Energy loss of Ar in a laser-produced C plasma

PLASMA-PHYSICS-01

Energy loss of Ar in a laser-produced C plasma A. Blažević1, A. Frank2, M. Günther2, K. Harres2, T. Hessling1, D.H.H. Hoffmann1,2, R. KnoblochMaas2, F. Nürnberg2, A. Pelka2, G. Schaumann2, A. Schökel2, M. Schollmeier2, D. Schuhmacher2, J. Schütrumpf2, M. Roth2 1

GSI, Darmstadt, Germany; 2TU Darmstadt, Germany.

Energy loss measurements of ions penetrating plasmas have become an interesting field of research, especially since the concept of the heavy ion driven Inertial Confinement Fusion had been developed. Earlier experiments of ions interacting with discharge or pinch plasmas [1] showed an increase of the projectile stopping power compared to cold matter. To extend the data base of experimental results to higher plasma densities and temperatures and hence test different theories, the plasma physics group at GSI has devoted its effort to the investigation of the interaction of ions penetrating laser generated hot and dense plasmas (Tpl > 150 eV, rfree electrons > 1020 cm-3). Therefore nhelix [2], a Nd:YAG laser with an energy of up to 100 J, a pulse length of 5 to 15 ns, focused to a large spot of 1 mm diameter, is irradiating a thin (0.5 to 2 µm) carbon foil to produce a carbon plasma. The Unilac ion beam, preferably consisting of Ar ions at an energy of 4 MeV/u, is probing the target. As the ion beam pulse length is several tens of microseconds, consisting of microbunches with a length of 3 ns (FWHM) at a frequency of 108 MHz the target is probed each 9 ns, beginning with cold solid matter over plasma creation, plasma expansion up to vacuum conditions with no target matter in the line of sight of the projectiles. By measuring the time of flight of the ion bunches one gets the energy loss of the projectiles in cold matter, in different plasma conditions during expansion and finally the initial beam energy in one single experiment. Several improvements in the experimental setup during the last years now lead to successful and reliable results. To generate a more homogeneous plasma for the ion beam a random phase plate was included into the laser beam line creating a homogeneous laser intensity profile over the large focal spot. Additionally the ion beam diameter could be reduced to 0.5 mm by developing a new diamond based semiconductor detector with a high sensitivity and sub nanosecond time resolution. Especially these two improvements led to an increase of the energy resolution of the experimental method to dE/E