Dynamics of electric fields driving the laser acceleration of multi-MeV protons

We present the first direct experimental measurement of the electric fields driving the acceleration of high energy protons from a thin foil irradiated by an intense (I $\sim $ 3.5 $\times $ 10$^{18}$ W/cm$^{2})$ and short (t$_{l} \quad \sim $ 1.5 ps) laser pulse. The measurement was performed employing an auxiliary laser-accelerated proton beam as a transverse charged particle probe. The initial sheath field at the target-vacuum interface and the predicted late time peak of the accelerating field at the expanding ion front are observed. The experimental results are in good agreement with Particle In Cell and fluid simulations of the expansion of a thin plasma into a vacuum.