15 MeV quasimonoenergetic electrons at 1 kHz with circularly polarized few-cycle pulses*

We compare the acceleration of quasi-monoenergetic electron bunches to 15 MeV using linearly (LP) and circularly polarized (CP) < 3 mJ, 5 fs pulses from a kHz laser system. Our previous laser wakefield acceleration (LWFA) experiments in the self-modulated regime, using 30-50 fs pulses, demonstrated that near-critical plasma densities reduced the needed power for relativistic self-focusing and enabled mJ-scale laser pulses to accelerate electrons to energies of a few MeV in a thermal spectrum [1, 2]. Our recent experiments, using few-cycle duration laser pulses driving wakefield bubbles at near-critical plasma densities, demonstrate acceleration of quasi-monoenergetic electron bunches to 15 MeV with low angular divergence of 7 mrad [3]. This result is obtained by mitigating the effect of carrier envelope phase (CEP) slip [4,5], due to the difference in group and phase velocity of the laser pulse in plasma, which can drive transverse oscillations of the bubble in the polarization plane and increase electron bunch divergence. We show that use of CP minimizes CEP-induced beam divergence compared to LP. [1]F. Salehi et al., Opt. Lett. 42, 215 (2017). [2]A. J. Goers et al. , Phys. Rev. Lett. 115, 194802 (2015). [3]F. Salehi et al., Phys. Rev. X 11, 021055 (2021) [4]E. N. Nerush and I. Yu. Kostyukov, Phys. Rev. Lett. 103, 035001 (2009). [5]J. Huijts et al.,  Phys. Plasmas 28, 043101 (2021)