International audienceThe past few years have seen remarkable progress in the development of laser-based particle accelerators. The ability to produce ultrabright beams of multi-megaelectronvolt protons routinely has many potential uses from engineering to medicine, but for this potential to be realized substantial improvements in the performances of these devices must be made. Here we show that in the laser-driven accelerator that has been demonstrated experimentally to produce the highest energy protons, scaling laws derived from fluid models and supported by numerical simulations can be used to accurately describe the acceleration of proton beams for a large range of laser and target parameters. This enables us to evaluate the laser parameters needed to produce high-energy and high-quality proton beams of interest for radiography of dense objects or proton therapy of deep-seated tumours

Proton and ion acceleration using high-intensity lasers is a field of rapidly growing interest. For possible applications of proton beams produced in laser-solid interactions, the generation of beams with controllable parameters such as energy spectrum, brightness, and spatial profile is crucial. Hence, the physics underlying the acceleration processes has to be well understood. After the first proof-of-principle experiments [1,2], systematical studies were carried out to examine the influence of target material and thickness [3,4]. To establish the influence of the main laser parameters such as intensity, pulse energy, and duration over a wide range, results from different laser systems have to be compared, since usually each system covers a small parameter range only. Besides these parameters, strength and duration of the prepulse due to amplified spontaneous emission (ASE) play an important role, too [3]. We report on experiments carried out to establish the influence of the laser prepulse due to ASE and the target thickness on the acceleration of protons from thin aluminum foils.The protons originate from water and hydrocarbon contaminations on the foil surfaces. We used the 6-TW ATLAS laser facility at MPQ delivering 150 fs pulses at 790 nm wave length containing up to 900 mJ of energy. The pulses are focused by an f /2.5 off-axis parabolic mirror onto aluminum foils of 0.8 . . .86 µm thickness to intensities in excess of 10 19 W/cm 2 .The duration of the ASE prepulse having a peak intensity of 8 × 10 11 W/cm 2 can be controled by means of an ultra-fast Pockels cell in the laser chain. The shortest prepulse duration is 500 ps and it can be extended to several ns. The protons accelerated from the foils are detected by a Thomson parabola positioned in normal direction of the target rear side. CR 39 plates are used as a detector. The proton pits made visible by etching the CR 39 in NaOH after the shot are counted by an optical microscope equipped with a pattern-recognition software.

We present measurements of a magnetic reconnection in a plasma created by two laser beams (1 ns pulse duration, 1 x 10(15) W cm(-2)) focused in close proximity on a planar solid target. Simultaneous optical probing and proton grid deflectometry reveal two high velocity, collimated outflowing jets and 0.7-1.3 MG magnetic fields at the focal spot edges. Thomson scattering measurements from the reconnection layer are consistent with high electron temperatures in this region.

Electron acceleration by laser-driven plasma waves 1,2 is capable of producing ultra-relativistic, quasi-monoenergetic electron bunches 3-5 with orders of magnitude higher accelerating gradients and much shorter electron pulses than state-of-the-art radio-frequency accelerators. Recent developments have shown peak energies reaching into the GeV range 6 and improved stability and control over the energy spectrum and charge 7 . Future applications, such as the development of laboratory X-ray sources with unprecedented peak brilliance 8,9 or ultrafast time-resolved measurements 10 critically rely on a temporal characterization of the acceleration process and the electron bunch. Here, we report the first real-time observation of the accelerated electron pulse and the accelerating plasma wave. Our time-resolved study allows a single-shot measurement of the 5.8 +1.9 −2.1 fs electron bunch duration full-width at half-maximum (2.5 +0.8 −0.9 fs root mean square) as well as the plasma wave with a density-dependent period of 12-22 fs and reveals the evolution of the bunch, its position in the surrounding plasma wave and the wake dynamics. The results afford promise for brilliant, sub-ångström-wavelength ultrafast electron and photon sources for diffraction imaging with atomic resolution in space and time 11 .The recent development in laser wakefield acceleration (LWFA) is made feasible by transverse breaking of the plasma wave 12 , which results in self-injection and trapping of electrons in the accelerating structure 2,13,14 . Whereas beam parameters such as the energy spectrum, accelerated charge, beam divergence and pointing are now being measured routinely with methods adopted from conventional accelerator technology, the duration of electron bunches arising from LWFA has so far defied accurate determination. Bunch duration measurements up to now have relied on techniques using THz radiation emitted by the electrons, yielding upper limits for the electron pulse length corresponding to the temporal resolution of ≥30 fs (refs 15-18). The plasma wave has been observed so far in single-shot, yet time-integrating schemes 19,20 . Thus, the measurements were incapable of providing insight into the relevant plasma wave dynamics and of timing the accelerated electron bunch with respect to the plasma wave.In this work we present snapshots of the magnetic field generated by the accelerated electron bunch and-simultaneously-of the plasma wave by the combination of two techniques: time-resolved polarimetry 21,22 and plasma shadowgraphy 23 . The novelty in our experimental investigation is the few-cycle duration of our laser pulses, which is even shorter than half of the plasma period. This, in combination with a high spatial resolution, allows