Nonlinear increase of X-ray intensities from thin foils irradiated with a 200 TW femtosecond laser

Abstract

We report, for the first time, that the energy of femtosecond optical laser pulses, E, with relativistic intensities I > 1021 W/cm2 is efficiently converted to X-ray radiation, which is emitted by “hot” electron component in collision-less processes and heats the solid density plasma periphery. As shown by direct high-resolution spectroscopic measurements X-ray radiation from plasma periphery exhibits unusual non-linear growth ~E4–5 of its power. The non-linear power growth occurs far earlier than the known regime when the radiation reaction dominates particle motion (RDR). Nevertheless, the radiation is shown to dominate the kinetics of the plasma periphery, changing in this regime (now labeled RDKR) the physical picture of the laser plasma interaction. Although in the experiments reported here we demonstrated by observation of KK hollow ions that X-ray intensities in the keV range exceeds ~1017 W/cm2, there is no theoretical limit of the radiation power. Therefore, such powerful X-ray sources can produce and probe exotic material states with high densities and multiple inner-shell electron excitations even for higher Z elements. Femtosecond laser-produced plasmas may thus provide unique ultra-bright X-ray sources, for future studies of matter in extreme conditions, material science studies, and radiography of biological systems.

Diagnosis of warm dense conditions in foil targets heated by intense femtosecond laser pulses using Kα imaging spectroscopy

“…4(b)] has a large blue wing as well as a significant emission at cold Kα 1 (4511 eV). This comparison led us to consider the T bulk distribution [18,24,29]. Mixed spectra with two different T bulk values, 5 eV + 15 eV in a 5:5 ratio, and 13 eV + 34 eV in 5:5 provide better agreements with the experimental data in Fig.…”

“…Kα emission occurs within the lifetime of fast electrons, which is typically shorter than the hydrodynamic expansion time [21]. Precedent works showed Kα line measurements could serve as a thermometer for the bulk of WDM samples created by high energy lasers [20,[22][23][24].…”

Experimental evidence for short-pulse laser heating of solid-density target to high bulk temperatures

“…These spectra components originate from the emission of KK and KL hollow atoms, respectively. In order to describe the full spectra containing both resonance and hollow atom lines, the concept of several plasma zones 30 with different electronic, radiation temperatures and electronic density should be applied. There, the hot over-ionized central zone irradiated by the laser is considered as a source of intense fast electron flux, which in turn generates ultra-bright X-rays in the keV photon energy range.…”

P3: An installation for high-energy density plasma physics and ultra-high intensity laser–matter interaction at ELI-Beamlines

“…While at sub-relativistic laser intensities the characteristic energies of hot electrons generated due to strong parametric instabilities (stimulated Raman scattering, and two-plasmon decay processes) typically achieve tens of keV [82], and at intensities above 10 20 W=cm 2 the valence electrons are rapidly field-ionized and accelerated to MeV energies. These suprathermal electrons further generate fast ions and strong X-ray fluxes in collisional and collision-less processes [163]. The interaction of energetic electrons, ions and photons with the target material results in the production of ions with simple or multiple vacancies in inner atomic shells (also called hollow ions, HI) [164].…”