Surface Ionization Source Research Articles

The Frankfurt MAXEBIS setup for advanced charge breeding experiments

The demand of exotic ions of higher charge states for the injection into an accelerator has driven the development of the charge breeding method. Existing facilities such as REX-ISOLDE or ISAC (TRIUMF) are already using a charge-state booster for the post acceleration of radioactive ions. Planned facilities such as MAFF, SPES, SPIRAL II, and EURISOL have identified the need for a breeding system because of the demand for highly charged ions for low-energy experiments and because of the available budget. Therefore, the exploration and optimization of existing charge-state breeders is mandatory and is supported by the I3-EURONS project. Some tasks of the joint research activity “charge breeding” are experiments with the Frankfurt MAXEBIS setup, which has been modified within the past years toward high current electron beam and external injection of alkaline ions by a surface ionization source. The electron gun, the inner electrode structure, and the collector of the MAXEBIS have been modified. The full MAXEBIS setup has been moved from Frankfurt to GSI, where the charge breeding measurements will be done. In addition the MAXEBIS will deliver highly charged ions for tests of the cooler Penning trap of the GSI Heavy Ion Trap project HITRAP. The setup, tests, and the planned breeding experiments will be discussed.

Status report of the radioactive ion beam production at TRIUMF (invited)

The ISAC radioactive ion beam (RIB) facility has been operational since November 1998. The ISAC Facility at TRIUMF utilizes up to 100μA proton intensity at 500MeV from the existing H− cyclotron to produce RIB using the isotopic separation on-line (ISOL) method. The science programs at ISAC range from nuclear astrophysics to fundamental symmetry. The facility can deliver RIB to the low energy area and a linear accelerator composed of a four-rod radio frequency quadrupole and a linear accelerator can provide beam from A=3to30amu with an energy range from 0.15to1.5AMeV. An accelerator upgrade is underway and will allow us to extend the mass range to A=150amu and the energy up to 6.5AMeV. The target/ion source assembly and heavy-ion optics components are located in a shield canyon under 2m of steel shielding allowing high proton beam intensity on a thick target. At the beginning the production target was operating at 2–5μA but rapidly the proton beam intensity was raised to 40μA. Now, we operate up to nearly 100μA proton beam using the new high power target design equipped with radial fins. While the production target capabilities have improved quite a lot, from 2to100μA, the ion sources development have not followed the same trend. Since the beginning of ISAC most of the RIB production is done using the surface ion source. A 2.45GHz electron cyclotron resonance ion source (ECRIS) was built and used on line with moderate success.

Direct 1+ to n+ method for production of radioactive alkaline ions

In the frame of the production of radioactive ion beams by isotope separator on line method, a new principle of target∕ion-source system has been developed at GANIL to produce multicharged ions of alkalis. The principle consists of a surface ionization source associated to a multicharged electron-cyclotron-resonance ion source (ECRIS) named NANOGAN III [R. Leroy et al., AIP Conf. Proc. 749, 137 (2004)] presently used to produce radioactive ions of gas on Système de Production d’Ions Radioactifs Accélérés en Ligne partie I. The singly charged ions are injected in the multicharged ion source without mass separation, through a very short beam line including several stages of acceleration, focusing, deceleration, and rf injection. The first tests showed a good behavior of the ECRIS with the new rf coupling. A study of the surface ionization source is in progress in order to improve its coupling to the 1+ beam line.

High current ion sources and injectors for induction linacs in heavy ion fusion

Heavy ion beam driven inertial fusion requires short ion beam pulses with high current and high brightness. Depending on the beam current and the number of beams in the driver system, the injector can use a large diameter surface ionization source or merge an array of small beamlets from a plasma source. In this paper, we review the scaling laws that govern the injector design and the various ion source options including the contact ionizer, the aluminosilicate source, the multicusp plasma source, and the metal vapor vacuum arc (MEVVA) source.

Design of a “two-ion source” Charge Breeder using ECR ion source in two frequency mode

The design of a charge breeder consisting of a surface ion source coupled to a dual frequency 14 and 10 GHz Electron Cyclotron Resonance ion-source (ECRIS) is presented. The simulations show that the decelerated 1 + beam could be focused to a spot size smaller than the radial dimensions of the plasma using a gradual deceleration scheme. A high capture and 1 + ⇒ n + conversion efficiency could be possible for radioactive ions using this design.

