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We present a summary of the beatwave particle acceleration program developed at Ecole Polytechnique. In dedicated experiments, plasma formation, plasma wave generation and saturation, and particle acceleration were successively studied and understood in detail. A maximum energy gain of 1.3 MeV was obtained, which is compatible with an accelerating gradient of 0.7 GV/m.

The possibility of using high-intensity laser-produced plasmas as a source of energetic ions for heavy ion accelerators is addressed. Experiments have shown that neon ions greater than 6 MeV can be produced from gas jet plasmas, and well-collimated proton beams greater than 20 MeV have been produced from high-intensity laser solid interactions. The proton beams from the back of thin targets appear to be more collimated and reproducible than are high-energy ions generated in the ablated plasma at the front of the target and may be more suitable for ion injection applications. Lead ions have been produced at energies up to 430 MeV. Index Terms--Heavy ion accelerators, ion accelerators, ion source, laser-plasma interactions.

A chirped pulse amplification (CPA) laser configuration capable of driving a plasma beat wave into saturation before modulation instabilities can grow is reported. The proposal is based on generating a single sub-ps, broad bandwidth pulse (~16 nm) and stretching and filtering to select two wavelength components (separation ~7 nm). The two spectral components are temporally stretched (to [is greater than] 100 ps) and separated (by ~1 ns). The pulses are then amplified sequentially in a single Nd : glass chain to greater than 15 J per pulse. Using a single-pass reflective grating compressor, the pulses are compressed (from 2 to 5 ps) and automatically synchronized. The system is capable of introducing a chirp, such that the compressed pulses can compensate for relativistic detuning of the plasma wave. The optimum laser amplification and recombination configuration for generating a saturated laser-driven beatwave is presented, and options for future work are discussed.

We report on experimental results regarding the propagation of ultraintense laser pulses in a preformed plasma channel. In this experiment, the long (4-mm) fully ionized plasma channel created by the amplified spontaneous emission (ASE) was measured by interferometry before and after the propagation of the short laser pulse. Forward spectra show a cascade of Raman satellites, which merge with one another when the laser power was increased up to critical power for relativistic self-focusing [P.sub.c]. The number of filaments measured by interferometry increases when the laser power increases. High conversion efficiency ([approximately equals] 10%) of second harmonic generation was observed in the interaction. Index Terms--Laser particale acceleration and plasma guiding, laser plasma interaction, relativistic self-focusing.

High-intensity laser-plasma interaction experiments were performed using high-power laser pulses (greater than 8 TW) at a wavelength of 527 nm. These pulses were focused to intensity greater than [10.sup.18] W/[cm.sup.2] into underdense helium plasma at a density of [n.sub.e] [is less than or equal to] 5 x [10.sup.19] [cm.sup.-3]. Systematic examination of the forward Raman scattering instability was performed by measuring the spectrum of the forward scattered light. It was found that the three-wave instability occurs first, which then proceeds to a nonlinear regime involving four-wave processes. Measurements of the accelerated electron spectrum suggest that higher phase velocity plasma waves are generated than when using lower frequency light. Index Terms--Electron accelerator, electromagnetic scattering by plasma media, plasma waves.

The interaction of an intense short pulse laser ([is greater than] 5 x [10.sup.18] W[cm.sup.-2]) with underdense plasma was extensively studied. The beam is found to be highly susceptible to the forward Raman scattering instability. At sufficiently high growth rates, this can lead to wavebreaking with the resultant production of a high flux of accelerated electrons ([is greater than] [10.sup.11] for E [is greater than] 2 MeV). Some electrons are found to be accelerated well above the dephasing energy, up to 94 MeV. Self-scattered images intimate the presence of high-intensity channels that extend more than 3.5 mm or 12 Rayleigh lengths. These filaments do not follow the axis of laser propagation, but are seen to be emitted within an f4 cone centered around this axis. Spectra of the self-scattered light show that the main contribution of the scattering is not from light captured within these filaments. But there is evidence for self-phase modulation from effects such as ionization and relativistic self-focusing. However, no clear correlation is observed between channel length and the number or energies of accelerated electrons. Evidence for high intensities within the channels is given by small-angle Thomson scattering of the plasma wave generated therein. With this method, the intensity is found to be of the order of [10.sup.18] W[cm.sup.-2] greater than 12 Rayleigh lengths from focus. Index Terms--Electromagnetic propagation in plasma media, electromagnetic scattering electron accelerators, plasma waves.

Picosecond resolution shadowgraphy is a powerful tool for measurement of high-intensity laser propagation instabilities through underdense plasma. It is shown that as the plasma density increases, the laser beam is subject to severe filamentation instabilities--likely due to the effect of stimulated Raman side-scattering. At higher plasma densities, scattering and absorption are so severe that the propagation distance of the high intensity laser pulse is reduced. Index Terms--Intense laser plasma.