P Koster

Experimental investigation of fast electron transport through Kα imaging and spectroscopy in relativistic laser–solid interactions

We report on experimental fast electron transport studies performed in the relativistic laser intensity interaction regime. The investigation has been carried out in the long-pulse (0.6 ps) regime relevant for the fast ignitor scheme in the inertial confinement fusion concept.Multilayer targets containing different materials were irradiated. Here we show the results concerning SiO2 or Al layers, respectively. The Kα radiation from a Cu tracer layer on the target rear side was found to be enhanced by a factor of about 8 with the irradiation of SiO2 targets with respect to the Al targets. The possible origin of this observation is discussed.

Observation of electron transport dynamics in high intensity laser interactions using multi-energy monochromatic x-ray imaging

We describe recent measurements in which a novel imaging technique was used to investigate the transport of high energy electrons produced by the interaction of a femtosecond laser pulse with a three-layer target at an intensity of 5 × 1019 W cm−2. The imaging system was configured to work in a single-photon detection regime to identify the energy of the x-ray photons and to discriminate among Kα photons generated in each target layer. Electrons emerging from the rear side after propagation through all the target layers were also detected using a custom developed detector. The results on fast electron propagation are combined with the information obtained from electron diagnostics and are modelled using analytical and numerical codes to obtain a detailed description of electron propagation dynamics.

Ray-tracing simulations of a bent crystal X-ray optics for imaging using laser-plasma X-ray sources

Ray-tracing simulations of an optical X-ray system based on a spherically bent crystal operating in Bragg configuration for monochromatic projection imaging of thin samples are presented, obtained using a code developed for that purpose. The code is particularly suited for characterizing experimental arrangements routinely used with laser-produced plasma X-ray sources. In particular, the spatial resolution of the imaging system was investigated and a careful study of the complex pattern of the X-ray backlighting beam was performed.

Small-scale laser based electron accelerators for biology and medicine: a comparative study of the biological effectiveness

Laser-driven electron accelerators based on the Laser Wakefield Acceleration process has entered a mature phase to be considered as alternative devices to conventional radiofrequency linear accelerators used in medical applications. Before entering the medical practice, however, deep studies of the radiobiological effects of such short bunches as the ones produced by laser-driven accelerators have to be performed. Here we report on the setup, characterization and first test of a small-scale laser accelerator for radiobiology experiments. A brief description of the experimental setup will be given at first, followed by an overview of the electron bunch characterization, in particular in terms of dose delivered to the samples. Finally, the first results from the irradiation of biological samples will be briefly discussed.

On the effect of rear-surface dielectric coatings on laser-driven proton acceleration

Laser-driven ion acceleration has been experimentally investigated by irradiating, with tightly focused femtosecond laser pulses at 5×1019 W/cm2, thin metal foils, which have been back-coated with a μm thick dielectric layer. The observation we report shows the production of MeV proton bunches with an unexpected highly uniform spatial cross section.

Experimental investigation of fast electron transport in solid density matter: Recent results from a new technique of X-ray energy-encoded 2D imaging

The development activity of a new experimental technique for the study of the fast electron transport in high density matter is reported. This new diagnostic tool enables the X-ray 2D imaging of ultrahigh intensity laser plasmas with simultaneous spectral resolution in a very large energy range to be obtained. Results from recent experiments are discussed, in which the electron propagation in multilayer targets was studied by using the Kα. In particular, results highlighting the role of anisotropic Bremsstrahlung are reported, for the sake of the explanation of the capabilities of the new diagnostics. A discussion of a test experiment conceived to extend the technique to a single-shot operation is finally given.

Focusing and stabilizing laser?plasma-generated electron beams with magnetic devices

External magnetic devices have been successfully tested to control the divergence and pointing stability of subrelativistic electron beams accelerated by ultrashort laser pulses in a nitrogen plasma (electron density of ?1019 cm?3). Different configurations of the magnetic devices have been studied, and their effects are discussed in detail. The analysis is also supported by the results of ray-tracing simulations using the first-order trajectory equation in the magnetic field configurations. This simple method of improving beam stability will be particularly useful for applying laser-generated ultrashort electron beams to high-dose radiobiological studies.

Fast electron generation for X‐ray production in ultrashort laser interactions with solids

This article reports on the study of fast electron generation leading to X‐ray emission from femtosecond laser interaction with solids presently in progress at the Intense Laser Irradiation Laboratory. The femtosecond laser source and custom developed diagnostics for the characterization of the fast electrons as well as of the X‐ray emission is first introduced. Experimental results obtained from the irradiation of Titanium foils are presented. The dependence of the X‐ray emission yield on the polarization of the laser light is also showed. Data from the electron diagnostics are then discussed, which shows the presence of energetic electrons accelerated in the forward direction normally to the target surface. Finally, these data are briefly compared with first results from hydrodynamic and PIC simulations.