D. K. Olsen

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.

Opportunities with accelerated radioactive ion beams

Abstract A discussion of the exciting and rapidly developing field of accelerated radioactive ion beam (RIB) production for nuclear and astrophysics research is presented. In particular, some scientific opportunities with RIBs are highlighted, the methods of RIB production are reviewed, and the existing and proposed facilities for this research are described. In addition, the ORNL RIB project, the only funded ISOL project employing an electrostatic accelerator, is described in some detail.

Toward a realistic and tractable model for negative-ion extraction from volume sources

A negative-ion source extraction model has been formulated and implemented that explicitly considers the motion of positive ions and the volume generation of negative ions. It is found that (1) for high-beam currents, the beam current is limited by a transverse space-charge limit, meaning that an increase in negative-ion density at the extraction sheath will result in a lower-beam current (this result is universally observed at high-beam current); (2) there is a saddle point with a potential barrier preventing most volume-produced negative ions from being extracted [the combination of (1) and (2) indicates that most of the negative ions being created do not find their way into the beam]; (3) the introduction of cesium may cause an increase in the transverse space-charge limit; (4) cesium also results in an increase in the fraction of volume-produced negative ions that are extracted; (5) cesium may also result in reduction of extracted electrons by producing a less negative bias on the plasma electrode wit...

Calibration of the HHIRF tandem accelerator energy-analyzing magnet☆

Abstract A description is given of the calibration of the Holifield Heavy Ion Research Facility tandem accelerator energy-analyzing magnet. Two methods were used to measure beam energies: a novel time-of-flight technique using a variable frequency buncher; and an alpha-particle reaction-energy technique. Calibrations, accurate to better than 0.04%, were obtained for 11 different magnet excitations corresponding to mass-energy products between 28 and 298.

Multipacting on the trailing edge of proton beam bunches in the PSR and SNS

The Proton Storage Ring (PSR) in Los Alamos has a fast intensity-limiting instability, which may result from an electron cloud interaction with the circulating proton beam leading to a transverse mode coupling instability. The most probable mechanism of the electron creation is multipacting. Though the effect depends on many parameters, a model is presented which predicts a large electron creation in the vacuum chamber. A comparison of this effect between the PSR in Los Alamos and the Spallation Neutron Source (SNS) in Oak Ridge is given. In addition, several possibilities to reduce multipactor are discussed.

The Oak Ridge Radioactive Ion Beam Facility

Abstract The Oak Ridge RIB Facility has been approved and is under construction. The general layout and capabilities of this facility are described, as is the possibility of studying nuclei near the 100 Sn double closed shell, new regions of exotic nuclear shapes, proton halo nuclei, and rp-process nucleosynthesis.

The Holifield Heavy Ion Research Facility

The Holifield Heavy Ion Research Facility has been in routine operation since July 1982. Beams have been provided using both the tandem accelerator alone and a coupled mode in which the Oak Ridge Isochronous Cyclotron is used as an energy booster for tandem beams. The coupled mode has proved to be especially effective and has allowed us to provide a wide range of energetic beams for scheduled experiments. In this report we discuss our operational experience and recent development activities.

Charge-state distributions of 100, 175, 275 and 352 MeV gold ions emerging from thin carbon foils

Abstract Ion charge-state distributions were measured for 100, 175, 275 and 352 MeV gold ions emerging from thin carbon foils. The 90° double-focusing energy analyzing magnet of the Holifield Heavy Ion Research Facility 25 MV tandem accelerator was used to separate the charge-states. Carbon foil thicknesses of 10, 20, 40 and 80 μg/cm 2 were used depending on the beam energy. At least two different foil thicknesses were used at each energy to insure that equilibrium distributions were measured. The measured mean charge was compared with the predictions of the Sayer, Nikolaev-Dmitriev, and Shima et al. semi-empirical charge-distribution formulas. The measurements are in general agreement with the semi-empirical predictions at 100 MeV but were systematically lower at the higher energies. The largest discrepancy was −4.9 charge-states at 352 MeV. These results are especially significant for the design of booster accelerators for large tandem accelerators and for the design of coupled cyclotron systems.

An RMS particle core model for rings

A self-consistent set of equations for the azimuthal variation of rms betatron oscillation amplitudes, including the effects of dispersion and space charge, is derived. These effective envelope equations can be integrated over the beam energy distribution to provide space charge forces in a particle core model for rings. The derivation of the envelope equations involves an accelerator ordering scheme for the beam dynamics and a statistical moments analysis of the canonical distribution function in the six-dimensional phase space of the beam Hamiltonian. The azimuthal variation of the second moments of the transverse canonical coordinates, xβ and zβ, integrated over the kinetic distribution function of the beam, provides the rms equations. These equations, at fixed beam energy, are integrated over the beam energy distribution to provide the overall space charge distribution and force. Because the envelope equations and dispersion function both depend upon and determine the space charge forces, the consiste...

Selection and design of ion sources for use at the Holifield radioactive ion beam facilitya)

The Holifield Radioactive Ion Beam Facility now under construction at the Oak Ridge National Laboratory will use the 25 MV tandem accelerator for the acceleration of radioactive ion beams to energies appropriate for research in nuclear physics; negative ion beams are, therefore, required for injection into the tandem accelerator. Because charge exchange is an efficient means for converting initially positive ion beams to negative ion beams, both positive and negative ion sources are viable options for use at the facility. The choice of the type of ion source will depend on the overall efficiency for generating the radioactive species of interest. Although direct‐extraction negative ion sources are clearly desirable, the ion formation efficiencies are often too low for practical consideration; for this situation, positive ion sources, in combination with charge exchange, are the logical choice. The high‐temperature version of the CERN‐ISOLDE positive ion source has been selected and a modified version of t...