Target normal sheath acceleration for laser-driven ion generation: advances in theoretical modeling

Recently, ultra-intense laser-driven ion acceleration has turned out to be an extremely interesting phenomenon, capable to produce ion beams which could potentially be suitable for applications as hadron therapy or dense matter diagnostics. The present PhD thesis is addressed to the study of Target Normal Sheath Acceleration (TNSA), namely the laser-based ion acceleration mechanism which dominates the presently accessible experimental conditions. The work is focused in particular on the theoretical modeling of TNSA, motivated by the need for an effective description which, by adopting proper approximations that can limit the required computational efforts, is capable to provide reliable predictions on the resulting ion beam features, given an initial laser-target configuration. Indeed, the development of a robust TNSA theoretical model would mean a deeper comprehension of the key physical factors governing the process, allowing at the same time to draw guidelines for potential experiments in the next future. In this dissertation, in order to achieve a significant advancement in the TNSA modeling field, the results of two original works are reported, the first is focused on a critical, quantitative analysis of existing descriptions, and the second, starting from the conclusions of such an analysis, is dedicated to the extension of a specific model, aiming at the inclusion of further, crucial, TNSA aspects. The quantitative analysis consists in the comparison of six well-known published descriptions, relying on their capability in estimating the maximum ion energy, which is tested over an extensive database of published TNSA experimental results, covering a wide range of laser-target conditions. Such a comparative study, despite the technical issues to be faced in order to reduce the arbitrariness of the results, allows to draw some interesting conclusions about the effectiveness of the six models considered and about TNSA effective modeling in general. According to the results, the quasi-static model proposed by M. Passoni and M. Lontano turns out to be the most reliable in predicting the ion cut-off energy, at the same time achieving such estimates through a self-consistent treatment of the accelerating potential. This work highlights also the limits of such a TNSA model, and of the main approximations usually adopted to obtain the different maximum ion energy estimates. Thus, starting from such considerations, an extension of this Passoni-Lontano model is proposed, including new crucial elements of TNSA physics within the description. In particular, further insights of the hot electron population dynamics are implemented, leading to a refined maximum energy prediction, which exhibits more solid theoretical bases, and which broadens the predicting capability of the original model to a larger range of system parameters. The resulting estimates are validated by means of literature experimental data and numerical simulations, demonstrating a remarkable agreement in most of the cases. The achieved model turns out to be particularly suitable in reproducing the maximum ion energy dependence on the target thickness, while some promising insights are obtained in the Mass Limited Targets (MLT) case. Nonetheless, further theoretical work is still required to attain a quantitative agreement with recently published experimental results on MLTs.