Investigation of Proton Focusing and Conversion Efficiency for Proton Fast Ignition

Author(s): Bartal, Teresa Jean | Abstract: Recent advances in generating high energy (g 50 MeV) protons from intense laser-matter interactions has opened up new areas of research, with applications in radiography, high energy density physics, and ion-proton beam fast ignition (FI). The ability to focus the proton beam has made these applications more attractive. Fast ignition (FI) is an evolved concept of conventional inertial confinement fusion (ICF). In proton FI, a collimated beam of protons is used to deliver the necessary ignition energy to the compressed Deuterium-Tritium (DT) fuel capsule instead of the original concept of a beam composed of relativistic electrons. In cone-guided FI, a cone is embedded into the side of the fuel capsule where the proton source foil is placed within the cone. The cone provides a clear path to the dense core and protects the proton source foil from radiation during the compression of the capsule. The proton source foil is a segment of a hemispherical shell target used to help focus the proton beam to the core to spark ignition. The viability of proton FI requires focusing of the generated proton beam to a 40 [mu]m spot at the compressed fuel and a laser to proton conversion efficiency of 15%. Here, proton focusing and the laser to proton conversion efficiency are investigated using flat foils and hemispherical shell targets. Experiments were conducted on the 200 TW short pulse laser at Los Alamos Laboratory. The 1053 nm laser pulse delivered 70-80 J on target in 500-600 fs focused by an f/8 parabolic mirror. The generated proton beam from the target was examined by placing a mesh downstream of the target, which the proton beam would pass though and then imaged with a pack of radiochromic film (RCF). A 3D ray-tracing technique was developed to determine the focal position and focal spot size of the generated proton beam by tracing the proton trajectories from the image of the mesh collected by the RCF back through the mesh to the central axis. The focal position calculated from the ray- tracing technique for the flat foils resulted in a real focus, contrasting the convention wisdom of a virtual focus. Investigation of the proton expansion from flat foils established that initially the protons are accelerated normal to the surface, due to the fact that the electrostatic sheath field generated by the escaping hot electrons is only a few microns beyond the rear surface of the foil. As time progresses and more electrons are accelerated into the target by the laser irradiation, the sheath expands away from the rear surface of the foil, developing a bell-shaped curvature. The protons are then accelerated normal to the sheath field, which is at the leading edge of the expansion. Due to the bell-shaped curvature, protons that are accelerated further away from the central axis of the laser interaction experience gradients within the expansion causing the protons to gain radial velocity, which changes the angle of divergence of the protons. The radial velocity gained by the protons affects the trajectory of the protons, resulting in a calculated real focal position when trajectories are calculated the ray-tracing technique. The trajectories of the protons are further affected by the mounting technique. When the foils are mounted to washers for stability, electrons accelerated in the foil escaped into the washer creating a field along the interior wall of washer. The field affects the proton trajectories near the wall and decreases the laser to proton conversion efficiency. With the understanding gained from the flat foil targets, proton focusing is further investigated using freestanding hemispherical shell targets. Using the 3D ray-tracing technique, the calculated focal position is determined to be located inside the radius of curvature of the hemisphere, which is less than the distance of 1.7 R (where R is the radius of curvature of the hemisphere, which is less than the distance of 1.7 R (where R is the radius of curvature of the hemispherical shell) determined from proton heating experiments. With the aid of particle-in-cell (PIC) simulations, using the code LSP (large-scale-plasma), it was determined that proton trajectories are not straight, but actually bend near the focal region. A hot electron pressure gradient in the expansion beam sets up a radial electric field, Er ? kTehot/R, where here R is the radial scale length of the beam and kTehot is the hot electron temperature. When the radial electric field surpasses the radial acceleration force, the proton trajectories are bent away from the focal axis. The first demonstration of the generation and focusing of a proton beam from a hemispherical shell in a FI geometry is presented, where the beam is generated from a curved focusing surface, which propagates and is channeled via surface fields through an enclosed cone structure. A segment of a hemispherical shell is placed within a novel cone-shaped target. The proton focusing and conversion efficiency are calculated for the structured targets and are compared to the freestanding hemispherical shells. Particle-in-cell (PIC) simulations are presented for further understanding. It is clearly shown that the focusing is strongly affected by the electric fields in the beam in both open and enclosed (cone) geometries, bending the trajectories near the axis. It is also reported that in the cone geometry, a sheath electric field effectively channels the proton beam through the cone tip, substantially improving the focusing properties. The sheath electric field on the wall of the cone is generated by electrons that escape the hemispherical shell and travel into the surrounding structure. Focusing of the proton beam is improved by the sheath electric field on the wall of the cone; however, the laser to proton conversion efficiency is decreased due to the hot electrons escaping the shell reducing the amount of energy available to accelerate the protons