Status of the laser ion source at IMP

A laser (Nd:YAG laser, 3 J, 1064 nm, 8-10 ns) ion source has been built and under development at IMP to provide pulsed high-charge-state heavy ion beams to a radio frequency quadrupole (RFQ) for upgrading the IMP accelerators with a new low-energy beam injector. The laser ion source currently operates in a direct plasma injection scheme to inject the high charge state ions produced from a solid target into the RFQ. The maximum power density on the target was about 8.4 × 10(12) W∕cm(2). The preliminary experimental results will be presented and discussed in this paper.     

Li+ ion emission from a hot-plate alumina-silicate source stimulated by flash heating with an infrared laser

The Neutralized Drift Compression Experiment-II accelerator under construction at Lawrence Berkeley National Laboratory has been designed to employ a lithium-doped alumino-silicate (Al-Si) hot-plate surface-ionization ion source. In order to achieve the design 1 mA∕cm(2) current density, the emitter must be constantly kept at a high temperature, leading to the accelerated loss of Li material as ions or neutrals. As a result, the estimated lifetime of the source is 50 h. This lifetime can be extended if the source is kept at low temperature during standby, and pulse heated to the high temperature during the ion extraction phase only. A pulsed heating technique based on an infrared laser (CO(2) gas discharge, λ = 10.6 μm) is described in this paper. The feasibility of ion current emission stimulated by flash heating with an infrared (IR) laser was demonstrated. High repeatability of the laser-stimulated ion current was observed, creating an opportunity for modulation and gating of the ion current with a laser pulse. It was found that with the available low power (≈115 W∕cm(2)) IR laser, current densities as high as 0.8 mA∕cm(2) could be achieved with a 2.8 mm diameter source. Various approaches for scaling to a larger (10 cm diameter) source and the application of short pulse, high power lasers are discussed. The results and conclusions of this paper may apply to various species of hot-plate ion sources.     

Laser ion sources for radioactive beams (invited)

Resonant ionisation laser ion sources are nowadays extensively used, when available, at many leading on-line facilities. Moreover, new laser ion sources are now under development in most of the recent on-line facility projects under construction worldwide. This success is mainly due to the reliability, the ionization efficiency and the high purity that this type of source can achieve for the production of radioactive species and for a large range of chemical elements. Laser ion sources for radioactive beams gather many different systems such as dye laser or all-solid state titanium:sapphire laser systems, high or low repetition rates, hot cavities or gas cells, additional selectivity by using chemical techniques, or the LIST technique (laser ion source trap). In this paper, the physics of laser ion sources will be described with the current limitations and challenges for the future. An overview of the laser ion source facilities will be given, with an emphasis on the ongoing developments and perspectives on LIS.     

Optimizing the front end test stand high performance H- ion source at RAL

The aim of the front end test stand project is to demonstrate that chopped low energy H(-) beams of high quality can be produced. The beam line currently consists of the ion source, a 3 solenoid low energy beam transport and a suite of diagnostics. A brief status report of the radio frequency quadrupole is given. This paper details the work to optimize the ion source performance. A new high power pulsed discharge power supply with greater reliability has been developed to allow long term, stable operation at 50 Hz with a 60 A, 2.2 ms discharge pulse and up to 100 A at 1.2 ms. The existing extraction power supply has been modified to operate up to 22 kV. Results from optical spectroscopy measurements and their application to source optimization are summarized. Source emittances and beam currents of 60 mA are reported.     

MeV negative ion source from ultra-intense laser-matter interaction

Experimental demonstration of negative ion acceleration to MeV energies from sub-micron size droplets of water spray irradiated by ultra-intense laser pulses is presented. Thanks to the specific target configuration and laser parameters, more than 10(9) negative ions per steradian solid angle in 5% energy bandwidth are accelerated in a stable and reliable manner. To our knowledge, by virtue of the ultra-short duration of the emission, this is by far the brightest negative ion source reported. The data also indicate the existence of beams of neutrals with at least similar numbers and energies.     

Upgrade of the resonance ionization laser ion source at ISOLDE on-line isotope separation facility: new lasers and new ion beams

The resonance ionization laser ion source (RILIS) produces beams for the majority of experiments at the ISOLDE on-line isotope separator. A substantial improvement in RILIS performance has been achieved through a series of upgrade steps: replacement of the copper vapor lasers by a Nd:YAG laser; replacement of the old homemade dye lasers by new commercial dye lasers; installation of a complementary Ti:Sapphire laser system. The combined dye and Ti:Sapphire laser system with harmonics is capable of generating beams at any wavelength in the range of 210-950 nm. In total, isotopes of 31 different elements have been selectively laser-ionized and separated at ISOLDE, including recently developed beams of samarium, praseodymium, polonium, and astatine.     

