Ion beams are relevant for radiobiological studies and for tumor therapy. In contrast to conventional accelerators, laser-driven ion acceleration offers a potentially more compact and cost-effective means of delivering ions for radiotherapy. Here, we show that by combining advanced acceleration using nanometer thin targets and beam transport, truly nanosecond quasi-monoenergetic proton bunches can be generated with a table-top laser system, delivering single shot doses up to 7 Gy to living cells. Although in their infancy, laser-ion accelerators allow studying fast radiobiological processes as demonstrated here by measurements of the relative biological effectiveness of nanosecond proton bunches in human tumor cells.

Experimental data on laser-driven carbon C 6+ ion acceleration with a peak intensity of 5 × 10 20 W cm −2 are presented and compared for opaque target normal sheath acceleration (TNSA) and relativistically transparent laser-plasma interactions. Particle numbers, peak ion energy and conversion efficiency have been investigated for target thicknesses from 50 nm to 25 µm using unprecedented full spectral beam profile line-out measurements made using a novel high-resolution ion wide-angle spectrometer. For thicknesses of about 200 nm, particle numbers and peak energy increase to 5 × 10 11 carbon C 6+ particles between 33 and 700 MeV (60 MeV u −1 ), which is a factor of five higher in particle number than that observed for targets with micron thickness. For 200 nm thick targets, we find that the peak conversion efficiency is 6% and that up to 55% of the target under the laser focal spot is accelerated to energies above 33 MeV. This contrasts with the results for targets with micron thickness, where surface acceleration with TNSA is dominant. The experimental findings are consistent with two-dimensional particle-in-cell simulations.

Here we present experimental results on laser-driven ion acceleration from relativistically transparent, overdense plasmas in the break-out afterburner (BOA) regime. Experiments were preformed at the Trident ultra-high contrast laser facility at Los Alamos National Laboratory, and at the Texas Petawatt laser facility, located in the University of Texas at Austin. 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

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.

A novel ion wide angle spectrometer (iWASP) has been developed, which is capable of measuring angularly resolved energy distributions of protons and a second ion species, such as carbon C(6 +), simultaneously. The energy resolution for protons and carbon ions is better than 10% at ∼50 MeV/nucleon and thus suitable for the study of novel laser-ion acceleration schemes aiming for ultrahigh particle energies. A wedged magnet design enables an acceptance angle of 30°(∼524 mrad) and high angular accuracy in the μrad range. First, results obtained at the LANL Trident laser facility are presented demonstrating high energy and angular resolution of this novel iWASP.

We report on experimental studies of divergence of proton beams from nanometer thick diamond-like carbon foils irradiated by a linearly polarized intense laser with high contrast. Proton beams with extremely small divergence (half angle) of 2° are observed in addition with a remarkably well-collimated feature over the whole energy range, showing one order of magnitude reduction of the divergence angle in comparison to the results from μm thick targets. Similar features are reproduced in two-dimensional particle-in-cell simulations with parameters representing our experiments, indicating a strong influence from the electron density distribution on the divergence of protons. Our comprehensive experimental study reveals grand opportunities for using nm foils in experiments that require high ion flux and small divergence.

We report on a Paul-trap system with large access angles that allows positioning of fully isolated micrometer-scale particles with micrometer precision as targets in high-intensity laser-plasma interactions. This paper summarizes theoretical and experimental concepts of the apparatus as well as supporting measurements that were performed for the trapping process of single particles.

We report on experiments irradiating isolated plastic spheres with a peak laser intensity of 2-3×10^{20}Wcm^{-2}. With a laser focal spot size of 10 μm full width half maximum (FWHM) the sphere diameter was varied between 520 nm and 19.3 μm. Maximum proton energies of ∼25 MeV are achieved for targets matching the focal spot size of 10 μm in diameter or being slightly smaller. For smaller spheres the kinetic energy distributions of protons become nonmonotonic, indicating a change in the accelerating mechanism from ambipolar expansion towards a regime dominated by effects caused by Coulomb repulsion of ions. The energy conversion efficiency from laser energy to proton kinetic energy is optimized when the target diameter matches the laser focal spot size with efficiencies reaching the percent level. The change of proton acceleration efficiency with target size can be attributed to the reduced cross-sectional overlap of subfocus targets with the laser. Reported experimental observations are in line with 3D3V particle in cell simulations. They make use of well-defined targets and point out pathways for future applications and experiments.