Order-of-magnitude increase in laser-target coupling at near-relativistic intensities using compound parabolic concentrators

Achieving a high conversion efficiency into relativistic electrons is central to short-pulse laser application and fundamentally relies on creating interaction regions with intensities $\gg {10}^{18}\mathrm{W}/{\mathrm{cm}}^2$. 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.

Figure 1

(a) Diagram of CPC cones with a 50-$\mu \mathrm{m}$-diameter cone tip (top) and a 25-$\mu \mathrm{m}$-diameter cone tip (bottom). (b) Au prism target. (c) Modeled, time-integrated ARC focal spot of a single beamlet. Dashed contours are shown at the entrance and exit apertures of the two cone geometries. Encircled energy in the cone tip region is 7% and 20% for the 25 $\mu \mathrm{m}$ and 50 $\mu \mathrm{m}$ tip, respectively.

Figure 2

Electron spectra measured from the (${90}^{\circ }$, ${78}^{\circ }$) line of sight from (a) flat and cone targets using four ARC beamlets at 10-ps pulse duration and (b) 50-$\mu \mathrm{m}$ cone tip and a single ARC beamlet at 2- and 10-ps pulse duration. (c) Electron temperatures and ponderomotive scaling (lines) as a function of laser intensity for the (${90}^{\circ }$, ${78}^{\circ }$) and (${90}^{\circ }$, ${315}^{\circ }$) lines of sight designated by square and circle markers, respectively, with colors corresponding to those in panels (a) and (b). All experimental parameters and results are given in Table 1.

Figure 3

Measured positron yield as a function of average laser intensity over the spot size (1/$e^2$). Diamond markers indicate current experiments. An analytic model, which assumes an electron temperature scaling of approximately 3 times ponderomotive (solid) and data from other facilities (dots) from Refs. [24] and [25], respectively.

Figure 4

(a) Electron density maps generated from hydrodynamic simulations and used as input for the PIC simulation at $t=0$ for cone (upper) and flat (lower) targets with contours corresponding to $n_c$, $n_c/4$, and $n_c/10$. The white region shows the initial location of the cone wall. (b) Electron spectra from cone, flat, and flat with cone preplasma. Exponential temperatures are fit to the high-energy region of the spectra.

Figure 5

Field intensity plots for (a) flat with cone preplasma and (b) cone targets with contours corresponding to $n_c$, $n_c/4$, and $n_c/10$. (c) Input laser intensity (dashed) and ratio of intensities for test and cone cases averaged between $-30<x\left(\mu \mathrm{m}\right)<30$ and within densities between $0.1<n_e/n_c<0.25$ (solid). (d) Fourier transform of transverse Poynting flux summed over the axial dimension for all cases.