  Direct Laser Acceleration (DLA) of electrons during a high-energy, picosecond
laser interaction with an underdense plasma has been demonstrated to be
substantially enhanced by controlling the laser focusing geometry. Experiments
using the OMEGA EP facility measured electrons accelerated to maximum energies
exceeding 120 times the ponderomotive energy under certain laser focusing,
pulse energy, and plasma density conditions. Two-dimensional particle-in-cell
simulations show that the laser focusing conditions alter the laser field
evolution, channel fields generation, and electron oscillation, all of which
contribute to the final electron energies. The optimal laser focusing condition
occurs when the transverse oscillation amplitude of the accelerated electron in
the channel fields matches the laser beam width, resulting in efficient energy
gain. Through this observation, a simple model was developed to calculate the
optimal laser focal spot size in more general conditions and is validated by
experimental data.
