We present experimental studies on ion acceleration from ultra-thin diamond-like carbon (DLC) foils irradiated by ultra-high contrast laser pulses of energy 0.7 J focussed to peak intensities of 5 × 10 19 W/cm 2 . A reduction in electron heating is observed when the laser polarization is changed from linear to circular, leading to a pronounced peak in the fully ionized carbon spectrum at the optimum foil thickness of 5.3 nm. Two-dimensional particle-in-cell (PIC) simulations reveal, that those C 6+ ions are for the first time dominantly accelerated in a phase-stable way by the laser radiation pressure.

We report on the acceleration of ion beams from ultrathin diamondlike carbon foils of thickness 50, 30, and 10 nm irradiated by ultrahigh contrast laser pulses at intensities of approximately 7 x 10;{19} W/cm;{2}. An unprecedented maximum energy of 185 MeV (15 MeV/u) for fully ionized carbon atoms is observed at the optimum thickness of 30 nm. The enhanced acceleration is attributed to self-induced transparency, leading to strong volumetric heating of the classically overdense electron population in the bulk of the target. Our experimental results are supported by both particle-in-cell (PIC) simulations and an analytical model.

Experiments on ion acceleration by irradiation of ultra-thin diamond-like carbon (DLC) foils, with thicknesses well below the skin depth, irradiated with laser pulses of ultra-high contrast and linear polarization, are presented. A maximum energy of 13 MeV for protons and 71 MeV for carbon ions is observed with a conversion efficiency of ∼ 10%. Two-dimensional particle-in-cell (PIC) simulations reveal that the increase in ion energies can be attributed to a dominantly collective rather than thermal motion of the foil electrons, when the target becomes transparent for the incident laser pulse.PACS numbers: 52.38. Kd, 41.75.Jv, 52.50.Jm, 52.65.Rr Recent experiments in the field of relativistic laserplasma interaction have shown that the conversion efficiency (CE) from the laser to the kinetic energy of the ion bunch as well as the maximal energy of the ions can be improved by the use of ultra-thin foil targets in the range of 100 nm [1,2,3,4]. Additionally, it was emphasized that besides the standard target normal sheath acceleration (TNSA) [5,6] other, radiation pressure dominated acceleration mechanisms become possible for ultrathin targets [7,8,9]. An ultra-high contrast is required to avoid substantial expansion of the targets before the interaction with the main pulse. These conditions can be achieved by the use of a specially designed laser system in combination with double plasma mirrors (DPM) [1,10,11]. The partial transmission of the intense laser pulse through an expanding target is expected to play a decisive role [12,13,14] and has been lately demonstrated in the regime of relativistic transparency [15].In this letter we present experimental results on ion acceleration from DLC foils of thicknesses ranging from 50 nm down to 2.9 nm. The targets are irradiated by linear polarized pulses of 45 fs FWHM duration focussed to a peak intensity of up to 5 × 10 19 W/cm 2 . We find an optimum in ion acceleration at a target thickness of 5.6 nm, where ion energies reach values of 13 MeV for protons and 71 MeV for carbon ions. Thus, for the ultrashort pulses used, the optimum foil thickness is well below the collisional skin depth l s ∼ 13 nm of the heated target, which is thus becoming transparent to the laser. The corresponding CE is reaching values of ∼ 1.6% for protons (E > 2M eV ) and ∼ 10% in the case of C 6+ (E > 5M eV ). By means of 2D PIC-simulations we find that the transmitted laser field imposes a dominantly collective rather than thermal motion of the foil electrons, leading to increased ion energies and enhanced CE. Our findings are supported by a semi-analytical model, which shows good agreement with the experimental results.The experiments were performed at the MBI -TW Ti:sapph laser of central wavelength 810 nm delivering 1.2 J in 45 fs FWHM pulses with an amplified spontaneous emission (ASE) contrast ratio smaller than 10 −7 up to ∼ 10 ps prior to the arrival of the main peak. By means of a re-collimating DPM [1], this contrast was increased by estimated four orders of magnitude [11], which is...

