Particle-in-Cell Simulations of Density Peak Formation and Ion Acceleration from Ultrashort Pulse Laser-Driven Ponderomotive Steepening*

We use particle-in-cell (PIC) simulations and simple analytic models to investigate the laser plasma interaction known as ponderomotive steepening. When laser light reflects at the critical density of a plasma, it is well known that the resulting standing electromagnetic wave will modify the electron density profile via the ponderomotive force, which creates peaks in the electron density separated by approximately half of the laser’s wavelength. What is less well studied is that this charge imbalance accelerates ions towards the electron density peaks, modifying the density profile of the plasma. Idealized PIC simulations with an extended under-dense plasma are used to isolate the dynamics of ion density peak growth, and exhibit counter-steaming populations of keV-scale ions. We identify an ‘ultrashort pulse laser regime,’ where the timescale for the resultant standing electromagnetic wave is shorter than the timescale for ions to reach an electron peak and equilibrate. We discuss conditions under which this phenomenon could be observed and how ion energy may depend on factors such as the laser’s wavelength, pulse duration, and intensity.