Evidence of relativistic laser beam filamentation in back-reflected images

The back-reflected image of a 100 TW laser incident on a long scale length plasma is measured. The plasma is deliberately preformed on a solid planar target in a controlled way. Multiple highly intense spots are observed insi de the original focal spot. These spots could be the experimental evidence for the laser beam relativistic filamentation in the plasma; Three-dimensio nal particle-in-cell (PIC) simulations for parameters close to the experime ntal values are performed. The experimental observations and the filamentat ion dynamics obtained in the PIC simulations are in a good agreement.