Title: Generation of Quasi-monoenergetic Protons by Laser Acceleration

Speaker: Xi Shao, Ph.D., University of Maryland, College Park, USA

Time and place: 2:00pm, May 17th, 2013

Abstract:

The acceleration of ions by laser irradiation of targets has been pursued actively via experiments, theory and simulations. In previous studies of the generation of highly energetic ion beams from laser-plasma interactions, foil targets with thicknesses ranging from a few to several tens of laser wavelengths have been employed, and target normal sheath acceleration was the predominant mechanism leading to the production of multi-tens of MeV ion beams. Recent studies of laser radiation pressure acceleration (RPA) of ultra-thin solid foil target shows promising aspects for efficient quasi-monoenergetic proton acceleration. The original scheme of RPA requires the target of suitable sub-wavelength thickness so that the whole foil trapping protons in it can be accelerated as a “light sail”. However, it is demonstrated with two-dimensional (2D) particle-in-cell (PIC) simulations that the Rayleigh-Taylor instability can limit the acceleration achieved by RPA and undesirably broaden the proton beam’s energy spectrum, so that fewer protons carry the desired energy [Liu et al., 2011, 2012]. Our recent work [Liu et al., 2013] found a new laser acceleration scheme using thin multi-ion foils to remedy the Rayleigh-Taylor instability and further accelerate the protons. The multi-ion thin foil can be made of carbon and hydrogen and the proton/carbon concentration needs to be below a critical ratio so that a self-organized triple layer of the electrons and the two different ion species can be formed. The acceleration of protons is boosted through a combination of laser radiation pressure acceleration and Coulomb repulsion of the protons by the carbon ion layer. The carbon layer helps to maintain the quasi-monoenergetic proton layer by delaying the disruption due to the Rayleigh–Taylor instability, and to accelerate the protons further by the electron-shielded Coulomb repulsion. The combined RPA and Coulomb repulsion acceleration therefore allows a much longer duration than the acceleration using single-ion hydrogen foils, and PIC simulations using modest laser power show a resulting quasi-monoenergetic proton energy about 7-8 times higher than using a single-ion hydrogen foil. Discussions on our recent work of laser thin gas target acceleration [He et al., 2012] and potential application of accelerated protons with good beam quality and a narrow energy spectrum in radiation therapy will also be presented.