A short-pulse laser beat wave scheme for advanced particle accelerator applications is examined. A short, intense (3-ps, >10(18)-W cm(-2)) two-frequency laser pulse is produced by use of a modified chirped-pulse amplification scheme and is shown to produce relativistic plasma waves during interactions with low-density plasmas. The generation of plasma waves was observed by measurement of forward Raman scattering. Resonance was found to occur at an electron density many times that expected, owing to ponderomotive displacement of plasma within the focal region.

International audienceExperiments to examine the generation of relativistic plasma waves via a high-intensity short-pulse beat-wave scheme are described in detail. The pulse stretcher of the Vulcan chirped-pulse amplification (CPA) laser system was modified to produce two frequency, 3 ps pulses focusable to intensities up to 10^18 W cm^−2. Short high-intensity pulses were used to avoid limitations to the plasma-wave amplitude due to the modulational instability. Two experiments were undertaken, at 3 and 10 TW, with the generation of plasma waves diagnosed by measuring the sidebands produced in the spectrum of the forward scattered beam. A resonance in the sideband signal was observed for an initial plasma density higher than expected for the given beat frequency. This resonance shift can be attributed to transverse ponderomotive expulsion of plasma electrons from the laser focal region. A monotonically increasing background was also observed, which was due to nonresonant cross-phase modulation

The direct side-pumping of Nd:YAG rods with laser diodes has been shown to be a cost effective scheme for scaling to simultaneous high average and high peak power operation. Careful control of the parameters that govern the performance of a multi-spatial mode MOPA system has led to the demonstration of reliable Q-switched average powers of up to 1600 W and peak powers in excess of 18 MW at the 1064 nm fundamental wavelength. The techniques used to refine the properties of an infrared stable resonator in order to optimise intra-cavity second harmonic generation are discussed. Average powers of up to 290 W and peak powers of up to 0.45 MW at 532 nm have been demonstrated from a single cavity. By polarisation multiplexing the outputs of two cavities emitting at 532 nm, we have achieved single-beam average and peak powers of up to 580 W and 0.9 MW respectively and gained flexible control of pulse duration and temporal shape.