Beams Of Highly-charged Ions Research Articles

Experimental investigations of two‐electron relaxation processes in Rydberg hollow atoms

AbstractThe two‐electron relaxation processes in Rydberg hollow atoms (RHA), formed in collisions of slow highly charged ions (HCI) of Xe (q = 23–36) with metallic Be surface, were studied experimentally by means of X‐ray spectroscopy. The pulsed beams of HCI were delivered by the electron beam ion source while the emitted X‐rays were observed by a silicon drift detector. From the measured spectra of M‐X‐rays, dominated by the Paschen series of nf3d one‐electron‐one photon radiative (R) transitions, the relaxation processes involving two electrons, namely, the internal dielectronic excitation, interatomic Coulombic decay, two‐electron‐one photon transitions and the Auger transition (A), were extracted. The dead‐time effect in X‐ray detector for a pulsed ion beam of HCI was considered in order to estimate the Auger decay rates. Finally, we demonstrate that the present results give access to estimate a time‐scale of relaxation of RHA in solids.

Studies of bremsstrahlung and characteristic X-Ray lines using the VENUS ECR

Though known for their production of high currents of highly-charged ion beams, advanced electron cyclotron resonance (ECR) ion sources like VENUS at Lawrence Berkeley National Laboratory (LBNL) also generate significant numbers of x-rays. The LBNL ECR ion source group has spent many years studying the x-rays emitted from VENUS in order to gain a better understanding of the ECR plasma. Based on the emitted bremsstrahlung continuum, a spectral temperature Ts can be calculated which is a relative indication of the temperature of the plasma electrons. We will present correlations between Ts with respect to parameters such as magnetic fields. In addition to the bremsstrahlung continuum, the plasma ions emit characteristic x-rays. We will show that additional information can be gained by investigating spectral line shifts. A summary of this recent research using VENUS will be presented and discussed.

Design of an energy analyzer for highly charged Ions HCI beams

The measurements of the energy spread and the average beam energy for highly charged ion (HCI) beams, such as those delivered from EBIS charge breeder ion source and ECR ion source, demand the development of an energy analyzer of highest measurement efficiency. A new prototype of a compact analyzer called retarding field energy analyzer RFEA capable to handle all kinds of HCI beams at RISP project is being designed. The RFEA's efficiency measurement has been evaluated via numerical simulations. The involvements of the beam emittance and, mainly, the space charge effect to degrade the RFEA's efficiency have been simulated. The simulations showed that the developed RFEA, with its improved optics system and for optimum voltages on the focusing cylinder, may lead to high efficiency energy resolutions not exceeding 0.57 eV for beam energy going up to 20 keV . Acceptable errors on the measured energy spread not exceeding 30 % are, likewise, achieved despite the presence of degradations stemming from the previous degrading effects. The measurement of the average energy showed large shifts of some hundred eV from the theoretical average beam energy due to the overlapping of the ensemble of beamlets forming the HCI beam. The 3D design of this analyzer and its components will be also presented. The material of these components and their positioning to avoid electrical breakdowns will be also discussed.

Physics book: CRYRING@ESR

The exploration of the unique properties of stored and cooled beams of highly-charged ions as provided by heavy-ion storage rings has opened novel and fascinating research opportunities in the realm of atomic and nuclear physics research. Since the late 1980s, pioneering work has been performed at the CRYRING at Stockholm (Abrahamsson et al. 1993) and at the Test Storage Ring (TSR) at Heidelberg (Baumann et al. 1988). For the heaviest ions in the highest charge-states, a real quantum jump was achieved in the early 1990s by the commissioning of the Experimental Storage Ring (ESR) at GSI Helmholtzzentrum fur Schwerionenforschung (GSI) in Darmstadt (Franzke 1987) where challenging experiments on the electron dynamics in the strong field regime as well as nuclear physics studies on exotic nuclei and at the borderline to atomic physics were performed. Meanwhile also at Lanzhou a heavy-ion storage ring has been taken in operation, exploiting the unique research opportunities in particular for medium-heavy ions and exotic nuclei (Xia et al. 2002).

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.

Analysis of Si crystal irradiated by highly-charged Ar ions using RBS-channeling technique

An ion-beam is one of the powerful tools applied in nanotechnology and nanoscience. It is expected that the application of highly-charged ion (HCI) beams, which have a higher reactivity in materials, would yield further developments in these fields. For effective applications of HCI beams, it is important to investigate any modifications of irradiated materials from a microscopic point of view. For this purpose, an irradiation-induced defect in a single crystal was analyzed by means of Rutherford backscattering-channeling (RBS) technique. In order to induce defects, Ar6+ and Ar9+ beams with an energy of 100keV were irradiated onto a single crystal of Si. By means of a simple analysis, the depth distribution of disordered Si atoms induced by the irradiation was extracted from the observed RBS-C spectra. The present result implies that the productivity of defects in a Si crystal is enhanced for Ar9+ ions compared with Ar6+ ions in a limited region of the surface.

Highly charged ion beam applied to lithography technique (invited)

In various fields of nanotechnology, the importance of nanoscale three-dimensional (3D) structures is increasing. In order to develop an efficient process to fabricate nanoscale 3D structures, we have applied highly charged ion (HCI) beams to the ion-beam lithography (IBL) technique. Ar-ion beams with various charge states (1+ to 9+) were applied to fabricate spin on glass (SOG) and Si by means of the IBL technique. The Ar ions were prepared by a facility built at Kochi University of Technology, which includes an electron cyclotron resonance ion source (NANOGAN, 10GHz). IBL fabrication was performed as a function of not only the charge state but also the energy and the dose of Ar ions. The present results show that the application of an Ar9+ beam reduces the etching time for SOG and enhances the etching depth compared with those observed with Ar ions in lower charged states. Considering the high-energy deposition of HCI at a surface, the former phenomena can be understood consistently. Also, the latter phenomena can be understood based on anomalously deep structural changes, which are remarkable for glasses. Furthermore, it has also been shown that the etching depth can be easily controlled with the kinetic energy of the Ar ions. These results show the possibilities of the IBL technique with HCI beams in the field of nanoscale 3D fabrication.

