We demonstrate a highly tunable, controlled injection laser-plasma-accelerator (LPA) through systematically varying parameters of a density shock injector. Beam energy, energy spread, charge and pointing can be controlled in the range of 50-200 MeV, with <10% energy spread, ~1.5 mrad divergence and < 1mrad pointing fluctuation. The beams are repeatable, suitable for high quality MeV Thomson photon sources or for injectors to staged systems.
Applications including compact production of near-monoenergetic x-rays by Thomson scattering and injectors for staged systems motivate high quality, reproducible electron bunches from laser-plasma-accelerators (LPAs). Thomson sources for example could open a range of applications including ultrafast x-ray science, radiation therapy, and homeland security. Recent Thomson source experiments based on ionization injection LPA beams have been reported . However, energy spread, collimation and repeatability of the electron beam have room to improve. Here we report a highly controllable, tunable LPA based on a shock-induced density down ramp injection. The LPA has energy spreads below 10% FWHM, divergence of 1.5mrad FWHM and central energy up to 200MeV. Calculation based on Ref.  show that hard (940 KeV) x-ray photons can be scattered from the optimized LPA beam with a 810nm laser pulse.
For these experiments, A 45fs, 1.8J, 810nm laser from the BELLA TW system was focused to a0 = 1.5 onto a 1-mm supersonic gas jet. A position-controlled razor blade created a sharp density transition over the jet. The charge, divergence, pointing, and energy spectra can be controlled through tuning shock position and angle, jet pressure, focus position and laser mode optical quality. As an example, stable acceleration with narrow divergence (1.5mrad) and energy spread (< 10%) was achieved over a range of straightened shock angle by clocking the jet/shock system. The laser mode was also found to play a significant role in e-beam quality. Details of parameter dependence allowing control of the accelerated beam will be presented. Correspondence of results to heuristic models will be discussed. The generation of electron beams with low energy spread, ~200MeV energies will benefit the prospects of high quality MeV Thomson photon sources and HEP injectors.
Work supported by the US DOE under Contract No. DE-AC02-05CH11231, and by the US DOE National Nuclear Security Administration, Defense Nuclear Nonproliferation R&D (NA22).
 Chen et al., Phys. Rev. Lett. 110 (2013)
 Tsai et al., Phys. Plasmas 22 (2015)
 Esarey et al., Phys. Rev. E 48 (1993)
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