Enhanced laser intensity and ion acceleration due to self-focusing in relativistically transparent ultrathin targets

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Enhanced laser intensity and ion acceleration due to self-focusing in relativistically transparent ultrathin targets

T. P. Frazer, R. Wilson, M. King, N. M. H. Butler, D. C. Carroll, M. J. Duff, A. Higginson, J. Jarrett, Z. E. Davidson, C. Armstrong, H. Liu, D. Neely, R. J. Gray, and P. McKenna

Phys. Rev. Research 2, 042015(R) – Published 19 October 2020

Abstract

Laser-driven proton acceleration from ultrathin foils is investigated experimentally using $f / 3$ and $f / 1$ focusing. Higher energies achieved with $f / 3$ are shown via simulations to result from self-focusing of the laser light in expanding foils that become relativistically transparent, enhancing the intensity. The increase in proton energy is maximized for an optimum initial target thickness, and thus expansion profile, with no enhancement occurring for targets that remain opaque, or with $f / 1$ focusing to close to the laser wavelength. The effect is shown to depend on the drive laser pulse duration.

Received 23 May 2020
Accepted 7 September 2020

DOI:https://doi.org/10.1103/PhysRevResearch.2.042015

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Laser driven ion acceleration Laser-plasma interactions Particle acceleration in plasmas

Plasma Physics

Authors & Affiliations

T. P. Frazer ¹, R. Wilson ¹, M. King¹, N. M. H. Butler¹, D. C. Carroll², M. J. Duff¹, A. Higginson ¹, J. Jarrett¹, Z. E. Davidson ¹, C. Armstrong^1,2, H. Liu^3,2, D. Neely^2,1, R. J. Gray¹, and P. McKenna ^1,*

¹SUPA Department of Physics, University of Strathclyde, Glasgow G4 0NG, United Kingdom
²Central Laser Facility, STFC Rutherford Appleton Laboratory, Oxfordshire OX11 0QX, United Kingdom
³Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

^*paul.mckenna@strath.ac.uk

Article Text

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Issue

Vol. 2, Iss. 4 — October - December 2020

Subject Areas

Plasma Physics

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Images

Figure 1
Schematic of the plasma mirror configurations used for (a) $f / 3$ and (b) $f / 1$ focusing, employing planar and ellipsoidal geometries, respectively. The spatially and energy-resolved dose distribution of the beam of protons accelerated from the ultrathin target foil was characterized using stacked radiochromic film (RCF). Example measured focal spot spatial-energy distributions (at low power) for (c) $f / 3$ and (d) $f / 1$ focusing. The measured encircled energy within the FWHM of each spot is stated.
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Figure 2
Experimental results. (a) Maximum proton energy ( $ε_{max}$ ) as a function of target thickness $ℓ$ . Open symbols correspond to 6- $μ m$ -thick Al reference targets. (b) Representative proton energy spectra for given $ℓ$ , for $f / 1$ and $f / 3$ focusing. (c) Laser-to-proton energy conversion efficiency ( $η_{p}$ ) and (d) hole-boring velocity ( $v_{hb}$ ) normalized to the speed of light in vacuum $c$ , as a function of $ℓ$ .
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Figure 3
2D PIC simulation results. (a) Spatial map of the instantaneous laser intensity for $f / 1$ focusing onto a $ℓ = 75$ nm CH target. The red lines highlight the focusing cone of the incident laser light. (b) Same, for the $f / 3$ focusing case, with the yellow lines highlighting the cone of the additional self-focusing. (c) Temporal evolution of the peak focused 2D laser intensity, for given $ϕ_{L}$ between 1.5 and 10 $μ m$ , propagating in vacuum. (d) Same, for the interaction with a $ℓ = 75$ nm CH target expanded to the point at which the target becomes relativistically transparent to the laser light. The peak 2D intensity reaches a similar magnitude for all $ϕ_{L}$ in the range 1.5–5 $μ m$ due to self-focusing. $t = 0$ ps is defined as the time at which the peak of the incident laser pulse interacts with the target.
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Figure 4
PIC simulation results. (a) 2D PIC: $ε_{max}$ scaling with $ϕ_{L}$ for $ℓ = 75$ nm and $ℓ = 200$ nm. Open symbols are for a $30 %$ reduction in encircled energy. (b) 2D PIC: Maximum intensity achieved as a function of $ℓ$ for the nominal $ϕ_{L} = 1.5$ $μ m$ and $ϕ_{L} = 5$ $μ m$ cases. Dotted lines indicate the peak nominal intensity and the dashed lines represent the maximum intensity achievable, corresponding to focusing to the diffraction limit. (c) 2D PIC: $ε_{max}$ as a function of $ℓ$ for both focusing cases. (d) 2D and 3D PIC: Peak intensity as a function of $ϕ_{L}$ for $τ_{L} = 400$ fs (blue) and $τ_{L} = 25$ fs (red) in 2D simulations. 3D PIC results for $τ_{L} = 25$ fs are shown in green. The target is an expanding $ℓ = 100$ nm foil in all cases. Dashed lines represent the nominal (vacuum) maximum intensity. The intensities of the 2D cases are scaled by the square of the enhancement over the nominal (vacuum) intensity, to enable direct comparison with the 3D results.
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