Radial Density Profile and Stability of Capillary Discharge Plasma Waveguides of Lengths up to 40 Centimeters

TL;DR

This study characterizes the stability and radial density profile of long capillary discharge plasma waveguides, demonstrating their suitability for high-energy laser wakefield acceleration with minimal parameter variations.

Contribution

It reports the longest capillary plasma waveguides to date (up to 40 cm) and provides detailed measurements of their stability, density profiles, and effects on laser pulse propagation.

Findings

Waveguide focusing strength stable to <0.2%

On-axis plasma density stable to <1%

Radial profile rises faster than parabolic

Abstract

We measured the parameter reproducibility and radial electron density profile of capillary discharge waveguides with diameters of 650 um to 2 mm and lengths of 9 to 40 cm. To our knowledge, 40 cm is the longest discharge capillary plasma waveguide to date. This length is important for >= 10 GeV electron energy gain in a single laser driven plasma wakefield acceleration (LPA) 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 LPA 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 are in excellent agreement with magneto-hydro-dynamic 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.