Plasma Collision in a Gas Atmosphere

We present a study on the impact of a gas atmosphere on the collision of two counterpropagating plasmas (gold and carbon). Imaging optical Thomson scattering data of the plasma collision with and without helium in between have been obtained at the Omega laser facility. Without gas, we observed large scale mixing of colliding gold and carbon ions. Once ambient helium is added, the two plasmas remain separated. The difference in ionic temperature is consistent with a reduction of the maximum Mach number of the flow from $M=7$ to $M=4$. It results in a reduction of a factor $\sim 10$ of the counterstreaming ion-ion mean free path. By adding a low-density ambient gas, it is possible to control the collision of two high-velocity counterstreaming plasma, transitioning from an interpenetrating regime to a regime in agreement with a hydrodynamic description.

Figure 1

(a) Schematic of the target for the gold-carbon case. On the target surfaces, the laser intensity is shown as calculated by a 3D thermal radiation and computer aided design code (visrad [16]); only a subset of laser beams are shown for clarity. (b) Schematic of the gasbag target. (c) Geometry of the Thomson probe. (d) Laser pulse shape used (blue) without and (red) with the helium gasbag.

Figure 2

(a) Spatially resolved Thomson scattering spectra in the gold-carbon case. (b)–(d) (Blue curves) Lineouts over Fig. 2 at various positions along the spatial axis. Distances are measured from the Au surface. (Black curve) Theoretical fits to experimental data. (e) Spatially resolved Thomson scattering spectra in the gold-helium-carbon case. (f)–(h) (Blue curves) Lineouts over Fig. 2 at various positions along the spatial axis. Distances are measured from the Au surface. (Black curve) Theoretical fits to experimental data.

Figure 3

Zero is the initial position of the gold ring, 1000 is the initial position of the carbon puck for all four plots. (a) species fraction for the gold-carbon case as a function of space. (b) Species fraction for the gold-helium-carbon case as a function of space. (c) Temperature (ion and electron) of the gold-carbon case as a function of space. (d) Temperature (ion and electron) of the gold-helium-carbon case as a function of space.

Figure 4

The gold-carbon case. Data were taken 400 m from the Au surface. Ion temperature and flow velocities as a function of time.