Ammonia activation by iron: state-specific reactions of Fe +( 6D, 4F) with ND 3 and the reaction of FeNH + with D 2

The kinetic energy dependence of the reactions of Fe +( 6D, 4F) with ND 3, FeNH + with D 2, and FeNH + with Xe are studied in a guided ion beam tandem mass spectrometer over the energy range of 0–10 eV. Only two products, both formed in endothermic processes, are observed in the former system, FeND 2 + and FeD +. By using both dc discharge/flow tube and surface ionization sources, state-specific reaction cross sections for these products are obtained. In the reaction of FeNH + with D 2, four ionic products, Fe +, FeNHD +, FeND 2 +, and FeD +, are observed. The latter three products are formed in endothermic reactions, whereas exothermic formation of Fe + + NHD 2 is shown to occur via a reaction barrier of 0.27 ± 0.05 eV. Analyses of the various endothermic reactions are used to obtain bond energies for Fe + NH 2 of 2.88 ± 0.12 eV and for Fe + NH of 2.76 ± 0.09 eV. Combining these various results permits a fairly complete experimental evaluation of the potential energy surface for ammonia activation by Fe +. The experimental information obtained is compared to calculated energetics and potential energy surfaces obtained using density functional theory (B3LYP/6-311++G(3df,3p)//B3LYP/6-311+G * and CCSD(T)/6-311++G(3df,3p)//B3LYP/6-311+G *) as well as previous experimental and theoretical results in the literature. Implications for the bond energy of FeOH + are also discussed.

Enhancement of ionization efficiency of surface, electron bombardment and laser ion sources by axial magnetic field application

Effect of an axial magnetic field influence on the ionization efficiency of surface and laser ion sources has been observed. The axial magnetic field applying in the range 500–800 G gives an increase of the ion current of atoms of all elements those are ionized in a hot cavity of the surface or laser ion source. Moreover, if for ionization of atoms with low ionization potentials such as K, Rb, and Cs the magnetic field applying effect is negligible, for elements with ionization potentials higher than 6 eV, such as Gd, Tm, Yb, Lu, and others it reaches value about 20 in the ion source temperature region of 2100–2500 °C. The results of on-line experiments are presented confirming the influence of the axial magnetic field on the ionization efficiency rise of neutron-deficient isotopes of rare-earth elements produced in the combined ion source-target assembly (ionizing target) and Eu isotopes resonantly ionized in the laser ion source and in the high temperature ionizing target.

An efficient negative surface ionization source for RIB generation

A high-efficiency, negative surface ionization source, equipped with spherical-sector LaB 6 ionizer, has been developed for generating radioactive beams of highly electronegative species for accelerator-based nuclear physics and nuclear astrophysics research. The source utilizes direct-surface ionization to form negative-ion beams resulting from interactions between highly electronegative atoms or molecules and a hot (∼1722 °C) LaB 6 surface. Even though this type of ionizer has a widely publicized propensity for being easily poisoned, no evidence of this effect was experienced during testing of the source. The source has been extensively evaluated off-line in terms of ionization efficiency for generating beams of Cl − and Br − by feeding AlCl 3 and AlBr 3 vapors at low-feed rates into the source. The geometry of the ionization volume is designed to minimize direct losses of neutral particles through the extraction aperture and to enhance the probability for striking the LaB 6 ionizer. The source is reliable, stable and easy to operate, and ionizes Cl − and Br − with efficiencies for formation and extraction of 30% and 20%, respectively. The design features, principles of operation, off-line performance, operational-parameter and beam-quality (emittance) data for the source are presented in this article.

Production of neutron-rich surface-ionized nuclides at PARRNe

Yields of neutron-rich isotopes produced by fast neutron induced fission of 238U were measured at the ISOL set-up Production d’Atomes Radioactifs Riches en Neutrons. A surface ion source was used to ionize selectively elements with low ionization potentials. In order to observe also the most n-rich isotopes, the identification was achieved by a combined measurement of β- and γ-rays and β-delayed neutrons. The flux of fast neutrons inducing fission was generated by a 1 μA beam of 26 MeV deuterons stopped in a graphite converter. The target was a standard ISOLDE type 238UC X /graphite (50 g/cm 2) heated by a tantalum oven to about 2050 °C. A large variety of radioactive isotopes could be determined quantitatively. Production rates measured for Rb and Cs isotopes are presented here.

