Production Of High Charge States Research Articles

Inner shell ionization of argon in ECRIS plasma

The volumetric Kα emission rate of argon emitted from the electron cyclotron resonance (ECR) heated plasmas of the JYFL (University of Jyväskylä, Department of Physics) 14 GHz ECR ion source (ECRIS) and the 14.5 GHz Grenoble Test Source (GTS) at iThemba Laboratory for Accelerator Based Sciences have been measured to gain an understanding of the influence of the ion source tune parameters on the absolute inner shell ionization rate. It was observed that the behaviour of the ionization rate and the extracted ion beam currents react differently, depending the parametric sweep performed. The neutral gas pressure and incident microwave power was found to have the strongest influence on the ionization rate. At high neutral gas pressure, the absolute inner shell ionization rate was found to saturate. This observation is as a result of the plasma energy content becoming insufficient to sustain the growth in ionization rate. It was also observed that the incident microwave power should be increased much more to counter the decrease in high charge state production as a result of the gas increase and subsequent increase in charge exchange. At low incident microwave, the ionization rate per unit absorbed microwave power was found to be high, which suggests that the inner shell ionization process is driven by the ion dynamics as opposed to the electron dynamics. The influence of the biased disc voltage and magnetic field configuration on the ionization rate was found to be minimal. This led to the suggestion that these two tune parameters does not directly impact the warm electron population of the ECRIS plasma during the parametric sweeps. The Kα emission rate can be used as an additional tool for benchmarking the results of numerical simulation codes on ECRIS plasmas.

Ionization of molecular hydrogen and stripping of oxygen atoms and ions in collisions of Oq++H2 ([formula omitted]): Data for secondary electron production from ion precipitation at Jupiter

Energetic oxygen and sulfur ion precipitation into the atmosphere of Jupiter is thought to produce an X-ray aurora as well as to contribute to ionization, heating, and dissociation of the molecules of the atmosphere. At high energy, stripping of electrons from these ions by atmospheric gas molecules results in the production of high charge states throughout a portion of this passage through the atmosphere. Therefore, to enable modeling of the effects of secondary electrons produced by this ion precipitation, from either the solar wind or magnetospheric sources such as the Galilean moons, a large range of ionization and stripping data is calculated and tabulated here that otherwise is not available. The present data are for the abundant precipitating species, oxygen, colliding with the dominant upper atmosphere gas, molecular hydrogen, and cover the principal reaction channels leading to secondary electron production (single and double ionization, transfer ionization, and double capture followed by autoionization, and single and double stripping of electrons from the projectile). Since the ions possess initial energies at the upper atmosphere in the keV to MeV range, and are then slowed as they pass through the atmosphere, results are calculated for 1–2000 keV/u Oq++H2 (q=0–8). In addition to the total cross sections for ionization and stripping processes, models require the distribution in energy and angle of the ejected electrons, so cross sections differential in these parameters are also calculated. The data may be used to model the energy deposited by ion precipitation in Jupiter’s atmosphere and thereby contribute to the elucidation of the ionosphere–atmosphere coupling.

On-line charge breeding using ECRIS and EBIS

The efficient and rapid production of a high-quality, pure beam of highly charged ions is at the heart of any radioactive ion beam facility. Whether an electron cyclotron resonance (ECR) ion source or an electron beam ion source (EBIS) is used to produce these highly charged ions, their operating characteristics will set the boundaries on the range of experiments which can be performed. In addition, time structure and duty cycle have to be considered when defining the operating parameters of the accelerator system as a whole. At Argonne National Laboratory (ANL), an ECR charge breeder was developed as part of the Californium Rare Ion Breeder Upgrade (CARIBU) program. The charge breeding efficiency and high charge state production of the source is at the forefront of ECR charge breeders, but its overall performance as part of the accelerator system is limited by pervasive background and relatively long breeding times. As such, an EBIS charge breeder has been developed and is running in an off-line configuration. It has already demonstrated good breeding efficiencies, shorter residence times, and reduced background and is scheduled to replace the ECR charge breeder in late 2015. The resultant change in duty cycle and time structure necessitates changes to the overall operation of the facility. The experiences with these breeders, as well as from several other facilities which already utilize an ECR or EBIS for charge breeding, help to define the operational characteristics of each technology – their strengths, their weaknesses, and the possible paths to improvement.

