In a forced laser-wakefield accelerator experiment (Malka et al 2002 Science298 1596) where the length of the pump laser pulse is a few plasma periods long, the leading edge of the laser pulse undergoes frequency downshifting and head erosion as the laser energy is transferred to the wake. Therefore, after some propagation distance, the group velocity of the leading edge of the pump pulse—and thus of the driven electron plasma wave—will slow down. This can have implications for the dephasing length of the accelerated electrons and therefore needs to be understood experimentally. We have carried out an experimental investigation where we have measured the velocity vf of the 'wave-front' of the plasma wave driven by a nominally 50 fs (full width half maximum), intense (a0 ≃ 1), 0.815 µm laser pulse. To determine the speed of the wave front, time- and space-resolved refractometry, interferometry and Thomson scattering were used. Although a laser pulse propagating through a relatively low-density plasma (ne = 1.3 × 1019 cm−3) showed no measurable changes in vf over 1.3 mm (and no accelerated electrons), a high-density plasma (ne = 5 × 1019 cm−3) generated accelerated electrons and showed a continuous change in vf as the laser pulse propagated through the plasma. Possible causes and consequences of the observed vf evolution are discussed.
Three-stage laser wakefield accelerator scheme for sub-Joule few-cycle laser pulses