Intense laser pulse interaction with a nanofoil and generation of energetic protons in underdense upstream plasma.

An analytical formalism is developed to study the radiation pressure hole boring ion acceleration of a CH foil in an underdense upstream plasma. The laser quickly converts the foil into an overdense plasma and exerts a strong forward ponderomotive force on the electrons. The electrons are pushed to the rear and detach from the ions to form a thin electron layer, followed by a hydrogen ion layer and a distant carbon ion layer. The triple layer of electrons, protons and carbon ions is accelerated by the radiation pressure; the space charge field plays an intermediary role in accelerating the ions. In the frame of the electron layer, moving with velocity v f z ^ , the upstream protons appear to be moving backward with velocity - v f z ^ . They get reflected back in the fast moving intense space charge field of the triple layer, acquiring velocity v f z ^ . In the lab frame, these protons have a velocity nearly twice the velocity of the electron layer. For a Gaussian (in time) laser pulse, a parabolic density profile of the upstream plasma is seen to produce a proton beam with narrow energy spread. For a normalized laser amplitude a 0 = 10 , one may obtain 180 MeV protons in a parabolically decreasing density profile of scale length 40 μ m. [ABSTRACT FROM AUTHOR]