We characterize the electron density distributions of preformed plasma for laser-accelerated proton generation. The preformed plasma of a titanium target 3 μm thick is generated by prepulse and amplified spontaneous emission (ASE) of a high-intensity Ti:sapphire laser and is measured with an interferometer using a second harmonic probe beam. High-energy protons are obtained by reducing the size of the preformed plasma by changing the ASE duration before main pulse at the front side (laser incidence side) of the target. Simulation results with two-dimensional radiation hydrodynamic code are close to the experimental results for low-density region ~4 × 1019 cm−3 at the front side. In the high-density region near to the target surface, the interferometry underestimates the density due to the substantial refraction. The characterization of hydrodynamic expansion with the interferometer and simulation is a useful tool for investigation of high-energy proton generation.

We observed a preformed plasma of an aluminum slab target produced by a high-intensity Ti:sapphire laser. The expansion length of the preformed plasma at the electron density of 3 × 1018 cm−3, which was the detection limit, was around 100 μm measured with a laser interferometer. In order to characterize quantitatively and to control the preformed plasmas, we perform a two-dimensional hydrodynamic simulation. The expansion length of the preformed plasma was almost the same as the experimental result, if we assumed that the amplified spontaneous emission lasted 3.5 ns before the main pulse arrived.

High-energy protons are generated by focusing an ultrashort pulsed high intensity laser at the Advanced Photon Research Center, JAERI-Kansai onto thin (thickness <10 μm) Tantalum targets. The laser intensities are about 4 × 1018 W/cm2. The prepulse level of the laser pulse is measured with combination of a PIN photo diode and a cross correlator and is less than 10−6. A quarter-wave plate is installed into the laser beam line to create circularly polarized pulses. Collimated high energy protons are observed with CH coated Tantalum targets irradiated with the circularly polarized laser pulses. The beam divergence of the generated proton beam is measured with a CR-39 track detector and is about 6 mrad.