  High frequency modulations appearing in the kinetic energy distribution of
laser accelerated ions are proposed for retrieving the acceleration field
dynamics at femtosecond time scale. Such an approach becomes possible if the
laser-cycling field modulates the particle density in the ion spectra and
produces quasi time stamps for analysis. We investigate target and laser
parameters determining this effect and discuss the dependencies of the observed
modulation. Our findings refine a basic mechanism, the Target Normal Sheath
Acceleration, where an intense and ultrafast laser pulse produces a very strong
electrical field at a plasma-vacuum interface. The field decays rapidly due to
energy dissipation and forms a characteristic spectrum of fast ions streaming
away from the interface. We show that the derived decay function of the field
is in accordance with model predictions of the accelerating field structure.
Our findings are supported by 2-dimensional particle-in-cell simulations. The
knowledge of the femtosecond field dynamics helps to rerate optimization
strategies for laser ion acceleration.
