Numerical simulation of beam deflection for smoothed laser beams

Abstract

When it reaches high energy density state, new features of laser propagation in plasma arises in the contrast to that of research field in classical optics. Such as beam deflection, a laser beam can change its propagation direction while it comes across a transverse plasma flow. On the other hand, employment of all sorts of smoothed laser beams becomes very common in high power laser facilities for high energy density physics experiments. Therefore, on what condition beam deflection comes into play for smoothed beams are necessary to be investigated. This paper presents numerical simulation results for that, which is performed by laser plasma interaction code LAP3D. It is a three dimensional massively parallel code, including a laser paraxial envelope solver and a nonlinear Eulerian hydrodynamics package, and models for filamentation, stimulated Raman scattering and stimulated Brillouin scattering, with beam smoothed by continuous phase plate (CPP), spectral dispersion (SSD), separately. For simplicity in this study, numerical simulations perform in a about 700 μm × 700 μm × 700 μm plasma using isotropic conditions (<i>T</i>_e = 3 keV, <i>T</i>_i = 1 keV, <i>n</i> = 0.1 nc) and only include refraction and diffraction effects, namely, with filamentation model excluding scattering models. Simulation employs the CPP and the SSD beam as representatives of spatial and temporal smoothed beams, respectively, and uses an oval like focused spot with extension in the long axis direction about 200 μm in the focus plane propagating through the left boundary into the simulation domain. Based on our previous investigations, we assume that beam deflection of a smoothed beam becomes effective when it satisfies two following conditions as that for a Gaussian beam, namely, suffering filamentation and facing a transverse plasma flow at ion sound speed. Simulation results of LAP3D confirm that both spatial and temporal smoothed beams suffer beam deflection when two above conditions are both satisfied. For the case of CPP smoothed beam, simulation results show that it suffers evident beam deflection under the conditions that it suffers filamentation when its average intensity is larger than that of filamentation threshold, and faces a transverse plasma flow at ion sound speed. For the case of SSD smoothed beam, simulation results show that the beam can avoid beam deflection even if it faces a transverse plasma flow at ion sound speed when filamentation is suppressed as beam bandwidth is much larger than the growth rate of filamentation, otherwise it suffers beam deflection.