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• High Power Laser Science and Engineering
• Vol. 9, Issue 2, 02000e17 (2021)
M. Turner1, A. J. Gonsalves1、*, S. S. Bulanov1, C. Benedetti1, N. A. Bobrova2, V. A. Gasilov2, P. V. Sasorov2、3, G. Korn3, K. Nakamura1, J. van Tilborg1, C. G. Geddes1, C. B. Schroeder1, and E. Esarey1
Author Affiliations
• 1Lawrence Berkeley National Laboratory, Berkeley, CA, USA
• 2Keldysh Institute of Applied Mathematics RAS, Moscow, Russia
• 3ELI Beamlines, Dolní Břežany, Czech Republic
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Abstract

We measured the parameter reproducibility and radial electron density profile of capillary discharge waveguides with diameters of 650 $\mathrm{\mu} \mathrm{m}$ to 2 mm and lengths of 9 to 40 cm. To the best of the authors’ knowledge, 40 cm is the longest discharge capillary plasma waveguide to date. This length is important for $\ge$10 GeV electron energy gain in a single laser-driven plasma wakefield acceleration stage. Evaluation of waveguide parameter variations showed that their focusing strength was stable and reproducible to $<0.2$% and their average on-axis plasma electron density to $<1$%. These variations explain only a small fraction of laser-driven plasma wakefield acceleration electron bunch variations observed in experiments to date. Measurements of laser pulse centroid oscillations revealed that the radial channel profile rises faster than parabolic and is in excellent agreement with magnetohydrodynamic simulation results. We show that the effects of non-parabolic contributions on Gaussian pulse propagation were negligible when the pulse was approximately matched to the channel. However, they affected pulse propagation for a non-matched configuration in which the waveguide was used as a plasma telescope to change the focused laser pulse spot size.

1 Introduction

Capillary discharge plasma waveguides[1,2], also used as active plasma lenses, allow control over laser pulse diffraction[3] and particle bunch divergence[47]. They are typically used to guide laser pulses and focus electron bunches, but may also prove useful in a ‘plasma telescope’ configuration to change laser system focal spot sizes without lengthy transport lines. Optical pulses are focused as a result of the waveguide’s radial variation of the plasma electron density[8]; charged particle bunches are focused by the magnetic field associated with the current[5]. Capillary plasma waveguides are compact, provide a strong gradient (or focusing)[9], and have a high damage threshold[10]. They allow for modification of the spot size evolution of intense particle beams and laser pulses and are thus of interest for many applications, including plasma wakefield acceleration[1114], high harmonic generation[1518], and X-ray lasing[19,20]. In this paper, we concentrate on waveguide properties that are mainly relevant for laser pulse propagation. Specifically, we measure the transverse electron density profile with sufficient accuracy to discuss the effect of non-parabolic contributions for applications, as well as the stability of the channel shape and position. These parameters are of great importance for the aforementioned applications.