Ionization efficiency studies with charge breeder and conventional electron cyclotron resonance ion source

Radioactive Ion Beams play an increasingly important role in several European research facility programs such as SPES, SPIRAL1 Upgrade, and SPIRAL2, but even more for those such as EURISOL. Although remarkable advances of ECRIS charge breeders (CBs) have been achieved, further studies are needed to gain insight on the physics of the charge breeding process. The fundamental plasma processes of charge breeders are studied in the frame of the European collaboration project, EMILIE, for optimizing the charge breeding. Important information on the charge breeding can be obtained by conducting similar experiments using the gas mixing and 2-frequency heating techniques with a conventional JYFL 14 GHz ECRIS and the LPSC-PHOENIX charge breeder. The first experiments were carried out with noble gases and they revealed, for example, that the effects of the gas mixing and 2-frequency heating on the production of high charge states appear to be additive for the conventional ECRIS. The results also indicate that at least in the case of noble gases the differences between the conventional ECRIS and the charge breeder cause only minor impact on the production efficiency of ion beams.

Oscillations of ECR ion source beam current along the beam transport of the JYFL K-130 cyclotron

A versatile measurement system has been developed to study the temporal characteristics of ion beams in millisecond time scale. The system is composed of data acquisition hardware and LabVIEW based measurement and analysis program. The measurement system and ion beam current oscillation results measured with a 14 GHz AECR-U type ion source at University of Jyv"askyl"a, Department of Physics (JYFL), are presented. It is shown that the ion beams exhibit periodic current fluctuations at frequencies from 100 Hz to 1.5 kHz with amplitudes ranging from 1 to 65 percent of the average beam current. It is argued that these oscillations originate from the ion source plasma since their characteristics depend on the ion source tuning. It is shown that increasing the ion source solenoid magnetic field increases the relative oscillation amplitude. Increasing the ion source extraction voltage has the opposite effect. The beam current oscillations were measured along the low energy beam transport (LEBT) connecting the ion source to the JYFL K-130 cyclotron as well as in the following high energy beamline. It is shown that the beam current oscillations originating from the ion source are preserved during the LEBT but are not observed after the cyclotron. The transmission efficiency from the ion source through the cyclotron was measured with two beams of comparable beam currents but different oscillation frequency and amplitude, corresponding with the two cases typically seen when the ion source is tuned for high charge state production. The transmission of the LEBT remained unaltered but the cyclotron performance was affected by the varying oscillation characteristics.

X-Ray FEL Induced Double Core-Hole and High Charge State Production

We report novel results on multiple photoionization of the N2 molecule by femtosecond x-ray pulses from the Linac Coherent Light Source free electron laser. Ion spectroscopy reveals the ionization and dissociation dynamics, including a molecular mechanism of frustrated absorption that suppresses the formation of high charge states at short pulse durations. Auger and photo-electron analysis of double core hole production (through absorption of two photons on the time scale shorter than the Auger decay) agrees well with theory for the case of both K-shell holes on the same N atom, and gives an experimental upper bound for the relative contribution in case of one core hole on each N atom.

Charge breeding simulations for radioactive ion beam production

The charge breeding technique is used for radioactive ion beam (RIB) production in order of optimizing the re-acceleration of the radioactive element ions produced by a primary beam in a thick target. Charge breeding is achieved by means of a device capable of increasing the ion charge state from 1+ to a desired value n+. In order to get high intensity RIB, experiments with charge breeding of very high efficiency could be required. To reach this goal, the charge breeding simulation could help to optimize the high charge state production efficiency by finding more proper parameters for the radioactive 1+ ions. In this paper a device based on an electron beam ion source (EBIS) is considered. In order to study that problem, a code already developed for studying the ion selective containment in an EBIS with RF quadrupoles, BRICTEST, has been modified to simulate the ion charge state breeding rate for different 1+ ion injection conditions. Particularly, the charge breeding simulations for an EBIS with a hollow electron beam have been studied.