Kinetic simulations of break-out-afterburner (BOA) ion acceleration from nm-scale targets are examined in a longer pulse length regime than studied previously. It is shown that when the target becomes relativistically transparent to the laser, an epoch of dramatic acceleration of ions occurs that lasts until the electron density in the expanding target reduces to the critical density in the non-relativistic limit. For given laser parameters, the optimal target thickness yielding the highest maximum ion energy is one in which this time window for ion acceleration overlaps with the intensity peak of the laser pulse. A simple analytic model of relativistically induced transparency is presented for plasma expansion at the time-evolving sound speed, from which these times may be estimated. The maximum ion energy attainable is controlled by the finite acceleration volume and time over which the BOA acts.

We propose to produce neutron-rich nuclei in the range of the astrophysical r-process (the rapid neutron-capture process) around the waiting point N = 126 [1,2,3] by fissioning a dense laser-accelerated thorium ion bunch in a thorium target (covered by a polyethylene layer, CH 2 ), where the light fission fragments of the beam fuse with the light fission fragments of the target. Via the 'hole-boring' (HB) mode of laser Radiation Pressure Acceleration (RPA) [4,5,6] using a high-intensity, short pulse laser, very efficiently bunches of 232 Th with solid-state density can be generated from a Th layer (ca. 560 nm thick), placed beneath a deuterated polyethylene foil (CD 2 with ca. 520 nm), both forming the production target. Th ions laser-accelerated to about 7 MeV/u will pass through a thin CH 2 layer placed in front of a thicker second Th foil (both forming the reaction target) closely behind the production target and disintegrate into light and heavy fission fragments. In addition, light ions (d,C) from the CD 2 production target will be accelerated as well to about 7 MeV/u, inducing the fission process of 232 Th also in the second Th layer. The laser-accelerated ion bunches with solid-state density, which are about 10 14 times more dense than classically accelerated ion bunches, allow for a high probability that generated fission products can fuse again when the fragments from the thorium beam strike the Th layer of the reaction target. In contrast to classical radioactive beam facilities, where intense but low-density radioactive beams of one ion species are merged with stable targets, the novel fissionfusion process draws on the fusion between neutron-rich, short-lived, light fission fragments both from beam and target. Moreover, the high ion beam density may lead to a strong collective modification of the stopping power in the target by 'snowplough-like' removal of target electrons, leading to significant range enhancement, thus allowing to use rather thick targets. Send offprint requests to:Using a high-intensity laser with two beams with a total energy of 300 J, 32 fs pulse length and 3 µm focal diameter, as, e.g., envisaged for the ELI-Nuclear Physics project in Bucharest (ELI-NP) [7], order-of-magnitude estimates promise a fusion yield of about 10 3 ions per laser pulse in the mass range of A = 180 − 190, thus enabling to approach the r-process waiting point at N=126. First studies on ion acceleration, collective modifications of the stopping behaviour and the production of neutronrich nuclei can also be performed at the upcoming new laser facility CALA (Center for Advanced Laser Applications) in Garching.

Research on fusion fast ignition (FI) initiated by laser-driven ion beams has made substantial progress in the last years. Compared with electrons, FI based on a beam of quasi-monoenergetic ions has the advantage of a more localized energy deposition, and stiffer particle transport, bringing the required total beam energy close to the theoretical minimum. Due to short pulse laser drive, the ion beam can easily deliver the 200 TW power required to ignite the compressed D–T fuel. In integrated calculations we recently simulated ion-based FI targets with high fusion gain targets and a proof of principle experiment [1]. These simulations identify three key requirements for the success of ion-driven fast ignition (IFI): (1) the generation of a sufficiently high-energetic ion beam (≈400–500 MeV for C), with (2) less than 20% energy spread at (3) more than 10% conversion efficiency of laser to beam energy. Here we present for the first time new experimental results, demonstrating all three parameters in separate experiments. Using diamond nanotargets and ultrahigh contrast laser pulses we were able to demonstrate >500 MeV carbon ions, as well as carbon pulses with ΔE/E < 20%. The first measurements put the total conversion efficiency of laser light into high energy carbon ions on the order of 10%.

We report on experimental studies of ion acceleration from spherical targets of diameter 15 microm irradiated by ultraintense (1x10(20) W/cm2) pulses from a 20-TW Ti:sapphire laser system. A highly directed proton beam with plateau-shaped spectrum extending to energies up to 8 MeV is observed in the laser propagation direction. This beam arises from acceleration in a converging shock launched by the laser, which is confirmed by 3-dimensional particle-in-cell simulations. The temporal evolution of the shock-front curvature shows excellent agreement with a two-dimensional radiation pressure model.

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