Transformation of the optical spectrum of a laser plasma when a surface is irradiated with a double pulse

An experimental investigation was made of the dependence of the contrast in the emission spectrum of a laser plasma on the degree of focusing of radiation from a neodymium laser on the surface of an aluminum target in the case of double-pulse irradiation. For an interpulse interval of 25 μs there was a range of optimal radiation intensities of 0.5–2.5 GW/cm² in which a fifteenfold increase in the contrast of the aluminum spectral lines was achieved. The principal mechanism for increasing the contrast and efficiency for transforming the laser radiation into radiation of the plasma spectral lines was found to be the flash absorption of the radiation of the second pulse in the leading edge of the gas–vapor cloud created on the target surface by the first pulse.