LAUSR

A novel practical and efficient way of obtaining intense attosecond pulses is proposed, where the near-critical-density (NCD) plasma target satisfying n0/a0nc ≈ 1 is used. The unique interaction dynamics in NCD plasmas have been identified theoretically and by particle-in-cell simulations, which show that three distinct dense electron nanobunches are formed each half a laser cycle and two of them can induce intense attosecond pulses in respectively the reflected and the transmitted directions by the so-called “coherent synchrotron emission” (CSE) mechanism [experimentally confirmed in Nat. Phys. 8, 804 (2012)]. Comparing with CSE in solids, here not only the required stringent conditions on laser and target are relaxed, but also the radiation intensities are enhanced by two orders of magnitude. It is shown that relativistically intense attosecond X-ray pulses with intensity 1019W/cm2 and duration ~50as can be robustly obtained in both directions by currently available driving lasers at intensities of 1020W/cm2.