Ion Low Energy Beam Transport Research Articles

CHOPPI: A Particle In Cell code for ion beam transport through beam chopper : A demonstration with experiments

A simulation package employing a Particle in Cell (PIC) method is developed to study a high current beam transport through the chopper system designed for producing pulsed ion beams at low energies. This package includes subroutines, which are well suited for high current beam transport dominated by space-charge with a complete 6-dimensional phase-space description including simultaneous dynamics of multiple ion species and electrons. The discretized form of the Poisson equation is generalized to include irregular boundary conditions required to simulated curved or irregular geometric components. In the following paper, the capabilities of CHOPPI are demonstrated through comparison with experimental results from low energy ion beam transport.

Experiments with low energy ion beam transport into toroidal magnetic fields

Abstract The stellarator-type storage ring for accumulation of multi-Ampere proton, and other intense ion beams with energies in the range of 100 A k e V to 1 A M e V is designed at Goethe university, Frankfurt am Main. The first proposal for beam confinement with high transversal momentum acceptance was presented in EPAC2006. This ring is typically suited for experiments in plasma physics and nuclear astrophysics. The accumulator ring with a closed longitudinal magnetic field is foreseen with a flux density up to 6 − 8 T . For demonstration, experiments were carried out with two toroidal segments operated at room temperature. The beam transport experiments in toroidal magnetic fields were first described in EPAC2008 within the framework of a proposed low energy ion storage ring. The test setup aims at developing a ring injection system with two beam lines representing the main beam line and the injection line. The primary beam line for the experiments was installed and successfully commissioned in 2009. A special diagnostics probe for “in situ” ion beam detection was installed. This modular technique allows online diagnostics of the ion beam along the beam path. In this paper, we present new results on beam transport experiments, with single segment and coupled configuration. The measurements are shown to be in agreement with the numerical model.

Dynamics of intense particle beam in axial-symmetric magnetic field

Axial-symmetric magnetic field is often used in focusing of particle beams. Most existing ion Low Energy Beam Transport lines are based on solenoid focusing. Modern accelerator projects utilize superconducting solenoids in combination with superconducting accelerating cavities for acceleration of high-intensity particle beams. Present article discusses conditions for matched beam in axial-symmetric magnetic field. Analysis allows us to minimize power consumption of solenoids and beam emittance growth due to nonlinear space charge, lens aberrations, and maximize acceptance of the channel. Expressions for maximum beam current in focusing structure, beam emittance growth due to spherical aberrations and non-linear space charge forces are derived.

Numerical simulation of gridded electrostatic lens

Gridded electrostatic lenses are frequently used in extraction systems and low energy ion beam transport line. Typically, for numerical simulation the grid is treated as a metal plate transparent for beam particles. The influence of real grid geometry on the beam dynamics in the gridded lens has been investigated by KOBRA-3d code. Beam emittance growth for different lens parameters has been investigated. Approximating expressions for obtained results are presented. The grid geometry providing minimal beam distortions is proposed.

Status of the ion sources developments for the Spiral2 project at GANIL

The SPIRAL 2 facility is now under construction and will deliver either stable or radioactive ion beams. First tests of nickel beam production have been performed at GANIL with a new version of the large capacity oven, and a calcium beam has been produced on the heavy ion low energy beam transport line of SPIRAL 2, installed at LPSC Grenoble. For the production of radioactive beams, several target∕ion-source systems (TISSs) are under development at GANIL as the 2.45 GHz electron cyclotron resonance ion source, the surface ionization source, and the oven prototype for heating the uranium carbide target up to 2000 °C. The existing test bench has been upgraded for these developments and a new one, dedicated for the validation of the TISS before mounting in the production module, is under design. Results and current status of these activities are presented.

Ion beam extraction and transport for high-field electron cyclotron resonance ion sources (abstract)

The next-generation, very high magnetic-field electron cyclotron resonance (ECR) ion sources, like VENUS, SERSE, or PHOENIX, strive for substantially (a factor of 10) higher extracted heavy ion beam intensities than currently achievable. Such high-intensity ion beams present significant challenges for the design and simulation of an ECR extraction and low-energy ion beam transport system. Extraction and beam formation take place in a strong (up to 3 T) axial magnetic field, which leads to significantly different focusing properties for the different ion masses and charge states of the extracted beam. Typically, beam simulations must take into account the contributions of up to 50 different charge states and ion masses. Space charge effects must be correctly included since the extraction and mass analyzing system have to be designed for a proton-equivalent current of ∼25 mA at 30 kV extraction voltage. The article discusses state of the art two-dimensional and three-dimensional simulation techniques for such ion beam extraction and transport systems. Furthermore, the main contributions to the ion beam emittance are discussed: (1) The induced beam rotation due to the strong axial magnetic field, (2) the concentration of high charge state ions at the source center axis, and (3) the ion beam temperature. A novel large-gap analyzing magnet design is described which allows efficient correction of higher-order aberrations for high-intensity heavy ion beams. Such a magnet limits emittance blow up, and is necessary if the analyzed beam has to be further transported or accelerated, e.g., in a radio frequency quadrupole.

