EGAS41 - Swansea University

41 st EGAS CP 156 Gdańsk 2009
Electron-impact double and triple ionization of tungsten atoms
and ions at low ionization stages
V. Jonauskas, S. Kučas, R. Karazija ∗
Institute of Theoretical Physics and Astronomy of Vilnius University,
A. Goštauto 12, Vilnius LT-01108, Lithuania
∗ Corresponding author: karazija@itpa.lt
Due to a very low sputtering rate of tungsten it is used as the first wall material in large
tokamaks, such as ASDEX Upgrade, and is planned to use in ITER. However, some its
ions migrate to the central region of discharge and may be the reason of large radiation
losses. Thus it is up-and-coming to investigate the role of the Auger decay of the states
with a vacancy and the direct multiple ionization in the production of tungsten ions during
their interactions with electrons.
The double and triple ionization of tungsten ions by electron impact at low ionization
stages was measured and described by semiempirical formula [1], but no ab initio
calculations were made. In this work the calculations of cross sections of these processes
are performed taking into account the Auger decay of states with the vacancy in 4d, 4f,
5s, 5p and 5d shells as well as the direct double ionization considered using sudden perturbation
model. The direct single ionization cross section is obtained in the relativistic
distorted wave or binary-encounter-dipole approximations; the contribution from the indirect
ionization by excitation-autoionization process is included too. The calculations
are performed using Flexible Atomic Code [2]. It is shown that both processes − Auger
decay and direct double ionization play an important role in the double ionization process.
The calculated cross sections are compared with experimental data.
The contributions from the Auger transitions and from the sudden perturbation to the
double ionization cross section are obtained of the same order, though the latter process
plays a smaller role, especially for the higher ions. Most of the Auger transitions end in
the configurations of the next ionization stage, thus the triple ionization cross section is
more determined by the direct ionization.
4
W 2+ W 4+
cross section, 10 -17 cm 2
3
2
1
0
30 100 1000
energy, eV
Figure 1: Electron-impact double ionization cross section of W 2+ ions. Experiment [1], points
with error bars. Results of calculation: dotted curve, single ionization following the Auger
transitions; full curve, total double ionization.
References
[1] M. Stenke et al., J. Phys. B 28, 4853 (1995)
[2] M.F. Gu, Astrophys. J. 582, 1241 (2003)
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