G. Lucchini

High-intensity ultrashort laser-induced ablation of stainless steel foil targets in the presence of ambient gas

Ablation depths of stainless steel targets irradiated by 80-fs laser pulses at a flux F ≤ 40 J/cm 2 (intensity ≤ 5 X 10 14 W/cm 2 ) in the presence of air at atmospheric pressure are experimentally measured. These values are lower than the theoretical predictions for metal targets in vacuum. Results are analyzed on the basis of the role of the ambient gas and of crater formation on the behavior of the ablated material.

Bright, point X-ray source based on a commercial portable 40 ps Nd:YAG laser system

AbstractWe present some experimental results on X-ray spectra obtained from plasmas produced using a compact Nd:YAG lasersystem. The beam was focused on different targets~Cu,Al,Ge,...!and both high resolution and low resolution X-rayspectra were recorded.Keywords: ps laser; X-ray plasma sources; X-ray spectroscopy 1. INTRODUCTIONLaser-plasma sources are very powerful laboratory-scaleX-raydeviceswherewavelengthtunabilityisfeasiblebecauseof the comparatively free choice of target material, laserwavelength, and intensity. They have already been success-fully applied for radiobiology ~Turcu et al., 1994a; Masiniet al., 1999; Milani et al., 1999; Bortolotto et al., 2000!,X-ray microscopy~Batani et al., 2000, 2002; Desai et al.2003;Polettietal.,2004!,micro-lithography~Bijkerketal.,1992; Turcu et al., 1994b!. For all these applications theknowledgeofX-rayspectraisveryimportant,eitherdirectlyorbecauseenergydepositionandradiationdosearestronglyenergy dependent.For instance in micro-lithography;1-keVX-ray photonsare required to achieve efficient radiation transfer throughthe X-ray mask membrane and sufficient energy depositiononto the resist coated Si wafer.In radiobiology, laser-plasma X-ray sources were used toirradiate biological specimens: V-79 Chinese hamster cells~Turcu et al., 1994a!, and study DNA damage, DNA repairand repair inhibition, or Saccharomices Cerevisiae yeastcells ~Masini et al., 1999; Milani et al., 1999! and studymetabolic damages. In the first case, X-rays at hn’1.2 keVwere produced using L-shell emission from Cu targets,which could guaranty penetration of X-rays to the cellnucleus, so that absorbed dose was really related to DNAdamage. In the second case, it was essential to produce lowenergy X-ray photons, which would mainly deposit the doseatthecellwall,inordertoproducedamagesatthemetaboliclevel without touching the cell nucleus.Finally in X-ray microscopy, the spectrum is importantbecause only X-rays in the “water window” ~between 22and 44 A ! will contribute to the formation of a clean image,being absorbed by biological material but not by water. Thepresence of other X-ray lines will diminish the image con-trast dramatically.Besides all this, the study of X-ray spectra from plasmasis very important in itself and it is a well developed field inphysics. Indeed on one side, X-ray spectra are very impor-tant for atomic physics because by recording X-ray emis-sion lines, it is possible to access the energy levels in atomsand in multi-charged ions~Rosmej et al., 1997; Stepanovet al., 1997; Vergunova et al., 1997; Biemont et al., 2000!.On the other side, X-ray spectroscopy is a key technique inplasma diagnostics ~Koenig et al., 1997; Magunov et al.,1998; Batani et al., 1999! since from measurements ofX-ray spectra we can calculate the plasma parameters: elec-tron density, electron temperature, ion charges, and evendeduce important information on plasma opacities and onthe exact shape of the electron distribution function~includ-ing the presence of fast electrons!A key problem for applications and for X-ray spectros-copy, would be the development of very compact X-raysources which would be cheap ~and be reproduced in many

Laser-induced ablation and crater formation at high laser flux

Laser ablation and crater formation have been studied on a copper target using a 10 Hz Nd:YAG laser system delivering pulses up to 100 mJ in 40 ps with a flux on target F ≤ 5000 J/cm2. Crater dimensions were measured using optical microscope or scanning electron microscope. In order to understand the process of crater formation, we considered various theoretical models present in the literature and revised them taking into account the occurrence of plasma phenomena, which are important at the intensities used in this experiment. We also compared our experimental results with other results obtained at the PALS laboratory, using a 0.44 μm wavelength laser and much higher laser intensities. Finally, we explore the possibility of extending the information derived from laser-produced craters to other types of craters.

