Ultrahigh Brilliance Multi-MeV $\gamma$-Ray Beams from Nonlinear Relativistic Thomson Scattering

We report on the generation of a narrow divergence (${\theta }_{\gamma }<2.5\mathrm{mrad}$), multi-MeV ($E_{\max }\approx 18\mathrm{MeV}$) and ultrahigh peak brilliance ($>1.8\times {10}^{20}\mathrm{photons}{\mathrm{s}}^{-1}{\mathrm{mm}}^{-2}{\mathrm{mrad}}^{-2}$ 0.1% BW) $\gamma$-ray beam from the scattering of an ultrarelativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude $a_0\approx 2$). The spectrum of the generated $\gamma$-ray beam is measured, with MeV resolution, seamlessly from 6 to 18 MeV, giving clear evidence of the onset of nonlinear relativistic Thomson scattering. To the best of our knowledge, this photon source has the highest peak brilliance in the multi-MeV regime ever reported in the literature.

Figure 1

Sketch of the experimental setup: a powerful laser pulse (Driver) is focussed by an $F/20$ OAP at the edge of a gas cell to generate an ultrarelativistic electron beam (first box: Laser-driven $\mathit{e}$-acceleration). A second laser beam (Wiggler) is focussed by a holed $F/2$ OAP counterpropagating to the electron beam (second box: High intensity int.). After interaction, the electron beam is deflected by a strong pair of magnets onto a LANEX scintillator screen (third box: Electron spectrometry) while the generated $\gamma$-ray beam propagates up to a Li-based spectrometer (fourth box: $\gamma$-ray detection). An $F/15$ hole in the $F/2$ OAP ensures unperturbed propagation of the scattered electron beam and generated $\gamma$-ray beam onto the detector, and minimizes backreflection of the laser in the amplification chain.

Figure 2

(a),(b) Electron spectra used for the first and second series of experiments, respectively. In frame (a), the dashed red line represents the approximated electron spectrum used as an input for the theoretical calculations. (c) Measured intensity distribution of the laser used for scattering (Wiggler in Fig. 1).

Figure 3

(a) Green band: $\gamma$-ray spectrum as measured during the interaction of the laser-driven electron beam [spectrum depicted in Fig. 2] with the high-intensity focal spot of a secondary laser beam [spatial distribution shown in Fig. 2]. The band represents the uncertainty associated with the experiment, as mainly resulting from the spectral resolution of the $\gamma$-ray spectrometer and the response of the detector. Solid and dashed brown lines depict theoretical expectations for the same electron and laser parameter as the experimental ones but with $a_0=2$ and $a_0=1$, respectively. (b) The green line represents the measured spectrum for optimized electron-laser overlap [same as green band in frame (a)], whereas dashed curves depict the measured spectra if an artificial misalignment of $\pm 20\mu \mathrm{m}$ is introduced. (c) The green line represents the measured spectrum for optimized electron-laser synchronization [for an electron spectrum as the one in Fig. 2] whereas dashed curves depict the measured spectra if an artificial desynchronization of $\pm 100\mathrm{fs}$ is introduced.

Figure 4

Comparison of the present $\gamma$-ray source (solid red circles) with other generation mechanisms reported in the literature: free-electron laser (PETRA III, black crosses, [17]), $k\text{−}\alpha$ (orange diamonds, [34]), solid-state undulators (ESRF, green crosses [35] and PSI, blue crosses [36]), betatron radiation (light blue crosses [18] and light green crosses [19]), bremsstrahlung radiation (green stars [10] and brown stars [5]), nonlinear relativistic Thomson scattering (NLTS, dark purple squares [24]), and linear relativistic Thomson scattering (LTS, yellow squares [20], grey squares [21], and light purple square [23]). Brilliance is expressed in units of photons ${\mathrm{s}}^{-1}{\mathrm{mm}}^{-2}{\mathrm{mrad}}^{-2}$ 0.1% BW.