Plasma formation and target preheating by prepulse of PW laser light*

An intense short pulse laser with intensity over $10^{21}$ W/cm$^2$ has become available, i.e. J-KAREN-P at QST. Although the contrast of the short pulse is improved to be of the order of $10^{-11}$, there is an unavoidable prepulse, which has multiple spikes (~ ps) on top of an exponential profile with intensity greater than $10^{14}$ W/cm$^2$ about 50 ps in front of the main pulse. The prepulse preheats the target and also produces tenuous plasmas in front of a target before the main pulse arrives. It is critical to understand such preheating of the target, where the nonlocal heat transport is essential at intensity $> 10^{14}$ W/cm$^2$, since the target condition might totally change before the interaction with the main pulse. Using a hydro code, FLASH, and a collisional particle-in-cell code, PICLS, we study the preplasma formation and target preheating over tens of picoseconds timescale, and discuss the prepulse effects on the main pulse interaction.