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

池上 将弘*; 中村 衆*; 岩下 芳久*; 白井 敏之*; 想田 光*; 田島 裕二郎*; 田辺 幹夫*; 頓宮 拓*; 伊東 宏之*; 新宅 宏基*; et al.

Physical Review Special Topics; Accelerators and Beams, 12(6), p.063501_1 - 063501_6, 2009/06

https://doi.org/10.1103/PhysRevSTAB.12.063501

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.