the emission properties of the different spectra obtained. The results show that n = 4-n = 4 and n = 4-n = 5 UTAs lead to two intense quasi-continuous emission bands in the 4.3–6.3 nm and 1.5–4.3 nm spectral regions. As a result of the different ion stage distributions in the plasmas induced by ps and ns laser irradiation the 1.5–4.3 nm UTA peak moves to shorter wavelength in the ps laser produced plasma spectra. For the ns spectrum, the most populated ion stage during the lifetime of this plasma that could be identified from the n = 4-n = 5 transitions was Re23+ while for the ps plasma the presence of significantly higher stages was demonstrated. For the n = 4-n = 4 4p64dN-4p54dN+1 + 4p64dN−14f transitions, the 4d-4f transitions contribute mainly in the most intense 4.7–5.5 nm region while the 4p-4d subgroup gives rise to a weaker feature in the 4.3–4.7 nm region. A number of previously unidentified spectral features produced by n = 4-n = 5 transitions in the spectra of Re XVI to Re XXXIX are identified.

Status of deceleration and laser spectroscopy of highly charged ions at HITRAP

Heavy few-electron ions are relatively simple systems in terms of electron structure and offer unique opportunities to conduct experiments under extremely large electromagnetic fields that exist around their nuclei. However, the preparation of highly charged ions (HCI) has remained the major challenge for experiments. As an extension of the existing GSI accelerator facility, the HITRAP facility was conceived as a multi-stage decelerator for HCI produced at high velocity. It is designed to prepare bunches of around 105 HCI and to deliver them at low energies to various experiments. One of these experiments is SpecTrap, aiming for laser spectroscopy of trapped, cold HCI. We present the latest results on deceleration of ions in a radio-frequency quadrupole, synchrotron cooling of electrons in a trap as a preparation step for the prospective electron cooling of the HCI decelerated in HITRAP, as well as laser cooling of singly charged Mg ions for sympathetic cooling of HCI in SpecTrap.

X-ray spectroscopy as a tool to enlighten the growth of Van der Waals nanoparticles in a supersonic jet

SynopsisThe clustering thermodynamics in a supersonic jet of argon is studied using x-ray emission induced by keV electrons and slow highly charged ions (HCIs). The high sensitivity of the HCI interaction dynamics allows to probe the very first steps of the argon bunch in a supersonic jet.

Combined linear polarization and angular distribution measurements of x-rays for precise determination of multipole-mixing in characteristic transitions of high-Z systems

By applying novel-type position sensitive x-ray detectors as Compton polarimeters we recently performed a study of the linear polarization of Lyman- radiation following radiative electron capture into initially bare uranium ions. It was found that a model-independent determination of the ratio of the E1 and M2 transition amplitudes, and consequently of the corresponding transition rates, is feasible by combining the linear polarization data with a measurement of the angular distribution of the emitted radiation. In this work a detailed description of the underlying experimental technique for combined measurements of the linear polarization and the angular distribution of characteristic transitions in high-Z ions is presented. Special emphasis is given to the application of two, two-dimensional position-sensitive x-ray detectors for Compton polarimetry of hard x-rays. Moreover, we demonstrate the polarimeter efficiency of such detector systems can be significantly improved if events, where the charge is spread over neighboring segments, are reconstructed to be used in the polarization analysis.

The source of X-rays and high-charged ions based on moderate power vacuum discharge with laser triggering

Abstract The source of X-ray radiation with the energy of quanta that may vary in the range hν = 1÷12 keV was developed for studies in X-ray interaction with matter and modification of solid surfaces. It was based on a vacuum spark discharge with the laser triggering. It was shown in our experiments that there is a possibility to adjust X-ray radiation spectrum by changing the configuration of the electrode system when the energy stored in the capacitor is varied within the range of 1÷17 J. A comprehensive study of X-ray imaging and quanta energy was carried out. These experiments were carried out for the case of both direct and reverse polarity of the voltage on the electrodes. Additionally, ion composition of plasma created in a laser-triggered vacuum discharge was analyzed. Highly charged ions Zn(+21), Cu(+20) and Fe(+18) were observed.

