Publications

Radiotherapy is a cornerstone of cancer management. The improvement of spatial dose distribution in the tumor volume by minimizing the dose deposited in the healthy tissues have been a major concern during the last decades. Temporal aspects of dose deposition are yet to be investigated. Laser-plasma-based particle accelerators are able to emit pulsed-proton beams at extremely high peak dose rates (similar to 10(9) Gy/s) during several nanoseconds. The impact of such dose rates on resistant glioblastoma cell lines, SF763 and U87-MG, was compared to conventionally accelerated protons and X-rays. No difference was observed in DNA double-strand breaks generation and cells killing. The variation of the repetition rate of the proton bunches produced an oscillation of the radio-induced cell susceptibility in human colon carcinoma HCT116 cells, which appeared to be related to the presence of the PARP1 protein and an efficient parylation process. Interestingly, when laser-driven proton bunches were applied at 0.5 Hz, survival of the radioresistant HCT116 p53(-/-) cells equaled that of its radiosensitive counterpart, HCT116 WT, which was also similar to cells treated with the PARP1 inhibitor Olaparib. Altogether, these results suggest that the application modality of ultrashort bunches of particles could provide a great therapeutic potential in radiotherapy.

Recent progress in laser-driven plasma acceleration now enables the acceleration of electrons to several gigaelectronvolts. Taking advantage of these novel accelerators, ultrashort, compact, and spatially coherent x-ray sources called betatron radiation have been developed and applied to high-resolution imaging. However, the scope of the betatron sources is limited by a low energy efficiency and a photon energy in the 10 s of kiloelectronvolt range, which for example prohibits the use of these sources for probing dense matter. Here, based on three-dimensional particle-in-cell simulations, we propose an original hybrid scheme that combines a low-density laser-driven plasma accelerator with a high-density beam-driven plasma radiator, thereby considerably increasing the photon energy and the radiated energy of the betatron source. The energy efficiency is also greatly improved, with about 1% of the laser energy transferred to the radiation, and the gamma-ray photon energy exceeds the megaelectronvolt range when using a 15 J laser pulse. This high-brilliance hybrid betatron source opens the way to a wide range of applications requiring MeV photons, such as the production of medical isotopes with photonuclear reactions, radiography of dense objects in the defense or industrial domains, and imaging in nuclear physics.

We demonstrate experimentally the control of the polarization (ellipticity and polarization axis) of femtosecond high-order harmonics. The method relies on a two-color (omega - 2 omega) configuration, where both omega and 2 omega generating fields have linear polarizations with a variable crossing angle. We correlate our measurements with the conservation rules of energy and linear momentum accounting for harmonic generation in a two-color field. We evidence that the generation process, and especially the number of omega and 2 omega photons absorbed for generating a given harmonic, strongly depends on the crossing angle. Finally, relying on a simple formalism, we derive analytical formulae for calculating both polarization axis and ellipticity of the separate harmonics. The model corroborates our results and represents a base for future investigations.

The achievable energy and the stability of accelerated electron beams have been the most critical issues in laser wakefield acceleration. As laser propagation, plasma wave formation and electron acceleration are highly nonlinear processes, the laser wakefield acceleration (LWFA) is extremely sensitive to initial experimental conditions. We propose a simple and elegant waveform control method for the LWFA process to enhance the performance of a laser electron accelerator by applying a fully optical and programmable technique to control the chirp of PW laser pulses. We found sensitive dependence of energy and stability of electron beams on the spectral phase of laser pulses and obtained stable 2-GeV electron beams from a 1-cm gas cell of helium. The waveform control technique for LWFA would prompt practical applications of centimeter-scale GeV-electron accelerators to a compact radiation sources in the x-ray and gamma-ray regions.

The quality of laser wakefield accelerated electrons beams is strongly determined by the physical mechanism exploited to inject electrons in the wakefield. One of the techniques used to improve the beam quality is the density transition injection, where the electron trapping occurs as the laser pulse passes a sharp density transition created in the plasma. Although this technique has been widely demonstrated experimentally, the literature lacks theoretical and numerical studies on the effects of all the transition parameters. We thus report and discuss the results of a series of particle in cell (PIC) simulations where the density transition height and downramp length are systematically varied, to show how the electron beam parameters and the injection mechanism are affected by the density transition parameters.

The Horizon 2020 Project EuPRAXIA ("European Plasma Research Accelerator with eXcellence In Applications") is preparing a conceptual design report of a highly compact and cost-effective European facility with multi-GeV electron beams using plasma as the acceleration medium. The accelerator facility will be based on a laser and/or a beam driven plasma acceleration approach and will be used for photon science, high-energy physics (HEP) detector tests, and other applications such as compact X-ray sources for medical imaging or material processing. EuPRAXIA started in November 2015 and will deliver the design report in October 2019. EuPRAXIA aims to be included on the ESFRI roadmap in 2020.

We briefly summarize the contributions that have been presented in the 5 working group 1 (WG1) sessions dedicated to electron beam from plasmas. (C) 2016 Elsevier B.V. All rights reserved.

Laser-based accelerators are gaining interest in recent years as an alternative to conventional machines [1]. In the actual ion acceleration scheme, energy and angular spread of the laser-driven beams are the main limiting factors for beam applications and different solutions for dedicated beam-transport lines have been proposed [2, 3]. In this context a system of Permanent Magnet Quadrupoles (PMQs) has been realized [4] by INFN-LNS (Laboratori Nazionali del Sud of the Instituto Nazionale di Fisica Nucleare) researchers, in collaboration with SIGMAPHI company in France, to be used as a collection and pre-selection system for laser driven proton beams. This system is meant to be a prototype to a more performing one [5] to be installed at ELI-Beamlines for the collection of ions. The final system is designed for protons and carbons up to 60 MeV/u. In order to validate the design and the performances of this large bore, compact, high gradient magnetic system prototype an experimental campaign have been carried out, in collaboration with the group of the SAPHIR experimental facility at LOA (Laboratoire d'Optique Appliquee) in Paris using a 200 TW Ti:Sapphire laser system. During this campaign a deep study of the quadrupole system optics has been performed, comparing the results with the simulation codes used to determine the setup of the PMQ system and to track protons with realistic TNSA-like divergence and spectrum. Experimental and simulation results are good agreement, demonstrating the possibility to have a good control on the magnet optics. The procedure used during the experimental campaign and the most relevant results are reported here.

The energy gain in laser wakefield accelerators is limited by dephasing between the driving laser pulse and the highly relativistic electrons in its wake. Since this phase depends on both the driver and the cavity length, the effects of dephasing can be mitigated with appropriate tailoring of the plasma density along propagation. Preceding studies have discussed the prospects of continuous phase-locking in the linear wakefield regime. However, most experiments are performed in the highly non-linear regime and rely on self-guiding of the laser pulse. Due to the complexity of the driver evolution in this regime, it is much more difficult to achieve phase locking. As an alternative, we study the scenario of rapid rephasing in sharp density transitions, as was recently demonstrated experimentally. Starting from a phenomenological model, we deduce expressions for the electron energy gain in such density profiles. The results are in accordance with particle-in-cell simulations, and we present gain estimations for single and multiple stages of rephasing. Published by AIP Publishing.

