EVIDENCE FOR MOVING FOCUS IN LASER INTERACTION WITH LONG SCALELENGTH PLASMAS

Laser interaction with a preformed expanding plasma has been experimentally studied. The plasma had an n(c)/10 bulk surrounded by a large low-density region. Time-resolved spectra of the light backscattered during interaction show that, after a short phase of laser coupling with the whole plasma, the laser ligth decouples from the main plasma bulk. In fact, the interaction region moves outside at supersonic velocity due to dynamical self-focusing of the laser beam. Beam smoothing with RPP does not modify appreciaby the effect. This observation is relevant to the general problem of laser energy deposition in a plasma.