A 1500 bar cryocompressor for the laser megajoule facility

The cryogenic targets of the Laser Megajoule Facility (LMJ) are hollow spheres. Their internal wall is covered by a solid cryogenic fuel layer. These spheres are filled by permeation through the material. The filling procedure is optimized at room temperature or more (350 K) due to the fact that the permeation coefficient is increases with temperature. At filling temperature, the required pressure is between 1000 and 1300 bar. A difficulty is to prevent the buckling of the shell under the outer pressure during the filling procedure. So the rate of increase in pressure must be limited. A cryogenic compressor has been built to meet this purpose. The pressure delivered by the compressor is governed by the temperature of the (double) condensing cell that has an overall capacity of 90 cc of liquid or ice. To resist the high level of pressure, each condensing cell, made out of stainless steel, consists of an annular shape with numerous small diameter (5 mm) cylindrical bores. An electron-beam-welded cover collects the bores to a tube in connection with HP circuit. An inner/outer copper jacket is hard soldered on the cells for thermal homogeneity. Cooling to 15 or 20 K may be provided either by a Gifford–MacMahon cryorefrigerator or by a liquid helium flow. The beginning of the cool-down is boosted by a liquid nitrogen forced-flow. The complete cool-down time from 300 to 10 K may be decreased to 4 h. To achieve the maximal pressure, the condensing cells may be warmed up to 300 K by means of a accurately driven 600 W heater. This heater may be controlled either by temperature (15–25 K) or by pressure (25–300 K). Depending on the volume to be filled, the total duration of pressure ramping is in the range of 3–4 to 48 h (or more if needed). The increase of pressure versus time may be linear or have another shape. A computer program can control this slope in the range of approximately 0.1–2 bar/min for a 270 cc receiver volume.