Laser-driven ion acceleration by intense, ultra-short, laser pulse has received increasing attention in recent years, and the availability of much compact and versatile ions sources motivates the study of laser-driven sources of energetic neutral atoms. We demonstrate the production of a neutral and directional beam of hydrogen and carbon atoms up to 200 keV per nucleon, with a peak flow of 2.7 x 10(13) atom s(-1). Laser accelerated ions are neutralized in a pulsed, supersonic argon jet with tunable density between 1.5 x 10(17) cm(-3) and 6 x 10(18) cm(-3). The neutralization efficiency has been measured by a time-of-flight detector for different argon densities. An optimum is found, for which complete neutralization occurs. The neutralization rate can be explained only at high areal densities (> 1 x 10(17) cm(-2)) by single electron charge transfer processes. These results suggest a new perspective for the study of neutral production by laser and open discussion of neutralization at a lower density.

An important limit for energy gain in laser-plasma wakefield accelerators is the dephasing length, after which the electron beam reaches the decelerating region of the wakefield and starts to decelerate. Here, we propose to manipulate the phase of the electron beam in the wakefield, in order to bring the beam back into the accelerating region, hence increasing the final beam energy. This rephasing is operated by placing an upward density step in the beam path. In a first experiment, we demonstrate the principle of this technique using a large energy spread electron beam. Then, we show that it can be used to increase the energy of monoenergetic electron beams by more than 50%.

The scheme of the x-ray free electron laser based on the optical undulator created by two overlapped transverse laser beams is analyzed. A kinetic theoretical description and an ad hoc numerical model are developed to account for the finite energy spread, angular divergence, and the spectral properties of the electron beam in the optical lattice. The theoretical findings are compared to the results of the one- and three-dimensional numerical modeling with the spectral free electron laser code PLARES.

The onset and evolution of magnetic fields in laboratory and astrophysical plasmas is determined by several mechanisms(1), including instabilities(2,3), dynamo effects(4,5) and ultrahigh-energy particle flows through gas, plasma(6,7) and interstellar media(8,9). These processes are relevant over a wide range of conditions, from cosmic ray acceleration and gamma ray bursts to nuclear fusion in stars. The disparate temporal and spatial scales where each process operates can be reconciled by scaling parameters that enable one to emulate astrophysical conditions in the laboratory. Here we unveil a new mechanism by which the flow of ultra-energetic particles in a laser-wakefield accelerator strongly magnetizes the boundary between plasma and non-ionized gas. We demonstrate, from time-resolved large-scale magnetic-field measurements and full-scale particle-in-cell simulations, the generation of strong magnetic fields up to 10-100 tesla (corresponding to nT in astrophysical conditions). These results open new paths for the exploration and modelling of ultrahigh-energy particle-driven magnetic-field generation in the laboratory.

Free-electron lasers (FELs) are a unique source of light, particularly in the x-ray domain. After the success of FELs based on conventional acceleration using radio-frequency cavities, an important challenge is the development of FELs based on electron bunching accelerated by a laser wakefield accelerator (LWFA). However, the present LWFA electron bunch properties do not permit use directly for a significant FEL amplification. It is known that longitudinal decompression of electron beams delivered by state-of-the-art LWFA eases the FEL process. We propose here a second order transverse beam manipulation turning the large inherent transverse chromatic emittances of LWFA beams into direct FEL gain advantage. Numerical simulations are presented showing that this beam manipulation can further enhance by orders of magnitude the peak power of the radiation.

The capability of plasmas to sustain ultrahigh electric fields has attracted considerable interest over the last decades and has given rise to laser-plasma engineering. Today, plasmas are commonly used for accelerating and collimating relativistic electrons, or to manipulate intense laser pulses. Here we propose an ultracompact plasma undulator that combines plasma technology and nanoengineering. When coupled with a laser-plasma accelerator, this undulator constitutes a millimetre-sized synchrotron radiation source of X-rays. The undulator consists of an array of nanowires, which are ionized by the laser pulse exiting from the accelerator. The strong charge-separation field, arising around the wires, efficiently wiggles the laser-accelerated electrons. We demonstrate that this system can produce bright, collimated and tunable beams of photons with 10-100 keV energies. This concept opens a path towards a new generation of compact synchrotron sources based on nanostructured plasmas.

Optimal regimes and physical processes at work are identified for the first round of laser wakefield acceleration experiments proposed at a future ClLEX facility. The Apollon-10P ClLEX laser, delivering fully compressed, near-PW-power pulses of sub-25 Is duration, is well suited for driving electron density wakes in the blowout regime in cm-length gas targets. Early destruction of the pulse (partly due to energy depletion) prevents electrons from reaching dephasing, limiting the energy gain to about 3 GeV. However, the optimal operating regimes, found with reduced and full three-dimensional particle-in-cell simulations, show high energy efficiency, with about 10% of incident pulse energy transferred to 3 GeV electron bunches with sub-5% energy spread, half-nC charge, and absolutely no low-energy background. This optimal acceleration occurs in 2 cm length plasmas of electron density below 10(18) cm(-3). Due to their high charge and low phase space volume, these multi-GeV bunches are tailor-made for staged acceleration planned in the framework of the ClLEX project. The hallmarks of the optimal regime are electron self-injection at the early stage of laser pulse propagation, stable self-guiding of the pulse through the entire acceleration process, and no need for an external plasma channel. With the initial focal spot closely matched for the nonlinear self-guiding, the laser pulse stabilizes transversely within two Rayleigh lengths, preventing subsequent evolution of the accelerating bucket. This dynamics prevents continuous self-injection of background electrons, preserving low phase space volume of the bunch through the plasma. Near the end of propagation, an optical shock builds up in the pulse tail. This neither disrupts pulse propagation nor produces any noticeable low-energy background in the electron spectra, which is in striking contrast with most of existing GeV-scale acceleration experiments. (C) 2013 Elsevier B.V. All rights reserved

Laser-wakefield acceleration constitutes a promising technology for future electron accelerators. A crucial step in such an accelerator is the injection of electrons into the wakefield, which will largely determine the properties of the extracted beam. We present here a new paradigm of colliding-pulse injection, which allows us to generate high-quality electron bunches having both a very low emittance (0: 17 mm . mrad) and a low energy spread (2%), while retaining a high charge (similar to 100 pC) and a short duration (3 fs). In this paradigm, the pulse collision provokes a transient expansion of the accelerating bubble, which then leads to transverse electron injection. This mechanism contrasts with previously observed optical injection mechanisms, which were essentially longitudinal. We also specify the range of parameters in which this new type of injection occurs and show that it is within reach of existing high-intensity laser facilities.

While laser-plasma accelerators have demonstrated a strong potential in the acceleration of electrons up to giga-electronvolt energies, few experimental tools for studying the acceleration physics have been developed. In this paper, we demonstrate a method for probing the acceleration process. A second laser beam, propagating perpendicular to the main beam, is focused on the gas jet few nanosecond before the main beam creates the accelerating plasma wave. This second beam is intense enough to ionize the gas and form a density depletion, which will locally inhibit the acceleration. The position of the density depletion is scanned along the interaction length to probe the electron injection and acceleration, and the betatron X-ray emission. To illustrate the potential of the method, the variation of the injection position with the plasma density is studied. (C) 2013 AIP Publishing LLC.

