Evolution of self-injected quasi-monoenergetic electron beams in a plasma bubble

Abstract

The evolution of electron beam injection and acceleration in the bubble regime of a laser wakefield is studied experimentally. When a 50 TW, 35 fs laser pulse is focused to a spot size of ∼22 μm (FWHM) in a tenuous 1-cm-long helium gas jet, well-collimated (a few milliradians) electron beams with energies in the sub-GeV level are produced. Multiple electron bunches are observed when the laser–plasma channel (i.e., interaction length) is long (<∼1 cm). However, only one bunch is generated when the channel is short (∼2 mm). These experimental results confirm the evolution scenarios of the electron bunches observed in 3D PIC simulations [refer to the publications: [6] N. Hafz et al., Nat. Photon. 2 (2008) 571 and [7] S. Kalmykov et al., Phys. Rev. Lett. 103 (2009) 135004], which suggested that multiple electron injections are a characteristic process for a long acceleration length and an evolving plasma bubble.