Hot-electron characterization fromKα measurements in high-contrast,p-polarized, picosecond laser-plasma interactions

Abstract

Strong Ka emission is observed in picosecond laser-plasma interactions with high-intensity-contrast, p-polarized, picosecond laser pulses. Ka emission from Si substrates overlaid with various thicknesses of Al is compared with a Monte Carlo simulation. The results show that the hot electrons which deposit their energy in the solid material have a 3-keV temperature and carry 10% of the incident laser energy. The shifted Ka emission indicates that the solid is heated to a temperature of 35 eV up to 5000 A from the target surface, which is consistent with the preheat of the hot electrons. PACS numbers: 52.40.Nk Short-pulse (~1 ps) laser-plasma interactions have aroused special interest because of the possibilities of producing ultrashortx-ray pulses [1-3] and near-soliddensity plasmas [4,5]. Recently, it has been shown that, for a high-intensity-contrast, picosecond, p-polarized laser pulse incident at a large oblique angle, the absorption can be as high as 60% [6]. Simulations predict that a large fraction of the absorbed laser energy can be carried by hot electrons in high-intensity, short-pulse laserplasma interactions [7,8]. Experimentally, the generation of hot electrons from the short-pulse laser plasma has been reported using an 80-fs pulse with a 3-ns pedestal (amplified spontaneous emission) at normal incidence [9].In this Letter, we report for the first time a quantitative measurement of the hot electrons generated in shortscale-length laser-plasma interactions by high-intensitycontrast (contrast of main pulse to the pedestal is greater than 10 ) picosecond laser pulses. This high contrast enables the main laser pulse to interact with a very shortscale-length (much less than the laser wavelength) plasma instead of interacting with a long-scale-length preformed plasma created by the prepulse. The results are significantly changed when a prepulse is present. The hot-electron energy was deduced from the Ka spectra of multilayered targets, a technique which has been successfully demonstrated for long-pulse laser plasmas [10,11]. The temperature of the hot electrons is measured to be 3 keV by fitting the experimental data by the Monte Carlo calculations and the total Ka yield shows that the hot electrons which deposit their energy in the solid target carry -10% of total laser energy assuming an isotropic Maxwellian distribution function. These hot electrons can preheat the target up to 100 eV producing a relatively cold solid-density plasma behind the hot surface plasma. This is consistent with the observed shifted Ka emission. A strong dependence of the Ka emission on the polarization of the laser pulse at 60 incidence angle is observed, consistent with resonance absorption in shortscale-length laser-plasma interactions [7,12].In the experiment, a 1.3-ps (FWHM), 1.05-pm pulse with an average energy of 10 m3 is provided by a Nd:glass laser system based on the chirped pulse amplification and compression (CPAC) technique [13,14]. With a saturable absorber (Kodak 9860 dye) after the compression gratings [13],a 5...

Hot surface ionic line emission and cold K-inner shell emission from petawatt-laser-irradiated Cu foil targets

“…The measurement of inner-shell emission lines is a valuable diagnostic to characterize the suprathermal electron distribution. 4,6,7,[22][23][24] Measurements of electron temperatures and temperature gradients provide important guidance for simulations to study energy transport in relativistic laser-solid density plasmas. The standard method to infer electron density and temperature in laser-produced plasmas is x-ray line spectroscopy; 25 this method has been applied to petawatt laserplasma experiments, e.g., Koch et al, using aluminum K-shell spectra.…”

Role of surface roughness in hard-x-ray emission from femtosecond-laser-produced copper plasmas

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Laser-plasma interactions relevant to Inertial Confinement Fusion

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