Laser wakefield acceleration experiments were performed using a 30 fs, 1 J laser pulse interacting with an underdense helium plasma. Temporally resolved polarimetry measurements demonstrate the presence of magnetic fields at the ionization front within the plasma which had a peak strength of similar to 2.8 MG and a radial extent of approximately 200 mu m. The field was seen to vary in strength over picosecond time-scales. The field is likely generated by return current generated in the plasma at the interface between plasma and neutral gas and which is caused by hot electrons produced in the wakefield during formation of a plasma 'bubble' and prior to the time of wave-breaking (beam injection). These effects are confirmed using particle-in-cell simulations. Such measurements can be useful as a diagnostic of bubble formation in laser wakefield accelerators.

Laser-plasma accelerators can produce high-quality electron beams, up to giga electronvolts in energy, from a centimetre scale device. The properties of the electron beams and the accelerator stability are largely determined by the injection stage of electrons into the accelerator. The simplest mechanism of injection is self-injection, in which the wakefield is strong enough to trap cold plasma electrons into the laser wake. The main drawback of this method is its lack of shot-to-shot stability. Here we present experimental and numerical results that demonstrate the existence of two different self-injection mechanisms. Transverse self-injection is shown to lead to low stability and poor-quality electron beams, because of a strong dependence on the intensity profile of the laser pulse. In contrast, longitudinal injection, which is unambiguously observed for the first time, is shown to lead to much more stable acceleration and higher-quality electron beams.

We show that electron bunches in the 50-100 keV range can be produced from a laser wakefield 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. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4792057]

Using a laser plasma accelerator, experiments with a 80 TW and 30 fs laser pulse demonstrated quasi-monoenergetic electron spectra with maximum energy over 0.4 GeV. This is achieved using a supersonic He gas jet and a sharp density ramp generated by a high intensity laser crossing pre-pulse focused 3 ns before the main laser pulse. By adjusting this crossing pre-pulse position inside the gas jet, among the laser shots with electron injection, more than 40% can produce quasi-monoenergetic spectra. This could become a relatively straight forward technique to control laser wakefield electron beams parameters.

A density perturbation in an underdense plasma was used to improve the quality of electron bunches produced in the laser-plasma wakefield acceleration scheme. Quasi-monoenergetic electrons were generated by controlled injection in the longitudinal density gradients of the density perturbation. By tuning the position of the density perturbation along the laser propagation axis, a fine control of the electron energy from a mean value of 60 MeV to 120 MeV has been demonstrated with a relative energy-spread of 15 +/- 3.6%, divergence of 4 +/- 0.8 mrad, and charge of 6 +/- 1.8 pC. 

This review article highlights the tremendous evolution of the research on laser plasma accelerators which has, in record time, led to the production of high quality electron beams at the GeV level, using compact laser systems. I will describe the path we followed to explore different injection schemes and I will present the most significant breakthrough which allowed us to generate stable, high peak current and high quality electron beams, with control of the charge, of the relative energy spread and of the electron energy. 

The decade research activity on laser plasma accelerator using the “salle jaune” laser has been very fruitful. Using the compact 30TW - 30fs laser system developed at LOA a number of important breakthroughs have been obtained. By exploring different regime of laser plasma interaction new approaches for controlling electrons injection into relativistic plasma wave have been achieved. The progresses of laser plasma accelerators have allowed us to produce high quality of energetic electron beams with controllable parameters. Because of their interesting properties such as shortness, brightness and spatial quality, these beams are promising for applications in many fields, including medicine, radiation biology, chemistry, physics and material science, security, and of course accelerator science. Thanks to the advent of compact and powerful lasers, with moderate costs and high repetition rate, this research field becomes more and more active.

We report on the characterization of recently developed submillimetric He gas jets with peak density higher than 10(21) atoms/cm(3) from cylindrical and slightly conical nozzles of throat diameter of less than 400 mu m. Helium gas at pressure 300-400 bar has been developed for this purpose to compensate the nozzle throat diameter reduction that affects the output mass flow rate. The fast-switching electro-valve enables to operate the jet safely for multi-stage vacuum pump assembly. Such gaseous thin targets are particularly suitable for laser-plasma interaction studies in the unexplored near-critical regime.

Development of x-ray phase contrast imaging applications with a laboratory scale source have been limited by the long exposure time needed to obtain one image. We demonstrate, using the Betatron x-ray radiation produced when electrons are accelerated and wiggled in the laser-wakefield cavity, that a high-quality phase contrast image of a complex object (here, a bee), located in air, can be obtained with a single laser shot. The Betatron x-ray source used in this proof of principle experiment has a source diameter of 1.7 mu m and produces a synchrotron spectrum with critical energy E-c = 12.3 +/- 2.5 keV and 109 photons per shot in the whole spectrum. 

Betatron x-ray radiation in laser-plasma accelerators is produced when electrons are accelerated and wiggled in the laser-wakefield cavity. This femtosecond source, producing intense x-ray beams in the multi-kiloelectronvolt (keV) range, has been observed at different interaction regimes using a high-power laser from 10 to 100 TW. However, none of the spectral measurements carried out were at sufficient resolution, bandwidth and signal-to-noise ratio to precisely determine the shape of spectra with a single laser shot in order to avoid shot-to-shot fluctuations. In this paper, the Betatron radiation produced using a 80 TW laser is characterized by using a single photon counting method. We measure in a single shot spectra from 8 to 21 keV with a resolution better than 350 eV. The results obtained are in excellent agreement with theoretical predictions and demonstrate the synchrotron-type nature of this radiation mechanism. The critical energy is found to be E-c = 5.6 +/- 1 keV for our experimental conditions. In addition, the features of the source at this energy range open up novel opportunities for applications in time-resolved x-ray science.

Thomas has recently derived scaling laws for x-ray radiation from electrons accelerated in plasma bubbles, as well as a threshold for the self-injection of background electrons into the bubble [A. G. R. Thomas, Phys. Plasmas 17, 056708 (2010)]. To obtain this threshold, the equations of motion for a test electron are studied within the frame of the bubble model, where the bubble is described by prescribed electromagnetic fields and has a perfectly spherical shape. The author affirms that any elliptical trajectory of the form x'(2)/gamma(2)(p) + y'(2) = R-2 is solution of the equations of motion (in the bubble frame), within the approximation p'(2)(y)/p'(2)(x). In addition, he highlights that his result is different from the work of Kostyukov et al. [Phys. Rev. Lett. 103, 175003 (2009)], and explains the error committed by Kostyukov–Nerush–Pukhov–Seredov (KNPS). In this comment, we show that numerically integrated trajectories, based on the same equations than the analytical work of Thomas, lead to a completely different result for the self-injection threshold, the result published by KNPS [Phys. Rev. Lett. 103, 175003 (2009)]. We explain why the analytical analysis of Thomas fails and we provide a discussion based on numerical simulations which show exactly where the difference arises. We also show that the arguments of Thomas concerning the error of KNPS do not hold, and that their analysis is mathematically correct. Finally, we emphasize that if the KNPS threshold is found not to be verified in PIC (Particle In Cell) simulations or experiments, it is due to a deficiency of the model itself, and not to an error in the mathematical derivation.

