  A strongly mismatched regime of self-guided nonlinear laser-plasma
acceleration in the bubble regime is modeled for optimization of Laser to
Particle energy efficiency with application to recently proposed laser positron
accelerator. The strong mismatch, in contrast with the matched condition,
arises from the incident laser spot-size being much larger than that needed for
equilibration of the laser ponderomotive and electron-ion charge-separation
force in the plasma bubble. This is shown to be favorable for optimization of
large self-injected electron charge and ultra-low transverse emittance. The
prominent signatures of the mismatched regime, strong optical-shock excitation
and bubble elongation, are validated using multi-dimensional Particle-In-Cell
simulations. This work thus uncovers a generalized regime that is shown to have
been favored by many laser-plasma acceleration experiments and opens a novel
pathway for a wide-range of future applications.