Production of highly ionized species in high-current pulsed cathodic arcs

Time resolved optical diagnostic techniques were used to study the production of highly ionized species in aluminum plasma produced by a centered-triggered high-current pulsed cathodic arc. Controlling the spacing between cathode spots enabled a correlation between a reduction in the mean charge state and an increase in the spacing of cathode spots to be observed. As the cathode current was increased, the distances between spots were reduced and these charge states were produced for longer times. Strong cathode spot coupling is proposed as a mechanism for the production of high charge states.

Experiments With Fundamental Aspects Performed in a Small ECR Ion Source for a New Carbon Therapy Facility

A small permanent magnet electron cyclotron resonance ion source (ECRIS) has been developed for the production of C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> ions for the third carbon-therapy facility in Japan. The emphasis of the design is on stability, easy operation and prevention of maintenance, where high charge state production is not a requirement. Tests with different feed-gases have shown that carbon-hydrogen compounds are favorable, in particular with carbon to hydrogen ratio of 1:1. A parallel study of (highly charged) C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5+</sup> production revealed that the various compounds of feed-gas are behaving as if feeding the two components separately. This includes the behavior of replacing hydrogen by deuterium. From these a plausible reason was found for the observed failure of hydrogen as a mixing gas. Other experiments with the small source were performed to study the effect of inserting an aluminum (-oxide) cylinder inside the electron cyclotron resonance plasma-chamber. Earlier research found an increase in production of high charge states, but a good explanation was still missing. By adapting an insulated cylinder that can be biased positively, it was shown that ion diffusion to the side walls can be reduced, in particular when the cylinder is made of strips covering the pole pieces of the hexapole magnet. The new tool can be used to study the mechanism in more detail and also has possibilities of further improving a source for higher charge state production.

ITEP Bernas ion source with additional electron beam

A joint research and development program is underway to develop steady-state intense ion sources for the two energy extremes of MeV and hundreds of eV. For the MeV range the investigations were focused on charge-state enhancement for ions generated by the modified Bernas ion sources. Based on the previously successful ITEP experience with the e-metal vapor vacuum arc ion source [e.g., Batalin et al., Rev. Sci. Instrum. 75, 1900 (2004)], the injection of a high-energy electron beam into the Bernas ion source discharge region is expected to enhance the production of high charge states. Presented here are construction details and studies of electron-beam influence on the enhancement of ion-beam charge states generated by the modified Bernas ion source.

Optimizing C4+ and C5+ beams of the Kei2 electron cyclotron resonance ion source using a special gas-mixing technique

With the prototype electron cyclotron resonance ion source for the next carbon therapy facility in Japan a series of measurements has been performed in order (a) to find the best condition for producing high beam currents of C4+ ions, and (b) to study the effect of “special” gas mixing by using a chemical compound as a feed gas. The effect would then appear as an increase in high charge state production in this case of C5+ ions. In “regular” gas-mixing experiments it is well known that an isotopic phenomenon occurs: a heavier isotope of the mixing gas is increasing the production of high charge states of the beam gas ions. A similar isotopic effect has been found in the present experiment: with deuterated methane (CD4 gas) the C5+ beam currents are about 10% higher than with regular methane (CH4 gas). The “mixing-gas” ratio D (or H) to C can be decreased by choosing, e.g., butane gas; in this case the isotopic effect for C5+ production is even stronger (&amp;gt;15%). For production of C4+ ions the isotopic effect appears to be absent. Clearly this is related to the much easier production. It turns out that the relative amount of carbon is much more important: butane gives about 10% higher C4+-ion currents than methane.