The present paper elaborates on the cold injection scheme, which was recently proposed in the context of laser wakefield acceleration (Davoine et al 2009 Phys. Rev. Lett. 102 065001). This scheme allows one to inject a bunch of electrons into a laser wakefield, which is possible thanks to the collision between the main and a counter-propagating laser pulse. Unlike in the conventional colliding pulse schemes, in this process, a beatwave is created during the collision, which allows the injection of electrons with negligible heating. In this paper, we show that the injection of on-axis electrons observed in simulations is well described by a one-dimensional (1D) model, as long as conditions given here are satisfied. Injection of off-axis electrons is also influenced by transverse effects, but the basic mechanisms remain the same. Then, a comparison with the conventional colliding pulse schemes shows that each scheme can occur in different regimes. In particular, cold injection proves to be more interesting regarding the energy spread issue. Indeed, the simulations demonstrate that electron bunches with sub-MeV absolute energy spreads can be injected, leading, after acceleration, to electrons at several GeV and relative energy spread below 1%.

The injection of quasimonoenergetic electron beams into a laser wakefield accelerator is demonstrated experimentally using density gradients at the edges of a plasma channel. In the experiment, two laser pulses are used; the main laser pulse drives a wakefield, while a second countercrossing laser beam produces a plasma whose expansion creates a channel with significant density gradients. Local injection of electrons in the wakefield is triggered by wave breaking in the density ramp. The injection is localized spatially and leads to the generation of collimated and narrow energy spread relativistic electron beams at the 100 MeV level, with charges in the range of 20-100 pC. The stability of this injection process is compared to the stability of the colliding pulse injection process and is found to be inferior for our experimental conditions. On the other hand, it is found that as the electron beam divergence is smaller in the case of gradient injection, the transverse emittance might be better.

The influence of pulse duration on proton acceleration using sub-ps (300-30 fs), ultra-intense (3.6 x 10(18) W/cm(2) to 3.6 x 10(19) W/cm(2)), constant energy (0.14J) laser pulses is studied using 2D simulations. The entire pulse duration is modelled, so that during the rising edge of the pulse a preplasma can naturally expand from the target front and rear sides into vacuum, altering, respectively, laser absorption and electrostatic field generation. In this paper, we study this effect for two target profiles sharp-edge profile and smooth density gradient at the front side-and we point out the existence of a weak optimum pulse duration for proton acceleration. For the different pulse durations we considered, we first show that the maximum proton energy variations are similar to those of the rear side electrostatic field amplitude. The energy variations, however, are smaller than expected from the field variations, and we explain this effect by considerations on the characteristic proton acceleration time. 

We use two-dimensional (2D) particle-in-cell simulations to study the interaction of short-duration, moderately relativistic laser pulses with submicrometric dense hydrogen plasma slabs. Particular attention is devoted to proton acceleration by the target normal sheath mechanism. We observed that improved acceleration due to relativistic transparency of the target is unlikely for the shortest pulses, even for ultra-thin (similar to 10 nm) targets. This mechanism would require either longer pulses or higher laser intensities. As the target density and thickness, pulse length, duration and polarization are varied, we see clear relationships between laser irradiance, hot electron temperature and peak proton energy. All these explain why, at a given incident laser energy level, the highest proton energy is not always obtained for the shortest-duration, highest-intensity pulse.

Low-intensity laser prepulses (<1013 W cm−2, nanosecond duration) are a major issue in experiments on laser-induced generation of protons, often limiting the performances of proton sources produced by high-intensity lasers (≈1019 W cm−2, picosecond or femtosecond duration). Depending on the intensity regime, several effects may be associated with the prepulse, some of which are discussed in this paper: (i) destruction of thin foil targets by the shock generated by the laser prepulse; (ii) creation of preplasma on the target front side affecting laser absorption; (iii) deformation of the target rear side; and (iv) whole displacement of thin foil targets affecting the focusing condition. In particular, we show that under oblique high-intensity irradiation and for low prepulse intensities, the proton beam is directed away from the target normal. Deviation is towards the laser forward direction, with an angle that increases with the level and duration of the ASE pedestal. Also, for a given laser pulse, the beam deviation increases with proton energy. The observations are discussed in terms of target normal sheath acceleration, in combination with a laser-controllable shock wave locally deforming the target surface.

Experimental measurements of proton acceleration with high intensity and high-contrast short laser pulses have been carried out over an order of magnitude range in target thickness and laser pulse duration. The dependence of the maximum proton energy with these parameters is qualitatively supported by two-dimensional particle-in-cell simulations. They evidence that two regimes of proton acceleration can take place, depending on the ratio between the density gradient and the hot electron Debye length at the rear target surface. As this ratio can be affected by the target thickness, a complex interplay between pulse duration and target thickness is observed. Measurements and simulations support unexpected variations in the laser absorption and hot electron temperature with the pulse duration and laser intensity, for which density profile modification at the target front surface is the controlling parameter.

The high current electron beam losses have been studied experimentally with 0.7 J, 40 fs, 6 10(19) W cm(-2) laser pulses interacting with Al foils of thicknesses 10-200 mu m. The fast electron beam characteristics and the foil temperature were measured by recording the intensity of the electromagnetic emission from the foils rear side at two different wavelengths in the optical domain, approximate to 407 nm ( the second harmonic of the laser light) and approximate to 500 nm. The experimentally observed fast electron distribution contains two components: one relativistic tail made of very energetic (T(h)(tail) approximate to 10 MeV) and highly collimated (7 degrees +/- 3 degrees) electrons, carrying a small amount of energy (less than 1% of the laser energy), and another, the bulk of the accelerated electrons, containing lower-energy (T(h)(bulk) = 500 +/- 100 keV) more divergent electrons (35 +/- 5 degrees), which transports about 35% of the laser energy. The relativistic component manifests itself by the coherent 2.0 emission due to the modulation of the electron density in the interaction zone. The bulk component induces a strong target heating producing measurable yields of thermal emission from the foils rear side. Our data and modeling demonstrate two mechanisms of fast electron energy deposition: resistive heating due to the neutralizing return current and collisions of fast electrons with plasma electrons. The resistive mechanism is more important at shallow target depths, representing an heating rate of 100 eV per Joule of laser energy at 15 mu m. Beyond that depth, because of the beam divergence, the incident current goes under 10(12) A cm(-2) and the collisional heating becomes more important than the resistive heating. The heating rate is of only 1.5 eV per Joule at 50 mu m depth.

In recent experiments, quasi-monoenergetic and well-collimated very-high energy electron (VHEE) beams were obtained by laser-plasma accelerators. We investigate their potential use for radiation therapy. Monte Carlo simulations are used to study the influence of the experimental characteristics such as beam energy, energy spread and initial angular distribution on the dose distributions. It is found that magnetic focusing of the electron beam improves the lateral penumbra. The dosimetric properties of the laser-accelerated VHEE beams are implemented in our inverse treatment planning system for intensity-modulated treatments. The influence of the beam characteristics on the quality of a prostate treatment plan is evaluated. In comparison to a clinically approved 6MV IMRT photon plan, a better target coverage is achieved. The quality of the sparing of organs at risk is found to be dependent on the depth. The bladder and rectum are better protected due to the sharp lateral penumbra at low depths, whereas the femoral heads receive a larger dose because of the large scattering amplitude at larger depths.

