Observation of polarization-dependent ponder-motive force in bubble regime

The quality of accelerated electron beams in a nonlinear laser wake-field acceleration regime (the bubble regime) depends on the shape of the produced bubble. The 2D-Particle-In-Cell (PIC) simulations show that the bubble shape suffers from some instability during the laser pulse propagation through the plasma. The outcomes reveal that the pulse self-chirped process induces an asymmetrical ponderomotive force, depending on the laser polarization direction, which pushes the electrons in a transverse direction anisotropically and initiates the instabilities in the surrounding sheath of the bubble. By tracing this effect and changing the polarization direction of light from the S mode to P-one, the instabilities were removed and the stable moving bubble was formed. The results indicate that the self-injection in the accelerating field of the bubble will be affected by the shape instabilities and so achieving the stable conditions play an important role to manipulate the beam properties and produce the quasi-monoenergetic electron bunch.