Experimental studies of interaction of intense long laser pulse with a laser–created Ta plasma L. L´ aska, K. Jungwirth, J. Kr´ asa, E. Krousk´ y, M. Pfeifer, K. Rohlena, J. Sk´ ala, J. Ullschmied, A. Velyhan Institute of Physics, ASCR, Na Slovance 2, 182 21 Prague 8, Czech Republic e-mail: laska@fzu.cz P. Kubeˇ s Czech Technical University, Technick´a 2, 16627 Prague 6, Czech Republic J. Badziak, P. Parys, M. Rosi´ nski, L. Ry´ c, J. Wo lowski Institute of Plasma Physics and Laser Microfusion, Hery St. 23, 00–908 Warsaw, Poland Received 2 May 2006 Characteristics of ions, emitted from the laser produced plasma, were systematically investigated using high power iodine laser system PALS, operating at 3 ω. A Ta target was irradiated perpendicularly with laser pulse energy 150 J. Changing the laser focus position, not only the laser intensity, but also the length (duration) of laser beam interaction with pre–formed plasma is changing. Self–focusing of laser beam in self–created plasma was proved, moreover with longitudinal structure with a period of ∼ 200 μm. PACS : 52.38.Kd, 52.38.Hb Key words : laser plasma, non–linear processes, self–focusing 1 Introduction The intense laser beam focused on a solid target creates a hot and a dense plasma, which emits ions with a large range of charge states and kinetic energies, as well as hard and soft X–ray radiation. At laser intensities above I L ∼ 10 14 W/cm 2 various non–linear interactions with pre–formed plasma may significantly increase the charge states and kinetic energy of the ions produced [1]. The necessary pre– formed plasma can be generated either by a suitable independent pre–pulse, pre- ceding the main laser pulse [2, 3], or by the front part of a sufficiently long main pulse (> 100 ps), the rest of which interacts with the self–created plasma [4]. De- pending on the laser pulse and plasma parameters, various forms of laser beam self–focusing, filamentation or channeling may occur. These effects are ascribed to the ponderomotive force expulsion of electrons from the laser pulse channel [5, 6, 7], to the presence of relativistic electrons [7]–[11], but also to the self–created magnetic field and a subsequent pinching at high current densities [12]–[14]. More than ten subgroups of ions have been distinguished when using the PALS high power iodine B506 Czechoslovak Journal of Physics, Vol. 56 (2006), Suppl. B