J.W. Stearns

Intense, mixed-energy hydrogen beams for CTR injection

Present high-current ion sources for neutral injection experiments accelerate a mixture of atomic and molecular hydrogen species that are converted into neutral particles with different energies and neutralization efficiencies. Beam composition can have important effects on injection system efficiency, vacuum design, and first-wall loading. Beam composition measurements of the 20-keV LBL high-current sources are used to calculate the relative power in the various hydrogen and deuterium beam components expected at higher energies.

Cross-sections for electron capture by 0.3- to 70-keV deuterons in H2, H2O, CO, CH4 and C8F16 gases

Electron-capture cross-section measurements are reported for 0.3- to 70-keV deuterons in H2, H2O, CO, CH4, and C8F16 gases. The experimental apparatus and the method employed – slow-ion collection – are described. The results are discussed, particularly the low-energy behaviour in molecular hydrogen.

Electron-capture and impact-ionisation cross sections for partially stripped iron ions colliding with atomic and molecular hydrogen

Cross sections for single-electron capture and for impact ionisation of Feq+ (q=10,15,20,25) ions incident on atomic hydrogen have been calculated in the energy range 50 to 1200 keV amu-1 using a classical-trajectory Monte Carlo method. Cross sections for the same processes for Feq+ ions incident on molecular hydrogen have been measured for q=11-22 at 1100 keV amu-1, for q=9 at 277 keV amu-1, and for q=12 and 14 at 262 keV amu-1. The experimental cross sections for molecular hydrogen divided by two are in excellent agreement with the calculated atomic hydrogen cross sections. Scaling laws for the cross sections with q, the charge state of the incident ion, are discussed. The scaling with q is not found to follow the q2 law predicted by the binary-encounter theory.

COLLISIONS OF FAST HIGHLY CHARGED IONS IN GAS TARGETS - IONIZATION, RECOIL-ION PRODUCTION, AND CHARGE-TRANSFER

Electron-capture, ionization, and recoil-ion-production cross-sections are measured and calculated for fast highly charged projectiles in hydrogen and rare-gas targets. Recoil-ion-production cross-sections are found to be large; the low energy and high charge states of the recoil ions make them useful for subsequent collision studies.

Single electron capture and loss cross sections for highly stripped Fe ions in hydrogen at 3.4 MeV/nucleon

Abstract Single-electron-capture and loss cross sections of 3.4 ± 0.1 MeV/nucleon iron ions in H 2 gas are reported for change states +20 through +25.

Resonant-transfer-and-excitation for highly charged ions (16 ≤ z ≤ 23) in collisions with helium

Abstract Significant new evidence is presented for resonant-transfer-and-excitation (RTE) in ion-atom collisions. This process occurs when a target electron is captured simultaneously with the excitation of the projectile followed by deexcitation via photon emission. RTE, which is analogous to dielectronic recombination (DR), proceeds via the inverse of an Auger transition, and is expected to be resonant for projectile velocities corresponding to the energy of the ejected electron in the Auger process. RTE was investigated by measuring cross sections for projectile K X-ray emission coincident with single electron capture for 15–200 MeV 16 S 13+ , 100–360 MeV 20 Ca 16+,17+,18+ and 180–460 MeV 23 V 19+,20+,21+ ions colliding with helium. Strong resonant behavior, in agreement with theoretical calculations of RTE, was observed in the coincidence cross sections.

Spin-dependent charge transfer in a polarised target

Polarisation transfer from an electron-spin-polarised alkali-vapour target to an incident hydrogen-ion beam is analysed. At energies below a few keV u-1, successive charge-changing collisions pump the beam to a state that is entirely neutral, and entirely electron- and nuclear-spin polarised. A significant fraction of the resultant H0 atoms can then be converted to nuclear-polarised H- ions by electron attachment in a second alkali-vapour target. This conversion is enhanced by spin-dependent electron attachment if the second target is also electron-spin polarised, but in the opposite sense. Calculations and results are given for 400 eV u-1 H+ and D+ beams in an electron spin-polarised caesium-vapour target.

IONIZATION AND CHARGE-TRANSFER IN HIGH-ENERGY ION ATOM COLLISIONS

Abstract Electron capture and loss by fast highly charged ions in a gas target, and ionization of the target by passage of the fast projectile beam, are fundamental processes in atomic physics. These processes, along with excitation, can be experimentally studied separately (“singles”) or together (“coincidence”). This paper is a review of recent results on singles measurements for electron capture and loss and for target ionization, for velocities which are generally high relative to the active electron, including recent ionization measurements for a nearly relativistic projectile.

K- and l-shell resonant transfer and excitation in ion-atom collisions

Recent experimental studies of resonant transfer and excitation (RTE) in ion-atom collisions are reviewed. In the RTE process correlated electron capture and projectile excitation occur together in a single encounter with a target atom. Measurements of Ca/sup q+/ + H/sub 2/ (q = 10 to 19) from 100 to 370 MeV establish the projectile charge-state dependence of K-shell RTE and provide a detailed test of the theory. Structure due to RTE is observed in the energy dependence of the total electron-capture cross sections for this collision system. A comparison of the Ca/sup 17 +/ + H/sub 2/ data with previous results for Ca/sup 17 +/ + He demonstrates the effect of the target-electron momentum distribution on the RTE process. Studies of 230 to 610 MeV Nb/sup 31 +/ + H/sub 2/ provide information about RTE involving agreement with theoretical calculations. 15 refs., 4 figs.

ELECTRON LOSS AND EXCITATION IN COLLISIONS OF HIGHLY CHARGED CALCIUM-IONS WITH H2 AND HE

Cross sections for projectile electron loss and excitation (LE) in single collisions of 150–360 MeV calcium ions with H2 and He have been measured for projectile charge states in the range 12 + to 19 +. The results establish that for the present collision partners projectile K X-ray production takes place through the excitation rather than removal of a ls electron.