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PETAL is an additional PW beamline to the LMJ. The kJ shots in the amplifier section, the compressor alignment and the 1.15 PW @ 850 J operations are detailed. Damage issues encountered are also addressed.

A multi-Petawatt high-energy laser PETAL coupled to the Ligne d’Int´ Laser (LIL) is under construction in the Aquitaine Region in France. This Petawatt laser will be dedicated to academic experiments in the fields of high energy density physics and ultra high intensity. Nd : glass laser chain coupled with the chirped pulse amplification (CPA technique allows delivery of high energy. Optical parametric CPA for pre-amplification and a new compression scheme will be implemented. PETAL is designed to deliver 3.6 kJ of energy in 500 fs on a target corresponding to 7.2 PW. The PETAL beam linked to the up to 60 kJ ns UV beams from the LIL will present new scientific research opportunities. (Some figures in this article are in colour only in the electronic version)

LMJ and LIL are two French high power lasers dedicated to fusion and plasma experiments. These laser beams involve hundreds of rather large optical components, the clear aperture of the beams being 400x400 mm². Among these components, an adaptative mirror is used to correct wavefront distortions in the amplification section. A simple design has been chosen with push/pull actuators glued on the backside of a thin glass plate (9 mm). To ensure the bonding mechanical steadiness, we need enough roughness on this backside. That is why it is ground. We noticed figure instabilities on several of these ground backside substrates. Those wavefront distortions can be of several hundreds of nanometers. We designed a specific mount to avoid the possibility of measurement discrepancies due to mechanical mounting. We noticed then significant evolutions over a time-scale of a few months. The possibility of slow stress variations in the ground backside has then been considered. It has been known for a long time that a ground surface is in a compressive state and consequently tends to take a convex shape, this effect being named Twyman effect after its discoverer. Anyway, as far as we know, there is still doubt on the physical mechanisms involved and no publication has been made on the fluctuations of this effect. We wish to expose here results that led us to believe that instabilities are also linked to the external stress which is seen during transport or storage. Finally, we present the experiments we put in place on samples to improve our kn owledge on this phenomenon and to test potential solutions. Keywords : wavefront, instability, mirror, Twyman effect, relaxation, viscoelasticity, glass

The laser Megajoule (LMJ) project was launched in 1995 by the French Atomic Energy Commission and is aimed at developing a facility to achieve inertial confinement fusion. The LMJ architecture is based on 240 laser beamlines to be focused onto a millimetre size target. To meet this challenge, one of the main issues consists in a proper control of the quality of the beam wavefront at the output of the laser. For this purpose, the LMJ makes use of a large-size adaptive mirror (n amed M1 in the actual laser baseline), with 400 by 400 mm² useful aperture. This paper provides an overview of the M1 prototype, using 39 mechanical actuators. This mi rror was made by the French company CILAS© together with the help of ISP© for the mechanical actuators. CEA, Cilas an d ISP have joined their efforts to complete the overall design. The prototype has been tested on the Laser Integration Line (LIL). This facility is fully consistent with the LMJ requirements, a complete laser chain which comprises 4 beamlines. After presenting the basic design principles, we focus on the demonstrated performance measured on a dedicated setup, as well as on LIL facility. Keywords: deformable mirror, adaptive optics, astatic actuator, mechanical actuator, wavefront control, wavefront correction, high power laser