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This paper presents the experimental investigation of laser-driven proton acceleration using a table top Ti:Sapphire laser system interacting with the thin-foil targets during the course of medical application of the laser-driven proton beam. The proton beam with maximum energy of upto 14~MeV is generated in 60 TW mode. The number of protons at ~10 MeV is estimated to be over 105proton/sr/MeV/shot with beam having half divergence angle of 5~degree. If 10 Hz operation is assumed 2 Gy dose is possible to irradiate during 10 min onto a ~1 mm tumor just under the skin. In contrast to the previous condition of our apparatus with which we demonstrated the DNA double-strand breaking by irradiating the laser-driven proton beam onto the human cancer cells in-vitrotest, the result reported here has significant meaning in the sense that pre-clinical in-vivotest can be started by irradiating the laser-driven proton beam onto the skin of the mouse, which is unavoidable step before the real radiation therapy.

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.

Neutron time of flight signals have been observed with a high resolution neutron spectrometer using the petawatt arm of the Vulcan laser facility at Rutherford Appleton Laboratory from plastic sandwich targets containing a deuterated layer. The neutron spectra have two elements: a high-energy component generated by beam-fusion reactions and a thermal component around 2.45?MeV. The ion temperatures calculated from the neutron signal width clearly demonstrate a dependence on the front layer thickness and are significantly higher than electron temperatures measured under similar conditions. The ion heating process is intensity dependent and is not observed with laser intensities on target below 1020?W?cm?2. The measurements are consistent with an ion instability driven by electron perturbations.