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Channel optimization of high-intensity laser beams in millimeter-scale plasmas

L. Ceurvorst, A. Savin, N. Ratan, M. F. Kasim, J. Sadler, P. A. Norreys, H. Habara, K. A. Tanaka, S. Zhang, M. S. Wei, S. Ivancic, D. H. Froula, and W. Theobald
Phys. Rev. E 97, 043208 – Published 20 April 2018
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  • INTRODUCTION
  • EXPERIMENTAL DESIGN
  • DENSITY RECONSTRUCTION
  • CHANNEL DEPTHS
  • TIMING DEPENDENCE
  • FOCAL POSITION DEPENDENCE
  • INTENSITY DEPENDENCE
  • LEADING FILAMENTS
  • SECONDARY DIAGNOSTICS
  • CONCLUSIONS
  • ACKNOWLEDGMENTS
    • References

    Abstract

    Channeling experiments were performed at the OMEGA EP facility using relativistic intensity (>1018W/cm2) kilojoule laser pulses through large density scale length (∼390–570 μm) laser-produced plasmas, demonstrating the effects of the pulse's focal location and intensity as well as the plasma's temperature on the resulting channel formation. The results show deeper channeling when focused into hot plasmas and at lower densities, as expected. However, contrary to previous large-scale particle-in-cell studies, the results also indicate deeper penetration by short (10 ps), intense pulses compared to their longer-duration equivalents. This new observation has many implications for future laser-plasma research in the relativistic regime.

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    • Received 3 February 2017
    • Revised 26 February 2018

    DOI:https://doi.org/10.1103/PhysRevE.97.043208

    ©2018 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Filamentation in plasmasHigh intensity laser-plasma interactionsHigh-energy-density plasmasInertial confinement fusionPonderomotive effectsSelf-focusing & filamentation in plasmas
    1. Physical Systems
    Near-critical & underdense plasmas
    1. Techniques
    Imaging & optical processing
    Plasma Physics

    Authors & Affiliations

    L. Ceurvorst1,*, A. Savin1, N. Ratan1, M. F. Kasim1, J. Sadler1, P. A. Norreys1,2, H. Habara3, K. A. Tanaka3,4, S. Zhang5, M. S. Wei6, S. Ivancic7, D. H. Froula8, and W. Theobald7

    • 1Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU United Kingdom
    • 2STFC Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX United Kingdom
    • 3Graduate School of Engineering, Osaka University, Suita, Osaka 5650871, Japan
    • 4ELI-NP/IFIN-HH, 30 Reactorului Street, Magurele, Ilfov County, P. O. Box MG-6, 077125 Romania
    • 5Department of Mechanical and Aerospace Engineering, University of California at San Diego, La Jolla, California 92093, USA
    • 6General Atomics, San Diego, California 92121, USA
    • 7Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14623, USA
    • 8Department of Physics and Astronomy, University of Rochester, Rochester, New York 14623, USA

    • *Present address: Université de Bordeaux-CNRS-CEA, Centre Lasers Intenses et Applications, UMR 5107, 33405 Talence, France; luke.ceurvorst@u-bordeaux.fr

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    Vol. 97, Iss. 4 — April 2018

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    Images

    • Figure 1
      Figure 1

      Schematic of experimental setup. A 3ω drive beam ablates the surface of a plastic target, generating a plasma plume. On completion of the driving phase, a 1ω channeling pulse begins propagation through the plasma normal to the target's surface. A 4ω probe pulse is used to observe the resulting channel formations.

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    • Figure 2
      Figure 2

      FLASH simulations for the channeling parameters experiment. As can be seen from these simulations, the expected density profile is expected to expand to a larger scale length 1.5ns after the end of the drive pulse (“Cold” simulations) compared to 0ns delay (“Hot” simulations). The electron temperature dramatically cools during this period as well.

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    • Figure 3
      Figure 3

      AFR density reconstruction for the channeling parameters experiment. Shown is a sample reconstructed density profile from a hot plasma in comparison to the corresponding FLASH simulations. While the scale lengths were comparable, the FLASH simulations were approximately half to a third of the AFR reconstruction for much of the interaction region.

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    • Figure 4
      Figure 4

      Channel penetration. The final distances from the original target surface (OTS) are plotted against the vacuum focal position for the 100ps and 10ps in both the hot and cold plasmas. Deeper penetration would therefore have a lower value. These measurements were made using the 4ω probe.

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    • Figure 5
      Figure 5

      AFR and shadowgraphy images for the channel depth timing dependence. Shadowgraphs are shown in the outer columns and AFR images are shown in the inner columns, each having been cropped to show the region of interest. The top row shows the results of the 100-ps beam in a hot plasma [(a) and (b)] compared to a cold plasma [(c) and (d)]. The bottom row does the same for the 10-ps beam with (f) showing the results of the hot plasma shot and (g) and (h) showing the cold plasma shot. Note that (e) is empty due to EMP disabling the shadowgraphy camera for this shot. The horizontal red lines are superimposed at the measured channel depth as plotted in Fig. 4. No superimposed line is present in (f) as the probe arrived prior to the channeling beam, meaning that the observed channel here is incomplete.

