Caption of Movie M1 : The four movies on the left show the temporal evolution of the beam intensity profile, I(r,z,t), in the four cases considered in Fig.3 of the main text. A common color scale is used for all images, so that the beam can no longer be observed when it defocuses and its intensity drops. The right panels show the corresponding plots of the on-axis intensity profile, I(r=0,z,t).