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Originally published in Science Express on 16 February 2006
Science 21 April 2006:
Vol. 312. no. 5772, pp. 410 - 413
DOI: 10.1126/science.1124412
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Reports

Ultrafast Laser-Driven Microlens to Focus and Energy-Select Mega-Electron Volt Protons

Toma Toncian1, Marco Borghesi2, Julien Fuchs3, Emmanuel d'Humières3,4, Patrizio Antici3, Patrick Audebert3, Erik Brambrink3, Carlo Alberto Cecchetti2, Ariane Pipahl1, Lorenzo Romagnani2 and Oswald Willi1*

1 Heinrich Heine Universität Düsseldorf, D-40225 Düsseldorf, Germany.
2 School of Mathematics and Physics, The Queen's University of Belfast, Belfast BT7 1NN, Northern Ireland, UK.
3 Laboratoire pour l'Utilisation des Lasers Intenses, UMR 7605 CNRS-CEA-Ecole Polytechnique-Université Paris VI, 91128 Palaiseau, France.
4 Centre de Physique Théorique, UMR 7644 CNRS-Ecole Polytechnique, 91128 Palaiseau, France.


Figure 1 Fig. 1. (A) Schematic of the microlens device. A proton beam is accelerated from a planar foil by the CPA1 laser pulse. The proton beam propagates through a hollow cylinder side irradiated by the CPA2 laser pulse. (B and C) Schematic of the operation's principle of the microlens. The CPA2 laser pulse injects hot electrons within the cylinder. These spread evenly onto the cylinder's inner wall. In the early stage (B), these electrons, confined over a Debye length on the cylinder's surface, generate a space-charge field (indicated by the radially pointing arrows), which then induces plasma expansion (C) from the cylinder's inner walls. The resulting radial electric field, still indicated by the arrows and decreasing in time as the plasma expands, focuses the protons. The field is peaked at the front but extends toward the cylinder's wall. [View Larger Version of this Image (35K GIF file)]
 

Figure 2 Fig. 2. (A) RCF layers showing the proton beam focusing effect due to a 3-mm-long, 700-µm-diameter dural (95% Al, 4% Cu, and 1% Mg) laser–irradiated cylinder with 50-µm wall thickness. For the energies reaching the Bragg peak in the two layers shown, the protons with an energy of 9 MeV pass through the cylinder before the electric field is present, showing no focusing, whereas the divergence of the protons with an energy of 7.5 MeV is strongly reduced by the electric field that has developed inside the cylinder. The shadow of the cylinder and of the 50-µm tungsten holding wire can be seen clearly. The cylinder was positioned far from the proton source intentionally to reduce the number of protons entering the cylinder and avoid RCF saturation. The distance between the entrance plane of the cylinder and the proton production foil was 4 mm. (B) Flux increase with respect to the unfocused flux for 7.5-MeV protons as deduced from the layer shown in (A). [View Larger Version of this Image (62K GIF file)]
 

Figure 3 Fig. 3. Evolution of the FWHM of the proton beam along its propagation, for protons with energy of 7.5 MeV (the propagation distance is calculated from the proton source). The black circles correspond to the case without the microlens (free-space divergence), the blue diamonds to the particle-tracer simulation in the fields given by the PIC simulation, and the red squares to the experimental results with the use of the microlens. The RCF shown on the right of Fig. 2A corresponds to the red point at 9.5 cm. [View Larger Version of this Image (36K GIF file)]
 

Figure 4 Fig. 4. Experimental proton spectra measured with a magnetic spectrometer without the microlens (black line) and with the microlens (green line), and proton spectra obtained from simulations (red and blue lines) performed with the use of the experimental proton beam parameters and the magnetic spectrometer parameters (i.e., distance from the source and slit characteristics). The red and blue curves were obtained with the use of energy bins of 0.2 and 0.1 MeV, respectively. The simulated spectrum was obtained by tracing, for each energy bin, 5000 protons through the fields predicted by the PIC simulation. [View Larger Version of this Image (32K GIF file)]
 

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