Bipolar pulse generator for intense pulsed ion beam accelerator

A new type of pulsed ion beam accelerator named "bipolar pulse accelerator" (BPA) has been proposed in order to improve the purity of intense pulsed ion beams. To confirm the principle of the BPA, we developed a bipolar pulse generator for the bipolar pulse experiment, which consists of a Marx generator and a pulse forming line (PFL) with a rail gap switch on its end. In this article, we report the first experimental result of the bipolar pulse and evaluate the electrical characteristics of the bipolar pulse generator. When the bipolar pulse generator was operated at 70% of the full charge condition of the PFL, the bipolar pulse with the first (-138 kV, 72 ns) and the second pulse (+130 kV, 70 ns) was successfully obtained. The evaluation of the electrical characteristics indicates that the developed generator can produce the bipolar pulse with fast rise time and sharp reversing time.     

A study of H+ production using metal hydride and other compounds by means of laser ion source

A laser ion source can provide wide variety of ion beams from solid target materials, however, it has been difficult to create proton beam efficiently. We examined capability of proton production using beeswax, polyethylene, and metal hydrides (MgH(2) and ZrH(2)) as target materials. The results showed that beeswax and polyethylene could not be used to produce protons because these targets are transparent to the laser wavelength of 1064 nm. On the other hand, the metal hydrides could supply protons. Although the obtained particle numbers of protons were less than those of the metal ions, the metal hydrides could be used as a target for proton laser ion source.     

Subcutoff microwave driven plasma ion sources for multielemental focused ion beam systems

A compact microwave driven plasma ion source for focused ion beam applications has been developed. Several gas species have been experimented including argon, krypton, and hydrogen. The plasma, confined by a minimum B multicusp magnetic field, has good radial and axial uniformity. The octupole multicusp configuration shows a superior performance in terms of plasma density (~1.3 x 10(11) cm(-3)) and electron temperature (7-15 eV) at a power density of 5-10 Wcm(2). Ion current densities ranging from a few hundreds to over 1000 mA/cm(2) have been obtained with different plasma electrode apertures. The ion source will be combined with electrostatic Einzel lenses and should be capable of producing multielemental focused ion beams for nanostructuring and implantations. The initial simulation results for the focused beams have been presented.     

Time-of-flight mass spectrometry for time-resolved measurements

A time-resolved time-of-flight mass spectrometer (TOF-MS) that can simultaneously monitor multiple species on the millisecond time scale has been constructed. A pulsed photolysis laser is used to initiate reaction, and then via a pinhole the reaction mixture is sampled by the TOF-MS. The ions are created by photoionization via either a discharge lamp or a pulsed laser. Comparison between the two ionization sources showed that the laser is at least an order of magnitude more efficient, based on the time to accumulate the data. Also, unlike the continuous lamp the pulsed laser is not mass limited. Frequency tripling the 355 nm output of a Nd:YAG laser provided a convenient laser ionization source. However, using a dye laser provided an equally intense laser ionization source with the ability to tune the vacuum ultraviolet (vuv) light. To show the versatility of the system the kinetics of the reaction of SO and ClSO radicals with NO(2) were simultaneously measured, and using the dye laser the vuv light was tuned to 114 nm in order to observe H(2)CO being formed from the reaction between CH(3)CO and O(2).     

Laser ablation-miniature mass spectrometer for elemental and isotopic analysis of rocks

A laser ablation-miniature mass spectrometer (LA-MMS) for the chemical and isotopic measurement of rocks and minerals is described. In the LA-MMS method, neutral atoms ablated by a pulsed laser are led into an electron impact ionization source, where they are ionized by a 70 eV electron beam. This results in a secondary ion pulse typically 10-100 μs wide, compared to the original 5-10 ns laser pulse duration. Ions of different masses are then spatially dispersed along the focal plane of the magnetic sector of the miniature mass spectrometer (MMS) and measured in parallel by a modified CCD array detector capable of detecting ions directly. Compared to conventional scanning techniques, simultaneous measurement of the ion pulse along the focal plane effectively offers a 100% duty cycle over a wide mass range. LA-MMS offers a more quantitative assessment of elemental composition than techniques that detect ions directly generated by the ablation process because the latter can be strongly influenced by matrix effects that vary with the structure and geometry of the surface, the wavelength of the laser beam, and the not well characterized ionization efficiencies of the elements in the process. The above problems attendant to the direct ion analysis has been minimized in the LA-MMS by analyzing the ablated neutral species after their post-ionization by electron impaction. These neutral species are much more abundant than the directly ablated ions in the ablated vapor plume and are, therefore, expected to be characteristic of the chemical composition of the solid. Also, the electron impact ionization of elements is well studied and their ionization cross sections are known and easy to find in databases. Currently, the LA-MMS limit of detection is 0.4 wt.%. Here we describe LA-MMS elemental composition measurements of various minerals including microcline, lepidolite, anorthoclase, and USGS BCR-2G samples. The measurements of high precision isotopic ratios including (41)K/(39)K (0.077 ± 0.004) and (29)Si/(28)Si (0.052 ± 0.006) in these minerals by LA-MMS are also described. The LA-MMS has been developed as a prototype instrument system for space applications for geochemical and geochronological measurements on the surface of extraterrestrial bodies.     

