Initial design of a beamline for ultra-intense laser-matter interactions at the BELLA-i PW laser user facility*

BELLA, the Berkeley Lab laser accelerator center hosts a 1 PW Ti:sapp laser with 1 Hz repetition rate, where electron acceleration to 4.5 GeV was demonstrated recently [1]. For electron acceleration, irradiances of up to 10$^{19}$ W/cm$^{2}$ are desired and these are implemented with a long focal length laser beamline and beam spots of w$_{\mathrm{0}}=$52$\mu $m. Much higher irradiances of 10$^{22}$ W/cm$^{2}$ can be achieved when the laser beam is focus more tightly, to a spot of w$_{\mathrm{0}}$\textless 5 $\mu$m in a shorter focal length beamline. A key requirement for many application of laser-matter interaction in this regime, such as laser-ion acceleration or the generation of relativistic surface high harmonics is the ultra-high intensity contrast of the laser pulse. We will describe our design for a short focal lengths beamline, BELLA-i, including multiple plasma mirrors for ultra-high contrast in the laser pulse. The resulting laser pulses will enable reliable access to many exciting aspects of high energy density laboratory physics and laser-matter interactions in the relativistic regime for a community of users.\\[4pt] [1] W. P. Leemans \textit{et al.,} PRL \textbf{113}, 245002 (2014)