Of these ndings passoni et al 2009 passoni bertagna

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Unformatted text preview: ling of proton energy cutoff with laser power and pulse duration. Squares are data from experiments performed with the DRACO laser at FZD (Dresden), showing a linear scaling with power in the short-pulse (30 fs) regime. Other points are data from other laboratories; see Zeil et al. (2010) for references and details. The fitting lines correspond to the static model by Schreiber et al. (2006) with different colors (labels) corresponding to different values of the pulse duration 1 as given in the legend. From Zeil et al., 2010. 770 Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . E. Experimental optimization After the first years of research, the combined vigorous development in both laser technology and advanced target manufacturing allowed the investigation of TNSA exploring a continuously increasing range of laser and target parameters. In most cases the two sets of parameters are intimately related. For example, the use of ‘‘extreme’’ geometrical target properties, such as thicknesses in the submicrometric range, requires the availability of extraordinarily clean, prepulse-free pulses to avoid early target evaporation and deformation. Such pulses can be obtained with recently developed techniques, such as plasma mirrors (Dromey et al., 2004; Fuchs et al., 2006a; Thaury et al., 2007, and references therein), optical parametric amplification (Shah et al., 2009), or crossed polarized wave (XPW) generation (Jullien et al., 2005; Zaouter et al., 2011, and references therein). 1. Energy cutoff enhancement Mackinnon et al. (2002) studied the dependence of ion acceleration on the target thickness, with the aim of addressing the role played by the electron temporal dynamics and its effect on the formation of the accelerating sheath electric field. The experimental results showed an increase in the peak proton energy from 6.5 to 24 MeV when the thickness of the Al foil target was decreased from 100 to 3 m. These data clearly indicate that an increase in the target thickness implies a lower mean density of the hot electrons at the surface and a consequent lowering of the peak proton energy. The influence of the laser prepulse due to amplified spontaneous emission (ASE) on the acceleration of protons in thin-foil experiments has been investigated in detail by Kaluza et al. (2004). In this experiment Al foils of different thickness (from 0.75 to 86 m) were used in the presence of an ASE prepulse whose duration could be controllably varied. The results indicated an optimal value for the target thickness, strongly depending on the prepulse duration, at which the TNSA process leads to the highest proton energies. For thinner targets, a prepulse-induced plasma formation at the rear side effectively suppressed TNSA, in agreement with the considerations developed in Secs. II.C and III.A. Related experimental work, where a wide range of laser parameters and different target materials have been considered, can be...
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