Fast ion beam-plasma interaction system

A device has been constructed for the study of the interaction between a fast ion beam and a target plasma of separately controllable parameters. The beam of either hydrogen or helium ions has an energy of 1-4 keV and a total current of 0.5-2 A. The beam energy and beam current can be varied separately. The ion source plasma is created by a pulsed (0.2-10-ms pulse length) discharge in neutral gas at up to 3 x 10(-3) Torr. The neutrals are pulsed into the source chamber, allowing the neutral pressure in the target region to remain less than 5 x 10(-5) Torr at a 2-Hz repetition rate. The creation of the source plasma can be described by a simple set of equations which predict optimum source design parameters. The target plasma is also produced by a pulsed discharge. Between the target and source chambers the beam is neutralized by electrons drawn from a set of hot filaments. Currently under study is an unstable wave in a field-free plasma excited when the beam velocity is nearly equal to the target electron thermal velocity (v(beam) approximately 3.5 x 10(7) cm/s, Te = 0.5 eV).     

Proton emission from a laser ion source

At intensities of the order of 10(10) W∕cm(2), ns pulsed lasers can be employed to ablate solid bulk targets in order to produce high emission of ions at different charge state and kinetic energy. A special interest is devoted to the production of protons with controllable energy and current from a roto-translating target irradiated in repetition rate at 1-10 Hz by a Nd:Yag pulsed laser beam. Different hydrogenated targets based on polymers and hydrates were irradiated in high vacuum. Special nanostrucutres can be embedded in the polymers in order to modify the laser absorption properties and the amount of protons to be accelerated in the plasma. For example, carbon nanotubes may increase the laser absorption and the hydrogen absorption to generate high proton yields from the plasma. Metallic nanostrucutres may increase the electron density of the plasma and the kinetic energy of the accelerated protons. Ion collectors, ion energy analyzer, and mass spectrometers, used in time-of-flight configuration, were employed to characterize the ion beam properties. A comparison with traditional proton ion source is presented and discussed.     

The development of the radio frequency driven negative ion source for neutral beam injectors (invited)

Large and powerful negative hydrogen ion sources are required for the neutral beam injection (NBI) systems of future fusion devices. Simplicity and maintenance-free operation favors RF sources, which are developed intensively at the Max-Planck-Institut fu?r Plasmaphysik (IPP) since many years. The negative hydrogen ions are generated by caesium-enhanced surface conversion of atoms and positive ions on the plasma grid surface. With a small scale prototype the required high ion current density and the low fraction of co-extracted electrons at low pressure as well as stable pulses up to 1 h could be demonstrated. The modular design allows extension to large source dimensions. This has led to the decision to choose RF sources for the NBI of the international fusion reactor, ITER. As an intermediate step towards the full size ITER source at IPP, the development will be continued with a half-size source on the new ELISE testbed. This will enable to gain experience for the first time with negative hydrogen ion beams from RF sources of these dimensions.     

One hundred anode microchannel plate ion detector

A one-hundred-anode microchannel plate detector is constructed on a 10 cm × 15 cm printed circuit board and attached to a homebuilt matrix assisted laser desorption ionization (MALDI) time-of-flight mass spectrometer. Ringing and cross talk between anodes have been successfully eliminated and preliminary mass spectra of peptide ions recorded. With one hundred anodes on the printed circuit board, spatial information about the ion beam can also be readily determined with this detector. During operation, the detector anode assembly loses sensitivity after ions strike it for a considerable period of time due to charging of the non-conductive regions between anodes. However, this effect can be minimized by deflecting matrix ions away from the detector.     

