Gev-Scale Accelerators Driven by Plasma-Modulated Pulses from Kilohertz Lasers

O. Jakobsson, S. M. Hooker, and R. Walczak

Phys. Rev. Lett. 127, 184801 – Published 26 October 2021

Abstract

We describe a new approach for driving GeV-scale plasma accelerators with long laser pulses. We show that the temporal phase of a long, high-energy driving laser pulse can be modulated periodically by copropagating it with a low-amplitude plasma wave driven by a short, low-energy seed pulse. Compression of the modulated driver by a dispersive optic generates a train of short pulses suitable for resonantly driving a plasma accelerator. Modulation of the driver occurs via well-controlled linear processes, as confirmed by good agreement between particle-in-cell (PIC) simulations and an analytic model. PIC simulations demonstrate that a 1.7 J, 1 ps driver, and a 140 mJ, 40 fs seed pulse can accelerate electrons to energies of 0.65 GeV in a plasma channel with an axial density of $2.5 \times 10^{17} {cm}^{- 3}$ . This work opens a route to high repetition-rate, GeV-scale plasma accelerators driven by thin-disk lasers, which can provide joule-scale, picosecond-duration laser pulses at multikilohertz repetition rates and high wall-plug efficiencies.

Received 17 June 2021
Revised 24 August 2021
Accepted 28 September 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.184801

Physics Subject Headings (PhySH)

High intensity laser-plasma interactions Laser applications Laser driven electron acceleration Laser wakefield acceleration Ultrashort pulses

Accelerators & BeamsPlasma PhysicsAtomic, Molecular & Optical

Authors & Affiliations

O. Jakobsson , S. M. Hooker , and R. Walczak ^*

John Adams Institute for Accelerator Science and Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, United Kingdom

^*roman.walczak@physics.ox.ac.uk

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Vol. 127, Iss. 18 — 29 October 2021

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Images

Figure 1
Schematic diagram of a LPA driven by plasma-modulated laser pulses. A long, high-energy, drive laser pulse is phase modulated in the modulator stage by its interaction with the plasma wave driven by a short, low-energy seed pulse. The modulation generates sidebands at $ω_{0} + m ω_{p 0}$ , although the temporal intensity profile of the drive pulse remains smooth. After leaving the modulator stage, the seed pulse is removed by a polarizing beam splitter, and the drive pulse is passed through (or reflected from) a dispersive optical system which removes the relative spectral phase of the sidebands, to form a train of short pulses spaced by $T_{p 0} = 2 π / ω_{p 0}$ . This pulse train is focused into an accelerator stage, which comprises a plasma channel with the same on-axis density as that of the modulator stage. The pulse train resonantly excites a strong plasma wave that can be used for particle acceleration.
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Figure 2
Particle-in-cell simulations. (a) PIC simulations of the modulator stage, with results shown for the end of the modulator ( $z = 120 mm$ ). The top panel shows the on-axis spectral intensity of the drive pulse, plotted against a frequency scale $m = (ω - ω_{0}) / ω_{p 0}$ . The middle panel shows the longitudinal intensity profiles of the seed and driver pulses and the relative amplitude of the plasma wave on axis ( $y = 0$ ). The bottom panel shows, for $z = 120 mm$ , a 2D plot of the electron density relative to the channel profile $δ n_{ch} / n_{ch} = (n_{e} - n_{ch}) / n_{ch}$ . The shading of the longitudinal profiles indicates the local effective frequency $- d ϕ / d t$ , where $ϕ$ is the temporal phase, using the same color scale as the top panel. (b) The modulus of the on-axis electric field of the drive pulse together with the red- and blueshifted components before and after application of a quadratic spectral phase $ψ^{(2)} = - 1480 {fs}^{2}$ . (c) The corresponding 2D intensity profiles. (d) The same plots as in (a) but at a distance $z = 50 mm$ into the accelerator stage.
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Figure 3
Comparison of the results of the 1D analytic model (dashed lines) and PIC simulations (solid lines), plotted as a function of the length of the modulator stage. (a) The relative transmitted energies of the drive pulse (black), and of its components in the central band (i.e., $| ω - ω_{0} | < ω_{p} / 2$ , green), and in the blueshifted (i.e., $| ω - ω_{0} | > ω_{p} / 2$ , blue) and redshifted ( $| ω - ω_{0} | < - ω_{p} / 2$ , red) sidebands. (b) The peak intensity (blue), and FWHM duration (orange), of the most intense pulse in the train generated by applying a quadratic spectral phase, optimized to yield the highest-intensity pulse train, on the drive pulse emerging from the modulator stage. (c) The peak accelerating electric field produced by injecting into the accelerator stage the pulse trains which would be generated by compressing the drive pulse at that point in the modulator.
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Figure 4
Performance of a scaled accelerator with seed and drive pulse energies increased to 140 mJ and 1.7 J, respectively. (a) The on-axis longitudinal profiles of the laser intensity and the relative electron density $δ n / n_{0}$ at $z = 2$ (top) and $z = 100 mm$ (bottom) in the acceleration stage. The color scale shows the local laser wavelength. (b) Evolution of the normalized spectral intensity of the drive laser with propagation distance $z$ in the acceleration stage. (c) Evolution of the normalized energy spectrum ${\hat{Q}}_{e} (W_{e}, z)$ of the injected electron bunch with $z$ .
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