Bright 5 - 85 MeV Compton gamma-ray pulses from GeV laser-plasma accelerator and plasma mirror

We convert a GeV laser-plasma electron accelerator into a compact femtosecond-pulsed $\gamma$-ray source by inserting a $100\mu$m-thick glass plate $\sim 3$ cm after the accelerator exit. With near-unity reliability, and requiring only crude alignment, this glass plasma mirror retro-reflected spent drive laser pulses (photon energy $\mathrm{\hslash }{\omega }_L=1.17$ eV) with $>50\mathrm{\%}$ efficiency back onto trailing electrons (peak Lorentz factor $1000 < \gamma_e < 4400$),creatinganopticalundulatorthatgenerated$\sim10^8 \gamma$-ray photons with sub-mrad divergence, estimated peak brilliance $\sim {10}^{21}$ photons/s/mm${}^2$/mrad${}^2$/$0.1\mathrm{\%}$ bandwidth and negligible bremsstrahlung background. The $\gamma$-ray photon energy $E_\gamma = 4\gamma_e^2 \hbar\omega_L$,inferredfromthemeasured$\gamma_e$ on each shot, peaked from 5 to 85 MeV, spanning a range otherwise available with comparable brilliance only from large-scale GeV-linac-based high-intensity $\gamma$-ray sources.