Optical and ion energy spectroscopy of laser-produced plasmas

Abstract

The work described in this thesis relates to the problem of observing population inversions in laserproduced plasmas, and to the associated problem of interpreting optically thick line intensities. In Part I we examine the conditions under which population inversions appropriate to soft X-ray transitions can occur in rapidly recombining plasmas, with particular reference to hydrogen-like transitions in a laser-produced carbon plasma. Computations have been performed for quasisteady state inversions between excited states of hydrogenlike ions, as well as for inversions with respect to the ground state. The effect of Lyman a self-absorption in reducing population inversions between quantum states n = 2 and 3 is examined in some detail. The emergent intensities of optically thick lines from an expanding plasma, such as a laser-produced plasma, are affected by the differential Doppler effect associated with ion streaming. As a first step in investigating this effect we have measured (in Part II) the velocity spectra of the streaming ions. A plasma was produced by focusing the pulse (4 ns duration) from a Neodymium laser onto a carbon target (power density in the range 3 x 10¹⁵ to 1.3 x 10¹⁶ Wm⁻² ) in vacuum. A 45° parallel plate electrostatic analyzer was built to record the energy spectra of the streaming ions. Space charge effects within the analyzer are examined quantitatively. Ion velocity spectra, deduced from the energy spectra and covering the range 1-5 x 10⁵ ms⁻¹ , are reported for C³⁺ , C⁴⁺ , C⁵⁺ and C⁶⁺ . The role of recombination in forming these spectra is discussed.