Efficient production of a collimated MeV proton beam from a polyimide target driven by an intense femtosecond laser pulse

Abstract

High-flux energetic protons whose maximum energies are up to 4 MeV are generated by an intense femtosecond titanium:sapphire laser pulse interacting with 7.5, 12.5, and 25 mu m thick polyimide tape targets. Laser pulse with an energy of 1.7 J and with a duration of 34 fs is focused with an f/3.4 parabolic mirror giving an intensity of 3x10(19) W cm(-2). The main pulse to amplified spontaneous emission (ASE) intensity contrast ratio is 2.5x10(7). The conversion efficiency from the laser energy into the proton kinetic energies is achieved to be similar to 3%, which is comparable to or even higher than those achieved in the previous works; using nanometer-thick targets, in combination with the short-pulse lasers that have almost the same pulse width and the intensity but different main pulse to ASE intensity contrast of similar to 10(10) [Neely , Appl. Phys. Lett. 89, 021502 (2006)], in which the authors claim that the main mechanism is target normal sheath acceleration; or using the 7.5 mu m thick polyimide target, in combination with the short-pulse laser, which has almost the same pulse width and the intensity, but the main pulse to ASE intensity contrast ratio was controlled to be 2.5x10(5) [Yogo , Phys. Rev. E 77, 016401 (2008)], in which the authors claim the efficient acceleration by the mechanism of the underdense plasma model. The contrast ratio of the present experiment is in between these two experiments. The possible mechanism of this regime is discussed. (c) 2008 American Institute of Physics.