Proton Acceleration with High-Intensity Ultrahigh-Contrast Laser Pulses T. Ceccotti, 1 A. Le ´vy, 1 H. Popescu, 1 F. Re ´au, 1 P. D’Oliveira, 1 P. Monot, 1 J. P. Geindre, 2 E. Lefebvre, 3 and Ph. Martin 1 1 Service des Photons, Atomes et Mole ´cules, Commissariat a ` l’Energie Atomique, DSM/DRECAM, CEN Saclay, 91191 Gif sur Yvette, France 2 LULI, UMR7605, CNRS-CEA-Ecole Polytechnique-Paris 6, 91128 Palaiseau, France 3 De ´partement de Physique The ´orique et Applique ´e, CEA/DAM Ile-de-France, BP 12, 91680 Bruye `res-le-Cha ˆtel, France (Received 9 March 2007; published 31 October 2007) We report on simultaneous measurements of backward- and forward-accelerated protons spectra when an ultrahigh intensity (5  10 18 W=cm 2 ), ultrahigh contrast (>10 10 ) laser pulse interacts with foils of thickness ranging from 0.08 to 105 m. Under such conditions, free of preplasma originating from ionization of the laser-irradiated surface, we show that the maximum proton energies are proportional to the p component of the laser electric field only and not to the ponderomotive force and that the characteristics of the proton beams originating from both target sides are almost identical. All these points have been corroborated by extensive 1D and 2D particle-in-cell simulations showing a very good agreement with the experimental data. DOI: 10.1103/PhysRevLett.99.185002 PACS numbers: 52.38.Kd, 52.50.Jm, 52.70.Nc The recent observations of intense energetic proton beams generated by interaction of high intensity (10 18 W=cm 2 ) sub picosecond lasers with solid targets [1– 3] have opened a new exciting field of research. These proton bunches allow for a better understanding of the underlying laser-solid interaction at very high intensities and are also considered for many practical purposes, such as high resolution probing of electric fields in plasmas [4], induction of nuclear phenomena [5], and fast ignition applications [6]. Up to now, most published works deal with protons emitted in the forward laser direction (FWD in the following) accelerated normally to the surface of the target, from the side opposite to laser irradiation. They generally exhibit a high laminarity, a low divergence ( <20  , decreasing with proton energy), and the estimated duration at the source lies in the picosecond range [7,8]. Protons originating from the laser-irradiated surface and accelerated in the backward direction (BWD in the follow- ing), are usually found much less energetic [9,10] and of less interest as a diagnostic or application tool. The admit- ted scenario for FWD proton acceleration involves three consecutive steps. First, the laser prepulse creates a thin plasma layer at the surface of the foil. Then, the intense part of the pulse interacting with this thin layer accelerates electrons toward the foil, essentially by the ponderomotive force. Finally, the electron beam reaches the rear surface and creates a strong electrostatic field which first ionizes and then accelerates protons and ions to high energies. This scenario, called Target Normal Sheath Acceleration (TNSA) [11] has been confirmed by several experiments [12]. Nevertheless, the influence of specific experimental conditions, for instance the laser contrast [13], on the electron acceleration toward the back of the target, and consequently on FWD and BWD emission characteristics, is not yet completely evident. This Letter reports a study of proton acceleration using thin Mylar foils of different thickness as targets, under low (10 6 ) and ultrahigh (10 10 ) laser contrast conditions (re- spectively ‘‘LC’’ and ‘‘HC’’ in the following). We per- formed simultaneous single shot measurements of proton emission behind the target in the laser direction (FWD) and in front of the target, opposite to laser direction (BWD). For both emission directions, the influence of the laser beam polarization on proton maximum energies is re- ported. We used 1D and 2D Particle-In-Cell (PIC) simula- tions to interpret our experimental data. Numerical results are in good agreement with collected data, pointing out both that increased electron confinement in the thinnest targets enhances the maximum proton energy and the dependence from the laser polarization. The experiment has been performed at the Saclay Laser Interaction Center Facility, using the UHI10 laser which delivers 10 TW ultrashort pulses (65 fs) at 10 Hz repetition rate. This Ti-Sapphire laser is based on the standard chirped-pulse-amplification (CPA) technique and operates at a central wavelength of 790 nm. The intrinsic 10 6 contrast of the beam is raised to 10 10 thanks to a ‘‘double plasma mirror’’ [14,15]. Under HC conditions, the spatial focal spot qualities are preserved while the laser energy is reduced by a factor of 2. The laser beam was focused to a spot size of 8 m (FWHM) using an off-axis f  300 mm parabola, under a 45  incidence angle and p polarization, on thin Mylar foils with thickness varying between 0.08 and 105 m. Maximum peak intensities close to 5  10 18 W=cm 2 (HC) and 10 19 W=cm 2 (LC) were reached. Proton spectra were recorded using two similar Thomson parabola spectrome- ters placed normally to the target surfaces at distances of 240 mm (BWD) and 600 mm (FWD). The entrance pin- hole diameter were respectively 100 m and 200 m. Once dispersed by the magnetic and electric fields of the spectrometer, protons and ions were detected by a two stage 40 mm diameter micro channel plate (MCP) coupled to a phosphor screen. This latter was imaged onto a 12 bit PRL 99, 185002 (2007) PHYSICAL REVIEW LETTERS week ending 2 NOVEMBER 2007 0031-9007= 07=99(18)=185002(4) 185002-1  2007 The American Physical Society