The necessity for reliable electron impact ionization rate coefficients is recognised in many areas (eg, astrophysics, plasma physics and controlled fusion) as well as in technical plasmas. There have been relatively few experimental checks on the theoretical calculations of electron impact ionization cross sections. For neutral atoms and ions of low charge state the crossed beam method has been used successfully. However, for ions in a higher charge state it is very difficult to obtain an intense enough beam of charged ions. In such cases the plasma spectroscopy method has been the principal experimental approach. Here a dedicated plasma source, most often a theta pinch, has been used to measure electron impact ionization ratecoefficients. Surprisingly the results obtained on the various machines differed from the theoretical results and from each other. This is considered most unsatisfactory. In the actual environment (ie, a plasma) for which the rates are needed, consistent results could not be obtained.
The project hoped to resolve this situation noting that 2 of the 4 working theta pinches were in Europe. A joint approach by the 3 participating institutes was proposed. The theta pinches at Strathclyde and Bochum are very different. It was envisaged that the Strathclyde group measure rates for carbon ions and the Bochum group for silicon ions and then each group would measure rates for neon ions. The lithium like stage was chosen as that to be studied primarily, not only because of available theory but also for the usefulness of isoelectronic comparison.
The laser scattering techniques as implemented at the 2 theta pinch sites were examined in situ by members of the consortium and the problems aired and solutions discussed. This was extended to include data capture and computer analysis. The estimating of uncertainties in the results and the implementation of independent checks were 2 of the topics raised. Since 1 group viewed the plasma radially and the o ther axially the advantages and disadvantages of each approach were discussed. The treatment of the problem of opacity was also explored in detail. The dynamics of the early phases of the discharges as a preheat mechanism.
Although the full experimental objectives of the contract had not been achieved, in that the same experiment be done in each machine (neon ions), it is felt that such experiments are not now totally justified in the light of the success of the collaboration in understanding the carbon and silicon experiments. It was concluded that for that particular experiment the influence on ionization by charge exchange involving hydrogen is rather unlikely. Following an analysis where allowance was made for the populations in low lying or metastable levels, effective rate coefficients in satisfactory agreement with previous compilations were obtained, especially when the propagation of uncertainties is considered. The low lying and metastables were included in the atomic model as was the measured structure of the plasma. Again agreement within the uncertainties was obtained between theory and experiment for the effective ionization rate coefficients.
The work has allowed studies to be undertaken to find why discrepant results were previously obtained for ionization rates as measured using different theta pinches. Experiments on the 2 European pinches have shown it is possible to obtain results which agree with theory for lithium like carbon and silicon ions when plasma structure and atomic level populations are properly recognized.
It is proposed to conduct joint experimental and theoretical studies of the measurement of electron impact ionisation rates in transient plasmas. Previously such measurements from different theta-pinch experiments have not supported each other, the spread of results being far greater than the experimental uncertainties. It is alarming that the measurements do not agree yet they are made in an environment corresponding most closely to that for which the ionisation rate coefficients are needed. Experiments will be done on each pinch for the same ion in the same plasma conditions. They will exploit the differences in plasma production and the similiraty of the diagnostics A concerted theoretical approach will attack weaknesses in the theoretical analysis of the experiments.
Funding SchemeCSC - Cost-sharing contracts
G1 1BA Glasgow