Capillary discharge plasma waveguides[1,2], also used as active plasma lenses, allow control over laser pulse diffraction and particle bunch divergence[4–7]. 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; charged particle bunches are focused by the magnetic field associated with the current. Capillary plasma waveguides are compact, provide a strong gradient (or focusing), and have a high damage threshold. 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[11–14], high harmonic generation[15–18], 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.
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