Stable laser–plasma accelerators at low densities

We report stable laser wakefield acceleration using 17–50 TW laser pulses interacting with 4 mm-long helium gas jet. The initial laser spot size was relatively large (28 μm) and the plasma densities were 0.48–2.0 × 1019 cm−3. High-quality 100–MeV electron beams were generated at the plasma density of 7.5 × 1018 cm−3, at which the beam parameters (pointing angle, energy spectrum, charge, and divergence angle) were measured and stabilized. At higher densities, filamentation instability of the laser-plasma interaction was observed and it has led to multiple wakefield accelerated electron beams. The experimental results are supported by 2D particle-in-cell simulations. The achievement presented here is an important step toward the use of laser-driven accelerators in real applications.