The present article reports on the generation of stable 50 pC low-divergence electron beams above 150 MeV from laser-driven wakefield acceleration using a continuous-flow gas target prototype tested at the 60 TW Salle Jaune facility at LOA. The gas target design is meant to be easily transported and integrated as an element of the beamline with a differential pumping system offering some 10−4 mbar pressure in the rest of the line. A dedicated gas injection system allows for the control of the gas mixture concentration and gas pressure in two different regions of the target within the frame of controlled ionisation injection schemes. The measured electron beam parameters show the importance of gas density profiles and longitudinal gas mixture confinement.

Betatron radiation is produced in Laser Plasma Accelerators when the electrons are accelerated and simultaneously wiggle across the propagation axis. The mechanisms of electron acceleration and X-ray radiation production follow different scaling laws, and the brightest X-ray radiation is often produced for an electron beam with a lower quality in terms of energy and divergence. Here, we report a laser-driven Betatron X-ray source where the plasma density profile is tailored in order to separate the acceleration and wiggler stages, which allows for the independent optimizations of acceleration and X-ray production. We demonstrate this concept experimentally, and show that the Betatron photon energy can be controlled by adjusting the length of the plasma wiggler. This scheme offers a path to overcome the limitations of conventional Betatron sources, enabling the production of bright, stable, energetic, and collimated X-ray beams.

Laser-plasma accelerators emerge as ultra-compact and versatile sources for numerous applications. Although the acceleration length is short (a few millimeters), they typically require large-scale infrastructures including ultra-high-power lasers, vacuum chambers and strict stability for temperature and humidity. As a result, most experiments are conducted in laboratories in large areas with controlled environments. Here, we present a highly compact (footprint of ~ 9 m²) and transportable system capable of generating electrons and photons in the MeV range at high repetition rates (up to 10 Hz) with average charge levels of 0.5 nC and up to 1 nC. This achievement shows the feasibility of performing laser-plasma acceleration outside of laboratory environments with a transportable system, significantly expanding the potential for practical applications.

Technology based on high-peak-power lasers has the potential to provide compact and intense radiation sources for a wide range of innovative applications. In particular, electrons that are accelerated in the wakefield of an intense laser pulse oscillate around the propagation axis and emit X-rays. This betatron source, which essentially reproduces the principle of a synchrotron at the millimeter scale, provides bright radiation with femtosecond duration and high spatial coherence. However, despite its unique features, the usability of the betatron source has been constrained by its poor control and stability. In this article, we demonstrate the reliable production of X-ray beams with tunable polarization. Using ionization-induced injection in a gas mixture, the orbits of the relativistic electrons emitting the radiation are reproducible and controlled. We observe that both the signal and beam profile fluctuations are significantly reduced and that the beam pointing varies by less than a tenth of the beam divergence. The polarization ratio reaches 80%, and the polarization axis can easily be rotated. We anticipate a broad impact of the source, as its unprecedented performance opens the way for new applications.

Betatron x-ray source from laser plasma interaction combines high brightness, few femtosecond duration and broad band energy spectrum. However, despite these unique features the Betatron source has a crippling drawback preventing its use for applications. Its properties significantly vary shot-to-shot and none of the developments performed so far resolved this problem. In this letter we present a simple method that allows to produce stable and bright Betatron x-ray beams. In addition, we demonstrate that this scheme provides polarized and easily tunable radiation. Experimental results show that the pointing stability is better than 10% of the beam divergence, with flux fluctuation of the order of 20% and a polarization degree reaching up to 80%

We performed laser wakefield electron acceleration experiments using laser powers up to 100TW in the 'bubble' regime. The measured angularly resolved energy spectra of the electron beam showed evidence of betatron oscillations during the acceleration process. Through diagnosis of these oscillations, electron injection into the wakefield could be controlled through adjustment of the shape of the laser focal spot or through changes in the plasma density. Several different acceleration regimes could be accessed including (i) injection of a single electron bunch into the wakefield 'bubble' (ii) multiple injection of several electron bunches and/or (iii) production of a transverse break up of the electron beam within the 'bubble' due to an asymmetry of the wakefield. We apply analytical formulae for electron motion in a wakefield to understand the experimental data.

