Laser physics studies with Phebus as part of the megajoule laser project

The requirements for target ignition with megajoule-class lasers (Nakatsuka et al., Second Annual International Conference on Solid-State Lasers for Application to ICF, Paris, October 22–25, 1995 [1]) create new constraints on the laser technology namely due to the propagation, frequency conversion and focusing of ultrahigh energy pulses. This paper emphasizes some advanced laser topics that arise with megajoule-class lasers such as frequency conversion with narrow and broadband pulses, energy losses due to mechanical sources of beam birefringence and due to a too long length of propagation in air. Wavefront distortion is measured and corrected and a new focusing scheme with high efficiency gratings is presented. The third beamline of the Phebus laser (Fleurot et al., Infrared Technology XVI, 1990, pp. 280–291 [2]) is dedicated to these laser physics investigations at an energy density level of 3–4 GW/cm2 that is close to the megajoule laser (LMJ) beam density value. This beamline delivers up to 1.2 kJ, 1ω light in ns square shaped pulses, i.e. typically 1 TW pulses. The output beam aperture is reduced from 23 to 18 cm to get the required energy density level. The bandwidth of the laser pulse is varied up to a few angstroms. The beam characteristics are recorded and analysed by means of the output sensor 1ω energy, time profile and spectrum diagnostics and by means of a specific set of diagnostics for the 1ω, 2ω and 3ω beam parameters, located in the switchyard on a workstation called BACF.