Search

With the recent advances in ultrahigh intensity lasers, exotic astrophysical phenomena can be investigated in laboratory environments. Collisionless shock in a plasma, prevalent in astrophysical events, is produced when a strong electric or electromagnetic force induces a shock structure in a time scale shorter than the collision time of charged particles. A near-critical-density (NCD) plasma, generated with an intense femtosecond laser, can be utilized to excite a collisionless shock due to its efficient and rapid energy absorption. We present electrostatic shock acceleration (ESA) in experiments performed with a high-density helium gas jet, containing a small fraction of hydrogen, irradiated with a 30 fs, petawatt laser. The onset of ESA exhibited a strong dependence on plasma density, consistent with the result of particle-in-cell simulations on relativistic plasma dynamics. The mass-dependent ESA in the NCD plasma, confirmed by the preferential reflection of only protons with two times the shock velocity, opens a new possibility of selective acceleration of ions by electrostatic shock. [ABSTRACT FROM AUTHOR]

Abstract: The evolution of electron beam injection and acceleration in the bubble regime of a laser wakefield is studied experimentally. When a 50TW, 35fs laser pulse is focused to a spot size of ∼22μm (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 (<∼1cm). However, only one bunch is generated when the channel is short (∼2mm). These experimental results confirm the evolution scenarios of the electron bunches observed in 3D PIC simulations [refer to the publications: N. Hafz et al., Nat. Photon. 2 (2008) 571 and 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. [Copyright &y& Elsevier]