Laser driven proton acceleration experiments from micron and submicron thick targets using high intensity (2 × 1021 W/cm2), high contrast (10−15) laser pulses show an enhancement of maximum energy when hydrogen containing targets were used instead of non-hydrogen containing. In our experiments, using thin (<1μm) plastic foil targets resulted in maximum proton energies that were consistently 20%–100% higher than when equivalent thickness inorganic targets, including Si3N4 and Al, were used. Proton energies up to 20 MeV were measured with a flux of 107 protons/MeV/sr.

We demonstrate the existence of the severe losses due ASE and ability EDP technique to suppress it and parasitic lasing. The optimal conditions that can deliver up to kJ level energy with existing technology are presented. Keywords: Ultrahigh power lasers; Laser amplifiers.

The generation of energetic electron and proton beams was studied from the interaction of high intensity laser pulses with pre-drilled conical targets. These conical targets are laser machined onto flat targets using 7–180 µJ pulses whose axis of propagation is identical to that of the main high intensity pulse. This method significantly relaxes requirements for alignment of conical targets in systematic experimental investigations and also reduces the cost of target fabrication. These experiments showed that conical targets increase the electron beam charge by up to 44 ± 18% compared with flat targets. We also found greater electron beam divergence for conical targets than for flat targets, which was due to escaping electrons from the surface of the cone wall into the surrounding solid target region. In addition, the experiments showed similar maximum proton energies for both targets since the larger electron beam divergence balances the increase in electron beam charge for conical targets. 2D particle in cell simulations were consistent with the experimental results. Simulations for conical target without preplasma showed higher energy gain for heavy ions due to ‘directed coulomb explosion’. This may be useful for medical applications or for ion beam fast ignition fusion.

Stimulated Raman side scattering of an ultrashort high power laser pulse is studied in experiments on laser wakefield acceleration. Experiments and simulations reveal that stimulated Raman side scattering occurs at the beginning of the interaction, that it contributes to the evolution of the pulse prior to wakefield formation, and also that it affects the quality of electron beams generated. The relativistic shift of the plasma frequency is measured.

An ablatively driven capillary discharge plasma waveguide has been used to demonstrate guiding of 30 fs, 35 TW laser pulses over distances up to 3 cm with incident intensity in excess of 4 x 10(18) W/cm(2). The plasma density range over which good guiding was observed was 1-3 x 10(18) cm(-3). The quality of the laser spot at the exit mode was observed to be similar to that at the entrance and the transmitted energy was similar to 25% at input powers of 35 TW. The transmitted laser spectrum typically showed blueshifting due to ionization of carbon and hydrogen atoms in the capillary plasma by the high intensity laser pulse. The low plasma density regime in which these capillaries operate makes these devices attractive for use in single stage electron accelerators to multi-GeV energies driven by petawatt class laser systems (C) 2009 American Institute of Physics. [doi:10.1063/1.3257909]

Laser wakefield acceleration (LWFA) in a supersonic gas-jet using a self-guided laser pulse was studied by changing the laser power and electron density. The recently upgraded HERCULES laser facility equipped with wavefront correction enables a peak intensity of 8 x 10(19) W/cm(2) at laser power of 100 TW to be delivered to the gas-jet using NO focusing optics. We found that electron beam charge was increased significantly with an increase of the laser power from 30 TW to 80 TW and showed density threshold behavior at a fixed laser power. Betatron motion of electrons was also observed depending on laser power and electron density.

X-rays generated by 0.1 - 0.5 GeV electron beams generated using a 100 TW laser are shown to have a low emittance, be spatially coherent and have a peak brightness comparable to 3(rd) generation synchrotron sources.

Purpose: To quantify the laser pulse requirements and target parameters required to achieve the Directed Coulomb Explosion (DCE) regime of laser‐target interaction for the acceleration of protons to therapeutic energies. Method and Materials: Particle‐in‐Cell (PIC) simulations of the planned experiments along the funded upgrade path of the HERCULES laser at the University of Michigan have predicted a new regime of attainable laser‐target interactions for proton acceleration. The laser was recently upgraded to 300 TW and a temporal pulse contrast ratio of 10−11, allowing intensities of 2×1022 W/cm2 to be achieved in a near diffraction limited, 1.3 micron, focal spot. The 2 ns long amplified spontaneous emission (ASE) pre‐pulse was suppressed by a factor 10−3 through the implementation of a cross‐polarized wave (XPW) pulse cleaner to prevent pre‐plasma creation on the front surface and preserve the physical integrity of the thin‐film target. Dual plasma mirrors are being characterized to reduce the prepulse at < 30 ps before the main pulse (caused by variations in the index of refraction through the optic path) below a contrast ratio of 10−11. This will allow experiments on thin foil targets (< 100 nm) up to 300TW with no significant pre‐pulse to compromise the target. Results: Preliminary measurements show an additional reduction of the contrast ratio of the ASE pre‐pulse by a factor of 10−3 after the addition of dual plasma mirrors. Additional work is required to optimize the setup and parameters of the plasma mirrors to account for polarization effects and wave front distortions and variable intensity levels. Conclusion: Implementation of dual plasma mirrors is progressing with promising results and will soon allow experimental implementation of the laser pulse characteristics required to test the DCE regime of proton acceleration.