To take full advantage of a laser-plasma accelerator, stability and control of the electron beam parameters have to be achieved. The external injection scheme with two colliding laser pulses is a way to stabilize the injection of electrons into the plasma wave, and to easily tune the energy of the output beam by changing the longitudinal position of the injection. In this Letter, it is shown that by tuning the optical injection parameters, one is able to control the phase-space volume of the injected particles, and thus the charge and the energy spread of the beam. With this method, the production of a laser accelerated electron beam of 10 pC at the 200 MeV level with a 1% relative energy spread at full width half maximum (3.1% rms) is demonstrated. This unique tunability extends the capability of laser-plasma accelerators and their applications.

A review of recent simulation and experimental studies of the colliding pulse injection scheme is presented. One dimensional particle in cell simulations show that when the colliding pulses have parallel polarizations, the dominant effects that have to be considered for modeling electron injection in plasma waves are (i) stochastic heating and (ii) wakefield inhibition at the collision. With cross polarized pulses, injection of an electron beam is still possible because stochastic heating still occurs. However, it is found numerically that the injection threshold is higher in this case. The simulations also underline the possibility of tuning the electron beam parameters by modifying the injection laser pulse. Experiments (i) validate these scenarios and show that stable and high quality electron beams are produced when two counterpropagating laser pulses collide in an underdense plasma and (ii) confirm very clearly the existence of a threshold for injection, which is higher with cross polarized pulses than with parallel polarized pulses.

The continuing development of powerful laser systems has permitted to extend the interaction of laser beams with matter far into the relativistic domain in which extremely high electric and magnetic fields are generated. Thanks to these tremendous fields, that only plasma can support and sustain, new and compact approaches for producing energetic particle beams have been recently achieved. The incredible progress of these laser-plasma accelerators has allowed physicists to produce high quality beams of energetic radiation and particles. These beams have interesting properties such as shortness, brightness and spatial quality, and could lend themselves to applications in many fields, including medicine (radiotherapy, proton therapy, imaging), radiation biology (short-time-scale), chemistry (radiolysis), physics and material science (radiography, electron and photon diffraction), security (material inspection), and of course accelerator science. Stimulated by the advent of compact and powerful lasers, with moderate costs and high repetition rate, this research field has witnessed considerable growth in the past few years, and the promises of laser-plasma accelerators are in tremendous progress. The recent years in particular have seen spectacular progress in the acceleration of electrons and of ions, both in terms of energy and in terms of quality of the beams. To cite this article: V Malka, P Mora, C R. Physique 10 (2009). 

The interaction of two laser pulses in an underdense plasma has been proven to be able to inject electrons into plasma waves, thus providing a stable and tunable source of electrons. Whereas previous works focused on the ‘beatwave’ injection scheme in which two lasers with the same polarization collide in a plasma, this present paper studies the effect of polarization and more specifically the interaction of two colliding cross-polarized laser pulses. It is shown both theoretically and experimentally that electrons can also be preaccelerated and injected by the stochastic heating occurring at the collision of two cross-polarized lasers and thus, a new regime of optical injection is demonstrated. It is found that injection with cross-polarized lasers occurs at higher laser intensities.

Two-dimensional particle-in-cell simulations are performed to study laser-induced proton acceleration from solid-density targets in the presence of laser-generated preformed plasma. The preplasma generation and hydrodynamics are described using a one-dimensional Lagrangian code. The electron acceleration mechanism is shown to depend on the plasma scale length, exhibiting a transition from jxB heating to standing wave heating as smoother and smoother profiles are considered. Accordingly, the relativistic electron temperature and the cutoff proton energy are found to increase with the preplasma characteristic length.

We present the results of an experimental investigation of the temporal evolution of plasmas produced by high power laser irradiation of various types of target materials. We obtained 'high-quality' interferometric data on the evolution of the plasma electronic profile, which can be directly compared with analytical models and numerical simulations. For aluminium and plastic targets, the agreement with 1D simulations performed with the hydrocode MULTI is excellent, at least for large times (t >= 400 ps). In this case, simulations also show that the effect of radiation transport is negligible. The situation is quite different for gold targets for which, in order to get a fair enough agreement, we must take radiation transport into account.

In this paper, we report on recent experimental results on electron acceleration using two counterpropagating ultrashort and ultraintense laser pulses. We demonstrate that the use of a second laser pulse provides enhanced control over the injection and subsequent acceleration of electrons into plasma wakefields. The experimental results show that the electron beams obtained in this manner are collimated, monoenergetic, tuneable, and stable. Simulations of the process emphasize the crucial role of kinetic effects and the need for self-consistent models.

During experiments performed on a laser-plasma-based accelerator, correlation of the electron output angle with the electron energy has been observed. These spectral oscillations of the electron beam centroid are attributed to betatron oscillations of the electron beam during its propagation. An analytical model for betatron oscillations including constant longitudinal acceleration is described and used to validate the scenario and retrieve physical parameters. The oscillations can arise from an off-axis injection of the electrons, which can be reproduced using an asymmetric laser intensity pro. le in Particle-In-Cell ( PIC) simulations. This study emphasizes the influence of non-ideal interaction conditions inherent to experiments.

We present the experimental results, and their analysis, connected to the possibility to control laser inhomogeneities exploiting the non-uniform electron density distribution created by the laser while propagating in a gas jet. The induced self-refraction in the plasma created in the gas medium results in re-distributing local over-intensities over larger surfaces. The experiment at the PALS laser facility has been performed creating a large non-uniformity by a wedge arrangement and recording the static and dynamic images with and without the coupling to an Al target.

An electron injector concept for a laser-plasma accelerator was developed by E. Esarey [Phys. Rev. Lett. 79, 2682 (1997)] and G. Fubiani [Phys. Rev. E 70, 016402 (2004)]; it relies on the use of counterpropagating ultrashort laser pulses. In the latter work, the scheme is as follows: the pump laser pulse generates a large-amplitude laser wakefield (plasma wave). The counterpropagating injection pulse interferes with the pump laser pulse to generate a beatwave pattern. The ponderomotive force of the beatwave is able to inject plasma electrons into the wakefield. In this paper, this injection scheme is studied using one-dimensional Particle-in-Cell simulations. The simulations reveal phenomena and important physical processes that were not taken into account in previous models. In particular, at the collision of the laser pulses, most plasma electrons are trapped in the beatwave pattern and cannot contribute to the collective oscillation supporting the plasma wave. At this point, the fluid approximation fails and the plasma wake is strongly inhibited. Consequently, the injected charge is reduced by one order of magnitude compared to the predictions from previous models. 

We present a series of experimental results, and their interpretation, connected to various aspects of the hydrodynamics of laser produced plasmas. Experiments were performed using the Prague PALS iodine laser working at 0.44 mu m wavelength and irradiances up to a few 10(14)W/cm(2). By adopting large focal spots and smoothed laser beams, the lateral energy transport and lateral expansion have been avoided. Therefore we could reach a quasi one-dimensional regime for which experimental results can be more easily and property compared to available analytical models.

For protons of energy up to a few MeV, the temporal evolution of etched latent tracks in CR-39 nuclear track detector has been numerically modeled by assuming that the electronic energy loss of the protons governs the latent track formation. The technique is applied in order to obtain the energy spectrum of high intensity laser driven proton beams, with high accuracy. The precise measurement of the track length and areal track density have been achieved by scanning short etched, highly populated CR-39 employing atomic force microscope. 

