Reduced fast electron transport in shock-heated plasma in multilayer targets due to self-generated magnetic fields

Fast electron transport has been studied in cold solid density CH, cold CH foam ($200\mathrm{mg}/{\mathrm{cm}}^3$), and CH plasma (40 eV $30\mathrm{mg}/{\mathrm{cm}}^3$) targets—the latter created by shocking the CH foam with a 1.2 kJ long pulse laser and allowing it to expand. The fast electrons were produced using the OMEGA EP laser pulse (800 J, 8 ps) incident on a Au flat target. With the CH plasma, the fluence of fast electrons reaching a Cu foil at the far side of the transport was reduced significantly ($25\times$ weaker peak $K\alpha$ emission). Particle-in-cell simulations using the osiris code modeled fast electron transport in the unshocked foam and plasma cases assuming fixed ionization and including source generation, transport in Au and CH layers, Coulomb collisions, and refluxing. Simulations indicate two main mechanisms which alter electron energy transport through the target between the foam and plasma cases, both due to the magnetic field: a collimating field in the CH region, caused by the resistivity of the return current and more prevalent in the foam; and an insulating field at the Au-CH interface, present only with the plasma.

Figure 1

Fast electrons from short pulse laser (red) interaction with a Au foil transported through three different CH layers: solid foil (a), foam (b), and foam heated to plasma by a 1.2 kJ beam [purple in (c) and (d)]. The Au foil is shown partially transparent in (b) to reveal the foam. A Cu foil was behind the transport layers. (d) The perspective of the Spherical Crystal Imager and also indicating the line of sight of the Zn Von Hamos spectrometer. Pinhole camera images of the interaction are shown in the inset.

Figure 2

Spherical Crystal Imager images of the Cu $K\alpha$ emission [(a)–(c), each normalized individually]. Filter-corrected lineouts are plotted (d) and the signals from the SCI and ZVH are analyzed (e). *The solid CH data have been scaled by laser power.

Figure 3

(Left column) Higher density “foam” case; (right column) lower density “plasma” case. (Top row) Forward heat flux at $t=6.3$ ps. (Bottom row) Magnetic field at $t=6.3$ ps. In the plasma case the strong field at the Au/CH interface is shown with enhanced color scale.

Figure 4

Total energy density in the electrons in the rear 20 $\mu \mathrm{m}$ of the simulation target averaged over 6.3 ps, for both plasma and foam cases.