It is generally believed  that the matched laser profile provides the optimal regime of electron acceleration. The laser spot size and the pulse duration evolve a little during laser propagation when the laser intensity, the plasma density and the laser profile are matched. The scaling laws predicted the electron energy as a function of the laser-plasma parameters have been formulated for the matched bubble regime [1,2]. However it has been shown in the recent experiments and numerical simulations [3,4] that the energy of the electrons accelerated in the mismatch regime can exceed the energy predicted by these scaling laws. The mismatch regime remains largely unexplored but looks attractive to increase the energy of the accelerated electrons.
We present recent experimental data on LWFA in the mismatch regime. Up to 20 J, 60fs laser pulses are focused with f/40 focusing system on the input of a gas cell. The laser intensity in the 40 um focal spot is enough to exceed the self-injection threshold. The extracted experimental dependencies are supported by full-scale PIC modeling of LWFA acceleration and by numerical simulation of gas distribution inside the gas cell. The accuracy of spectra reconstruction affected by not perfect pointing stability of the accelerated electron beam are discussed. The electron spectra with cut-off energies beyond 1 GeV and beyond of the predictions of the similarity theory of the bubble regime are demonstrated.
 W. Lu et al., Phys. Rev. ST Accel. Beams 10, 061301(2007).
 S. Gordienko and A. Pukhov, Phys. Plasmas 12, 043109 (2005).
 K. Poder et al., Multi-GeV scale electron acceleration with self guided laser wakefield accelerators, 17th Advanced Accelerator Concepts Workshop.
 A.A. Sahai et al., Laser-driven plasma acceleration in a regime of strong-mismatch between the incident laser envelope and the nonlinear plasma response, arXiv: 1704.02913.
|Working group||Laser-driven electron acceleration|