  Achieving a high conversion efficiency into relativistic electrons is central
to short-pulse laser application and fundamentally relies on creating
interaction regions with intensities ${\gg}10^{18}$~W/cm$^2$. Small focal
length optics are typically employed to achieve this goal; however, this
solution is impractical for large kJ-class systems that are constrained by
facility geometry, debris concerns, and component costs. We fielded
target-mounted compound parabolic concentrators to overcome these limitations
and achieved nearly an order of magnitude increase to the conversion efficiency
and more than tripled electron temperature compared to flat targets.
Particle-in-cell simulations demonstrate that plasma confinement within the
cone and formation of turbulent laser fields that develop from cone wall
reflections are responsible for the improved laser-to-target coupling. {These
passive target components can be used to improve the coupling efficiency for
all high-intensity short-pulse laser applications, particularly at large
facilities with long focal length optics.

Plasma Physics