Diagnostics of a spark-discharge coupled to laser aluminum plasma by optical emission spectroscopy and ion time-of-flight

摘要

Optical emission and ion time-of-flight are measured for a spark-coupled laser aluminum plasma. A Q-switched Nd:YAG laser (wavelength lambda = 1064 nm, pulse width tau similar to 7 ns, pulse energy E-p = 260 mJ, intensity I = 15 x 10(9) W/cm(2)) generates the Al plasma, while a synchronized spark-discharge enhances the ion flux and charge state. Time-integrated, spatially-resolved optical spectra are used to obtain the plasma excitation temperature T-e and density n(e). The coupling of 2.4 J of spark-discharge energy to the laser plasma enhances the optical emission line intensity. The effective ion temperature T-i is calculated from a shifted Maxwell-Boltzmann distribution fit of the time-of-flight signal deconvolved for each ion charge. For I = 3.5 x 10(9) W/cm(2) , T-i is similar to 15eV. For a spark energy of 2.4 J coupled to the laser plasma, T-i increases to similar to 50eV and up to Al8+ is identified from the ion time-of-flight signal. The T-i obtained from the ion time-of-flight is much larger than T-e obtained from optical spectroscopy, although the plasma is considered to be in local thermodynamic equilibrium. This result is explained in view of the temporal development of the ablation plume and the different plasma regions probed by the two methods.