Fabrication of cryogenic targets for laser fusion

Current understanding of future targets for a laser fusion reactor is to use cryogenic targets which have a liquid or solid fuel layer on the inner surface of the fuel container. To provide the cryogenic target at a wide range of temperature is very valuable in a view point of studying the implosion physics of the cryogenic targets. The cryogenic target at liquid phase can provide the central void with gas fuel at variable density by controlling the temperature. This gaseous fuel would provide a hot core to ignite the main fuel. The authors have developed two alternative techniques, one is the foam method in which they utilize a foam shell to sustain liquid fuel and the other is plasma layering method to make a uniform solid fuel layer with an aid of plasma. Key points in the foam method are to fabricate a low density, low atomic number foam shell with fine structure and to cover the foam shell with a tough gas barrier to prevent the liquid fuel boiling off in the vacuum. No gap is allowed between the foam layer and the gas barrier. They have successfully developed a technique to coat the foam shell with the gas barrier using interfacial polycondensation method. They proposed a plasma layering technique in which a nonuniform solid fuel layer is redistributed into uniform one with an aid of a glow discharge. When the inner surface of the ice layer is uniformly heated by the plasma and the target is located in an isothermal condition, higher surface temperature at a thick ice area induces sublimation of the ice and evaporated vapor condenses on the cold, thin ice area. As the results, ice layer turns to be uniform after certain discharge time. They demonstrated the plasma layering technique by using Kr gas and liquid nitrogen as substitutions for deuterium fuel and liquid helium