A layout of the source is shown in fig. 1. The source chamber (diameter 150 mm, length 300 mm with two elliptical domes) is made from zircalloy. It has volume 6 l. The chamber has double walls (2x0.5 mm) with flowing cold helium between them from cryogenic refrigerator (capacity 150W at 4.5K). At cooling the deuterium from a tank (volume 6 m3) supplies the chamber. The deuterium is condensed and come to a solid state in the chamber.
A special technical solution is necessary to decrease the deuterium temperature lower 10-12K. The problem is a loss of a thermal contact between cold chamber wall and the solid deuterium because of a deuterium volume decreases at cooling. A cooling is possible at some torrs of a vapor pressure. For example, at 12K the saturated vapor pressure is 0.75 torr, but at 10K - only 5x10-2 torr. A possible technical solution is to place a spiral tube on inner chamber wall to achieve a good thermal contact. However, at first step we did experiments with a simple design. The second step will be with the same design, but with deuterium containing some helium quantity for heat exchange. Only third step will include more complicated chamber design if it will be necessary.
Next step of investigation is decreasing of the source temperature down to 6-7K with aim to study capability of increasing of UCN yield. It is necessary to note, that at 6-7K the heat conductivity of the solid ortho-deuterium increases in order of value. It makes better the source capacity to heat load. The result of investigation can be used for new projects of solid deuterium sources at high flux reactors with heavy water reflectors. A heavy water is a good shield from high energy neutrons and gamma rays. That allows to have a low level of heat load at high flux of thermal neutrons (reactor PIK, Gatchina, reactor ILL, Grenoble). Another possibility is using of solid deuterium sources at neutron spallation sources, where the relation of heat load to neutron flux is better than one for reactors. For example, the solid deuterium source for UCN production is planning to be installed at 1MW spallation source in Los Alamos (USA).