  Collision between relativistic electron sheets and counter-propagating laser
pulses is recognized as a promising way to produce intense attosecond X-rays
through coherent Thomson backscattering (TBS). In a double-layer scheme, the
electrons in an ultrathin solid foil are first pushed out by an intense laser
driver and then interact with the laser reflected off a second foil to form a
high-density relativistic electron sheet with vanishing transverse momentum.
However, the repulsion between these concentrated electrons can increase the
thickness of the layer, reducing both its density and subsequently the coherent
TBS. Here, we present a systematic study on the evolution of the flying
electron layer and find that its resulting thickness is determined by the
interplay between the intrinsic space-charge expansion and the velocity
compression induced by the drive laser. How the laser driver, the target areal
density, the reflector and the collision laser intensity affect the properties
of the produced X-rays is explored. Multi-dimensional particle-in-cell
simulations indicate that employing this scheme in the nonlinear regime has the
potential to stably produce soft X-rays with several GW peak power in hundreds
of TW ultrafast laser facilities. The pulse duration can be tuned to tens of
attoseconds. This compact and intense attosecond X-ray source may have broad
applications in attosecond science.