Developing high brightness and high current beams for HIF injectors

The U.S. Heavy Ion Fusion Virtual National Laboratory is continuing research into ion sources and injectors that simultaneously provide high current (0.5–1.0 A) and high brightness (normalized emittance better than 1.0 π-mm-mr). The central issue of focus is whether to continue pursuing the traditional approach of large surface ionization sources or to adopt a multiaperture approach that transports many smaller “beamlets” separately at low energies before allowing them to merge. For the large surface source concept, the recent commissioning of the 2-MeV injector for the High Current eXperiment has increased our understanding of the beam quality limitations for these sources. We have also improved our techniques for fabricating large diameter aluminosilicate sources to improve lifetime and emission uniformity. For the multiaperture approach, we are continuing to study the feasibility of small surface sources and a RF-induced plasma source in preparation for beamlet merging experiments, while continuing to run computer simulations for better understanding of this alternate concept. Experiments into both architectures will be performed on a newly commissioned ion source test stand at Lawrence Livermore National Laboratory called STS-500. This stand test provides a platform for testing a variety of ion sources and accelerating structures with 500-kV, 17-μs pulses. Recent progress in these areas is discussed as well as plans for future experiments.

Performance of the K+ ion diode in the 2 MV injector for heavy ion fusion

Heavy ion beam inertial fusion driver concepts depend on the availability and performance of high-brightness high-current ion sources. Surface ionization sources have relatively low current density but high brightness because of the low temperature of the emitted ions. We have measured the beam profiles at the exit of the injector diode, and compared the measured profiles with EGUN and WARP-3D predictions. Spherical aberrations are significant in this large aspect ratio diode. We discuss the measured and calculated beam size and beam profiles, the effect of aberrations, quality of vacuum, and secondary electron distributions on the beam profile.

Efficient negative-ion sources for radioactive ion beam applications (abstract)

Direct extraction, negative-surface ionization, and sputter-type sources have been designed to efficiently ionize specific members of the group VII A elements (F, Cl, Br, I, and At) for use in the nuclear structure physics and nuclear astrophysics research programs at the Holifield Radioactive Ion Beam Facility (HRIBF). A negative surface ionization source that utilizes a solid, spherical geometry, LaB6 ionizer (φ≅2.7 eV) for negatively ionizing Cl, Br, l, and At. During off-line evaluation, the ionization efficiency for Cl− generation ranged between 15% and 20% and for Br− generation, between 15% and 25%. Chemically active elements, such as fluorine, are often released from refractory host materials in a variety of molecular forms. Consequently, the LaB6 surface ionization source, described above, is ineffective for simultaneously dissociating and negatively ionizing this element. To overcome this problem, a new concept source, referred to as the kinetic ejection negative ion source (KENIS), was developed. The source utilizes the Cs-sputter technique to effectively dissociate molecular carriers and ionize F at efficiencies in excess of 7%. The source has been used on-line to inject >3×107 17F−/s into the tandem accelerator and deliver ∼3×106 17F9+/s (fully stripped) to the research station for completion of important nuclear-astrophysics experiments. In this article, emphasis will be placed on the design details, materials of construction, ion optics, operational parameters, thermal transport properties, emittances, and measured ionization efficiencies for these sources.

Development of high current density surface ionization sources for heavy ion fusion applications

Results are presented on the maximum current density obtained in pulsed (2 μs) contact ionization sources that utilize potassium and cesium atoms on porous tungsten substrates. Maximum values of 89 and 48 mA/cm2 for K+ and Cs+, respectively, were obtained in space charge limited mode for the source geometry used. Similar results were observed for potassium aluminosilicate (K20⋅Al2O3⋅4SiO2) sources. Neutral emission rates were measured in the temperature range of interest to heavy ion fusion (1000–1150 °C). Evidence is presented regarding a diffusion-limited flow of alkali atoms towards the emitting surface, which would be a determining factor in the source lifetime.

Zinc Isotope Separation by Ligand Exchange Chromatography Using Cation Exchange Resin.

Zinc isotope effects in Zn-EDTA ligand exchange were studied by using a macroporous strongly acidic cation exchange resin packed in columns. Both front and rear boundaries of a zinc band were sharp after 28m migration at 60°C. Zinc isotope ratios of 66Zn/64Zn were measured by a mass spectrometer with surface ionization source. The enrichment of the heavier isotope, 66Zn, was found at the front band boundary region. Isotope separation coefficient per unit mass, ε/ΔM, and height equivalent to a theoretical plate were calculated as 8.0×10-5 and 0.9mm, respectively. Discussion is extended to the profile analysis on the chromatographic isotope separation. The results were compared with previously reported isotope effects in the ligand exchange reaction of other elements.

