Investigating the Dynamics of Filamentation in Underdense Plasmas*

In the filamentation instability, the ponderomotive and thermal ejection of electrons from the high-intensity regions of a laser beam within a plasma create modulations in the plasma density and refractive index, which lead to self-focusing and filamentation of the beam. We present an experiment that utilizes the joint operation of the OMEGA 60 and OMEGA EP laser systems at the University of Rochester’s Laboratory for Laser Energetics to investigate the growth rate of the filamentation instability. 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 of the filamented short-pulse beam is recorded as a time-integrated 2-D image while plasma parameters are determined via Thomson scattering. By modifying the incident short-pulse beam pulse duration, we can limit the growth of the instability and record the beam spray at discrete steps in its temporal evolution. The inferred radial-growth rate is then compared with simulations, theoretical predictions, and scaling with incident beam intensity and plasma temperature.