The interaction of high-intensity laser pulses with matter releases instantaneously ultra-large currents of highly energetic electrons, leading to the generation of highly-transient, large-amplitude electric and magnetic fields. We report results of recent experiments in which such charge dynamics have been studied by using proton probing techniques able to provide maps of the electrostatic fields with high spatial and temporal resolution. The dynamics of ponderomotive channeling in underdense plasmas have been studied in this way, as also the processes of Debye sheath formation and MeV ion front expansion at the rear of laser-irradiated thin metallic foils. Laser-driven impulsive fields at the surface of solid targets can be applied for energy-selective ion beam focusing.

We propose a focusing mechanism of high-energy ions by an electron cloud produced by a laser interaction with slab plasma. In our 2.5-dimensional (2.5D) particle-in-cell simulations, the laser intensity is 2 × 1020 W/cm2, the laser wavelength λ is 1.053 μm, and the laser spot size is 2.5λ. When the high intensity laser irradiates slab plasma, electrons are accelerated, oscillate around the plasma and produce the electron cloud locally at the sides of the plasma. Because the electrons are localized transversely, a static electric potential is formed to focus ions and at the same time the ions are accelerated longitudinally. Though the longitudinal ion acceleration has been studied well, the ion focusing effect is reported for the first time in this paper. In our calculations, the maximum energy and intensity of the protons are 8.61 MeV and 1.89 × 1017 W/cm2, and the diameter of the proton bunch accelerated are focused to 71.2% of its initial size.