Radial focusing and energy compression of a laser-produced proton beam by a synchronous rf field

Ikegami, Masahiro*; Nakamura, Shu*; Iwashita, Yoshihisa*; Shirai, Toshiyuki*; Soda, Hikaru*; Tajima, Yujiro*; Tanabe, Mikio*; Tongu, Hiromu*; Ito, Hiroyuki*; Shintaku, Hiroki*; Yamazaki, Atsushi*; Daido, Hiroyuki; Yogo, Akifumi; Orimo, Satoshi; Mori, Michiaki; Nishiuchi, Mamiko; Ogura, Koichi; Sagisaka, Akito; Pirozhkov, A. S.; Kiriyama, Hiromitsu; Kanazawa, Shuhei; Kondo, Shuji; Yamamoto, Yoichi*; Shimomura, Takuya; Tanoue, Manabu*; Nakai, Yoshiki; Akutsu, Atsushi; Bulanov, S. V.; Kimura, Toyoaki; Oishi, Yuji*; Nemoto, Koshichi*; Tajima, Toshiki; Noda, Akira*

The dynamics of a MeV laser-produced proton beam affected by a radio frequency (rf) electric field has been studied. The proton beam was emitted normal to the rear surface of a thin polyimide target irradiated with an ultrashort pulsed laser with a power density of 410 W/cm. The energy spread was estimated to be compressed to less than 11% at the full width at half maximum (FWHM) by an rf field. Focusing and defocusing effects of the transverse direction were also observed. These effects were analyzed and reproduced by Monte-Carlo simulations. The simulation results show that the transversely focused protons had a broad continuous spectrum, while the peaks in the proton spectrum were defocused. Based on this new information, we propose that a further improvement of the quality of laser produced protons is possible by utilizing a focal length difference between the continuous spectral protons and the protons included in the spectral peak.