Compact tunable Compton x-ray source from laser wakefield accelerator and plasma mirror

Compton backscatter (CBS) x-rays have been generated from laser wakefield accelerator (LWFA) electron beams by retro-reflecting the LWFA drive pulse with a plasma mirror (PM) [1] and by backscattering a secondary pulse split from the driver pulse [2]. However, tunable quasi-monoenergetic CBS x-rays have been produced only by the latter method, which requires challenging alignment. Here we demonstrate quasi-monoenergetic ($\sim$ 50{\%} FWHM), bright (5 x 10$^{6}$ photon per shot) CBS x-rays with central energy tunability from 75 KeV to 200 KeV by combining a PM with a tunable LWFA. 30 TW, 30-fs (FWHM), laser pulses from the UT$^{3}$ laser system were focused (f/12) to spot diameter 11 micron, intensity $\sim$ 6x10$^{18}$ W/cm$^{2}$ (a$=$1.5) at a 1-mm long Helium gas jet, yielding quasi-monoenergetic relativistic electrons. A thin plastic film near the gas jet exit efficiently retro-reflected the LWFA driving pulse into oncoming electrons to produce CBS x-rays without detecting bremsstrahlung background. By changing gas jet backing pressure, electron energy was tuned from 60 to 90 MeV, thereby tuning the CBS x-ray energy, which was determined by measuring transmission through a metal filter pack. The x-ray beam profiles recorded on an image plate had 5-10-mrad divergence. \\[4pt] [1] K. Ta Phuoc \textit{et al}., \textit{Nature Photonics} \textbf{6}, 308 (2012).\\[0pt] [2] N.D. Power et al., \textit{Nature Photonics }\textbf{8}, 28--31(2014).