Ion sources and injectors for HIF induction linacs

Ion source and injector development is one of the major parts of the HIF program in the USA. Our challenge is to design a cost effective driver-scale injector and to build a multiple beam module within the next couple of years. In this paper, several current–voltage scaling laws are summarized for guiding the injector design. Following the traditional way of building injectors for HIF induction linac, we have produced a preliminary design for a multiple beam driver-scale injector. We also developed an alternate option for a high current density injector that is much smaller in size. One of the changes following this new option is the possibility of using other kinds of ion sources than the surface ionization sources. So far, we are still looking for an ideal ion source candidate that can readily meet all the essential requirements.

Development of the 2-MV Injector for HIF

The 2-MV Injector consists of a 17-cm-diameter surface ionization source, an extraction diode, and an electrostatic quadrupole (ESQ) accelerator, with maximum current of 0.8 A of potassium beam at 2 MeV. Previous performance of the Injector produced a beam with adequate current and emittance but with a hollow profile at the end of the ESQ section. We have examined the profile of the beam as it leaves the diode. The measured nonuniform beam density distribution qualitatively agrees with EGUN simulation. Implications for emittance growth in the post acceleration and transport phase will be investigated.

Design of a two-ion-source (2-IS) beam transport line for the production of multi charged radioactive ion beams

A “two-ion-source” beam transport line between a surface ionization source and a 6.4 GHz on-line Electron Cyclotron Resonance Ion Source (ECRIS) for the production of multi-charged radioactive ions has been designed. The 1 + ions from the surface ionization source are decelerated and focused onto the ECRIS plasma so that they can be efficiently trapped there and further ionized to charge state q>1 +. A scheme for stepwise and gradual deceleration of the 1 + ion beam consisting of a multi-electrode decelerator and a tuning electrode placed before the ECRIS has been optimized. The beam dynamics calculations show that the 1 + beam decelerated to energies of 20–50 eV could be focused to a spot size smaller than the radial dimensions of the ECR plasma zone.

Commissioning and initial operation of a radioactive beam ion source at ISAC

In November of 1998, the ISAC radioactive beam facility at TRIUMF started delivering on-line isotope separated radioactive beams to experiments. A surface ionization source developed for ISAC has been used to commission the mass separator and beam transport systems and is providing radioactive beams to the first generation of ISAC experiments. The ion source is integral with the radioactive beam production target and is designed to be simple, radiation hard, inexpensive, and easily exchanged by remote-handling techniques. The ion source and its extraction column are suspended at the bottom of ∼2 m of steel shielding incorporated in the target module. The module is suspended in a vacuum tank with primary and secondary vacuum systems. All services for the target/ion source and beam extraction system are ducted through the module shielding. The first sets of beam transport elements and beam diagnostic devices are similarly suspended in vacuum at the bottom of two additional shielded modules. Ion beam characteristics can be routinely monitored during on-line operation by a system of Faraday cups, wire scanners, “harp” monitors, and a novel emittance measurement apparatus that can measure beam emittance in both horizontal and vertical planes. The diagnostics devices are capable of resolving beam signals down to the 10 pA range.

Surface Ionization Ion Mobility Spectrometry

A surface ionization (SI) source was designed and constructed for ion mobility spectrometry (IMS). Compared with a conventional (63)Ni source, the surface ionization source is as simple and reliable, has an extended dynamic response range, is more selective in response, and does not have regulatory problems associated with radioactive ionization sources. The performance of this SI-IMS was evaluated with several different classes of compounds. Triethylamine was employed for studying the behavior of the ionization source under different source conditions and gaseous environments. Amines, tobacco alkaloids, and triazine herbicides were also investigated. Picogram level detection limits were achieved for target compounds with a response dynamic range of 5 orders of magnitude. Selective monitoring by IMS was also demonstrated. While the surface ionization source does not have the universality of response that is obtained with a (63)Ni ionization source, it is an excellent nonradioactive alternative for the ionization and ion mobility detection of those compounds to which it responds.

Ion sources for heavy ion fusion induction linacs

Depending on the type of beam transport selected at the front end of a heavy ion fusion driver, the optimum injector current (per beam) can vary from 0.5 to 25 A of cesium ions. The average current density should be ≥10 mA/cm 2. So far, surface ionization sources have been used in most HIF driver designs. These sources can efficiently produce singly charged alkali ions such as Cs + and K +. Nevertheless, the gas sources and the metal vapor vacuum arc (MEVVA) sources are also potentially useful for HIF applications. Recent experimental results on ion source development are summarized.