Tera-FLOP particle-in-cell simulations of rapid ionization front expansion on a target due to a short-pulse ultra intense laser*

Recent experiments utilizing a novel shadowgraphy diagnostic have allowed for time resolved measurements on a sub-picosecond time scale of the ionization of a sub micron thick target after irradiation by an intense (5×10¹⁸ W/cm²) short-pulse laser. The shadowgraphs show filaments in the ionization, suggesting that Weibel-like instabilities play a role in ionization dynamics once the laser has interacted with the target. In order to model this experiment, fully three dimensional particle-in-cell simulations, with high spatial resolution (50nm to 10nm cells) over a relatively large section of the target (40 microns by 40 microns) were required. Simulations using the Large-Scale Plasma code were performed which pushed the code's limits, requiring processing of terabytes of data and concurrent computation in the regime of a hundred trillion of floating point operations per second (Tera-FLOP). These simulations demonstrate filamentation of the magnetic field in the target yielding a density pattern consistent with the experimental observation, and this instability is seeded while the laser is still interacting with the target.