The use of curved targets to generate focused TNSA ion beams

Target-normal sheath acceleration (TNSA) is a common technique in high-energy-density laser-driven experiments, that can be used in a wide range of applications from radiography to generation of warm dense matter and cancer therapy. The application of focus for this study is neutron radiography, where a neutron source can be generated from deuterium breakdown, DD- fusion and (p,n) reactions when a TNSA deuterium and proton beam collides with a secondary target. This generates a neutron source with characteristic energy and divergence proportional to the incident ion beam, atop an isotropic neutron component. Laser-driven neutron sources have shorter duration than conventional reactor-based sources, making them ideal for radiographing evolving experiments or for tomography. An optimal radiography source has high brightness and a small source size, to increase contrast and resolution capability respectively. If the ion beam can be focused to a smaller area, more ions may impact the secondary target (increasing neutron brightness) in a smaller area (decreasing neutron source size and increase resolution capability). Literature in high energy density (HED) physics has used curved (hemispherical) TNSA targets to generate warm dense matter at the focus of the ion beam, but not studied the ion dynamics post-focus.

Here, we investigate the use of curved primary targets to focus a TNSA ion beam onto the secondary target, with the aim of increasing neutron yield and decreasing the neutron source size by manipulating the number of ions and ion spot size on the secondary target. We evaluate the post-focus ion beam profile and characteristics, which will allow the secondary target standoff to be larger and away from the laser interaction. The PIC code WARPX, coupled with OMEGA-EP experiments, is used to explore the laser parameters and geometries required to generate focusing ion beams and to characterize them as a function of target curvature.