Bulk acceleration of ions in intense laser interaction with foams

It is suggested that bulk acceleration of ions can occur in a target with density discontinuities when hot electrons are transported through the target. A foam target just belongs to such a kind of target, which is composed of irregular lamellar layers distributed randomly. To simplify the problem, we study the interaction of a high intensity laser pulse with a target consisting of regular micro-thin layers separated with a thickness of around a micrometre. Particle-in-cell simulations suggest that localized electrostatic fields with multi-peaks around the surfaces of the thin layers inside are induced when fast electrons produced are transported through such a target. These fields inhibit hot electron transport and simultaneously accelerate ions from the thin layers inside the target, forming a bulk acceleration in contrast to the surface acceleration at the front and rear sides of a thin solid target. Bulk acceleration can produce a large number of ions of moderate energy, which may be useful for applications such as fast ignition by fast protons. Experimental evidence of bulk acceleration is found with low-density foams irradiated by ultra-intense laser pulses.