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1.1962 TO THE MEGAJOULE LASERShortly after the discovery of the laser by T Maiman in 1960 [1], scientists had the idea of using these energetic and intense optical sources to heat plasmas [2, 3]. This was the beginning of the development and operation of multiple generations of high energy and/or power laser facilities worldwide. At CEA in France, eleven generations of main high-energy laser systems were used to study hot plasmas (see Fig 1). These facilities were located at CEA Limeil (from 1962 to 1999) and since then at CEA Cesta. Figure 1:Main generation of high energy, high power laser in operation at CEA from 1962 to 2024. LMJ laser is represented at first light (one bundle = 8 beamlines in 2014) and in tis 2024 configuration (11 bundles = 88 beamlines)  Some of the main performance, as well as important laser results, are detailed below:
2.THE MEGAJOULE LASER ERA.The Laser Megajoule (LMJ) is one of the most powerful inertial confinement fusion laser facilities in the world, alongside the National Ignition Facility (NIF) in the United States. The project started with a first laser test bundle called the LIL (Laser Integration Line), which operated from 2004 to 2014. The LIL was used for qualification purposes and as an experimental facility for laser plasma experiments [12]. The construction of the LMJ facility started in 2003 and the first photons were emitted by laser bundle #28 in 2014. Currently, 11 laser bundles, each consisting of 88 large-aperture laser beams of 0.35×0.35 m², are in operation. These beams deliver up to 330 kJ of energy per day at a wavelength of 351 nm to a target positioned at the center of a 10 meters diameter vacuum chamber. The LIL/LMJ beamline incorporates many innovations compared to the previous generation of lasers: large 0.35x0.35 square apertures, 4-pass main amplifiers, deformable mirror for wavefront correction, optically addressable light valve for spatial contrast reduction, high damage resitance manufacturing processes for optics, beam steering and focusing gratings… A short-pulse high-intensity CPA laser beam, called PETAL, adds a PW capacity to the experimental facility [13]. Today, the LMJ laser is operated at half energy and half power to allow user experiments to be carried out on the same type of additional bundles to be assembled and commissioned. Laser campaigns are also performed since 2021 (QPerf2021 and QPerf 2023) to prepare for full energy and power operation of the facility [14]. These tests include various improvements in terms of laser beam performance (spatial and temporal modulation reduction) and UV optics high damage manufacturing processes. For these experiments, performed with a limited number of laser beams and shots, we have designed a specific optic to investigate damage growth [15]. This optic was used to calibrate our instrument for measuring damage size from measured integrated intensity. This technic is much more accurate than the dimension estimated from the number of pixels [16]. ACKNOWLEDGEMENTThe PETAL project has been performed by the CEA (“maître d’oeuvre”) under the financial auspices of the New-Aquitaine Region (project owner), the French Government and the European Union and with the scientific support of Association Lasers & Plasmas (ALP). REFERENCESMaiman, T.,
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