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    • Figure 6
      Figure 6

      AFR and shadowgraphy images for the channel depth focal position dependence. Shadowgraphs for the 100-ps pulse in hot plasmas are shown for focal positions (a) 1.5mm, (b) 0.9mm, and (c) 0.3mm. Due to issues with the shadowgraphy camera on the high-power shots, AFR images are shown instead for the 10-ps pulse at focal positions (d) 1.5mm, (e) 0.9mm, and (f) 0.3mm. Superimposed horizontal red lines indicate the measured channel depth. Note that (a) and (d) are reproduced from Figs. 5 and 5, respectively, for ease of viewing.

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    • Figure 7
      Figure 7

      Channeling beam profile. Strong speckling of the channeling beam at focus may be observed. The right image shows the nominal fluence on a log-scale with 80% of the energy contained within an 18.4-μm radius as indicated by the black dotted line.

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    • Figure 8
      Figure 8

      Simulated channeling beam profile. The left image shows the nominal fluence of the focal spot at the target plane. A horizontal lineout was taken at Y=0μm and fitted with five Gaussian functions. The results of this lineout and fit are shown in the bottom plot.

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    • Figure 9
      Figure 9

      Simulated intensity profile of imperfect beam in vacuum. (a) The spatially resolved electromagnetic energy of the vacuum simulation is shown after 2.5ps in (a). A lineout at Z=392μm was taken and is plotted in (b). As can be seen, the simulation did not successfully capture the fitted spot shown in Fig. 8, but instead generated two broad peaks at focus.

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    • Figure 10
      Figure 10

      Simulated channel profiles for a perfect and a speckled beam. Shown are the electron density profiles after [(a) and (c)] 2.5ps and [(b) and (d)] 5.0ps for [(a) and (b)] the perfect beam and [(b) and (d)] the imperfect beam.

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    • Figure 11
      Figure 11

      Shadowgraphy images for the channel depth intensity dependence. Shadowgraphs are shown for the three relevant intensity comparisons. The left column shows the channels formed by the 100-ps pulse while the right column corresponds to the 10-ps pulse. Panels (a) and (b) are in a hot plasma with focal positions at Z=0.9mm, (c) and(d) are in a hot plasma with focal positions at Z=0.3mm, and (e) and (f) are in a hot plasma with focal positions at Z=1.5mm. Superimposed horizontal red lines indicate the measured channel depth. Note that several of these images are reproduced from Figs. 5 and 6 for ease of comparison.

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    • Figure 12
      Figure 12

      Integration of channeling profile equation for experiment's pulses. Equation (1) was integrated using the pulse parameters found in this experiment through the end of the pulses' durations.

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    • Figure 13
      Figure 13

      Integration of channeling profile equation at various intensities. Equation (1) was integrated until the 3D density profile was fully evacuated at various intensities. Red (thick) lines indicate the integration was performed in 2D while blue (thin) lines indicate 3D. Dotted lines correspond to a0=1 at t=3.758ps, dashed lines to a0=3 at t=1.732ps, and solid lines to a0=10 at t=0.907ps.

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    • Figure 14
      Figure 14

      Channeling velocity dependence on intensity and self-focusing ratio. Plotted is the ratio of channeling velocity when a pulse experiences a self-focusing ratio of R. As can be seen, this has little effect at lower intensities but increases until plateauing at R1/2.

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    • Figure 15
      Figure 15

      Examples of leading filaments. Cropped shadowgraphs are shown for (a) 10-ps pulse at half energy, focused at Z=1.5mm, (b) 100-ps pulse focused at Z=0.9mm, and (c) 100-ps pulse focused at Z=1.5mm. Superimposed horizontal cyan lines indicate the critical surface based on the AFR reconstruction.

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    • Figure 16
      Figure 16

      Von Hamos spectrometer output and Cu Kα yield. An example von Hamos spectrometer signal is shown in (a) with white dashed lines indicating the Cu Kα region and dotted lines indicating the Cu Kβ region. An integrated lineout is shown in (b) along with the background profile. The Kα region was then integrated within a 0.15-keV range of its peak to calculate the total Cu Kα yield plotted in (c) for each hot plasma shot.

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    • Figure 17
      Figure 17

      Polarimetry of the channels. The raw signal for the 10-ps pulse focused at Z=0.9mm is shown in (a). Superimposed white boxes indicate the S (solid lines) and P (dashed lines) images. Panel (b) shows the cropped calculation of the beam rotation angle, θ without any normalizations. Panel (c) shows the same calculation after normalizing the S and P images using the plasma-free shot's images. The axes on (b) and (c) are not exact due to the image transformations involved in aligning the two raw images but are well matched to the shadowgraphy results and therefore provide a reasonable estimate.

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