Space- and time-resolved observation of single filaments propagation in an underdense plasma and of beam coupling between neighbouring filaments

We have performed a systematic study of beam propagation (400 ps, I = 10¹⁰–10¹⁴ W cm⁻²) in underdense plasmas (n_e = 10¹⁹–10²⁰ cm⁻³) at a level of reduced complexity compared with the smoothed beams currently used in inertial confinement fusion studies, using one or two well-controlled filaments. These experiments have been performed on the LULI 100 TW laser facility. The use of well-controlled, diffraction-limited single filaments is possibly due to the use of adaptative optics. We have used either a single filament or two filaments having variable distance, delay, intensity ratio and polarization. The single filament configuration allows to study basic beam propagation and reveals occurrence of filamentation at low intensity levels. The use of two filaments demonstrates the occurrence of beam coupling and merging, and the importance of cross-talk effects supported by the plasma.