The experimental demonstration of laser acceleration of ions to multi-MeV energies with short, intense laser pulses has spurred the prospect of using this ion source for medical isotope production. Using numerical models for laser-plasma interaction and ion acceleration, then for ion transport and isotope production, we compute the isotope yields that could be expected from such sources, and their variations with interaction parameters such as target thickness and laser intensity. Using 36 fs, 4x10(20) W/cm(2) pulses at kilohertz repetition rate, more than 100 GBq of F-18 are expected after irradiation for 1 h. 

The use of ultra short laser pulses delivered by powerful laser systems has permitted the emergence of new approaches for generating energetic particle beams. By focusing these laser pulses onto matter. extremely large electric fields can be generated, reaching the TV/rn level well in excess than those produced by conventional accelerators. As a result, the distance over which particles extracted from the target can be accelerated to hundreds of MeV is reduced to distances on the order of millimetres. These laser-produced electron beams have a number of interesting properties and could lead themselves to applications in many fields, including medicine (radiotherapy), radiobiology (short-time-scale, low dose irradiation), chemistry (radiolysis), non-destructive material inspection by radiography, and accelerator physics.

The concepts of the laser-plasma based accelerator and injector are discussed here. The recent tests done at LOA as well as design studies of high-quality GeV electron beam production with low energy spread (1%) are presented. These laser-produced particle beams have a number of interesting properties and could lend themselves to applications in many fields, including medicine ( radiotherapy), chemistry (radiolysis), and accelerator physics. They could be used as a source for the production of gamma ray beams for nondestructive material inspection by radiography, or for future compact X-free electron laser machines.

We report on some recent experimental results on proton production from ultra-intense laser pulse interaction with thin aluminium and plastic foil targets. These results were obtained at Laboratoire d'Optique Appliquee with the 100TW 'salle jaune' laser system, delivering 35 fs laser pulses at 0.8 mu m, reaching a maximum intensity on target of a few 10(19) W/cm(2).In such extreme interaction conditions, an intense and collimated relativistic electron current is injected from the plasma created on the laser focal spot into the cold interior of the target. Its transport through dense matter, ruled by both collisions and self-induced (electro-magnetic) field effects, is the driving mechanism for proton acceleration from the rear side of thin foils: when reaching and leaving the foil rear-side, the fast electrons create a large charge separation and a huge electrostatic field with a maximum value of few TV/m, capable of accelerating protons.A parametric study as a function of the laser driver and target parameters indicates an optimal value for target thickness, which strongly depends on the laser prepulse duration. In our experiments, we did irradiate targets of various materials (CH, Al, Au) changing the prepulse duration by using fast Pockels cells in the laser chain. CR-39 nuclear track detectors with Al filters of different thickness and a Thomson parabola were used to detect proton generation. The best results were obtained for 2 mu m Al targets, leading to the generation of proton energies with energies up to 12 MeV.

We present here results obtained in an experiment carried out using the CPA beam of the "Salle Jaune" laser system at Laboratoire d'Optique Appliquee (LOA). The generation of high energy electrons and protons escaping from the plasma has been investigated in the interaction of a 2 J, 30 fs laser with CH or metallic foils. The energy and angular distributions of the supra-thermal electrons produced with different targets are characterized by using both an electron spectrometer and bremsstrahlung induced (gamma,n) reactions. We measured simultaneously the number of energetic protons produced using (p,n) reactions. A correlation between the electrons and the protons production is observed together with a dependence of the number of supra-thermal electrons on the atomic number of the target element.

Laser plasma accelerators are currently able to produce ultra-short, quasi-monoenergetic and collimated electron beam of major interest for the study of ultra-fast phenomena, material science and medicine. A laser based accelerator designed to produce GeV electrons with 2% bandwidth and low emittance (0.02 mm mrad) is presented. This compact approach (cm scale for the plasma) and tens of meters length for the whole facility should permit a miniaturization and cost reduction of future accelerators and associated X-Free Electrons Lasers. 

The implementation of gas jet targets appears to be a very attractive alternative to pre-exploded thin foils targets for relativistic laser plasma interactions. In this article, we report a review of recent results obtained by focusing a nanosecond laser beam onto a gas jet target. The benefit of gas jet targets for the generation of homogeneous and large scale plasmas, which are of interest for a MJ program will be presented. Additionally, laser beam smoothing, the studies of parametric instabilities (Raman and Brillouin) and applications in atomic physic in warm and dense plasmas will be discussed.

A new method for accelerating proton beams, based on the use of intense and short laser pulses, is presented. It is shown that by focusing an ultrashort laser pulse onto a thin foil target, a proton beam with energy LIP to tens of MeV is produced. Due to the original properties of these beams unexplored fields in science will be discovered. In addition, this compact and low cost approach would probably be of great interest for medical applications. 

The transport of an intense electron beam produced by ultrahigh intensity laser pulses through metals and insulators has been studied by high resolution imaging of the optical emission from the targets. In metals, the emission is mainly due to coherent transition radiation, while in plastic, it is due to the Cerenkov effect and it is orders of magnitude larger. It is also observed that in the case of insulators the fast-electron beam undergoes strong filamentation and the number of filaments increases with the target thickness. This filamented behavior in insulators is due to the instability of the ionization front related to the electric field ionization process. The filamentary structures characteristic growth rate and characteristic transversal scale are in agreement with analytical predictions.

Experiments to examine the generation of relativistic plasma waves via a high-intensity short-pulse beat-wave scheme are described in detail. The pulse stretcher of the Vulcan chirped-pulse amplification (CPA) laser system was modified to produce two frequency, 3 ps pulses focusable to intensities up to 10(18) W cm(-2). Short high-intensity pulses were used to avoid limitations to the plasma-wave amplitude due to the modulational instability. Two experiments were undertaken, at 3 and 10 TW, with the generation of plasma waves diagnosed by measuring the sidebands produced in the spectrum of the forward scattered beam. A resonance in the sideband signal was observed for an initial plasma density higher than expected for the given beat frequency. This resonance shift can be attributed to transverse ponderomotive expulsion of plasma electrons from the laser focal region. A monotonically increasing background was also observed, which was due to nonresonant cross-phase modulation.

The recent and continuous development of powerful laser systems has permitted the emergence of new approaches for generating energetic electron beams. By focusing light pulses containing a few joules of energy in a few tens of femtoseconds onto gas jets, extremely large electric fields can be generated, reaching the terravolts per metre level. Such fields are 10 000 times greater than those produced in the radio-frequency cavities of conventional accelerators. As a result, the length over which electrons extracted from the target can be accelerated to hundreds of MeV is reduced to a few millimetres. The reduction of the size and the cost of laser-plasma accelerators is a promising consequence, but these electron beams also reveal original properties, which make them a wonderful tool for science. By adjusting the interaction parameters, the electron energy distribution can be tuned from a maxwellian-like distribution to a quasi-monoenergetic one. The new properties of these laser-based particle beams are well suited to many applications in different fields, including medicine (radiotherapy), chemistry (ultrafast radiolysis), material science (non-destructive material inspection using radiography) and, of course, for accelerator physics.

The possibility of accelerating electrons to the GeV level using a Petawatt laser focused in a uniform plasma is investigated. The proposed scheme relies on the wakefield acceleration of an electron bunch from a state-of-the-art radio-frequency accelerator. Using an analytical model as well as numerical simulations performed with WAKE [P. Mora and T. M. Antonsen, Phys. Plasmas 4, 217 (1997)], a systematical study of the injector parameters is carried out. In particular, it is found that the quality of the accelerated electron bunch-in terms of bunch length and energy spread-depends crucially on the injection energy. Injection energies of a few MeV lead to a GeV electron beam with sub-100 fs bunches and 10% energy spreads. Most of the features of the acceleration process can be explained within the linear response framework, including both the reduction of energy spread and bunch length at low injection energies. The role of nonlinear effects is discussed.

