STABLE ELECTRON BEAMS WITH LOW ABSOLUTE ENERGY SPREAD FROM A LASER WAKEFIELD ACCELERATOR WITH PLASMA DENSITY RAMP CONTROLLED INJECTION∗

Laser wakefield accelerators produce accelerating gradi-ents up to hundreds of GeV/m, and recently demonstrated 1-10 MeV energy spread at energies up to 1 GeV using electrons self-trapped from the plasma. Controlled injec-tion and staging may further improve beam quality by cir-cumventing tradeoffs between energy, stability, and energy spread/emittance. We present experiments demonstrating production of a stable electron beam near 1 MeV with hundred-keV level energy spread and central energy sta-bility by using the plasma density profile to control self-injection, and supporting simulations. Simulations indicate that such beams can be post accelerated to high energies, potentially reducing momentum spread in laser accelera-tors by 100-fold or more