Investigating the Dynamics of Short-Pulse Laser Beam Filamentation in Underdense Plasmas

The filamentation instability results from the ponderomotive and thermal ejection of electrons from the high-intensity regions of a laser beam within a plasma. This process creates modulations in the plasma density and the refractive index that can lead to self-focusing and filamentation of the beam. The resulting intensified beam and modulated plasma can inhibit the propagation of the beam and limit the efficacy of laser-plasma interactions such as Thomson scattering, Raman amplification, and laser guiding structures. We present experimental and simulation results investigating the growth rate of the filamentation instability. The experiment utilizes the joint operation of the OMEGA 60 and OMEGA EP laser systems at the University of Rochester’s Laboratory for Laser Energetics. In our experiment, a 1ω short-pulse (1—100-ps) laser beam from OMEGA EP is coupled into a preheated plasma on the OMEGA 60 laser-plasma interaction platform. The resulting beam spray profile of the filamented short-pulse beam is recorded, while plasma parameters are determined via spatially-resolved Thomson scattering. We compare experimental results with 2D, axisymmetric simulations utilizing a paraxial electromagnetic wave solver coupled to a single-fluid nonlinear hydrodynamic solver. The simulations model the experiment based on the plasma condition retrieved from the Thomson scattering data and the transmitted nearfield measurements of a vacuum-propagated beam. By modifying the incident short-pulse beam pulse duration in the experiment and simulations, we limit the growth of the instability allowing us to connect the recorded beam spray to the underlying physics of filamentation at discrete steps in its temporal evolution.

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