D. G. Seely

Production, Characterization, and Measurement of H(D) Beams on the ORNL Merged‐Beams Experiment

Total cross section measurements of electron capture processes are being studied for low‐energy, Aq++H(D) collisions using the Ion‐Atom Merged‐Beams apparatus at the Multicharged Ion Research Facility (MIRF) at Oak Ridge National Laboratory (ORNL). On this apparatus, a modified Faraday cup detector is used to measure the intensity of the neutral beam. The conversion of the measured electrical current to the true neutral particle beam current is necessary to accurately determine the true cross section values. Inherent in this conversion process is the number of secondary electrons (γ) emitted from the surface of the detector upon impact of an atom. The method employed to determine γ and its role in the absolute electron capture measurements at ORNL‐MIRF are presented. With a recent upgrade to the apparatus, the neutral beam H(D) production technique has been improved and is discussed in detail in this paper.

Isotope Effects in Low Energy Ion‐Atom Collisions

Isotope effects for charge transfer processes have recently received increased attention. The ion‐atom merged‐beams apparatus at Oak Ridge National Laboratory is used to measure charge transfer for low energy collisions of multi‐charged ions with H and D and is therefore well suited to investigate isotope effects. The apparatus has been relocated and upgraded to accept high velocity beams from the 250 kV High Voltage Platform at the Multi‐Charged Ion Research Facility. The intense higher velocity multi‐charged ion beams allow, for the first time, measurements with both H and D from keV/u down to meV/u collision energies in the center‐of‐mass frame. When charge transfer occurs at relatively large inter‐nuclear distances (via radial couplings) the ion‐induced dipole attraction can lead to trajectory effects, causing differences in the charge transfer cross sections for H and D. A strong isotope effect (nearly a factor of two) has been observed in the cross section for Si4++H(D) below 0.1 eV/u. However, litt...

Low-energy charge transfer between C{sup 5+} and atomic hydrogen

Charge transfer (CT) with carbon ions has been identified for a long time as important in both magnetic fusion plasma devices and more recently in solar wind interactions with comets, planets, or neutrals in the heliosphere. A merged-beams technique is used to measure the absolute total charge transfer cross section for C5+ and atomic H over four orders of magnitude in collision energy, from 12,000 eV/u to 0.64 eV/u. The present measurements are compared with previous measurements using an atomic hydrogen target and benchmark available classical trajectory Monte-Carlo and molecular-orbital close-coupling calculations. An increasing cross section below 10 eV/u is attributed to trajectory effects due to the ion-induced dipole attraction between reactants.

Solar Wind Charge Exchange Studies Of Highly Charged Ions On Atomic Hydrogen

Accurate studies of low‐energy charge exchange (CX) are critical to understanding underlying soft X‐ray radiation processes in the interaction of highly charged ions from the solar wind with the neutral atoms and molecules in the heliosphere, cometary comas, planetary atmospheres, interstellar winds, etc.. Particularly important are the CX cross sections for bare, H‐like, and He‐like ions of C, N, O and Ne, which are the dominant charge states for these heavier elements in the solar wind. Absolute total cross sections for single electron capture by H‐like ions of C, N, O and fully‐stripped O ions from atomic hydrogen have been measured in an expanded range of relative collision energies (5 eV/u–20 keV/u) and compared to previous H‐oven measurements. The present measurements are performed using a merged‐beams technique with intense highly charged ion beams extracted from a 14.5 GHz ECR ion source installed on a high voltage platform at the Oak Ridge National Laboratory. For the collision energy range of 0....

Production of Long‐Lived H2−, HD−, and D2− during Grazing Scattering Collisions of H2+, H3+, D2+, D3+ and D2H+ Ions with KBr, KCl, and LiF Surfaces

We have investigated atomic and molecular anion production from singly charged atomic and molecular hydrogen, deuterium, and mixed isotope beams during grazing interactions with large area KBr, KCl, and LiF single crystal targets in the incident energy range 4–22.5 keV. Electron capture and, in the case of incident molecular ions, dissociation occur during the grazing interactions without appreciable angular straggling or change in velocity. As a result, atomic and molecular cation and anion interaction products are strongly peaked in the specular reflection direction, and, in case of dissociation products, at the fractional kinetic energies determined by the product fragment mass to incident mass ratios. A large‐acceptance electrostatic analysis and detection system is used to collect the charged scattering products with high efficiency. Of particular interest is the production of metastable molecular ions H2−, HD−, and D2−. By comparing molecular anion yields obtained from incident hydrogen, deuterium a...