Development of a low-energy oxygen 14 ion beam (abstract)

At the 88 Inch Cyclotron at the Lawrence Berkeley National Laboratory (LBNL) we have developed an intense (3×107 pps), low-energy O14 ion beam for precision tests of the standard model. The 70 s half-life of O14 requires on-line production of the isotope. O14 is produced in the form of C1412O in a high-temperature carbon aerogel target using a 20 MeV He3 beam from the LBNL 88 Inch Cyclotron via the reaction C12(3He,n)14O. In order to minimize the background radiation for the experiments, the O14 atoms must be separated from the other radioactive isotopes produced in the carbon target. For this purpose, we have developed an experimental setup including the target, transfer line, the electron cyclotron resonance ion source IRIS, and a low-energy ion beam transport line. The major components of the setup are described. The release and transport efficiency for the CO molecules from the target through the transfer line was measured for various target temperatures. The on-line and off-line ion source efficiencies for radioactive and stable oxygen beams are presented.

Deceleration and ion beam optics in the regime of 10–200 eV

A deceleration lens system for low energy ion beam transport applications in the region of 10–200 eV has been designed, constructed and experimentally characterized. In this ion beam system, the target is required to be in an almost field-free region at the exit of the lens to allow for maximum target maneuverability. The lens design was optimized using the simulation data obtained with chden, a computer program for calculating ion trajectories with space-charge included. The design chosen in this work was a five-electrode lens which can be operated in either a simplified two-electrode deceleration mode or, when the best system performance is required, in a five-electrode decelerator-focusing mode. The performance of the lens was experimentally evaluated by using a 3 keV pure argon ion beam with a beam size of about 1 mm2 and beam current of about 10μA which was generated by a modified Colutron Ion Gun Model G-2 unit. The lens decelerated the incident beam to 10–200 eV and transported it to either a Faraday cage for beam current and beam profile measurements or to a quadrupole mass spectrometer equipped with a cylindrical ion energy analyzer for beam energy and mass characterization. The results indicated that the lens could deliver an ion current density of approximately 100 μA/cm2 at ion energies higher than 50 eV. The available current density dropped with decreasing energies but remained at more than 10 μA/cm2 at 10 eV.

High accuracy ion optics computing

Computer simulation of focused ion beams for surface analysis of materials by SIMS, or for microfabrication by ion beam lithography plays an important role in the design of low energy ion beam transport and optical systems. Many computer packages currently available, are limited in their applications, being inaccurate or inappropriate for a number of practical purposes. This work describes an efficient and accurate computer programme which has been developed and tested for use on medium sized machines. The programme is written in Algol 68 and models the behaviour of a beam of charged particles through an electrostatic system. A variable grid finite difference method is used with a unique data structure, to calculate the eletric potential in an axially symmetric region, for arbitrary shaped boundaries. Emphasis has been placed upon finding an economic method of solving the resulting set of sparse linear equations in the calculation of the electric field and several of these are described. Applications include individual ion lenses, extraction optics for ions in surface analytical instruments and the design of columns for ion beam lithography. Computational results have been compared with analytical calculations and with some data obtained from individual einzel lenses by Harting and Read ( Electrostatic Lenses, Elsevier, Amsterdam ( 1976)).

Ion–surface interactions during thin film deposition

The interactions of ion beams with surfaces are strongly dependent on the incident kinetic energy and the amount of ionic charge. Fundamental effects of ion bombardment by atomic and cluster ion beams have been elucidated in terms of kinetic energy of the ion beams. These fundamental effects include enhancement of adatom migration, desorption of physically absorbed atoms, displacement of the surface atoms, sputtering, shallow ion implantation, and trapping of impinging atoms. The importance of low energy ion beams for film formation is shown by taking into account the binding energies of the atoms in a solid. Emphasis has been placed on interactions of ionized cluster beams (ICB) during film formation because ICB offers unique capabilities for deposition due to the properties of the clusters and also to characteristic low energy ion beam transport over a range from thermal energy to a few hundred eV.

Low energy ion beam transport by permanent magnets

The guiding of a 300 eV divergent hydrogen ion beam by a surface magnetic field generated by samarium cobalt magnets has been investigated. It was found that magnets arranged in a multi-ring-cusp configuration produced the best beam transport efficiency.

Low energy ion beam transport through apertures

Abstract Argon ion transport through a beam defining aperture has been investigated at energies between 0.1 and 15 keV. It was found that the current which can be fed through the aperture depends strongly upon the operation parameters of the ion source. For optimum source performance the maximum current transmitted is limited by space charge expansion of the beam. This interpretation of the experimental results is supported by simple calculations. The measurements additionally provide information about the plasma potential.

Use of an electrostatic particle guide for large angle bends

A cylindrical electrostatic particle guide incorporating a 90 degrees bend has been constructed and tested. Transmission efficiency and beam profile are investigated for 500 eV. electrons and the results suggest that the technique will be particularly suited to low energy ion beam transport over nonrectilinear trajectories.