Advanced magnetic calculations for high magnetic field compact ion source

The design of the advanced electronic cyclotronic resonance ion source (ECRIS) requires relatively high axial and radial magnetic inductions to allow the ECR frequency increase and to take advantage of the subsequent density increase (scaling laws). The last improvements of the commercial rare-earth magnet characteristics open new opportunities for ECRIS and enable the design of very high hexapolar magnetic fields for next generation compact ECRIS. Moreover, the high temperature superconducting (HTS) wires allow designing reliable and compact axial field coils (30 K cooled) at a very effective cost. It is thus very relevant to study a compact hybrid ECRIS using high remanence magnet and HTS technologies. In such a design, the volume of the plasma chamber is a free parameter that can be adjusted to the user requirement. It can be dedicated to very high ionic current production or high charge state production, pulsed, or cw operations. This paper presents the three-dimensional overall simulation of a 3 T axial magnetic field compact ECRIS with a high radial field sextupole composed with several magnet types and reaching ∼1.9 T in front of the radially magnetized magnets. This design study will lead to the building of the 28–40 GHz A-PHOENIX source at the laboratory which will deliver its first beam by the end of 2004.

Charge state breeding applications with the ECR PHOENIX source: From low to high current production

The electron cyclotron resonance (ECR) charge state breeding method based on the PHOENIX source is first dedicated to the continuous high charge state radioactive ion production for cyclotrons, due to their q/m requirements, and to the high charge state production efficiency. It can be used as an injector for RFQ, linacs, and synchrotrons either in cw or pulsed operation. The efficiency of the 1+ to the n+ ion production and the charge breeding time are the fundamental parameters of the method, their dependence with regard to the 1+ ion beam emittance injected has been measured and will be discussed with regard to the radioactive ion beam acceleration. The efficiency of the process slightly decreases when injecting ion beam emittances higher than 30 π mm mrad, and remains constant in the 1+ radioactive intensity range available today and in the future. The ECR ion trap tuning associated with the afterglow mode permits the ionization, the accumulation, and the extraction of the injected primary beam keeping interesting efficiencies suitable to pulsed accelerators.

Influence of plasma-wall interaction on high charge state production in electron cyclotron resonance ion sources

In contrast to the widely accepted model of ion cooling, which is supposed to fully explain the gas mixing effect, we recently presented an alternative model of self-enhanced sputtering of wall material. In order to prove our model further, we numerically simulated different plasma parameters in the vicinity of plasma limiting edges of the discharge chamber (walls) via particle in cell and Monte Carlo methods. Both experimental investigations and simulations give clear evidence that the optimization of an electron cyclotron resonance ion source with respect to the high charge state production is sensitively influenced by the contamination of the plasma with heavy atoms sputtered off the walls. Any optimization of the charge state distribution in physical terms means nothing else than balancing the ratio of hot and cold electrons.

Production of high-charge states of Xe in a femtosecond laser pulse

In a recent experiment for ionization of the neutral Xe atom with a femtosecond Ti:sapphire laser pulse, large yields of high charge states up to sixfold ionizations have been observed. Here, we present a dynamical model of laser-induced nonsequential multiple ionization of a complex atom. The calculated results for the ion yields are found to agree remarkably well for the six charge states measured. Analyses show a significant presence of nonsequential (or direct) escapes of up to four electrons in the experimental data. For future experimental references, predictions for the sevenfold and eightfold ionizations are also made.

State Selective Electron Capture Studies: The Contribution of M1- and E2-Transitions to the Lyman Radiation of H-like Uranium

The heavy-ion storage ring ESR provides a unique possibility to decelerate highly charged ions up to bare uranium to energies which are far below the energy required for the efficient production of high charge states. This technique allows for a detailed study of atomic collision dynamics and, in particular, for an (n,l,j)-sensitive investigation of electron capture processes by means of x-ray spectroscopy. The results of a first state selective capture investigation performed for decelerated bare uranium ion are discussed and their relevance for Lamb shift investigations on high-Z hydrogenlike ions is emphasized.

