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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 $textit{online}$ time-of-flight spectrometer, which provides $textit{shot-to-shot}$ proton-energy distributions immediately after the irradiation of a laser pulse having an intensity of $$sim 10^{18}$$ W/cm$$^2$$ onto a 5-$$mu$$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 $$mu$$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.

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Category:Physics, Fluids & Plasmas

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