I.Yu. Tolstikhina

Stripping of fast heavy low-charged ions in gaseous targets

Abstract Projectile-ionization (stripping) cross-sections and beam lifetimes have been calculated for heavy low-charged ions of Pb, Bi and U colliding with H, He, Be, Li, F, N, Ar and Xe atoms in the E =1–100 MeV/u energy range. Calculations have been performed for single-electron processes in the Born approximation using the LOSS computer code accounting for the atomic structure of the target. Results are compared with available experimental data, CTMC calculations and Z 2 + Z scaling where Z is the nuclear charge of the target atom. In the case of Pb-like ions (Pb 0+ ,Bi 1+ ,…,U 10+ ), the scaling law for the stripping cross-sections on the projectile charge and the target nuclear charge is obtained. In the energy range considered, the calculated cross-sections are proportional to Z 1.4 due to the screening of the target nucleus by its electrons which differs markedly from the Z 2 -dependence given by the first-order perturbation theory. The role of multi-electron processes is briefly discussed.

The target-density effect in electron-capture processes

The influence of the target density on the electron-capture (EC) processes in collisions of fast ions with atoms and molecules is considered. The partial EC cross sections σn on the principal quantum number n of the scattered projectile, as well as the total σtot = Σnσn values, are calculated for highly charged ions interacting with gaseous and solid targets in the energy range of E = 100 keV u−1 to 10 MeV u−1. It is shown that with the target density increasing, the populations of the excited states of the scattered projectiles, formed via the EC channel, are drastically suppressed due to projectile ionization by the target particles and, as a result, the total EC cross sections decrease by orders of magnitude at low energies, while the reduction is less prominent at high energies.

Influence of theTarget — Density Effects on Electron — Capture Processes

The influence of the target density on the electron‐capture (EC) processes in collisions of fast ions with atoms and molecules is considered. The partial EC cross sections σn on the principal quantum number n of the scattered projectile, as well as the total σtot values are calculated for highly charged ions interacting with gaseous and solid targets in the energy range of E = 100 keV/amu to 10 MeV/amu. It is shown that with the target density increasing, the population of the excited states of the scattered projectiles, formed via the EC channel, is suppressed due to projectile ionization by the target particles and, as a result, the effective EC cross sections drastically decrease.

Electron Loss and Capture Processes in Collisions of Heavy Many-Electron Ions with Neutral Atoms

The present status and properties of charge-changing processes—electron capture and electron loss—are considered for heavy many-electron ions colliding with neutral atoms over a wide energy range E = 10 keV/u–100 GeV/u. The role of single- and multiple-electron charge-changing processes is discussed, and a brief description of available computer codes for calculation of the corresponding cross sections is presented. Experimental data for electron-loss and capture cross sections for germanium, xenon, lead, and uranium ions colliding with H, N, Ne, Ar, and Xe targets are given in comparison with numerical calculations applying different theoretical models as well as semiempirical formulae.

Calculation of low-Z impurity pellet induced fluxes of charge exchange neutral particles escaping from magnetically confined toroidal plasmas

Measurements of energy- and time-resolved neutral hydrogen and helium fluxes from an impurity pellet ablation cloud, referred to as pellet charge exchange or PCX experiments, can be used to study local fast ion energy distributions in fusion plasmas. The estimation of the local distribution function f(i)(E) of fast ions entering the cloud requires knowledge of both the fraction F(0)(E) of incident ions exiting the cloud as neutral atoms and the attenuation factor A(E,rho) describing the loss of fast atoms in the plasma. Determination of A(E,rho), in turn, requires the total stopping cross section sigma(loss) of neutral atoms in the plasma and the Jacobian reflecting the measurement geometry and the magnetic surface shape. The obtained functions F(0)(E) and A(E,rho) enter multiplicatively into the probability density for escaping neutral particle kinetic energy. A general calculation scheme has been developed and realized as a FORTRAN code, which is to be applied for the calculation of f(i)(E) from PCX experimental results obtained with low-Z impurity pellets.

Loss of positive ion beam by interaction with residual gases during acceleration

Abstract Electron-capture and -loss processes of fast highly charged ions colliding with residual-gas atoms and molecules in an accelerator are considered. A special attention is paid to the temporal variation of the ion-beam intensity while the ions are accelerated from the initial energy to the final higher energy with a constant acceleration rate. Interactions of C4+, Au53+ and U73+ ions in the MeV/u-energy regime with different residual-gas mixtures and pressures are considered in detail. Based upon the present analysis, the estimated cross sections and lifetimes of ions in the accelerators are favourably compared with the experimentally observed values.