On the electric dipole moment of the electron and the P,T-odd electron–nucleus interaction in highly-charged heavy ions

It is demonstrated that the effect of the electric dipole moment of electron and of the P,T-odd electron–nucleus interaction can be well distinguished in the experiments with H-like (or Li-like) highly charged ions in storage rings. These two effects cannot be separated in the experiments with a certain atom or molecule. Possible experiments for the search of P,T-odd effects with H-like ions in storage rings are discussed and the parameters of these experiments (observation time, field control) are estimated.

High-resolution x-ray spectroscopy to probe quantum dynamics in collisions of Ar17+,18+ ions with atoms and solids, towards clusters

We report on studies of projectile excited states produced by electron capture in both low and high velocity regimes, and when highly charged ions (HCIs) collide either with dilute or dense matter. Quantum effects in the interaction dynamics are probed via high-resolution x-ray spectroscopy for Ar17+ at 7 keV u−1 and for Ar18+ at 13.6 MeV u−1 on Ar, N2 or CH4 gas targets and on carbon solid foils. Relevant comparison between those two collision velocity regimes, and between gaseous and solid targets reveal specific features. In particular, the effect of multiple capture process occurring within a single-collision with gaseous target can be compared with the consequence of multistep collisions arising at surfaces and in solid-bulk at low velocity. At high velocity, beside evidence for collective response of the target electrons due to the wake field induced by HCI passing through the solid-bulk, we demonstrate that excitation and ionization collision processes damp the populations of projectile excited states for long ion transit times. The evolution of the np population as a function of n in solid is at variance from the 1/n3 law found in gas, and the disagreement increases with solid target thickness. We have also tackled studies of HCIs in collision with clusters showing that x-ray spectroscopy provides a powerful tool to sign the presence of clusters in a supersonic gas jet.

Double-stage soft x-ray laser pumped by multiple pulses applied in grazing incidence

In this paper we report on results obtained with a compact double-stage molybdenum x-ray laser (XRL), operated with a total pump energy of 600 mJ. The two gain regions were pumped using the double-pulse grazing incidence pumping technique, which includes travelling wave excitation for both the seed- and the amplifier-target. In addition, the influence of an additional pre-pulse has been studied. Seeded XRL operation has been demonstrated in both schemes, resulting in XRL pulses with a divergence of 2 × 2 mrad. The peak brilliance of the amplified XRL of 4 × 1023 photons/s/mm2/mrad2 in 5 × 10−5 relative bandwidth was more than two orders of magnitude larger compared to the original seed pulses. The presented experimental concept provides an alternative approach to the currently more common use of high-order harmonic pulses as a seed source, well suited for applications like laser spectroscopy of highly-charged ions at a storage ring.

Ground-state excitation of heavy highly-charged ions

We have studied the excitation of H-like and He-like uranium (U91+ and U90+) in relativistic collisions with gaseous targets by observing the subsequent x-ray emission. The experiment was conducted at the ESR storage ring of the GSI accelerator facility in Darmstadt, Germany. The measurements were performed with a newly developed multi-phase target at different collision energies. This enabled us to explore the proton (nucleus) impact excitation as well as the electron impact excitation processes in the relativistic collisions. The large fine-structure splitting in uranium allowed us to unambiguously resolve excitation to different L-shell levels. Moreover, information about the population of different magnetic sublevels has been obtained via an angular differential study of the decay photons associated with the subsequent de-excitation process. The experimental results are compared with calculations performed within the relativistic framework including excitation mechanisms due to both protons (nucleus) and electrons.