Plasma-based accelerators have been proposed for the next generation of compact accelerators because of the huge electric fields they can support. However, it has been difficult to use them efficiently for applications because they produce poor quality particle beams with large energy spreads. Here, we demonstrate a dramatic enhancement in the quality of electron beams produced in laser-plasma interaction: an ultrashort laser pulse drives a plasma bubble which traps and accelerates plasma electrons to a single energy. This produces an extremely collimated and quasi-monoenergetic electron beam with a high charge of 0.5 nanocoulomb at energy 170 + 20 MeV.

A method for obtaining quantitative information about electric field and charge distributions from proton imaging measurements of laser-induced plasmas is presented. A parameterised charge distribution is used as target plasma. The deflection of a proton beam by the electric field of such a plasma is simulated numerically as well as the resulting proton density, which will be obtained on a screen behind the plasma according to the proton imaging technique. The parameters of the specific charge distributions are delivered by a combination of linear regression and nonlinear fitting of the calculated proton density distribution to the measured optical density of a radiochromic film screen changed by proton exposure. It is shown that superpositions of spherical Gaussian charge distributions as target plasma are sufficient to simulate various structures in proton imaging measurements, which makes this method very flexible.

The fundamental importance of understanding the primary effects of ionizing radiations in liquid phase and solutions is emphasized in fields such as electron transfer reactions, radical chemistry and radiobiology. With the advent of ultrashort optical pulses and powerful laser systems (TW lasers), ultrafast spectroscopic investigations might conjecture the direct observation of primary events induced by low-energy (photons) and high-energy (relativistic electrons) radiations. The different points discussed in this paper concern the investigation of short-time solvent caging effects on elementary radical reactions in homogeneous liquid phase and nascent spurs.

An electron beam from a laser-plasma accelerator is converted into a γ-ray source using bremsstrahlung radiation in a dense material. The γ-ray beam has a pointlike source size because it is generated by a high quality electron beam with a small source size and a low divergence. Using this γ-ray source, the radiography of complex and dense objects with submillimeter resolution is performed. It is the first evidence of a γ-ray source size of a few hundreds micrometers produced with laser-driven accelerators. This size is consistent with results from Monte Carlo simulations.

The synchrotron x-ray source based on the betatron oscillation of a relative electron in a laser-produced ion channel was investigated. Nonlinear Thompson scattering radiation emitted by the electrons of the plasma oscillating in the intense laser field was also proposed. The brightness of produced x-ray radiation was estimated from the pulse duration and the size of the x-ray source. The results show that the ultrashort-duration pulses of x-ray beams could be produced by the synchrotron radiation mechanism in a compact device.

With the recent advent of table-top terawatt Ti:Sa laser amplifier systems, laser plasma interactions provide high-energy, femtosecond electron bunches, which might conjecture direct observation of radiation events in media of biological interest. We report on the first femtolysis studies using such laser produced relativistic electron pulses in the 2.5-15 MeV range. A real-time observation of elementary radical events is performed on water molecules and media containing an important disulfide biomolecule. The primary yield of a reducing radical produced in clusters of excitation-ionisation events (spurs) has been determined at t ∼3.5 10-12 s. These data provide important information about the initial energy loss and spatial distribution of early radical events. Femtolysis studies devoted to a disulfide biomolecule is noteworthy as it is the first time that a primary ionisation event can be controlled by an ultrafast radical anion formation in the prethermal regime. This innovating domain foreshadows the development of new applications in radiobiology (microdosimetry at the nanometric scale). In the near future, electron femtolysis studies would clearly enhance the understanding of radiation-induced damages in biological confined spaces (aqueous groove of DNA and protein pockets).

The forthcoming laser installations related to inertial confinement fusion, Laser Mégajoule (LMJ) (France) and National Ignition Facility (NIF) (USA), require multidimensional numerical simulation tools for interpreting current experimental data and to perform predictive modeling for future experiments. Simulations of macroscopic plasma volumes of the order of 1 mm 3 and laser exposure times of the order of hundreds of picoseconds are necessary. We present recent developments in the PARAX code towards this goal. The laser field is treated in a standard paraxial approximation in three dimensions. The plasma response is described by single-fluid, two-temperature, fully nonlinear hydrodynamical equations in the plane transverse to the laser propagation axis. The code also accounts for the dominant nonlocal transport terms in spectral form originating from a linearized solution to the Fokker-Planck equation. The simulations of interest are hohlraum plasmas in the case of indirect drive or the plasma corona for direct drive. Recent experimental results on plasma-induced smoothing of RPP laser beams are used to validate the code.

The transverse emittance of a relativistic electron beam generated by the interaction of a high-intensity laser with an underdense plasma has been measured with the “pepper-pot” method. For parameters pertaining to the forced laser wakefield regime, we have measured an emittance as low as (2.7±0.9) π   mm mrad for (55±2)   MeV electrons. These measurements are consistent with 3D particle-in-cell simulations of the experiment, which additionally show the existence of a relatively large halo around the beam core.

Proton beams of up to 10 MeV have been obtained by the interaction of a 10 Hz “table-top” laser, focused to intensities of 6×1019 W/cm2, with 6-μm-thin foil targets. Such proton beams can be used to induce 11B(p,n)11C reactions, which could yield an integrated activity of 13.4 MBq (0.36 mCi) after 30 min laser irradiation. This can be extended to GBq levels using similar lasers with kilohertz repetition rates, making this positron-emission tomography isotope production scheme comparable to the one using conventional accelerators.

The interaction of short and intense laser pulses with plasmas is a very efficient source of relativistic electrons with tunable properties. In low-density plasmas, we observed bunches of electrons up to 200 MeV, accelerated in the wakefield of the laser pulse. Less energetic electrons (tens of megaelectronvolt) have been obtained, albeit with a higher efficiency, during the interaction with a pre-exploded foil or a solid target. When these relativistic electrons slow down in a thick tungsten target, they emit very energetic Bremsstrahlung photons which have been diagnosed directly with photoconductors, and indirectly through photonuclear activation measurements. Dose, photoactivation, and photofission measurements are reported. These results are in reasonable agreement, over three orders of magnitude, with a model built on laser-plasma interaction and electron transport numerical simulations.

The interaction of intense laser pulses (power>30 TW) with underdense plasmas  has been studied. In the regime where the pulse length is much longer than the plasma  period (τl≫2πω−1p),(τl≫2πωp−1), the laser pulse is found to be self-modulated at the plasma frequency by the forward Raman scattering instability. Wavebreaking of the resulting plasma wave results in energetic electrons being accelerated to more than 100 MeV. Reducing the pulse length so that τl∼2πωp−1, but retaining the same power, also leads to wavebreaking. This is a direct result of a combination of  laser beam self-focusing, front-edge laser pulse steepening and relativistic lengthening of the plasma wave wavelength, which can result in a forced growth of the wakefield plasma wave, even for initially nonresonant laser pulses (τl≠πωp−1). Since, in this forced laser wakefield regime, the interaction of the plasma wave and the bunch of accelerated electrons with the laser pulse is reduced, this can result in higher energy gain (to beyond 200 MeV) and better beam quality.

