H. Tawara

W. Fritsch

National Institute for Fusion Science (NIFS-Repository)

§7. Theoretical Study of Electron Transitions in Slow Collisions of Ions with He Fritsch, W. (Bereich Theorie, Hahn-Meitner-lnstitut, D-14091 Berlin, Germany) Tawara, H. In theoretical studies of electron transitions in slow ion-atom collisions, the close-coupling method is established [1] as reliable and flexible. Many features of the collisions, once guessed from intu-itive physical pictur,es, can be investigated quanti-tatively by choosing appropriate· basis sets for the time-evolution of the electron wavefunction. We have set out to investigate electron trans-fer, excitation, as well as two-electron transitions in He2+, Be4 +, C6 + - He collisions in the energy range 1 - 100 ke V fu. With large basis sets of atomic or-bitals including pseudostates, we aim at the pre-diction of cross sections for the population of spe-cific final states. Information on all these processes is needed for charge exchange and beam emission plasma diagnostics methods which are in wide use at fusion facilities. It also contributes to a quantita-tive understanding of processes in the plasma edge region. This work is conducted in close contact with the needs of the experimental programme at JET, Culham, England, and with related laboratory ex-periments at Groningen, The Netherlands. For the investigation of He2+- He collisions, elec-tron transfer and transfer-excitation channels are included up to the n = 4 shell of, respectively, the projectile and the target. Double-electron capture and single-excitation channels are included up to the n = 3 shells. A close analysis of the electron dy-namics in slow collisions as well as the numerical re-sults show that partial single-transfer and transfer-excitation cross sections to a given state in the tar-get and in the projectile are about the ~arne. This is, at first sight, a surprising feature since the two-electron process of transfer excitation is generally weak. In this symmetric collisions system, however, the distinction between target and projectile is lost in slow collisions. We illustrate this point in a plot of the calculated and measured 4-3 line emission cross section ratios, see Fig. 1. Work on this system has been completed. A full 244 1.0 '? He2+- He b 0.8 + b .......... .......... ~ tf·! b 0.6 ~ 0 L c 0.4 ~ u v fl) fl) fl) 0.2 0 L u 0.0 1 2 J 4 5 10 2 J 4 5 100 2 energy (keV /u) Fig. 1. RatiO of 4-3 line emission cross sections, in He2+ -He collisions, from the target (transfer-excitation) to the cross sections from both target and projectile (single transfer and transfer-excitation). Shown are results from this work (line) and recent data [2] from Groningen. account is given in a forthcoming publication (3]. Calculations on the systems of Be4+, C6 + - He are in progress. Here we include the single-transfer channels of up to n = 7, the single excitation chan-nels to the n = 2 states of He, as well as the most important transfer-excitation and double:transfer channels. This work has led to an assessrp.ent of the scaling properties, with respect to the projectile charge st.ate, of transfer-excitation cross sections [ 4]. The calculations within this work have been per-formed in part on a HP Apollo 735 workstation which is operated by NIFS Nagoya. We are in-debted to the staff of the Computer Center for tech-nical support. Partial financial support by JET is also gratefully acknowledged. References [1] Fritsch, W., Lin, C.D., Phys. Rep. 202 (1991) 1. [2] Folkerts, H.O., et al, J. Phys. B 26 (1993) 1619. [3] Fritsch; W., J. Phys. B (1994) in press. [4] Fritsch, W., Phys. Lett. A (1994) in press. 

Theoretical Study of Electron Transitions in Slow Collisions of Ions with He

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