The new-type Pomeranchuk cooling machine

Pomeranchuk cooling is now a commonly used cooling method to obtain the temperature region of a few millikelvin. This one-shot method has not been used as a refrigerator to cool specimens other than the working substance 3He in the device, and this is a big drawback as compared with a continuously operating dilution refrigerator. On the other hand, below about 5 mK, Pomeranchuk cooling has larger cooling power than the dilution method when the molar rate of production of solid is the same as the circulation rate of liquid 3He. In order to realize a Pomeranchuk refrigerator with sufficiently large cooling capacity, it is necessary to scale up the device to overcome the weak point of the single-cycle method as usually done in the demagnetization cooling. The difficulty arises, however, that unreasonably long time is necessary in pre-cooling a large amount of liquid 3He in a Pomeranchuk cell. This difficulty may be avoided by utilizing the Leiden dilution refrigerator (L.D.R.). In the new-type Pomeranchuk cooling machine, compressive cooling of 3He takes place directly in the mixing chamber of the L.D.R. The advantages are that large amount of 3He may be pre-cooled directly in a short time, and that this pre-cooling can be performed under pressure. This new-type machine consists of two L.D.R.s: the first one is used for Pomeranchuk cooling and the second is as a thermal guard. The construction of this apparatus is given. The specification, the function of each part of this machine and the characteristics are explained. The initial temperature of 3 mK and the capacity of 1.7 x 105 erg/mole of Cu are required. (Nakai, Y