Enhancements in laser-generated hot-electron production via focusing cone targets at short pulse and high contrast

We report on the increase in the accelerated electron number and energy using compound parabolic concentrator (CPC) targets from a short-pulse ($\sim 150$ fs), high-intensity ($>{10}^{18}$ W/${\mathrm{cm}}^2$), and high-contrast ($\sim {10}^8$) laser-solid interaction. We report on experimental measurements using CPC targets where the hot-electron temperature is enhanced up to $\sim 9$ times when compared to planar targets. The temperature measured from the CPC target is $〈T_e〉=4.4\pm 1.3$ MeV. Using hydrodynamic and particle in cell simulations, we identify the primary source of this temperature enhancement is the intensity increase caused by the CPC geometry that focuses the laser, reducing the focal spot and therefore increasing the intensity of the laser-solid interaction, which is also consistent with analytic expectations for the geometrical focusing.

Figure 1

A top-down schematic of the experimental setup showing the target, laser, and electron spectrometer. A 3D drawing of the CPC, tantalum substrate, and the incoming laser is also shown.

Figure 2

(a) A sample focal spot on a logarithmic intensity scale. The outer white line represents the acceptance aperture of the CPC, and the inner green circle is the tip of the CPC. The blue circle represents the radius at which 50% of the energy is enclosed. (b) The geometry of the CPC used on the experimental campaign. The opening aperture is 805 $\mu \mathrm{m}$, and the tip is 65 $\mu \mathrm{m}$. (c) A sample of the electron spectra retrieved from different individual shots using the electron spectrometer on CPCs (green) or planar (blue) targets. The dashed lines represent the regions where the electron temperature is fitted. There is a clear enhancement in electron spectra when using CPC targets. (d) The hot-electron temperature from planar and CPC targets plotted against the incident laser intensity. Lower-intensity points correspond to the pulse duration scan that was performed. Both planar and CPC targets follow a ponderomotive scaling, with the CPCs having a factor of 9 enhancement. The shaded region represents the enhanced electron temperature using the ponderomotive scaling and a purely geometric focusing that increases intensity.

Figure 3

(a) Hot-electron spectra from 2D planar, 2D CPC, and 3D CPC PIC simulations. The dashed lines represent temperature fits to spectra. (b) The intensity and hot-electron temperature extracted from the 2D and 3D PIC simulations. The intensity enhancements due to the focusing nature of the CPCs directly affects the temperature of the accelerated electrons.

Figure 4

Intensity profiles from the (a) 2D and (b) 3D PIC simulations respectively for the same tip size (5 $\mu \mathrm{m}$). The intensity achieved in the 3D simulation is greater than that achieved in the 2D simulation due to the extra focusing dimension.

Figure 5

Electron spectra produced when the laser is pointed in the middle and with 15 and 25 $\mu \mathrm{m}$ displacement in a 2D PIC simulation. The inset shows the change in angular distribution of electrons with energies greater than 1 MeV. The arrows represent the directions of the experimental detection of the electron spectrum.