Measurement of the laser-pulse group velocity in plasma waveguides

Electrically discharged plasma channels can guide laser pulses, extending the laser-plasma interaction length to many Rayleigh ranges. In applications such as the laser-plasma accelerator, the laser group velocity in the channel plays a critical role. The laser travel time (and thus the averaged group velocity) was measured through two-pulse frequency-domain interferometry and was found to depend on the on-axis plasma density and laser spot size. The data is in agreement with theory.