G. D. Alton

Design studies for an advanced ECR ion source

An innovative technique: for increasing ion source intensity is described which, in principle, could lead to significant advances in ECR ion source technology for multiply charged ion beam formation. The advanced concept design uses a minimum-B magnetic mirror geometry which consists of a multi-cusp, magnetic field, to assist in confining the plasma radially, a flat central field for tuning to the ECR resonant condition, and specially tailored min-or fields in the end zones to confine the plasma in the axial direction. The magnetic field is designed to achieve an axially symmetric plasma ``volume`` with constant mod-B, which extends over the length of the central field region. This design, which strongly contrasts w h the ECR ``surfaces`` characteristic of conventional ECR ion sources, results in dramatic increases in the absorption of RF power, thereby increasing the electron temperature and ``hot`` electron population within the ionization volume of the source.

An axial geometry cesium sputter negative ion source with continuous tungsten surface ionizer

Abstract A high brightness axial geometry negative ion source has been designed, developed, and used as an “on-line” source for routine operation of the Holifield Heavy Ion Research Facility tandem accelerator. The present source utilizes a continuous surface, solid tungsten ionizer to effect ionization of cesium vapor which, in turn, is used to sputter a negatively biased probe containing the material of interest. Operational experience with the source indicates that the source is reliable, long lived, stably operating and a prolific producer of a wide spectrum of negative ions. To date the source has been used to produce more than eighteen negative ion species including Ag − , Au − , B − , CaH 3 − , Cl − , CrH 2 − , Cu − , Lu − , MgH 3 − , Mo − , Ni − , O − , S − , Si − , Sn − , TiH 3 − , Tm − , and Yb − . Details of the mechanical design features and computational techniques utilized in arriving at the final electrode configuration are presented in the text. Data pertinent to source operation, the dependence of negative ion yields on certain source operational parameters, and examples of intensities typical of particular negative ion species are also given.

Further evaluation of the high intensity plasma sputter heavy negative ion source

In this report, we provide more detailed information on the high intensity heavy negative ion source described previously. Intensity vs time spectra, mass distribution data and source operational data are presented for Au, Cu, Ni and CuO sputter probes. Sputter probe voltage limited beam intensities of 10.2, 8.2, 5.1 and 4.5 mA, respectively have been realized from these sputter probes. The results of emittance measurements for Au and Ni probes indicate a rather strong dependence on beam intensity, as expected from space charge considerations. The source, when operated in pulsed mode, holds considerable promise for use in conjunction with tandem electrostatic accelerator/synchrotron injection applications. The high intensity capabilities of the source make it a viable candidate for generating mA intensity level, cw ion beams for a variety of other applications, including ion implantation.

A versatile high intensity plasma sputter heavy negative ion source

Abstract A multicusp magnetic field plasma surface ion source, normally used for H − ion beam formation, has been utilized for the generation of high intensity, pulsed, heavy negative ion beams suitable for a variety of uses including tandem electrostatic accelerator/synchrotron injection applications. Sputter probe voltage limited total ion currents of 5.5, 8.2, 5.1 and 4.5 mA (peak intensity) have been produced from Au, Cu, Ni and CuO sputter probes, respectively. The mass distributions of these ion beams are found to be dominated by Au − , Cu − , Ni − and O − atomic species, respectively. The source offers the interesting prospect of providing cw negative ion beams at mA intensity levels of the commonly used semiconducting material dopants (e.g. B − , P − , As − and Sb − ) as well as O − for isolation barrier formation. Illustrative examples of intensity versus time and the mass distribution of ion beams extracted from a Ni sputter probe, along with brief descriptions of the source, experimental apparatus and procedures followed during the course of these investigations, are presented in this report.

Criteria for selection of target materials and design of high-efficiency-release targets for radioactive ion beam generation

Abstract In this report, we define criteria for choosing target materials and for designing, mechanically stable, short-diffusion-length, highly permeable targets for generation of high-intensity radioactive ion beams (RIBs) for use at nuclear physics and astrophysics research facilities based on the ISOL principle. In addition, lists of refractory target materials are provided and examples are given of a number of successful targets, based on these criteria, that have been fabricated and tested for use at the Holifield Radioactive Ion Beam Facility (HRIBF).

