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...

We report on an experimental demonstration of laser wakefield electron acceleration using a sub-TW power laser by tightly focusing 30 fs laser pulses with an 8 mJ pulse energy on a 100 µm scale gas target. The experiments are carried out at an unprecedented 0.5 kHz repetition rate, allowing 'realtime' optimization of accelerator parameters. Well-collimated and stable electron beams with quasi-monoenergetic peaks in the 100 keV range are measured. Particle-in-cell simulations show excellent agreement with the experimental results and suggest an acceleration mechanism based on electron trapping on the density downramp, due to the time-varying phase velocity of the plasma waves.

Coherent control of a system involves steering an interaction to a final coherent state by controlling the phase of an applied field. Plasmas support coherent wave structures that can be generated by intense laser fields. Here, we demonstrate the coherent control of plasma dynamics in a laser wakefield electron acceleration experiment. A genetic algorithm is implemented using a deformable mirror with the electron beam signal as feedback, which allows a heuristic search for the optimal wavefront under laser-plasma conditions that is not known a priori. We are able to improve both the electron beam charge and angular distribution by an order of magnitude. These improvements do not simply correlate with having the 'best' focal spot, as the highest quality vacuum focal spot produces a greatly inferior electron beam, but instead correspond to the particular laser phase front that steers the plasma wave to a final state with optimal accelerating fields.

International audienceWe show that electron bunches in the 50-100 keV range can be produced from a laser wake-field accelerator using 10 mJ, 35 fs laser pulses operating at 0.5 kHz. It is shown that using a solenoid magnetic lens, the electron bunch distribution can be shaped. The resulting transverse and longitudinal coherence is suitable for producing diffraction images from a polycrystalline 10 nm aluminum foil. The high repetition rate, the stability of the electron source and the fact that its uncorrelated bunch duration is below 100 fs make this approach promising for the development of sub-100 fs ultrafast electron diffraction experiments

Recent development in laser-based accelerators is finally offering the possibility of building metre-size electron-positron colliders with specifications comparable to those based on conventional acceleration techniques. Electron beams with energies exceeding the GeV have been experimentally demonstrated [1] with the possibility of approaching 100 GeV with the next generation of laser systems [2]. It is thus timely to study the feasibility of generating laser-driven positron beams with similar characteristics. Here we report on the experimental demonstration of table-top, all-optical generation of short (beam duration ~ 30fs), ultra-relativistic and collimated positron beams with peak energies approaching the GeV and divergencies in the mrad range. Plasma-based afterburners [3] to further accelerate these beams will also be discussed. The reported results represent the first experimental step towards the generation of metre-scale all-optical electron-positron colliders. The intrinsic synchronisation of these sources with a high-intensity laser will prove fundamental for the study of highly non-linear photon-lepton interactions and the testing of matter-antimatter symmetry in a highly non-linear regime.

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Sarri et al. Reply: The Comment [1] reports on independent numerical simulations of the experimental results in Ref. [2]. Positron yields and spectra similar to those in Ref. [2] are numerically obtained, but with a larger divergence. A larger positron source size at the rear of the converter is hence inferred, resulting in a smaller positron density. Since the publication of the Letter, comprehensive Monte Carlo simulations of the characteristics of such positron beams have already been published by members of our collaboration [3]. The simulations discussed in the Comment are only a specific subset of this more extensive work, and largely agree with it on the divergence.

High intensity, short pulse lasers can be used to accelerate electrons to ultra-relativistic energies via laser wakefield acceleration (LWFA) [T. Tajima and J. M. Dawson, Phys. Rev. Lett. 43, 267 (1979)]. Recently, it was shown that separating the injection and acceleration processes into two distinct stages could prove beneficial in obtaining stable, high energy electron beams [Gonsalves et al., Nat. Phys. 7, 862 (2011); Liu et al., Phys. Rev. Lett. 107, 035001 (2011); Pollock et al., Phys. Rev. Lett. 107, 045001 (2011)]. Here, we use a stereolithography based 3D printer to produce two-stage gas targets for LWFA experiments on the HERCULES laser system at the University of Michigan. We demonstrate substantial improvements to the divergence, pointing stability, and energy spread of a laser wakefield accelerated electron beam compared with a single-stage gas cell or gas jet target.