Experimental data on laser-driven carbon C 6+ ion acceleration with a peak intensity of 5 × 10 20 W cm −2 are presented and compared for opaque target normal sheath acceleration (TNSA) and relativistically transparent laser-plasma interactions. Particle numbers, peak ion energy and conversion efficiency have been investigated for target thicknesses from 50 nm to 25 µm using unprecedented full spectral beam profile line-out measurements made using a novel high-resolution ion wide-angle spectrometer. For thicknesses of about 200 nm, particle numbers and peak energy increase to 5 × 10 11 carbon C 6+ particles between 33 and 700 MeV (60 MeV u −1 ), which is a factor of five higher in particle number than that observed for targets with micron thickness. For 200 nm thick targets, we find that the peak conversion efficiency is 6% and that up to 55% of the target under the laser focal spot is accelerated to energies above 33 MeV. This contrasts with the results for targets with micron thickness, where surface acceleration with TNSA is dominant. The experimental findings are consistent with two-dimensional particle-in-cell simulations.
Ne to Ne 8 ionization yields in 10 14 W=cm 2 to 10 18 W=cm 2 laser fields are reported over a 10 9 dynamic range. A 3D relativistic rescattering model incorporating e; 2e and e; 3e electron impact ionization, single-and double-excitation is compared to the data. For double ionization the agreement is excellent; however, for higher charge states the model accounts for only 15% of multielectron nonsequential ionization. Rescattering is not affected by the laser magnetic field until 10 17 W=cm 2 .Atomic ionization and radiation processes in strong fields have remained at the forefront of time resolved dynamics and laser science for the past 15 years. Recent interest has focused on quantum [1] and classical mechanisms [2] behind multielectron, multiphoton ionization and the new area of attosecond science [3,4]. These studies, which often address multielectron nonsequential ionization (NSI) [5] and high harmonic generation (HHG) [6], involve a field driven rescattering mechanism. As the field increases, higher charge states, relativistic effects, and the laser magnetic field (B laser ) will affect NSI, HHG, and rescattering physics. This Letter begins to address these topics with a precision, ultrahigh field ionization experiment and relativistic, semiclassical ionization and rescattering model. Rescattering [7] occurs when a photoelectron, which is oscillating in the continuum with the laser electric field (E laser ), is driven back toward the parent ion. For laser intensities from 10 13 W=cm 2 to 10 15 W=cm 2 , inelastic rescattering between the photoelectron and the parent ion may result in collisional ionization (NSI) or radiation (HHG). Two-electron NSI has been observed [8] to exceed by 10 5 the expected doubly ionized species from a sequential ionization (SI) mechanism, in which the laser field ionizes the atom one electron at a time. Recent theoretical [9-12] and experimental efforts have made significant progress towards understanding two-electron NSI mechanisms including resonantly enhanced multiphoton ionization [13,14] and rescattering impact excitation and ionization [15].Beyond two-electron NSI of the neutral atom, multielectron NSI has been reported [16] but is not well understood at this time. Fully differential rates for the multiphoton ionization of ground state Ne [17], correlated electron emission measurements for multiphoton double ionization [18], and ion momentum measurements [19] indicate rescattering is likely to become a dominant NSI mechanism in ultrahigh fields with high charge states. Chowdhury [20] and Dammasch [21] measured multielectron ionization from 10 15 W=cm 2 to >10 17 W=cm 2 and found NSI decreased at higher intensities. Diminished NSI was at first believed to result from a reduction in rescattering due to the Lorentz force on the photoelectron. In the ''Lorentz force paradigm,'' B laser and the significant photoelectron velocity force the rescattering into the laser propagation direction (k v B) and away from the parent ion. As we will show, B laser does not play as large of a ...
Kinetic simulations of break-out-afterburner (BOA) ion acceleration from nm-scale targets are examined in a longer pulse length regime than studied previously. It is shown that when the target becomes relativistically transparent to the laser, an epoch of dramatic acceleration of ions occurs that lasts until the electron density in the expanding target reduces to the critical density in the non-relativistic limit. For given laser parameters, the optimal target thickness yielding the highest maximum ion energy is one in which this time window for ion acceleration overlaps with the intensity peak of the laser pulse. A simple analytic model of relativistically induced transparency is presented for plasma expansion at the time-evolving sound speed, from which these times may be estimated. The maximum ion energy attainable is controlled by the finite acceleration volume and time over which the BOA acts.
Ion acceleration from relativistic laser solid interactions has been of particular interest over the last decade. While beam profiles have been studied for target normal sheath acceleration (TNSA), such profiles have yet to be described for other mechanisms. Here, experimental data is presented, investigating ion beam profiles from acceleration governed by relativistic transparent laser plasma interaction. The beam shape of carbon C 6+ ions and protons has been measured simultaneously with a wide angle spectrometer. It was found that ion beams deviate from the typical Gaussian-like shape found with TNSA and that the profile is governed by electron dynamics in the volumetric laser-plasma interaction with a relativistically transparent plasma; due to the ponderomotive force electrons are depleted from the center of the laser axis and form lobes affecting the ion beam structure. The results are in good agreement with high resolution three-dimensional-VPIC simulations and can be used as a new tool to experimentally distinguish between different acceleration mechanisms.
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