Investigation of hot and warm electrons in solid-density plasmas irradiated by super-intense laser pulses

Abstract

By applying ultraintense and ultrafast laser pulses (~ 10^21 W/cmrocess that can be helpful in furthering our understanding of the energy transport and the rapid heating processes occurring in the bulk materials. The relativistic electrons—accelerated by an intense laser beam at the front surface of the target— interact with the atoms of the target material during their transport through it. As a result of this interaction, the characteristic K-shell line emissions in the X-ray regime are generated. When these electrons reach the boundary of the target, the transition radiations (TR) could also be generated in the optical regime. Here, we utilized both; the Kα x-ray imaging spectroscopy and the optical transition radiation (OTR) diagnosis, to investigate the electron properties at various temperatures. The temperature distribution of the warm bulk electrons in a titanium foil, irradiated by a laser pulse with a peak intensity of 5 × 10^18 W/cm^2, is measured using the x-ray imaging spectroscopy technique. Spatially resolved Kα spectra are obtained using a toroidally bent GaAs crystal and a x-ray charge-coupled device. The radially resolved Kα spectra by Abel-inversion are compared with the calculated K-shell emission spectra using the atomic-kinetics spectroscopy simulation code SCFLY. The Kα line-shapes are strongly affected by the charge states of the bulk target and can be used to determine the distribution of electron temperatures in the energy range of 5–40 eV. Our measurements show a broad range of temperature distribution of electrons in the targets. The bulk electron temperature distribution is dependent on the target thickness. In thinner targets (~1 μm), it was found that an off-central regime can be heated to a higher temperature than the laser-irradiated spot. This result infers a significant refluxing effect for the lateral transport of hot electrons in the thin foil targets. For temporal characteristics of the micro-bunches of the relativistic electrons, the OTR spectra from the rear surface of targets with various thicknesses (100 nm to 5 μm) are collected by irradiating these targets with intense laser pulses of intensities up to 1020 W/cm2. The spectral modulations are observed depending on the target thickness and the laser intensities. To gain physical insights into the relativistic electron transport, the 2-D particle-in-cell simulations were performed and a theoretical method to calculate the OTR spectra was developed. The radiation power distribution is strongly affected by the temporal coherence of the electron micro-bunches, namely hot electron energy, dispersion, and the bunch separation. With these effects accounted for, the spectral modulation of the OTR can prove helpful in understanding the acceleration and deceleration of hot electrons, and the dephasing of micro-bunching structures at a very early stage of target heating by the relativistic laser pulses.