Duration-controlled amplified spontaneous emission with an intensity of 10(13) W/cm(2) is used to convert a 7.5-microm -thick polyimide foil into a near-critical plasma, in which the p -polarized, 45-fs , 10(19) -Wcm (2) laser pulse generates 3.8-MeV protons, emitted at some angle between the target normal and the laser propagation direction of 45 degrees . Particle-in-cell simulations reveal that the efficient proton acceleration is due to the generation of a quasistatic magnetic field on the target rear side with magnetic pressure inducing and sustaining a charge separation electrostatic field.

Absolute energy distribution of hard x rays produced in the interaction of a kilohertz femtosecond laser with tantalum targets Rev. Sci. Instrum. 77, 093302 (2006); Fast protons are observed by a newly developed online time-of-flight spectrometer, which provides shot-to-shot proton-energy distributions immediately after the irradiation of a laser pulse having an intensity of ϳ10 18 W/cm 2 onto a 5-m-thick copper foil. The maximum proton energy is found to increase when the intensity of a fs prepulse arriving 9 ns before the main pulse increases from 10 14 to 10 15 W/cm 2 . Interferometric measurement indicates that the preformed-plasma expansion at the front surface is smaller than 15 m, which corresponds to the spatial resolution of the diagnostics. This sharp gradient of the plasma has the beneficial effect of increasing the absorption efficiency of the main-pulse energy, resulting in the increase in the proton energy. This is supported by the result that the x-ray intensity from the laser plasma clearly increases with the prepulse intensity.

The dependence on laser intensity and pulse duration in energetic proton acceleration by irradiation of ultrashort laser pulses on a 5μm thick copper tape target was measured. The laser intensity was varied from 8.5×1017W∕cm2 to 1.1×1019W∕cm2, and the pulse duration from 55 fs to 400 fs. The maximum proton energy increased as the pulse duration was increased while the laser intensity was kept constant. The dependence of the maximum proton energy on laser intensity and pulse duration was in good agreement with an analytical plasma-expanding model.

A thin tape target driver for laser ion acceleration was developed. The driver can move a copper tape of 5 μm thickness with a positioning reproducibility of less than 30 μm (peak to valley), which is sufficient for a laser irradiation target. Using this tape target and laser pulses of energy 350 mJ and duration 60 fs, protons of energies of over 1 MeV were accelerated in the forward direction.

The results of experiments are presented for the single laser pulse interaction with a very low density gas target, under the conditions when the generated wake wave is below the wave-breaking threshold and the laser pulse power is lower than the critical power for relativistic self-focusing. A quasi-monoenergetic electron beam is found to be stably generated for various laser pulse intensity values by controlling the acceleration length. The results of two-dimensional particle-in-cell simulations show that for the electron acceleration an additional mechanism of electron injection into the acceleration phase is required. It is demonstrated that the longitudinal inhomogeneity of the plasma density leads to the electron injection.