Measurement of the laser pulse phase velocity in plasma channel for DLA optimization

We demonstrate a novel, direct method for measuring the phase velocity vϕ of an intense laser pulse within a plasma channel—the crucial parameter that controls the resonance condition in direct laser acceleration (DLA). The technique exploits the second harmonic (SH) radiation generated at the channel sheath—a phenomenon previously observed in laser-wakefield acceleration experiments. The SH emission angle is governed by a phase-matching condition that directly depends on vϕ. Experimental measurements performed using a 1 TW, 10 Hz Ti:Sa laser system yield phase velocities in the range vϕ=(1.010−1.030)c for plasma electron densities in the range ne=(0.01−0.06)ncr. The diagnostic is validated through quasi-3D particle-in-cell simulations that reproduce the experimental conditions. This work provides a way to optimize DLA schemes by enabling in situ measurement of the laser pulse phase velocity in plasma channels.