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The Mégajoules Laser (LMJ) is comprised of 240 separate beam lines that each produces about 20 kJ of 1 .053 microns light. Simply stated the architecture consists of 240 optical pulse generation system that each delivers a 10 nJ shaped and modulated pulse. Next, 240 preamplifier modules amplify the tailored pulses to 1 J, whereupon they are transported to the large amplifier chains where the laser energy is increased to the 20 kJ level. The preamplifier modules contain a regenerative amplifier ( regen), a spatial beam shaping module and a fourpass amplifier. The specifications for the regen include a total gain of i0, an output energy of 10 mJ, operating at a 1 Hz rate, a square-pulse distortion < 2, a signal-to noise-ratio < i0, and a pulse-to-pulse output energy stability ofbetter than 3%. In case the Mégajoules pulses are smoothed with a multimode fiber, we designed and tested a multimode regenerative amplifier that is described in this paper. The current design of this cavity is shown on Fig 1 . The 4.25 long cavity consists of 2 lenses and 2 spherical mirrors that make it equivalent to a confocal cavity. A square and a circular diaphragms are used in order to have the required Abbe's invariant and to shape the beam in the cavity; the square plane will correspond to the pupil, whereas the circle plane will correspond to the focus plane of the laser. Near a pupil plane is the 4 mm diameter LNA rod amplifier, that is side-pumped by laser diodes. The laser diodes deliver 2 J in a 400 ts pulse, and about 500 mJ are stored in the upper laser state. The total avalaible energy for the cavity modes is about 150 mJ. For the experiment, the input of the regenerative amplifier was a 1 0 pJ pulse of 20 ns duration, delivered by a LNA microchip laser coupled to a multimode fiber. The output beam was mode matched to the regen with 2 diaphragms (square and circular ) and 2 lenses. The s-polarized input was then introduced into the cavity via reflection on a GLAN polarizer. After one round trip the Pockels cell is turned on, trapping the pulse in the cavity untill saturation is reached. The pulse is switched out by energizing one more time the Pockels cell. We measured an output energy of 30 mJ; gain and transmission were determined using Findlay-Clay (relaxation oscillations ), and found to be respectively equal to 20 and 0.91 per round trip. The signal-to-noise ratio was determined by comparing the pulse buildup in the cavity when the input seed is blocked and unblocked. It lead us to a ratio equal to 300, so we can expect 3.1 5 with 10 nJ injected. The amount of square pulse distortion was found to be less than 3.
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