Closed Cycle Cryocooler Principles of Operation
Closed Cycle Cryocooler Components
The major components of the closed cycle cryostat are the expander, compressor, vacuum shroud, and radiation shield. The expander, commonly referred to as the coldhead or cold finger, is where the Gifford-McMahon refrigeration cycle takes pace. It is connected to a compressor by two gas lines and an electrical power cable. One of the gas lines supplies high pressure helium gas to the expander, the other gas line returns low pressure helium gas from the expander. The compressor provides the necessary helium gas flow rate at the high and low pressure for the expander to convert into the desired refrigeration capacity. The vacuum shroud surrounds the cold end of the expander in vacuum limiting the heat load on the expander caused by conduction and convection. The radiation shield is actively cooled by the first stage of the expander and insulates the second stage from the room temperature thermal radiation being emitted from the vacuum shroud.
In addition to these major components the closed cycle cryocooler is often accompanied by several support systems. Typically laboratory systems will have an instrumentation skirt, which provides a vacuum port and electrical fedthroughs, as well as a temperature controller to measure and adjust the sample temperature. The system also requires electricity, cooling water for the compressor, and a vacuum pump for the sample space.
The Gifford-McMahon Refrigeration Cycle
The ARS closed cycle cryocoolers operate on a pneumatically driven Gifford-McMahon refrigeration cycle, often shortened to GM Cycle or GM cooler. The pneumatically driven GM cooler is different from mechanically driven GM coolers in that it uses an internal pressure differential to move the displacer instead of a mechanical piston. This results in smaller vibrations.
The refrigeration cycle of the ARS closed cycle cryostat starts with the rotation of the valve disk that opens the high pressure path. This allows the high pressure helium gas to pass through the regenerating material and into the expansion space. Second, the pressure differential drives the displacer "up", allowing the gas at the bottom to expand and cool. Third, the rotation of the valve disk next opens the low pressure path, allowing the cold gas to flow through the regenerating material which removes heat from the system. Finally, the pressure differential returns the displacer to its original position, and the cycle is completed.
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FIG. 1. Block diagram of the internal structure of an ARS closed cycle cryocooler expander.