We demonstrate the highest intensity - 300 TW laser by developing booster amplifying stage to the 50-TW-Ti:sapphire laser (HERCULES). To our knowledge this is the first multi-100TW-scale laser at 0.1 Hz repetition rate.

The scheme of a simultaneous multiple pulse focusing on one spot naturally arises from the structural features of projected new laser systems, such as the Extreme Light Infrastructure (ELI) and High Power laser Energy Research (HiPER). It is shown that the multiple pulse configuration is beneficial for observing e+ e- pair production from a vacuum under the action of sufficiently strong electromagnetic fields. The field of focused pulses is described using a realistic three-dimensional model based on an exact solution of the Maxwell equations. The e+ e- pair production threshold in terms of electromagnetic field energy can be substantially lowered if, instead of one or even two colliding pulses, multiple pulses are focused on one spot. The multiple pulse interaction geometry gives rise to subwavelength field features in the focal region. These features result in the production of extremely short e+ e- bunches.

The generation of ultra-relativistic positron beams with short duration (τ e + ≤ 30 fs), small divergence (θ e + 3 mrad), and high density (n e + 10 14 − 10 15 cm −3 ) from a fully optical setup is reported. The detected positron beam propagates with a high-density electron beam and γ-rays of similar spectral shape and peak energy, thus closely resembling the structure of an astrophysical leptonic jet. It is envisaged that this experimental evidence, besides the intrinsic relevance to laserdriven particle acceleration, may open the pathway for the small-scale study of astrophysical leptonic jets in the laboratory.Creating and characterizing high-density beams of relativistic positrons in the laboratory is of paramount importance in experimental physics, due to their direct application to a wide range of physical subjects, including nuclear physics, particle physics, and laboratory astrophysics. Arguably, the most practical way to generate them is to exploit the electromagnetic cascade initiated by the propagation of an ultra-relativistic electron beam through a high-Z solid. This process is exploited to generate low-energy positrons in injector systems for conventional accelerators such as the Electron-Positron Collider (LEP) [1]. In this case, an ultra-relativistic electron beam (E e − ≈ 200 MeV) was pre-accelerated by a LINAC and then directed onto a tungsten target. The resulting positron population, after due accumulation in a storage ring, was further accelerated by a conventional, large-scale (R ≈ 27 km), synchrotron accelerator up to a peak energy of 209 GeV. The large cost and size of these machines have motivated the study of alternative particle accelerator schemes. A particularly compact and promising system is represented by plasma devices which can support much higher accelerating fields (of the order of 100s of GV/m, compared to MV/m in solid-state accelerators) and thus significantly shorten the overall size of the accelerator. Laser-driven generation of electron beams with energies per particle reaching [2][3][4][5], and exceeding [6], 1 GeV have been experimentally demonstrated and the production of electron beams with energies approaching 100 GeV is envisaged for the next generation of highpower lasers (1 -10 PW) [7]. Hybrid schemes have also been proposed and successfully tested in first proof-ofprinciple experiments [8,9]. On the other hand, laserdriven low energy positrons (E e + ≈ 1−5 MeV) have been first experimentally obtained by C. Gahn and coworkers [10] and recently generated during the interaction of a picosecond, kiloJoule class laser with thick gold targets [11][12][13][14]. Despite the intrinsic interest of these results, the low energy and broad divergence reported (E e + ≤ 20 MeV and θ e + ≥ 350 mrad , respectively) still represent clear limitations for future use in hybrid machines.The possibility of generating high density and high energy electron-positron beams is of central importance also for astrophysics, due to their similarity to jets of long gamma-ray bursts (GRBs), whic...