This thesis documents the experimental and theoretical investigation of laser pulse evolution in relativistic laser-plasma interactions for plasma-wakefield acceleration and ion acceleration experiments.

Power amplification of the Astra Gemini laser in a plasma was observed, with the compression of an initially 55 fs, 180 TW pulse down to 14 fs, with a peak power of 320 TW. This was achieved in a laser-driven plasma wakefield operating just below the self-injection threshold density for a propagation distance of 15 mm. Self-guiding of the laser pulse was observed, while pulse depletion was characterised as a function of density and propagation distance, showing that the pulse evolution scales equally with both. These measurements displayed good agreement with a depletion model based on pulse front etching. Particle-in-cell simulations were seen to closely reproduce the experimental results, which were concluded to be predominantly dependent on the longitudinal properties of the laser and wakefield. The simulations also revealed a new wakefield instability that is driven by the far red-shifted component of the laser pulse.

In the case of high-contrast solid-density interactions, oscillations of the front surface of the plasma were seen to result in the generation of the second harmonic of the driving laser for a p-polarised interaction. Conversion efficiencies of 22% into the second harmonic were measured, while the total plasma reflectivity into the first and second harmonics remained relatively constant at 65% over the intensity range of 1E17 - 1E21 W/cm2.

For normal incidence interactions with sub-micron thickness foils, the cycle-averaged surface motion was measured using a FROG diagnostic. Targets of a few nanometers in thickness underwent an acceleration away from the laser, but the measured surface velocities did not match the expected hole-boring velocities or the measured ion energies, due to the thermal expansion of the plasma. 2D simulations revealed that studying target motion in this way is affected by the scale length of the plasma and photon acceleration that can occur in the tenuous plasma in front of the laser-reflecting surface.