Scintillation light produced by low-energy beams of highly-charged ions

Measurements have been performed of scintillation light intensities emitted from various inorganic scintillators irradiated with low-energy beams of highly-charged ions from an electron beam ion source (EBIS) and an electron cyclotron resonance ion source (ECRIS). Beams of xenon ions Xe q+ with various charge states between q = 2 and q = 18 have been used at energies between 5 and 17.5 keV per charge generated by the ECRIS. The intensity of the beam was typically varied between 1 and 100 nA. Beams of highly charged residual gas ions have been produced by the EBIS at 4.5 keV per charge and with low intensities down to 100 pA. The scintillator materials used are flat screens of P46 YAG and P43 phosphor. In all cases, scintillation light emitted from the screen surface was detected by a CCD camera. The scintillation light intensity has been found to depend linearly on the kinetic ion energy per time deposited into the scintillator, while up to q = 18 no significant contribution from the ions’ potential energy was found. We discuss the results on the background of a possible use as beam diagnostics, e.g. for the new HITRAP facility at GSI, Germany.

Ion beam lithography by using highly charged ion beam of Ar

The ion beam lithography (IBL) method is one of the promising techniques that can fabricate 3D nano-structures. In order to develop IBL further, highly charged ion (HCI) beams have been applied to IBL method in our research group. Considering a high reactivity of HCI beams in an irradiated material, it is expected that HCI beams enhance a productivity of damage in irradiated materials compared with singly charged ion beams. This effect will be observed as a change in etching speed of IBL process. In the present study, the HCI effect on IBL was investigated by irradiating Ar9+ and Ar1+ with the fluence of 6.3×1012 to 4.7×1014ptcl./cm2, which is lower than that in our previous study (6.3×1014 to 3.1×1015ptcl./cm2). The Ar9+ and Ar1+ ion beams with E=90keV, which were prepared by an irradiation facility of HCI beams at Kochi University of Technology, were irradiated onto spin-on-glass (SOG) through a stencil mask. In order to investigate an etching process by using BHF solution, the fabrication depth of SOG surface was measured as a function of an etching time. The depth measurements show that an irradiation of HCI beams enhances an etching speed and the fabrication depth. For example, the fabrication depth with Ar9+ beams, which is at least 100nm deeper than that with Ar1+ beams, was achieved. The present result shows a priority of HCI beams to fabricate deeper 3D-structures and gives information required to optimize the fabrication process with keeping a good precision.

Production of a nm sized slow HCI beam with a guiding effect

We have developed a method to produce a nm sized slow highly-charged ion beam based on a self-organized charge up inside a single tapered-glass capillary. In order to investigate the characteristics of the obtained beams, the transmission of 8 keV Ar8+ beam through the capillary of 5-cm long with 800/24 μm inlet/outlet inner diameter was examined. The transmitted beam had the same size as the outlet with the beam-density enhancement of approximately 10. The initial beam was guided through a capillary tilted by as large as ±100 mrad and it still kept the incident charge. A focused ion beam of Ga+ was employed to manufacture the capillary outlet as small as several hundreds nm in diameter and fabricate a thin glass window at the capillary outlet for biological applications.

Nano-scale surface modification of materials with slow, highly charged ion beams

Some results on surface modification of Si and graphite with highly charged ions (HCIs) are presented. Modified surfaces were observed using scanning tunneling microscopy. Crater-like structure with a diameter in nm region is formed on a Si(111)-(7×7) surface by the incidence of a single HCI. The protrusion structure is formed on a highly oriented pyrolytic graphite surface on the other hand, and the structure becomes an active site for molecular adsorption. A new, intense HCI source and an experimental apparatus are under development in order to process and observe aligned nanostructures created by the impact of collimated HCI beam.

Highly charged ion beams from the Tokyo EBITfor applications to nano-science and -technology

We report present status of a beam line for transportation of highly charged ions(HCIs) extracted from the Tokyo EBIT. We have produced continuous beams of 2.5 × 105 cps forXe44+ through a 1 mm aperture. With slightly high energy operation (electron beam energy: 78keV) of the Tokyo EBIT, we have also obtained 103 ions/pulse for Ta70+ HCIs extracted by apulse mode (trapping time: 3 sec). We are going to apply such HCI beams to nano-processes onsolid surfaces by utilizing some useful characteristics of the HCI-interactions. Future perspectiveof HCI-based nano-science and -technology is presented.

Atomic Collision Experiments Utilizing Low-Velocity, Highly-Charged Ion Beams

Intense beams of highly-stripped ions are now routinely produced at low velocities using the Brookhaven dual MP-tandems in a unique four-stage accel/decel mode. This mode of operation combines three stages of acceleration, stripping at high energy, and one stage of deceleration to near-zero velocity. To date, experiments have used 10-100 nA beams of bare and few-electron heavy ions at energies as low as 0.2 MeV/amu, and upgrades of the facility should push the lower limit below 0.1 MeV/amu. Recent experiments, such as measurements of charge transfer and x-ray production for S6-16+ on He and Ar at 6-20 MeV and P(b) measurements for MO x-rays produced in Cl16+ + Ar collisions at 20, 10 and 5 MeV have demonstrated the usefulness of highly-stripped, low-velocity projectiles. These experiments and a few possibilities for future experiments are discussed.