Experimental study of the ion thermalization at a Z-pinch at stagnation

Abstract

Summary form only given. The time-history of the ion-kinetic energy Ek ion throughout the stagnation phase of a neon-puff, 500 ns, 600 kA, Z-pinch implosion was determined. The X-ray spectroscopic system provides a resolving power of 6700 and four consecutive time gated (~1 ns) spectra. A simultaneous axial imaging allows for studying the ion kinetic energy at 0.1-mm-resolution along the pinch column. Ek ion in the stagnating plasma is obtained from the Doppler contribution to the line shapes of the Lyalpha satellites, verified to be optically thin. The line shapes give the ion velocity distribution just before stagnation (non Gaussian) and throughout the 10-ns-long stagnation (Gaussian-like). Ek ion was found to be sime12 keV early at stagnation, dropping down during the stagnation to the electron thermal energy (sime300 eV). The time scale of ion-kinetic energy loss is longer (cong2 ns) than expected from the ion and electron collisional thermalization time (cong0.1 ns). A plausible explanation of the data is that upon reaching the pinch axis, the stagnating plasma develops a turbulent flow, in which most of the implosion energy is stored. The turbulent motion then dissipates into ion heat more slowly than the ion-electron energy equilibration time, which causes Tion to be low, resulting in a slowing down of the ion energy transfer to electrons and to radiation. Detailed study of the experimental line shapes is used to examine this explanation. Axially-resolved measurements of the time-dependent stagnating-plasma properties, and the absolute total neon K radiation show that, within the experimental uncertainties, the observed total ion-kinetic energy accounts for the total radiation emitted from this plasma. These findings, and assuming the explanation given above, can be used to discriminate between the thermal and the turbulent ion kinetic energies throughout the stagnation. Results on the thus-inferred Tion will be presented. Comparisons will be made to implosion velocities and time-resolved line-widths observed in wire-array implosions on the Z machine.