A new 14 GHz electron-cyclotron-resonance ion source for the heavy ion accelerator facility ATLAS

A 14 GHz electron-cyclotron-resonance (ECR) ion source has been designed and built at Argonne National Laboratory. The source is a modification of the AECR [D. J. Clark, C. M. Lyneis, and Z. Q. Xie, 14th Particle Accelerator Conference (PAC), IEEE Conference 91 CH3038-7, 1991 (unpublished), p. 2796 and C. M. Lyneis, Z. Q. Zie, D. J. Clark, R. S. Lam, and S. A. Lundgren, 10th International Workshop on ECR Ion Sources, Oak Ridge, ORNL CONF-9011136, 1990 (unpublished), p. 47.] at Berkeley and incorporates the latest results from electron-cyclotron-resonance (ECR) developments to produce intense beams of highly charged ions, including an improved magnetic confinement of the plasma electrons with an axial mirror ratio of 3.5. The aluminum plasma chamber and extraction electrode as well as a biased disk on axis at the microwave injection side donate additional electrons to the plasma, making use of the large secondary electron yield from aluminum oxide. The source is capable of ECR plasma heating using two different frequencies simultaneously to increase the electron energy gain for the production of high charge states. The main design goal is to produce several eμA of at least U35+238 in order to accelerate the beam to coulomb-barrier energies without further stripping. First charge state distributions for gaseous elements have been measured and 210 eμA16O7+ has been achieved. A normalized, 90% emittance from 0.1 to 0.2 π mm mrad for krypton and oxygen beam has been found.

Differential cross sections for the multiple ionization of Ne and Ar by protons.

Differential cross sections d${\mathrm{\ensuremath{\sigma}}}^{\mathit{q}+}$(E,cphi)/d\ensuremath{\Omega} for multiple ionization of Ne and Ar by protons have been measured for final charge states q=1--6, impact energies E=0.4--3 MeV, and scattering angles cphi=0.28--6.5 mrad. Within these ranges of energies and scattering angles, the angular dependence of the differential cross section for single ionization at constant energy follows a power law, d${\mathrm{\ensuremath{\sigma}}}^{+}$/d\ensuremath{\Omega}\ensuremath{\propto}${\mathit{cphi}}^{\mathrm{\ensuremath{-}}\mathit{k}}$. The power k is found to be near 3.8 for Ne and 3.65 for Ar. A monotonic increase of all the multiple-ionization cross sections d${\mathrm{\ensuremath{\sigma}}}^{\mathit{q}+}$/d\ensuremath{\Omega}, q\ensuremath{\ge}2 relative to the single-ionization cross section d${\mathrm{\ensuremath{\sigma}}}^{+}$/d\ensuremath{\Omega}, is observed for increasing scattering angle. This behavior is most likely due to the combined effect of two multiple-ionization mechanisms. At the most distant collisions, the multiple ionization is dominated by the direct removal of valence electrons by the projectile. At the closest collisions, inner-shell ionization, followed by vacancy cascades, adds to the production of high-charge states. For Ne, this interpretation is supported by qualitative agreement with a model calculation based on the assumption that the multiple ionization of each shell follows binomial statistics. Quantitative agreement between experiment and theory requires a more thorough analysis of the many-body problem.

Multiple ionisation of Ar atoms by single-electron impact at energies near threshold

Multiple ionisation of Ar atoms by single-electron impact is investigated in the energy range from threshold up to about 600 eV. Partial cross sections for the production of Arq+ with 3<or=q<or=6 are measured. In the near-threshold region the ionisation curves can be fitted according to power laws sigma varies as (Eel-Ethr)K, where the exponent K is found to be smaller than q for high charge states. At higher electron energies the ionisation cross sections show a significant influence of inner-shell ionisation, which becomes the dominant mechanism for the production of high charge states.

High Charge State Heavy Ion Sources

A review is given of existing sources of highly charged heavy ions. The performance of the sources, predominantly of the P.I.G. type, are summarized and compared. Some mechanisms for the production of high charge states are described. There is a brief description of new developments, particularly of devices using ion trapping.