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Electron density structure measurements with scattered intense laser beam

K. Sakai, K. Himeno, S. J. Tanaka, T. Asai, T. Minami, Y. Abe, F. Nikaido, K. Kuramoto, M. Kanasaki, H. Kiriyama, A. Kon, K. Kondo, N. Nakanii, W. Y. Woon, C. M. Chu, K. T. Wu, C. S. Jao, Y. L. Liu, T. A. Pikuz, H. Kohri, A. O. Tokiyasu, S. Isayama, H. S. Kumar, K. Tomita, Y. Fukuda, Y. Kuramitsu

TL;DR

This work introduces an imaging diagnostic based on scattering of an intense drive laser to map the local electron-density structure at laser–plasma interaction regions with high temporal and spatial resolution. By polarization-resolved imaging and simple forward modeling, the authors show that pre-pulse effects create a global density profile and that strong ponderomotive forces evacuate density near the focal spot, reducing scattered light there. Complementary 2D PIC simulations reproduce the observed features, including upstream density against background and a density cavity at focus, and demonstrate consistency with the measured focal-spot suppression after accounting for finite optical resolution. The method offers a practical route to visualize electron-density dynamics on sub-pulse timescales and motivates future spectroscopic extensions to extract fuller plasma-state information.

Abstract

Short-pulse intense lasers have the potential to model extreme astrophysical environments in laboratories. Although there are diagnostics for energetic electrons and ions resulting from laser-plasma interactions, the diagnostics to measure velocity distribution functions at the interaction region of laser and plasma are limited. We have been developing the diagnostics of the interaction between intense laser and plasma using scattered intense laser. We performed experiments to measure electron density by observing the spatial distributions and ratio of horizontal to vertical polarization components of scattered laser beam using optical imaging. The observed ratio of polarization components is consistent with the drive laser beam indicating the observed light originates from the drive laser. Imaging of the scattered light shows the structure of electron density, the zeros moment of electron velocity distribution function, interacting with the intense laser. We observed the change of structure due to the laser pre-pulse that destroys the target before the arrival of the main pulse.

Electron density structure measurements with scattered intense laser beam

TL;DR

This work introduces an imaging diagnostic based on scattering of an intense drive laser to map the local electron-density structure at laser–plasma interaction regions with high temporal and spatial resolution. By polarization-resolved imaging and simple forward modeling, the authors show that pre-pulse effects create a global density profile and that strong ponderomotive forces evacuate density near the focal spot, reducing scattered light there. Complementary 2D PIC simulations reproduce the observed features, including upstream density against background and a density cavity at focus, and demonstrate consistency with the measured focal-spot suppression after accounting for finite optical resolution. The method offers a practical route to visualize electron-density dynamics on sub-pulse timescales and motivates future spectroscopic extensions to extract fuller plasma-state information.

Abstract

Short-pulse intense lasers have the potential to model extreme astrophysical environments in laboratories. Although there are diagnostics for energetic electrons and ions resulting from laser-plasma interactions, the diagnostics to measure velocity distribution functions at the interaction region of laser and plasma are limited. We have been developing the diagnostics of the interaction between intense laser and plasma using scattered intense laser. We performed experiments to measure electron density by observing the spatial distributions and ratio of horizontal to vertical polarization components of scattered laser beam using optical imaging. The observed ratio of polarization components is consistent with the drive laser beam indicating the observed light originates from the drive laser. Imaging of the scattered light shows the structure of electron density, the zeros moment of electron velocity distribution function, interacting with the intense laser. We observed the change of structure due to the laser pre-pulse that destroys the target before the arrival of the main pulse.
Paper Structure (8 sections, 4 equations, 8 figures)

This paper contains 8 sections, 4 equations, 8 figures.

Table of Contents

  1. Introduction
  2. Experiment
  3. Polarization measurement
  4. Pre-pulse dependence
  5. Non-uniform target
  6. Model calculation
  7. Numerical simulation
  8. Discussion and summary

Figures (8)

  • Figure 1: (a) Schematic illustration of the experimental setup. (b) Side view of the setup. (c) Top view of the setup.
  • Figure 2: (a) View from the observation direction. (b) 2D image of the scattered light within the dashed square in (a) with the argon gas target. The upper and lower show the horizontal and vertical polarizations on the detector, respectively.
  • Figure 3: 2D images of scattered light using the hydrogen gas target (a) with and (b) without PM.
  • Figure 4: (a) 2D images of scattered light with the hydrogen cluster target. (b) An enlarged view of the cluster close to the focal spot in (a). (c) horizontal and vertical profiles of (b).
  • Figure 5: Schematic illustration of the imaging measurement and the line-of-sight integral in Eq. \ref{['eq:sint']}. The inset shows the shape of an F/1.35 Gaussian laser beam around the focal spot with the spot size of 2µ m and the scattered intensity along the $x$ axis assuming a uniform density given by Eq. \ref{['eq:ismodel']}.
  • ...and 3 more figures