Apparatus and method for achieving temperature stability in a two-stage cryocooler

A hybrid two stage cryocooler comprising:

• a first-stage Stirling expander comprising
• an expansion volume having an expander inlet, a first-stage regenerator, and an outlet, and
• a displacer which forces a working gas through the expander inlet and the first-stage regenerator, and into the expansion volume;
• a thermal-energy storage device in thermal communication with the expansion volume of the first-stage Stirling expander;
• a second-stage pulse tube expander comprising
• a pulse tube inlet,
• a pulse tube gas volume in gaseous communication with the pulse tube inlet, the gas volume including a second-stage regenerator, a pulse tube gas column, and a surge volume, and
• a second-stage heat exchanger in thermal communication with the second-stage regenerator and the pulse tube gas column;
• the gas flow path establishing gaseous communication between the outlet of the expansion volume of the Stirling expander and the pulse tube inlet, and
• a flow-through heat exchanger disposed along the gas flow path between the output of the expansion volume of the Stirling expander and the pulse tube inlet.

A method for cooling a heat load, comprising the steps of providing a cryocooler comprising

• a first-stage Stirling expander having
• a first-stage interface,
• a displacer,
• a first-stage regenerator,
• a motor that drives the displacer, and
• a Stirling expander outlet,
• a thermal-energy storage device in thermal communication with first-stage interface,
• a second-stage pulse tube expander having a pulse tube inlet, the second-stage pulse tube expander being in thermal contact with the heat load;
• a motor controller for the motor of the first-stage Stirling expander, the motor controller being operable to vary a relative cooling power of the first-stage Stirling expander and the second-stage pulse tube expander,
• a gas flow path extending between the Stirling expander outlet and the pulse tube inlet, and
• a heat exchanger in thermal contact with the gas flow path;
• operating the motor controller to increase the relative cooling power of the second-stage pulse tube expander for a large heat load, and thereafter to decrease the relative cooling power of the second-stage pulse tube expander.