13th International Workshop on the Fast Ignition of Fusion Targets

September 18, 2014

13th International Workshop on the Fast Ignition of Fusion Targets


The High Energy Density Physics (HEDP) research group is represented at the 13th International Workshop on the Fast Ignition of Fusion Targets, 2014 by graduate student Sheng Jiang.




Enhancing Resistive Guiding of Hot Electrons Using Front-Surface Target Structures 

     Resistive gradient magnetic fields have been identified as a means to narrow the hot electron angular distribution [1] in the fast ignition scheme for inertial confinement fusion. We propose a multi-beam configuration to enhance this effect with novel front-surface structured targets which are used to seed the process. First, an ultra-fast (10’s fs), ultra-high intensity (>5*1020 W/cm2) laser with moderate energy (10’s J) interacts with the structured targets which rapidly grows the resistive magnetic fields which collimate the hot electrons generated by the longer, higher energy laser proposed for the standard fast ignition scheme.

     Novel front-surface structured targets (shown in Figure 1a) have recently been found with 3D particle-in-cell (PIC) modeling to significantly narrow the angular distribution of hot electrons [2] produced by an intense short-pulse laser compared to that for a standard flat target (Figure 1). Furthermore, the peak fast electron current density (as well as the return current) inside the structured-target is found to be 4-5 times higher than for a comparable flat target. As a result, the temperature gradient due to resistive heating by the return current is significantly enhanced by these structured targets.

     Because the electrical resistivity of plasma initially increases with temperature [3] from room temperature, the enhanced temperature gradient due to the structured targets will in turn produce larger collimating magnetic fields due to the increased resistivity gradient. Preliminary experimental results with these structured targets as well as PIC simulations exploring this enhancement are presented.

Figure 1:  (a) 10 µm long “spikes” target, (b) and (c) fast electron (>1MeV) angular distribution 5 μm into solid density after laser (15 J, 30 fs, peak intensity of 5 x 1021 W/cm2, polarized in the x direction) excitation of flat (b) and spikes (c) Al targets [2].

[1] Y. Sentoku, E. d’Humieres, et al., Phys. Rev. Lett. 107, 135005 (2011)
[2] S. Jiang, A. G. Krygier, et al., Phys. Rev. E 89, 013106 (2014)
[3] H. M. Milchberg, R. R. Freeman, et al., Phys. Rev. Lett. 61, 2364 (1988)