Near-critical plasmas from supersonic gas jets for laser-driven ion acceleration

High-energy (~10-100 MeV) ion sources driven by ultra-intense laser pulses are interesting for many applications such as ultrafast radiography, creation of warm dense matter, isotope production, intense neutron sources, etc. So far, laser-driven ion acceleration has been mainly investigated in solid targets in the so-called target normal sheath acceleration (TNSA) regime [1]. Fewer studies have addressed the case of near-critical plasmas (ne ~ nc) due to the technical difficulty of achieving such gas densities. Previous experiments have targeted solid foils [2] or CO2 lasers with gas jet targets [3]. Near-critical plasmas are predicted to give rise to new ion acceleration regimes combining TNSA and collisionless shock acceleration (CSA) [4], as well as enhanced hot-electron production, beyond the standard ponderomotive scaling. Simulations carried out using the PIC code CALDER predict efficient volumetric electron heating due to phase mixing between bulk electrons and electrons trapped in laser-induced plasma waves. The strong electron pressure gradients that ensue in the nonuniform plasma profile can then trigger an electrostatic shock, enhancing ion acceleration [5]. We report here on a recent laser-gas experiment conducted at the VEGA-2 laser facility (CLPU, Salamanca Spain). The objective was to understand the interaction of the state-of-the-art SourceLab gas jet [6] with the 30 fs 200 TW VEGA-2 laser pulse. At the same time, we aimed at characterizing the gas jet target and testing its operating system. The results from three diagnostics consisting of CR39 particle tracking, radiochromic film analysis, and time-of-flight signals are presented. Finally, we outline a future experiment at the 1 PW VEGA-3 laser facility, for which we show preliminary PIC simulations (performed with the CEA developed PIC code CALDER [7]) that highlight the sensitivity of the collisionless shock formation to the density profile.


[1] A. Macchi et al., Rev. Mod. Phys. 85, (2013)

[2] L. Willingale et al., Phys. Rev. Lett. 96, 245002 (2009)

[3] D. Haberberger et al., Nat. Phys. 8, 95-99 (2012)

[4] F. Fizua et al., Phys. Plasmas 20, 054304 (2012)

[5] A. Debayle et al., New. J. Phys. 19, 123013 (2017)

[6] F. Sylla et al., Rev. Sci. Instrum. 83, 033507 (2012)

[7] E. Lefebvre et al., Nucl. Fusion 43, 629 (2003)