• M. Shverdin, F. Albert, S.G. Anderson, C.P.J. Barty, A.J. Bayramian, M. Betts, T.S. Chu, C.A. Ebbers, D.J. Gibson, F.V. Hartemann, R.A. Marsh, D.P. McNabb, M. J. Messerly, H.H. Phan, M.A. Prantil, C. Siders, S.S.Q. Wu
  LLNL, Livermore, California

Generation of mono-energetic, high brightness gamma-rays requires state of the art lasers to both produce a low emittance electron beam in the linac and high intensity, narrow linewidth laser photons for scattering with the relativistic electrons. Here, we overview the laser systems for the 3rd generation Monoenergetic Gamma-ray Source (MEGa-ray) currently under construction at Lawrence Livermore National Lab. We also describe a method for increasing the efficiency of laser Compton scattering through laser pulse recirculation. The fiber-based photoinjector laser will produce 50 uJ temporally and spatially shaped UV pulses at 120 Hz to generate a low emmittance electron beam in the X-band RF photoinjector. The interaction laser generates high intensity photons that focus into the interaction region and scatter off the accelerated electrons. This system utilizes chirped pulse amplification and commercial diode pumped solid state Nd:YAG amplifiers to produce 0.5 J, 10 ps, 120 Hz pulses at 10⁶⁴ nm and up to 0.2 J after frequency doubling. A single passively mode-locked Ytterbium fiber oscillator seeds both laser systems and provides a timing synch with the linac.