Selection and design of the Oak Ridge radioactive ion beam facility target/ion source

Abstract The high-temperature version of the CERN ISOLDE source has been selected as the first source to be used for the generation of radioactive ion beams at the Oak Ridge Radioactive Ion Beam Facility (ORRIBF) because of its low emittance, relatively high ionization efficiency, and capability for producing a broad range of radioactive species. Of equal importance, the source has been cleverly engineered for remote installation, removal and servicing as required for safe handling of highly radioactive contaminated sources, source components, and ancillary equipment. The source design also permits easy modification to lower-temperature versions and conversion from electron impact ionization to either thermal or positive and negative surface ionization sources. The reasons for choosing the CERN ISOLDE source and design features of the source are discussed in this paper.

Collisional cooling of negative-ion beams

Abstract Studies have been conducted to determine the feasibility of using collisional cooling for reducing emittances and energy spreads in negative-ion beams to levels commensurate with effective isobaric purification with conventional high-resolution electromagnetic isobar separators as required for use at the Holifield Radioactive Ion Beam Facility (HRIBF). We have designed a gas-filled radio frequency quadrupole ion cooler equipped with provisions for retarding energetic negative-ion beams to energies below thresholds for electron detachment at injection and for re-acceleration to initial energies after the cooling process. The device has been used to cool several ion beams with initial energy spreads, ΔE>10 eV to final energy spreads, ΔE∼2 eV FWHM, including O − and F − . Overall transmission efficiencies of ∼14% for F − beams have been obtained. Experimental results show that electron detachment is the major loss mechanism for negative ions.

Targets and ion sources for RIB generation at the Holifield Radioactive Ion Beam Facility

In this report, the authors describe the performance characteristics for a selected number of target ion sources that will be employed for initial use at the Holifield Radioactive Ion Beam Facility (HRIBF) as well as prototype ion sources that show promise for future use for RIB applications. A brief review of present efforts to select target materials and to design composite target matrix/heat-sink systems that simultaneously incorporate the short diffusion lengths, high permeabilities, and controllable temperatures required to effect fast and efficient diffusion release of the short-lived species is also given.

The emittances and brightnesses of high-intensity negative ion sources

Abstract The emittances of high-intensity ion beams extracted from cesium sputter negative ion sources equipped with cylindrical and ellipsoidal solid tungsten and spiral-wound tantalum (General Ionex Corporation, Model 860) cesium surface ionizers have been measured for several ion species, including 12 C − , 28 Si − , 58 Ni − , and 197 Au − . While certain sets of data from the ellipsoidal and cylindrical geometry ionizer sources suggest a moderate growth in emittance with increasing negative ion beam intensity I over the range of intensities investigated (5 ⩽ I ⩽ 60 μ A) of perhaps 20%, not all data exhibit this dependence, especially those from the Model 860 source. As well, no evidence of a monotonic dependence of emittance on ion mass was found. The emittances of ion beams at the 80% intensity levels from the Model 860 source are found to be higher on the average by factors of 1.8 and 1.7, respectively, than those from sources equipped with ellipsoidal and cylindrical geometry cesium surface ionizers. Brief descriptions of the emittance-measuring device and the sources investigated are given. Emittance and brightness data for dc operation of each of the sources considered are presented and comparisons made of the results.

The sputter generation of negative ion beams

A brief review is given of recent progress toward a quantitative understanding of negative ion formation by sputtering from surfaces covered with fractional layers of highly electropositive adsorbates. Practical models developed for estimating changes in work functions {Delta}{phi} by electropositive adsorbates are described. The secondary negative ion generation process is examined through the use of composite energy/velocity dependent analytical models. These models are used to illustrate the effect of work function on the energy distributions of negative ions sputter ejected from a polycrystalline molybdenum surface covered with fractional layers of cesium. Predictions are also made of the functional dependence of the probability for negative ion formation on cesium coverage. The models predict energy distributions which are in basic disagreement with experimental observations, implying their inappropriateness for describing the sputter negative ion generation process. We have also developed a model for calculating sputter ratios based on the use of simple scaling procedures to bring Sigmund theory into close agreement with experimental observation accounting for the threshold effect. Scaling factors for projectile energies E > 1000 eV are found to be independent of energy while those for projectile energies E{sub th} < E < 1000 eV were found to be energy dependent. In this study, the model and scaling techniques utilized to bring Sigmund theory into agreement with experiment are discussed in detail and several examples provided which illustrate the versatility, accuracy and utility of the model. In the present report, we describe the model and apply it to the case of sputtering a selected number of metals with energetic cesium ions. In particular, we present sputter ratio information for a number of Cs-projectile/metal-target combinations; the targets are bombarded at normal incidence to the surface.