Filamentation in long scale length plasmas: Experimental evidence and effects of laser spatial incoherence

Signatures of filamentation have been observed in preformed plasmas using complementary diagnostics: time-resolved images of the transmitted laser light, dark field imaging, and time-resolved spectra of Raman light. This last diagnostic clearly shows the presence of small channels inside the plasma with temporal evolution in agreement with the formation of filaments. The filamentary structures disappeared when random phase plates were used in the laser beam. This result is in agreement with a theoretical analysis showing that filamentation does not grow when the speckle size is smaller than the perturbation wavelength which maximizes, in the coherent case, the filamentation growth.