Quasi-classical approximation in quantum-defect calculations of continuous plasma absorption

A quasi-classical approach is proposed to the calculation of the photoionizational and inverse bremsstrahlung continua of low-temperature plasma, within the framework of the quantum-defect method. An analytical expression is obtained for the absorption coefficient, which, in the particular case of a hydrogen- like atom, reduces to the well-known Kramers–Unsold formula. The limits of applicability are investigated. The results are compared with Peach's numerical calculation and the Biberman–Norman approximation.