Laser wakefield acceleration (LWFA) was studied using ablated plasmas as the target medium. A low density laser-ablated plasma (carbon and fluorine) was produced by focusing a 100 mJ, 10 ns pulse from a Nd : YAG laser onto the surface of a plastic target to a peak intensity of 3 x 10(10) W cm(-2). A 30 fs interaction pulse from the HERCULES Ti : sapphire laser system with 30 TW laser power subsequently irradiated the plasma at a peak intensity of 3 x 10(18) W cm(-2). The plasma density profile was varied in situ by changing the time delay between the two laser pulses. It was observed that electron energies up to 120 MeV with monoenergetic features were observed. For larger delays, the electron beam charge increases while the transmittance of the interaction pulse decreases. This correlation suggests that pump depletion occurs due to wake excitation. The use of an ablated plasma target enables LWFA operation at much higher repetition rates due to the fast plasma dynamics and adds flexibility of plasma parameters such as temperature, charge state and ion composition.

Laser wakefield acceleration (LWFA) in plasmas has been demonstrated with gradients which are orders of magnitude greater than the limit on conventional Radio Frequency accelerators. However, the acceleration length is limited by two factors, the dephasing length and the Rayleigh range of the laser pulse. Dephasing length is the distance in which electrons overtake the laser pulse and can be increased by decreasing plasma density. Alternatively the interaction length can be extended by orders of magnitude by using ablative wall discharge capillary targets, in which a plasma is preformed with a transverse density profile capable of guiding the focused laser. We have demonstrated guiding of high intensity laser pulses from the HERCULES laser over 3 cm for powers up to 35 TW. The quality of the laser spot can be retained and the intensity remains high even at the exit of the capillary. The transmitted laser spectrum shows blueshifting due to field ionization by the laser pulse. This ionization might enhance electron injection at low electron density for LWFA GeV accelerators. The field ionization affects carbon atoms and ions from the ablated capillary, which are not present in hydrogen-filled capillaries. This creates an additional challenge to guiding compared to hydrogen-filled capillaries. However, the setup and materials are easier to come by. The use of these capillary targets may also be of interest to other high intensity laser-plasma interactions requiring long interaction lengths such as high harmonic generation from gases and plasmas, or x-ray lasing in underdense plasmas.

Laser wakefield acceleration (LWFA) in a supersonic gas-jet using a self-guided laser pulse was studied by changing laser power and plasma electron density. The recently upgraded HERCULES laser facility equipped with wavefront correction enables a peak intensity of 6.1x10(19) W/cm(2) at laser power of 80 TW to be delivered to the gas-jet using F/10 focusing optics. We found that electron beam charge was increased significantly with an increase of laser power from 30 TW to 80 TW and showed density threshold behavior at a fixed laser power. We also studied the influence of laser focusing conditions by changing the f-number of the optics to F/15 and found an increase in density threshold for electron production compared to the F/10 configuration. The analysis of different phenomena such as betatron motion of electrons, side scattering of the laser pulse for different focusing conditions, the influence of plasma density down ramp on LWFA are shown.

A relativistic plasma shutter technique is proposed and tested to remove the sub-100 ps pedestal of a high-intensity laser pulse. The shutter is an ultrathin foil placed before the target of interest. As the leading edge of the laser ionizes the shutter material it will expand into a relativistically underdense plasma allowing for the peak pulse to propagate through while rejecting the low intensity pedestal. An increase in the laser temporal contrast is demonstrated by measuring characteristic signatures in the accelerated proton spectra and directionality from the interaction of 30 TW pulses with ultrathin foils along with supporting hydrodynamic and particle-in-cell simulations.