Efficient aberrations pre-compensation and wavefront correction with a deformable mirror in the middle of a petawatt-class CPA laser system

In this paper, we describe the experimental validation of the technique of correction of wavefront aberration in the middle of the laser amplifying chain. This technique allows the correction of the aberrations from the first part of the laser system, and the pre-compensation of the aberrations built in the second part. This approach will allow an effective aberration management in the laser chain, to protect the optical surfaces and optimize performances, and is the only possible approach for multi-petawatt laser system from the technical and economical point of view. This approach is now possible after the introduction of new deformable mirrors with lower static aberrations and higher dynamic than the standard devices.

Pressure amplification in thermal X-ray irradiated foam layered gold targets

We have studied the interaction of soft X-ray thermal radiation with foam-layered metal targets. The X-radiation was produced by focusing a high energy laser inside a small size hohlraum. An increment in shock pressure was observed with the foam layer as compared to bare metal targets.

Wavefront correction and aberrations pre-compensation in the middle of Petawatt-class CPA laser systems

We describe preliminary experiences to validate correction of wavefront aberrations in middle of laser chain. This technique allows correction of aberrations from first part, and the pre-compensation of aberrations built in second part of laser.

Potential effects of Plasma on the Development of Laser Produced Craters

Laser ablation and crater formation have been studied on a copper target using a 10 Hz, Nd:YAG laser system delivering pulses up to 100 mJ in 40 ps with a flux on target F⩽ 5000 J/cm2. Crater dimensions were measured using optical microscope or SEM (Scanning Electron Microscope). In order to understand the process of crater formation, we considered various theoretical models present in the literature and revised them taking into account the occurrence of plasma phenomena, which are important at the intensities used in this experiment. We also compared our experimental results with other results obtained at the PALS laboratory, using a 0.44 μm wavelength laser and much higher laser intensities. Finally we explore the possibility of extending the information derived from laser produced craters to other types of craters.

Generation and characterization of a bright X-ray source using picosecond laser

High-resolution soft X-ray spectra of H-like and He-like ions were produced from laser irradiated silicon and aluminum targets. Plasma size was about 100 μm. X-ray spectra were analyzed to determine plasma parameters. We compared the line shape of resonance transitions and their intensity ratios to corresponding dielectronic satellites and the intensities of the inter combination lines of He-like ions, with the results of model calculations. Such comparison gave average values of the electron density N e=(1−1.9)×1021 cm−3 and the electron temperature T e=460–560 eV for Si plasmas and about 560 eV for Al plasmas produced by the first and the second laser harmonics. According to our estimations, more than 1012 photons were produced within the resonance line spectral width and in the solid angle 2π steredian during the total decay period.

Carbon hugoniot at megabar pressures driven by laser-induced shocks

Experiments on the Equation of State (EOS) of Carbon were performed at the PALS and LULI laboratories. We used Carbon samples with two different value of initial density, in order to explore a wider region of the phase diagram. We obtain experimental data for carbon Hugoniot at Megabar pressures induced by laser-driven shock waves. The target rear side emissivity due to the shock unloading was recorded from (two-materials two-steps) targets (Al-C) with space and time resolution. By applying the impedance mismatch method, a direct determination of relative EOS points was obtained. Experimental data are compared with previous experiments and with theoretical models. Our results indicate a higher compressibility of carbon at Megabar pressures compared to theoretical models.

Laser-driven shock experiments at PALS

Here we discuss the results of the experiments performed using the Prague Asterix Laser System (PALS) of wavelength 0.44 μm (3ω of Iodine laser) and energy ≈ 250 J in 450 ps (FWHM). Two sets of experiments were carried out, firstly, generation of high quality shocks which were steady in time and uniform in space using Phase Zone Plates (PZP), to establish the scaling laws of shock pressure Vs. laser intensity for aluminum foil target of thickness 8 μm. Our results show a good agreement with the delocalized laser absorption model. Secondly, measurements of the Equation of State of carbon compressed by shocks at megabars of pressure have been realized. Equation of State were obtained for carbon using the impedance mismatch technique. Step targets allowed the simultaneous measurements of shock velocity in two different materials. Aluminum was used as a reference material and relative EOS data for carbon have been obtained up to ≈ 14 Mbar pressure.