Development of electron beam ion source charge breeder for rare isotopes at Californium Rare Isotope Breeder Upgrade

Recently, the Californium Rare Isotope Breeder Upgrade (CARIBU) to the Argonne Tandem Linac Accelerator System (ATLAS) was commissioned and became available for production of rare isotopes. Currently, an electron cyclotron resonance ion source is used as a charge breeder for CARIBU beams. To further increase the intensity and improve the purity of neutron-rich ion beams accelerated by ATLAS, we are developing a high-efficiency charge breeder for CARIBU based on an electron beam ion source (EBIS). The CARIBU EBIS charge breeder will utilize the state-of-the-art EBIS technology recently developed at Brookhaven National Laboratory (BNL). The electron beam current density in the CARIBU EBIS trap will be significantly higher than that in existing operational charge-state breeders based on the EBIS concept. The design of the CARIBU EBIS charge breeder is nearly complete. Long-lead components of the EBIS such as a 6-T superconducting solenoid and an electron gun have been ordered with the delivery schedule in the fall of 2011. Measurements of expected breeding efficiency using the BNL Test EBIS have been performed using a Cs(+) surface ionization ion source for external injection in pulsed mode. In these experiments we have achieved ～70% injection∕extraction efficiency and breeding efficiency into the most abundant charge state of ～17%.     

Laser ion source with a double pulse laser system

To extend an ion beam pulse of a laser ion source, multiple laser shots could be used. To check the feasibility of this idea, we tested double laser irradiations on an iron target. When the interval of the two laser shots is longer than 10 μs, the obtained ion current profile was expressed as a sum of two individual expanded laser plasmas. However, if the interval is too close, a current reduction was observed. This technique can be effectively applied to low charge state ion production.     

New laser setup for the selective isotope production and investigation in a laser ion source at the IRIS (Investigation of Radioactive Isotopes on Synchrocyclotron) facility

New laser installation for the resonance ionization spectroscopy in a laser ion source and for rare isotope production has been recently put into operation at the IRIS (Investigation of Radioactive Isotopes on Synchrocyclotron) facility (Petersburg Nuclear Physics Institute, Gatchina). This is a significant improvement of a previous target-laser ion source device of the IRIS mass-separator, working on-line with 1 GeV proton beam of PNPI (Petersburg Nuclear Physics Institute) synchrocyclotron. It makes possible for us to get the isobarically clean radioactive isotope beams of a great number of chemical elements. New laser setup provides the two- or three-resonance step ionization in the range of wavelength of 265-850 nm. The first results obtained at the laser setup for Tl isotopes are presented.     

Mass filter and focusing ion detector for use with time-of-flight velocity distribution measurements

A novel mass filter and focusing ion detector for time-of-flight studies of dissociative ionization fragments is described. The filter-dector combination is particularly suited for measuring the kinetic energy released in the molecular dissociation process. The mass filter utilizes a computer-controlled, time-dependent retarding potential synchronized with, and located a distance L from, a pulsed electron-bombardment ion source. Ions arriving at the filter at time t have a kinetic energy of mL(2)/2t(2). The retarding potential varies with time as mL(2)/2et(2), thus deflecting ions with masses less than m. Operation of the filter in a mode which provides unity mass resolution at 16 amu has been demonstrated. The detector consists of a channel electron multiplier and a focusing electrode configuration approximating that of a charged sphere above a grounded conducting plane. The planar input aperture of the detector is located just behind the mass filter along the ion flight path. The detector features high efficiency, large input aperture, low background, and simple operation. Ion transit times through the detector are small and essentially independent of the initial ion kinetic energy.     

An ultra-low energy (30-200 eV) ion-atomic beam source for ion-beam-assisted deposition in ultrahigh vacuum

The paper describes the design and construction of an ion-atomic beam source with an optimized generation of ions for ion-beam-assisted deposition under ultrahigh vacuum (UHV) conditions. The source combines an effusion cell and an electron impact ion source and produces ion beams with ultra-low energies in the range from 30 eV to 200 eV. Decreasing ion beam energy to hyperthermal values (≈10(1) eV) without loosing optimum ionization conditions has been mainly achieved by the incorporation of an ionization chamber with a grid transparent enough for electron and ion beams. In this way the energy and current density of nitrogen ion beams in the order of 10(1) eV and 10(1) nA/cm(2), respectively, have been achieved. The source is capable of growing ultrathin layers or nanostructures at ultra-low energies with a growth rate of several MLs/h. The ion-atomic beam source will be preferentially applied for the synthesis of GaN under UHV conditions.