Laser prepulse dependency of proton-energy distributions in ultraintense laser-foil interactions with an online time-of-flight technique

Yogo, Akifumi; Daido, Hiroyuki; Fukumi, Atsushi*; Li, Z.*; Ogura, Koichi; Sagisaka, Akito; Pirozhkov, A. S.; Nakamura, Shu*; Iwashita, Yoshihisa*; Shirai, Toshiyuki*; Noda, Akira*; Oishi, Yuji*; Nayuki, Takuya*; Fujii, Takashi*; Nemoto, Koshichi*; Choi, I. W.*; Sung, J. H.*; Ko, D.-K.*; Lee, J.*; Kaneda, Minoru*; Ito, Akio*

Fast protons are observed by a newly-developed time-of-flight spectrometer, which provides proton-energy distributions immediately after the irradiation of a laser pulse having an intensity of W/cm 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 to 10 W/cm. 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 makes a beneficial effect on 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.