Evolution and progress of the cryogenic target shroud remover prototypes developed for the LMJ facility

The Laser Megajoule (LMJ) cryogenic target is protected from ambient thermal radiation by a thermal shroud. When the cryo-target, held by the cryo-target positioner, is at the LMJ chamber center, the thermal shroud has to be removed just before the shot to allow the laser beams to reach the laser entrance hole of the cavity. The shroud remover, PET, will have to disconnect the thermal shroud from the cryogenic target base without disturbing the target base temperature regulation (≅18 K ± 2 mK), which guarantees the needed cryogenic target conditions to reach the ignition. The shroud withdrawal is divided into two successive phases: a slow withdrawal for the thermal disconnection between shroud and target base and a fast withdrawal for a quick extraction of the shroud out of the laser beam ways pointing onto the cavity. The slow shroud withdrawal must be handled within 30 min to respect laser pointing stability. After the final target alignment at the chamber center, the shroud must be ejected 0.5 m away from the source point in ≤0.1 s before the shot. To cope with all these issues, a prototype of the shroud remover, PPET, has been first built and developed at CEA-Grenoble, at INAC/SBT, before being tested at CEA-CESTA on the DEMOCRYTE setup, a prototype of the cryogenic target charger and holder. The experimental results mainly obtained at CEA-CESTA in 2008 and 2009 on two generations of target bases and shrouds are presented in this paper. (authors)