Effect of Target Material on Fast Electron Transport
In cone-guided fast ignition (FI) inertial confinement fusion, successful ignition relies on the efficient transport of a relativistic electron beam (REB) through a solid density cone tip to a high-density fuel core. A variety of physics mechanisms affect the quality of beam transport, and these effects vary with tip material. This thesis presents a systematic study of the effect of tip material on REB transport.
An experiment was performed using the Titan laser (150 J, 0.7 ps pulse duration, 1 $\mu$m wavelength) at LLNL on multilayered targets with varying transport layers. A more collimated electron beam was consistently observed using high- or mid-Z transport layers as compared to low Z layers, without a significant loss in forward-going electron energy flux. PIC simulations agreed well with experiments, showing the formation of strong resistive magnetic channels ($\sim$80 MG ) enveloped by a global B-field that collimate initially divergent fast electrons (in high-Z targets). These results illustrated the dynamic competition between stopping and collimation that is essential to understand in order to optimize electron flux levels.
Hybrid-PIC simulations further investigated transport in various materials at Titan laser conditions. REB energy loss from stopping was similar in low- and mid-Z materials (21 - 27 \%), and much higher in Au (54 \%), dominated by ohmic stopping. Resistive magnetic field growth was shown to depend on the dynamic competition between the resistivity and resistivity gradient source terms in Faraday's Law. Resistivity evolution, in addition, was shown to depend on the Spitzer-like competition between the ionization state and temperature growth rates. Results suggest that, at Titan conditions, mid-atomic number materials like Cu and Ag are optimal for collimation.
This work has significant implications for fast ignition. At FI conditions, more energy will be injected into the cone tip very quickly, leading to faster ionization and heating rates. Higher atomic number materials may be favorable at these conditions as ionization can continue for a longer period during a $\sim$20 ps FI pulse. These results motivate further computational and experimental work to investigate how multilayer targets can be exploited to maximize fast electron beam collimation whilst minimizing deposition rates.