Intense laser-driven ion beams in the relativistic-transparency regime: acceleration, control and applications*

Laser-plasma interactions in the novel regime of relativistically-induced transparency have been harnessed to generate efficiently intense ion beams with average energies exceeding 10 MeV/nucleon (\textgreater 100 MeV for protons) at ``table-top'' scales. We have discovered and utilized a self-organizing scheme that exploits persisting self-generated plasma electric (\textasciitilde 0.1 TV/m) and magnetic (\textasciitilde 10$^{\mathrm{4}}$ Tesla) fields to reduce the ion-energy ($E_{\mathrm{i}})$ spread after the laser exits the plasma [1], thus separating acceleration from spread reduction. In this way we routinely generate aluminum and carbon beams with narrow spectral peaks at $E_{\mathrm{i}}$ up to 310 MeV and 220 MeV, respectively, with high efficiency ($\approx $ 5{\%}). The experimental demonstration has been done at the LANL Trident laser with 0.12 PW, high-contrast, 0.65 ps Gaussian laser pulses irradiating planar foils up to 250 nm thick. In this regime, $E_{\mathrm{i}}$ scales empirically with laser intensity ($I)$ as $I^{\mathrm{1/2}}$. Our progress is enabled by high-fidelity, massive computer simulations of the experiments. This work advances next-generation compact accelerators suitable for new applications. $E.g$., a carbon beam with $E_{\mathrm{i}}$ $\approx $ 400 MeV and 10{\%} energy spread is suitable for fast ignition (FI) of compressed DT [2]. The observed scaling suggests that is feasible with existing target fabrication and PW-laser technologies, using a sub-ps laser pulse with $I \approx $ 2.5 \texttimes 10$^{\mathrm{21}}$ W/cm$^{\mathrm{2}}$. These beams have been used on Trident to generate warm-dense matter at solid-densities [3], enabling us to investigate its equation of state and mixing of heterogeneous interfaces purely by plasma effects distinct from hydrodynamics. They also drive an intense neutron-beam source [4] with great promise for important applications such as active interrogation of shielded nuclear materials. Considerations on controlling ion-beam divergence for their increased utility are discussed. [1] S. Palaniyappan, C. Huang, \textit{et al.,} Nature Comm. \textbf{6}, 10170 (2015) [2] J. C. Fern\'{a}ndez, \textit{et al.,} Nucl. Fus. \textbf{54}, 054006 (2014) [3] W. Bang, \textit{et al.}, Sci. Rep. \textbf{5}, 14318 (2015) [4] M. Roth, \textit{et al.}, PRL \textbf{110}, 044802 (2013)