Physics Division 1996 to 1998
Physics Division 1996 to 1998The principal activities of the atomic physics program are threefold: atomic collisions research, atomic data compilation, and advanced plasma diagnostic development. The atomic collision physics research can be broadly characterized in two parts: accelerator-based atomic physics and atomic collisions of low-energy, highly charged ions as supplied by an ECR source.
The scientific objective of the accelerator-based atomic physics program is development of detailed understanding of the interactions of charged and neutral atoms and molecules with electrons, atoms, surfaces, and solids through investigations employing beams of particles accelerated to high energies. Facilities at ORNL used for the experimental phase of this research include the EN Tandem Van de Graaff and the Holifield Radioactive Ion Beam Facility (HRIBF). Off-site facilities such as the Texas A&M K500 Superconducting Cyclotron and the Super Proton Synchrotron (SPS) at CERN, Geneva, Switzerland, are also used to extend the range of energies examined. Electron-molecular ion interactions are being studied at the CRYRING heavy-ion storage ring in Stockholm, Sweden. Taken together, these facilities permit measurements over a range of collision energies from milli-electron volts to tens of terra-electron volts, a span of sixteen orders of magnitude, from thermal to ultrarelativistic. Present focii of interest include, at the high-energy end, atomic collisions of ultrarelativistic lead ions (33 TeV) which leads to pair production and to electron capture from pairs. Also of interest is a new effect, the reduction of stopping power due to nuclear size effects. At the low energy end, dissociative recombination of molecular ions such as H3+ and HeH+ are studied down to energies of 10-4 eV using storage ring techniques with ~20-MeV ions. The results here have direct impact on astrophysical models.
The goal of the ECR-based program is improved understanding of the collisional interactions of low-energy molecular and atomic ions (singly and multicharged) with electrons, atoms, ions, and surfaces. A major part of the program focuses on collision systems found in the edge and divertor plasma of present and future magnetically confined fusion devices that are important in determining, e.g., ionization balance, and energy and particle transport, or the interpretation of edge plasma diagnostics. A crossed beams approach is applied to cross-section measurements for ionization of multicharged ions, and for dissociation of molecular ions, both by electron impact. A collaborative electron-ion merged-beams experiment is underway to measure cross sections for electron-impact excitation of multiply charged ions by electron energy-loss spectroscopy. Merged beams are also being used to measure total and state-selective charge exchange and ionization cross sections for multiply charged ions colliding with hydrogen (deuterium) atoms at very low kinetic energies. Analysis of ejected electrons and scattered ions is being performed to study the interaction of low energy multiply charged ions with solid surfaces. The theoretical effort has focused on the development of new techniques for the calculation of heavy particle and electron collision processes such as charge transfer, elastic scattering, and electron impact excitation or ionization that are either of fundamental interest or needed in support of modeling or simulation efforts in the magnetically confined fusion program. This theoretical work is closely coordinated with the activities of the Controlled Fusion Atomic Data Center (CFADC). The plasma diagnostics program concentrates on the development of advanced diagnostics for magnetic fusion experiments. The current emphasis is on the development of a collective Thomson scattering diagnostic to measure density and velocity distributions of alpha particles.