  Intense laser-plasma interactions are an essential tool for the laboratory
study of ion acceleration at a collisionless shock. With two-dimensional
particle-in-cell calculations of a multicomponent plasma we observe two
electrostatic collisionless shocks at two distinct longitudinal positions when
driven with a linearly-polarized laser at normalized laser vector potential a0
that exceeds 10. Moreover, these shocks, associated with protons and carbon
ions, show a power-law dependence on a0 and accelerate ions to different
velocities in an expanding upstream with higher flux than in a single-component
hydrogen or carbon plasma. This results from an electrostatic ion two-stream
instability caused by differences in the charge-to-mass ratio of different
ions. Particle acceleration in collisionless shocks in multicomponent plasma
are ubiquitous in space and astrophysics, and these calculations identify the
possibility for studying these complex processes in the laboratory.
