Characterization of Expanding Laser Plasma Produced by Laser Ablation

This work is devoted to characterizing the plasma produced by nanosecond laser ablation. The expanding plasma was generated by the interaction of a home made XeCl excimer laser with a Cu target. The time-of-flight method was utilized as diagnostic tool of the plasma charged particles. By this method, in fact, we were able to measure the ion and electron currents, the charge-states and the energy spectra of ions utilizing very suitable charge collectors. Furthermore, the plasma density was characterized during its vacuum expansion in order to estimate the role of the recombination processes on the plasma charge loss. Introduction The interaction of laser light with solid materials and the properties of the plasma produced by the laser ablation have been investigated for many years [1] and many problems are still open. Nevertheless, the generation of high density and high temperature plasmas by focusing high power laser radiation onto a solid target is interesting for the scientific implications, basic science, engineering and material processing technology [2, 3]. In particular, plasmas generated by pulsed lasers are characterized by a high concentration of ionized matter, which, in turn, can generate pulsed high current ion beams, also known as laser ion sources. The latter are very important due to their suitability for getting solid material vaporisation and deposition, as well as ion implantation with or without an additional ion acceleration [4,5]. In particular, by focusing ultraviolet laser onto solid target, it is possible to obtain ablation plasma characterized by ions of low-charged states such as 1+ and 2+ [6] and low temperature [7, 8]. The advantages to obtain these kinds of ion beams consist in minimization of the space charge effects for a fixed particle density of the beam. Moreover, the plasma with a low temperature leads to low emittance of the beam. The common feature of all the corpuscular diagnostics is the fact that they give information about the plasma parameters at long distances from the target. It is also important to study the plasma propagation in vacuum and the role of the recombination processes on the decreasing ion and electron density. After the end of the recombination processes, the “freezing” of plasma charge states occurs and a steady plasma, expanding adiabatically into the vacuum, is observed. Beyond this distance, the local changes in the ion current density and in the total charge carried by the ions, obey the power law for a three-dimensional expansion of laser plasma far from the target. The details of the recombination zone are important when the extraction of ions from the plasma is required, in particular on the way towards the development of the laser ion source. Experimental set-up The experimental set-up employed a XeCl laser (λ = 308 nm, τL = 20 ns, pulse energy on the target ≈ 70 mJ). A 15 cm focal length lens was used to focus the laser beam onto a Cu target having a purity of 99.99%. The laser spot area was approximately 0.01 cm and thus the resulting fluence average value was of about 7 J/cm. The interaction angle between the normal to the target surface and the laser beam was 70°. A stainless-steel vacuum chamber, equipped with two symmetric quartz windows for the beam entrance and evacuated up to a pressure of 10 mbar, was used as the interaction chamber. The WDS'06 Proceedings of Contributed Papers, Part II, 75–80, 2006. ISBN 80-86732-85-1 © MATFYZPRESS