1.  Field of the Invention
This invention relates to cryogenic cooling systems and, in particular, to an apparatus and a method for extracting cooling power from a cooling system regenerator. 
2.  Description of the Related Art
It is often desirable to cool devices, e.g., semiconductor electronics, superconducting electronics, superconducting magnets, sub-Kelvin cooling stages, and the like, to low temperatures, such as temperatures near absolute zero.  The cooling systems that provide cooling to such devices are inherently thermally linked to a room-temperature environment and/or intermediate temperature environments via various structures, e.g., mechanical structures, electrical cabling and leads.  The cooling capacity of such systems is also impacted by thermal radiation from the environment.  These extraneous thermal sources result in a parasitic thermal load on the cooling system in addition to the thermal load created by the device or devices to be cooled.  Additional thermal loads can cause power loss, cooling inefficiencies, and other problems that could be detrimental to a process or manufacturing operation. 
Generally, such cooling systems are generally two-stage pulse tube, Stirling, or Gifford-McMahon type cooling systems having a first stage operating within a range of about 40K to about 100K and a second stage operating in the liquid helium temperature range, i.e., about 2K to about 6K. It is generally desirable to reduce the parasitic heat load on the lowest temperature cooling stage to increase the overall efficiency of the system.  Conventionally, this problem has been addressed by operating the first stage of the cooling system at the lowest achievable temperature, resulting in less heat being transferred to the second, or lower temperature, stage.  Success by this method, however, is generally limited by the cooling capacity of the first, or upper temperature, stage.  Furthermore, more inefficiency (e.g., power and thermal inefficiencies) may result from this approach. 
The problem has also been addressed by utilizing a three-stage cooling system having a second stage operating in the range of about 10K to about 20K. Such a system, however, is more costly and complex than a two-stage cooler and may have lower reliability. 
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.