MeV-Energy X Rays from Inverse Compton Scattering with Laser-Wakefield Accelerated Electrons

We report the generation of MeV x rays using an undulator and accelerator that are both driven by the same 100-terawatt laser system. The laser pulse driving the accelerator and the scattering laser pulse are independently optimized to generate a high energy electron beam ($>200\mathrm{MeV}$) and maximize the output x-ray brightness. The total x-ray photon number was measured to be $\sim 1\times {10}^7$, the source size was $5\mu \mathrm{m}$, and the beam divergence angle was $\sim 10\mathrm{mrad}$. The x-ray photon energy, peaked at 1 MeV (reaching up to 4 MeV), exceeds the thresholds of fundamental nuclear processes (e.g., pair production and photodisintegration).

Figure 1

All-laser-driven Compton $\gamma$-ray source. Schematic of the experimental setup. Left inset shows the electron energy spectrum as recorded by the LANEX screen, which was imaged by CCD camera. Right inset shows the $\gamma$-ray beam profile measured by pixelated CsI(Tl) scintillator imaged by EMCCD camera.

Figure 2

Interaction geometry defining beam parameters.

Figure 3

$\gamma$-ray spectrum. (a) Simulated on axis $\gamma$-ray spectrum, with corresponding $e$-beam spectrum (inset). (b) Simulated $\gamma$-ray beam profile transmitted through quad filter (not attenuated at top-left corner, 1.7 mm lead at top-right, 3.4 mm lead at bottom-left, 5.1 mm lead at bottom-right). (c) Measured $\gamma$-ray beam profile transmitted through quad filter. (d) Comparison between simulated and measured $\gamma$-ray transmittances through different filters, as a function of divergence angle. The solid line represents the simulated transmittance and squares represent the measured transmittance.

Figure 4

$\gamma$-ray and electron radiation source size measurements. Normalized $\gamma$-ray yield as a function of the laser pulse vertical position. Each data point represents the $\gamma$-ray yield obtained by integrating over a single-shot background-subtracted $\gamma$-ray beam profile image. The solid line is a Gaussian fit with an rms width of $11\pm 1\mu \mathrm{m}$. By deconvolving the cross-correlation trace, we estimate the rms width of the $e$-beam size at the interaction point to be $6\pm 3\mu \mathrm{m}$.