Proton radiography of petawatt-driven channel formation in a plasma gradient

Channel formation by ultra-intense laser pulses in underdense plasmas is a challenging simulation problem with direct relevance to many areas of current research. Recent experiments at the Orion laser facility have used high-energy proton radiography (\textgreater 40 MeV) driven by a 1$\omega $ petawatt beam to directly probe the interaction of another petawatt beam with a well-characterized plasma density gradient. The interaction plasma was generated using a 3$\omega $ long-pulse beam and diagnosed using a 2$\omega $ optical probe, simultaneously imaged onto four gated optical imagers and two streak cameras. The unique capabilities of the Orion facility allowed a comparison of the channels generated by intense 1$\omega $ (1 $\mu$m, 100-500 J, 0.6 ps, 10$^{21}$ W/cm$^{2}$, f/3 parabola) and 2$\omega $ (0.5 $\mu$m, 100 J, 0.6 ps, 10$^{20}$ W/cm$^{2}$, f/6 parabola) pulses. Proton radiographs of these channels are presented along with PIC simulations performed using the EPOCH code, supported by K-$\alpha $ measurements of hot electron beam divergence and magnetic spectrometer data. Together these provide a solid foundation for improvements to hydrodynamic and PIC simulations, further developing the predictive capabilities required to optimize future experiments.