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Electronic and photonic interactions involving highly charged tungsten ions

Subject Area Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Term from 2011 to 2018
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 192988978
 
Final Report Year 2017

Final Report Abstract

Within the present project interactions of tungsten ions with photons and with free electrons in a wide range of ion charge states have been studied using colliding-beams techniques. A crossedbeams setup at Giessen university was employed and merged-beams arrangements at the heavyion storage ring TSR in Heidelberg and at the synchrotron-radiation light source ALS in Berkeley were used. Processes that have been investigated include single and multiple photoionization of tungsten ions Wq+ in low charge states q = 1, 2, 3, 4, 5, electron-ion recombination of tungsten ions in moderately high charge states q = 18, 19, 20, 21, and electron-impact single and double ionization of tungsten ions with q = 1, 2, ..., 19. The goals of this research were twofold. Prime interest has been in the understanding of many-electron effects on the structure and on the collisions of complex atoms and atomic ions. Tungsten ions with open d and f subshells are ideal candidates for such studies. Equally important was the aspect of tungsten being an indispensible plasma-chamber wall material in controlled nuclear fusion. Tungsten-impurity concentrations of the order of 0.001% in a fusion plasma can prevent ignition and, hence, the economic production of fusion energy. Tungsten atoms inevitably enter the reaction volume from the surfaces of plasma limiters as a consequence of heavy bombardment with energetic particles. Understanding the behaviour of tungsten in a fusion plasma is only possible on the basis of atomic physics insights and of atomic data for processes such as ionization and recombination. The bulk of the data can only be provided by theoretical calculations. Clearly, theory has to be benchmarked by experiments and that has been possible by the experiments carried out in this project. Electron-impact ionization of tungsten ions in charge states up to q=19 can be described fairly well by distorted-wave theory, provided a sufficiently large number of autoionizing states is included in the calculation of excitation-autoionization channels - many more than what has been previously assumed. A striking result in the recombination measurements was the observation of excessive recombination rates at low electron-ion collision energies. This finding stirred up substantial interest in the scientific community and initiated new theoretical descriptions of the recombination process. A further surprise was that the most sophisticated relativistic Dirac-Coulomb R-matrix approximation could not readily provide a decent prediction for the photoionization cross section of W5+ ions even with a very large set of basis states in calculations using many hours on the world’s most powerful supercomputers. This was totally unexpected given the relatively high charge state of the ion and the experience with other complex ions including the lower charge states of tungsten. This problem is presently under investigation. In summary, the present tungsten project has been extremely fruitful with respect to our understanding of fundamental many-particle effects on atomic structure and collisions of complex many-electron atoms or ions and with respect to data needs for controlled nuclear fusion. In particular, the measurements of storage-ring recombination rate coefficients and the derived cross sections and plasma rate coefficients have stimulated new theoretical developments which finally succeeded in providing satisfactory agreement with the experimental results obtained within the present project. Theory on photoionization still has to be critically reviewed.

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