Free-free emission continuum of weakly non-ideal argon plasmas

Abstract

The optical emission of plasmas of moderate temperature and pressure ( T ≈ 10 4 K, N ≈ 10 17 cm −3) has been and still is the subject of many experimental investigations. It results mainly from the free-bound and free-free transitions of electrons in the fields of ions, the contribution of the bremsstrahlung increasing with wavelength. For shorter wavelengths, the emission agrees well with calculations using simple model potentials but shows an excess for longer wavelengths, the cause of which is not yet clear. Since these plasmas are weakly non-ideal (non-ideality parameter Г ⩽ 0.1) and the theory refers to the ideal case, at least part of the difference might be due to a change in the absorption cross sections caused by the plasma interaction. In this paper, we restrict ourselves to the calculation of free-free transitions in the field of an argon ion, using a potential which combines consistently a scaled Thomas-Fermi potential for the ion core with the Debye-Hückel potential for the plasma interaction. The results are given in the form of temperature-averaged Gaunt factors, which can be related to those for an ideal plasma by means of an approximation formula, first given by Green, for one-electron systems with small screening. The changes in the resultant free-free emissivity are small and show a decrease rather than an increase with pressure, so that they cannot explain the specified discrepancy.