Soft X-ray spectral analysis of laser produced molybdenum plasmas using the fundamental and second harmonics of a Nd:YAG laser.

Our measurement of the soft X-ray emission of Mo plasmas produced by picosecond Nd:YAG lasers emitting on the fundamental (1064 nm, 150 ps) and second (532 nm, 130 ps) harmonics is presented. The contrast in intensity between spectral peaks and the intensity outside them is lower for the second harmonic produced plasmas probably due to the presence more intense satellite emission and higher optical thickness. The measured spectra are absolutely calibrated and the observed output photon flux was (7 - 9) × 1013 photons/sr in the water-window (2.3 - 4.4 nm) spectral range for a laser energy of 160 mJ independent of laser wavelength. However, in the short wavelength range 1.5 - 2 nm, the emission using the second harmonic is strongly enhanced and is even higher than for the maximum energy of 220 mJ of the fundamental wavelength, so despite inevitable energy losses, laser wavelength conversion may lead to emission enhancement in certain spectral ranges. This enhancement is attributed to higher absorption of short wavelength laser light and higher charge state generation in denser plasmas.