RevModPhys.85.751

Hot electrons moving with a component of the velocity

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Unformatted text preview: f the acceleration of the most energetic ions. This transverse refluxing can result in a hotter, denser, and more homogeneous electron sheath at the target-vacuum interface. A significant increase in the maximum proton energy (up to threefold), as well as increased laser-to-ion conversion efficiency, can be obtained with these conditions, as shown in Fig. 21. Similar results, Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . FIG. 21 (color online). Experimentally observed (a) cutoff proton energies and (b) conversion efficiency (for >1:5 MeV protons) for 2 m thick Au targets as a function of surface area, evidencing the effect of electron refluxing. The laser pulse had 400 fs duration, 2 Â 1019 W cmÀ2 intensity, 45 incidence, and P polarization. From Buffechoux et al., 2010. obtained with different laser and target parameters, have been found by Tresca et al. (2011), who also measured an increase in the maximum energy of protons accelerated from the edges of the target with decreasing target area. Several other attempts have been made to increase the energy density of the hot electrons in the sheath and, consequently, the maximum proton energy. Following from the indications of Kaluza et al. (2004), McKenna et al. (2008) investigated whether there exists an optimum density profile at the front of the target which maximizes the laser absorption. The proton cutoff energy was increased by 25% with respect to a sharp interface case at ‘‘intermediate’’ plasma scale length (tens of m). Under such conditions, the higher conversion efficiency into fast electrons was attributed to self-focusing of the driver pulse. Other studies of controlled prepulse effects on ion acceleration have been reported by Flacco et al. (2008) and Batani et al. (2010). Recently, an energy cutoff increase up to 67.5 MeV, 35% higher than for comparator flat foil shots, was demonstrated by Gaillard et al. (2011) using specially devised targets, namely, flattop hollow microcones (Flippo et al., 2008), which are a modification of conical targets used in fast ignition experiments (see Sec. V.C). The laser pulse is focused inside the target and starts interacting with the walls of the cone that it grazes while focusing down toward the flattop section. The reported result, obtained with 80 J of laser energy on the Trident laser at Los Alamos National Laboratory (LANL), is attributed to an efficient mechanism of electron acceleration taking place on the inner cone walls, named ‘‘direct laser-light-pressure acceleration.’’ The resulting increase in the number of highenergy electrons results in the increase of the maximum proton energy. The use of targets with various structures has also been investigated with the particular aim to increase the ion energy already at relatively low laser intensities (below 1018 W cmÀ2 ), using, e.g., double layer targets (Badziak et al., 2001) and more recently nanowire...
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This document was uploaded on 09/28/2013.

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