Laser pulse group velocity in electrically-discharged capillary waveguides

Plasma channels are critical in maintaining high intensity laser fields over extended distances, such as required in efficient laser-plasma accelerators (LPAs). In LPAs, the background plasma electron density is a critical parameter as it influences the dephasing length - the distance of optimum acceleration - as well as laser guiding, accelerating wakefield amplitude and particle injection. In this talk a novel method is presented that measures the group velocity in the plasma through two-pulse spectral interferometry, from which the on-axis electron density can be determined. We will show results on parabolic plasma channels created in discharged capillaries, for a range of pressures as relevant for current-day LPAs. The obtained knowledge on the group velocity allows us to improve the design of guiding channels and accelerator structures.