The interaction of a 600 ps laser pulse at 0.53 μm wavelength with helium gas jet at an electron density of 8×1019 cm-3 was analyzed. Time resolved Thomson scattering was used to define the plasma parameters for density and temperature. Thomson scattering diagnostics were used to get quantitative values of the local and instantaneous reflectivity.

The interaction of an intense laser pulse (intensity, greater than 1019 Wcm−2) has been studied with underdense deuterium plasmas. The laser pulse is found to be self-modulated at the plasma frequency by the Raman forward-scatter instability. Wave breaking of the resulting plasma wave causes high-energy electrons to be accelerated beyond 100MeV.

Energy and angular distributions of the fast outgoing electron beam induced by the interaction of a 1 J, 30 fs, [formula presented] 10 Hz laser with a thin foil target are characterized by electron energy spectroscopy and photonuclear reactions. We have investigated the effect of the target thickness and the intensity contrast ratio level on the electron production. Using a [formula presented] polyethylene target, up to [formula presented] electrons with energies between 5 and 60 MeV were produced per laser pulse and converted to [formula presented] rays by bremsstrahlung in a Ta secondary target. The rates of photofission of U as well as photonuclear reactions in Cu, Au, and C samples have been measured. In optimal focusing conditions, about 0.06% of the laser energy has been converted to outgoing electrons with energies above 5 MeV. Such electrons leave the target in the laser direction with an opening angle of [formula presented].

Several (1.0±0.2)×10
6
thermonuclear fusion neutrons were generated from the interaction of an intense laser pulse with an underdense plasma. The neutrons were generated by D(d, n)
3
He reactions in the plasma which is heated to thermonuclear fusion temperatures. As a result, it was possible to measure the ion temperature of the underdense plasma, which was found to be 1 keV.

Plasmas irradiated by an intense laser beam have recently been demonstrated to be sources for particles (electrons, ions, positrons). During this interaction, it has been found that these charged particles can be efficiently accelerated. An overview of these results as well as some perspectives are presented here.

An experiment investigating laser self-focusing in underdense plasmas is presented. It was shown experimentally that the critical power for relativistic self-focusing Pc is not the only relevant parameter, in particular when the laser pulse duration is comparable to plasma particle motion times: ω−1p for electrons and ω−1pi for ions. Using time resolved shadowgraphy, it was demonstrated that: (i) a pulse does not relativistically self-focus if its duration is too short compared to ω−1p, even in the case where the power is greater than Pc. This is due to defocusing by the longitudinal wake which is generated by the laser pulse itself. (ii) For pulses longer than ω−1pi, self-focusing can occur even for powers lower than Pc. This is due to the radial expansion of ions, creating a channel whose effect combines with relativistic focusing and helps the pulse to self-focus.

The optimization of a cylindrical nozzle design to generate a uniform density profile for laser plasmas studies has been investigated using numerical simulations. In addition gas jet flows have been characterized using a Mach-Zehnder interferometer. The experimental results are in very good agreement with the simulation.

Measurements of Raman instabilities with subpicosecond time resolution in the referential frame of the laser pulse were carried out. It was found that Raman 30° side scattering and forward scattering occur at the back of the pulse. A simple model was developed that allows to calculate RFS spectra with a good accuracy.

The possibility of using high-intensity laser-produced plasmas as a source of energetic ions for heavy ion accelerators is addressed. Experiments have shown that neon ions greater than 6 MeV can be produced from gas jet plasmas, and well-collimated proton beams greater than 20 MeV have been produced from highintensity laser solid interactions. The proton beams from the back of thin targets appear to be more collimated and reproducible than are high-energy ions generated in the ablated plasma at the front of the target and may be more suitable for ion injection applications. Lead ions have been produced at energies up to 430 MeV.

We report on experimental results regarding the propagation of ultraintense laser pulses in a preformed plasma channel. In this experiment, the long (4-mm) fully ionized plasma channel created by the amplified spontaneous emission (ASE) was measured by interferometry before and after the propagation of the short laser pulse. Forward spectra show a cascade of Raman satellites, which merge with one another when the laser power was increased up to critical power for relativistic self-focusing P-c. The number of filaments measured by interferometry increases when the laser power increases. High conversion efficiency (approximate to 10%) of second harmonic generation was observed in the interaction.

The propagation of an ultra-intense laser pulse in a preformed plasma channel was investigated experimentally. Different regimes of propagation were observed when the pulse duration was varied. For a long pulse and powers lower than the critical power for self-focusing, PL/PC<1 (I0 = 2×1017 W/cm2), the laser pulse was guided by the preformed plasma channel over three Rayleigh lengths (4 mm) and a longitudinal plasma wave was generated by envelope self-modulation of the pulse. For a short pulse and PL/PC≫1, the interaction was dominated by self-focusing and Raman instabilities. Numerical simulations were run for the latter case, giving results comparable to the experiment. The simulations were also used to investigate the dynamics of the instabilities at high power. They showed that strong Raman side scattering first occurs at the beginning of the interaction and is then followed by self-focusing and envelope self-modulation.

We report on a detailed study of channel formation in the interaction of a nanosecond laser pulse with a He gas jet. A complete set of diagnostics is used in order to characterize the plasma precisely. The evolution of the plasma radius and of the electron density and temperature are measured by Thomson scattering, Schlieren imaging, and Mach-Zehnder interferometry. In gas jets, one observes the formation of a channel with a deep density depletion on axis. Because of ionization-induced defocusing which increases the size of the focal spot and decreases the maximum laser intensity, no channel is observed in the case of a gas-filled chamber. The results obtained in various gas-jet and laser conditions show that the channel radius, as well as the density along the propagation axis, can be adjusted by changing the laser energy and gas-jet pressure. This is a crucial issue when one wants to adapt the channel parameters in order to guide a subsequent high-intensity laser pulse. The experimental results and their comparison with one-dimensional (1D) and two-dimensional hydrodynamic simulations show that the main mechanism for channel formation is the hydrodynamic evolution behind a supersonic electron heat wave propagating radially in the plasma. It is also shown from 2D simulations that a fraction of the long pulse can be self-guided in the channel it creates. The preliminary results and analyses on this subject have been published before [V. Malka et al., Phys. Rev. Lett. 79, 2979 (1997)].

We demonstrate efficient generation of picosecond narrow-bandwidth pulses by frequency mixing of broadband opposite chirped pulses in a type I doubling crystal. This procedure allows us to produce picosecond pulses that are perfectly synchronized with femtosecond pulses. The experiment shows a decrease of the initial bandwidth by a factor of more than 30, while a high conversion efficiency is maintained.

The dependence of the stimulated Brillouin scattering (SBS) reflectivity on both the focusing aperture and the incident laser intensity has been experimentally studied in the case of a spatially smoothed beam. The experiment was performed in millimeter size, homogeneous, stationary plasmas created by irradiating a helium gas jet. SBS was excited by a 1.053-μm wavelength, 600-ps interaction beam at intensities up to 4×1014W/cm2. The saturation level of SBS reflectivities was of the order of 10%. A good agreement between the experimental SBS thresholds and the theoretical ones obtained from the one-dimensional stochastic convective SBS model [Phys. Plasmas 2, 1804 (1995)] is observed.