Response functions of imaging plates to photons, electrons and 4He particles.

Imaging plates from Fuji (BAS-SR, MS, and TR types) are phosphor films routinely used in ultra high intensity laser experiments. However, few data are available on the absolute IP response functions to ionizing particles. We have previously measured and modeled the IP response functions to protons. We focus here on the determination of the responses to photons, electrons, and (4)He particles. The response functions are obtained on an energy range going from a few tens of keV to a few tens of MeV and are compared to available data. The IP sensitivities to the different ionizing particles demonstrate a quenching effect depending on the particle stopping power.

Response functions of Fuji imaging plates to monoenergetic protons in the energy range 0.6-3.2 MeV.

We have measured the responses of Fuji MS, SR, and TR imaging plates (IPs) to protons with energies ranging from 0.6 to 3.2 MeV. Monoenergetic protons were produced with the 3.5 MV AIFIRA (Applications Interdisciplinaires de Faisceaux d'Ions en Région Aquitaine) accelerator at the Centre d'Etudes Nucléaires de Bordeaux Gradignan (CENBG). The IPs were irradiated with protons backscattered off a tantalum target. We present the photo-stimulated luminescence response of the IPs together with the fading measurements for these IPs. A method is applied to allow correction of fading effects for variable proton irradiation duration. Using the IP fading corrections, a model of the IP response function to protons was developed. The model enables extrapolation of the IP response to protons up to proton energies of 10 MeV. Our work is finally compared to previous works conducted on Fuji TR IP response to protons.

NATALIE: a 32 detector integrated acquisition system to characterize laser produced energetic particles with nuclear techniques.

We present a stand-alone system to characterize the high-energy particles emitted in the interaction of ultrahigh intensity laser pulses with matter. According to the laser and target characteristics, electrons or protons are produced with energies higher than a few mega electron volts. Selected material samples can, therefore, be activated via nuclear reactions. A multidetector, named NATALIE, has been developed to count the ß(+) activity of these irradiated samples. The coincidence technique used, designed in an integrated system, results in very low background in the data, which is required for low activity measurements. It, therefore, allows a good precision on the nuclear activation yields of the produced radionuclides. The system allows high counting rates and online correction of the dead time. It also provides, online, a quick control of the experiment. Geant4 simulations are used at different steps of the data analysis to deduce, from the measured activities, the energy and angular distributions of the laser-induced particle beams. Two applications are presented to illustrate the characterization of electrons and protons.

Fast electron transport and induced heating in solid targets from rear-side interferometry imaging.

Fast adiabatic plasma heating of a thin solid target irradiated by a high intensity laser has been observed by an optical fast interferometry diagnostic. It is driven by the hot electron current induced by the laser plasma interaction at the front side of the target. Radial and longitudinal temperature profiles are calculated to reproduce the observed rear-side plasma expansion. The main parameters of the suprathermal electrons (number, temperature, and divergence) have been deduced from these observations.

High flux of relativistic electrons produced in femtosecond laser-thin foil target interactions: characterization with nuclear techniques.

We present a protocol to characterize the high energy electron beam emitted in the interaction of an ultraintense laser with matter at intensities higher than 10(19) W cm(-2). The electron energies and angular distributions are determined as well as the total number of electrons produced above a 10 MeV threshold. This protocol is based on measurements with an electron spectrometer and nuclear activation techniques, combined with Monte Carlo simulations based on the GEANT3 code. The method is detailed and exemplified with data obtained with polypropylene and copper thin solid targets at a laser intensity of 2x10(19) W cm(-2). Special care is taken of the different sources of uncertainties. In particular, the reproducibility of the laser shots is considered.

Direct evidence of strongly inhomogeneous energy deposition in target heating with laser-produced ion beams.

We report on strong nonuniformities in target heating with intense, laser-produced proton beams. The observed inhomogeneity in energy deposition can strongly perturb equation of state (EOS) measurements with laser-accelerated ions which are planned in several laboratories. Interferometric measurements of the target expansion show different expansion velocities on the front and rear surfaces, indicating a strong difference in local temperature. The nonuniformity indicates at an additional heating mechanism, which seems to originate from electrons in the keV range.

Electron acceleration by a wake field forced by an intense ultrashort laser pulse.

Plasmas are an attractive medium for the next generation of particle accelerators because they can support electric fields greater than several hundred gigavolts per meter. These accelerating fields are generated by relativistic plasma waves-space-charge oscillations-that can be excited when a high-intensity laser propagates through a plasma. Large currents of background electrons can then be trapped and subsequently accelerated by these relativistic waves. In the forced laser wake field regime, where the laser pulse length is of the order of the plasma wavelength, we show that a gain in maximum electron energy of up to 200 megaelectronvolts can be achieved, along with an improvement in the quality of the ultrashort electron beam.

Relativistic electron generation in interactions of a 30 TW laser pulse with a thin foil target.

Energy and angular distributions of the fast outgoing electron beam induced by the interaction of a 1 J, 30 fs, 2 x 10(19) W/cm(2), 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 6-microm polyethylene target, up to 4 x 10(8) electrons with energies between 5 and 60 MeV were produced per laser pulse and converted to gamma 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 2.5 degrees.