Radiative Recombination and Photoionization Data for Tungsten Ions. Electron Structure of Ions in Plasmas

Theoretical studies of tungsten ions in plasmas are presented. New calculations of the radiative recombination and photoionization cross-sections, as well as radiative recombination and radiated power loss rate coefficients have been performed for 54 tungsten ions for the range W6+–W71+. The data are of importance for fusion investigations at the reactor ITER, as well as devices ASDEX Upgrade and EBIT. Calculations are fully relativistic. Electron wave functions are found by the Dirac–Fock method with proper consideration of the electron exchange. All significant multipoles of the radiative field are taken into account. The radiative recombination rates and the radiated power loss rates are determined provided the continuum electron velocity is described by the relativistic Maxwell–Jüttner distribution. The impact of the core electron polarization on the radiative recombination cross-section is estimated for the Ne-like iron ion and for highly-charged tungsten ions within an analytical approximation using the Dirac–Fock electron wave functions. The effect is shown to enhance the radiative recombination cross-sections by ≲20%. The enhancement depends on the photon energy, the principal quantum number of polarized shells and the ion charge. The influence of plasma temperature and density on the electron structure of ions in local thermodynamic equilibrium plasmas is investigated. Results for the iron and uranium ions in dense plasmas are in good agreement with previous calculations. New calculations were performed for the tungsten ion in dense plasmas on the basis of the average-atom model, as well as for the impurity tungsten ion in fusion plasmas using the non-linear self-consistent field screening model. The temperature and density dependence of the ion charge, level energies and populations are considered.

Characterization of ion Coulomb crystals for fundamental sciences

We performed classical molecular dynamics (MD) simulations in order to search the conditions for efficient sympathetic cooling of highly charged ions (HCIs) in a linear Paul trap. Small two-component ion Coulomb crystals consisting of laser-cooled ions and HCIs were characterized by the results of the MD simulations.We found that the spatial distribution is determined by not only the charge-to-mass ratio but also the space charge effect. Moreover, the simulation results suggest that the temperature of HCIs do not necessarily decrease with increasing the number of laser-cooled ions in the cases of linear ion crystals. We also determined the cooling limit of sympathetically cooled 165Ho14+ ions in small linear ion Coulomb crystals. The present results show that sub-milli-Kelvin temperatures of at least 10 Ho14+ ions will be achieved by sympathetic cooling with a single laser-cooled Be+.

Penning trap mass measurements utilizing highly charged ions as a path to benchmark isospin-symmetry breaking corrections inRb74

Penning trap mass measurements of neutron-deficient Rb isotopes have been performed at TRIUMF's Ion Trap for Atomic and Nuclear Science (TITAN) facility by utilizing highly charged ions (HCIs). As imperative for a new approach with significant gain in measurement precision, experimental procedures, and systematic uncertainties are discussed in detail. Among the investigated nuclides, the superallowed nuclear $\ensuremath{\beta}$ emitter $^{74}\mathrm{Rb}$ will especially benefit from the advantage offered by HCI because the limited attainable precision owing to its short half-life $({T}_{1/2}=65$ ms) represents a challenge for conventional Penning trap mass spectrometry. Motivated by an updated ${Q}_{\mathrm{EC}}$ value for $^{74}\mathrm{Rb}$ of 10 416.8(3.9) keV and its large isospin-symmetry breaking corrections, we present a new test to benchmark the consistency between theoretical models of isospin-symmetry breaking corrections in superallowed decays, the conserved vector current hypothesis, and experimental data.

Towards a fast calculator for the radiation characteristics of radiative recombination and radiative electron capture

The radiative capture of free electrons (radiative recombination) and bound electrons (radiative electron capture) are among the most important charge changing processes for fast, highly-charged ions. While total cross sections can be obtained by an approximate formula with reasonable accuracy, the estimation of angular distributions and polarization properties of the emitted radiation requires a fully relativistic treatment that is numerical expensive. Therefore we recently started the development of a fast calculator for these radiation characteristics. The program is based on a grid of rigorously calculated data points for free- electron capture into bare ions, between which interpolation is performed to obtain radiation characteristics for specific collision systems. Also capture into few-electron systems is taken into account in an approximate way. We present first results from this development.