A micrometre long pre-plasma leads to a three-fold proton energy enhancement in Target Normal Sheath Acceleration by enabling an improved laser-to-electron coupling

Target Normal Sheath Acceleration has attracted much interest over the past twenty years as a promising, compact and affordable ion source. Important challenges still affecting this consolidated laser-driven ion acceleration scheme regard the increase of the maximum ion energy and the improvement of the beam quality.

In this work, we present novel experimental results demonstrating a three-fold energy enhancement obtained by pre-expanding in a controlled way the front surface of not-so-thin solid targets (Gizzi et al. 2021 Sci. Rep. 11, 13728). Using a pre-pulse, a 5-10 μm scale-length pre-plasma has been created on the front surface of 25 μm Titanium targets, leading to proton cutoff energies up to 12 MeV as opposite to 4 MeV achieved without pre-plasma. Particle-In-Cell simulations modelling the experimental conditions show a very complex and rich laser dynamics in the long underdense region with the creation of standing waves. This causes electrons to undergo stochastics motion thus enabling a more efficient heating mechanism. As a result, protons from the contaminant layer on the back surface of the target are accelerated to higher energies. Interestingly, the presence of a long-scale pre-plasma seems also beneficial to reduce the divergence of the accelerated ions.