**5. Conclusions**

In the present paper, the problem of direct spectroscopic monitoring of the laser induced plasma in water confined LSP processes is discussed as a feasible procedure aimed at answering the existing need for the development of process monitoring and control in industrial-scope applications. The authors present a procedure of diagnosis in relevant LSP conditions (confining water flow) using the spectroscopic possibilities offered by the Stark broadening and shift of the Hα-line (656.27 nm) to estimate electron densities and temperatures. The procedure has been demonstrated for the case of Al2024-T351 alloy using the luminescence of Mg II lines emitted by the traces of this element present in the alloy.

A Q-switched laser of Nd:YAG (2.5 J per pulse, 10 ns of pulse duration) was focused on an aluminum sample (Al2024-T351) used as the demonstrator. The Stark width and shift of the Balmer Hα-line (656.27 nm) was obtained with different delay times after the pulse of the laser (2–5 μs) and with several gate times (100, 200, 300, 500, and 1000 ns). The electron densities and electronic temperatures were estimated from this experimental data, the procedure being reproducible for any alloy containing Mg traces without lack of generality.

The measurement of the electronic temperature was performed directly under realistic LSP conditions. In the relevant experiments, the electron density and the temperature ranged between 1.2 × 10<sup>17</sup> and 3.5 × 10<sup>17</sup> cm<sup>−</sup><sup>3</sup> and between 10,000 and 16,000 K, respectively. These values are similar to those obtained by other authors in similar experiments without water. The best time of integration of the light was found to be about 500 ns. The rest of measured windows present different types of problems due to a lack of statistics or because they exceed the plasma evolution times (in the case of the 1000 ns gate time), this parameter being adjustable for each particular case of interest.

The novelty of the presented work stems from its proved capability to estimate the mentioned parameters under realistic water-confined LSP conditions using an improved procedure over previous developments, allowing single, direct determinations and shorter delay times from the laser pulse incidence. This procedure to estimate both electronic density and temperature density by a single experimental determination is considered an improvement on the existing plasma monitoring of LSP processes and opens the door for the much needed real-time monitoring of plasma behavior.

**Author Contributions:** Conceptualization: C.C., A.A.-M., J.L.O., M.I.d.A.-G.; Methodology: C.C., A.A.-M., J.L.O., J.A.P., M.I.d.A.-G., C.M.-D.; Investigation: C.C., A.A.-M., J.L.O., J.A.P., M.I.d.A.-G., C.M.-D., I.A.; Writing—review and editing: C.C., M.I.d.A.-G.

**Funding:** This work was financially supported by the Spanish DGI project MAT2015- 63974-C4-2-R.

**Conflicts of Interest:** The authors declare no conflict of interest.
