Helium Recirculation Cryostat for Sub-2 K Specimen Cooling
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Solution Overview
Problem
Current cryogenic apparatuses, such as Gifford-McMahon coolers, have limitations in reaching temperatures below 4 K efficiently and quickly, and often require direct contact with the cold head, which is not always convenient for specimen measurement.
Innovation Solution
A cryogenic apparatus comprising a two-stage Gifford-McMahon cooler with a helium gas extraction flow duct and liquid helium recirculation circuit, including heat exchangers and a heat shield to minimize radiant heat transfer, allowing for efficient cooling of specimens to temperatures below 2 K without direct contact with the cold head.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a two-stage Gifford-McMahon cooler is used to achieve temperatures below 4 K, then the cooling temperature is improved, but the cooling speed and efficiency deteriorate
Solution Approach 1:
The cooling system is divided into two independent stages: a first Gifford-McMahon cooler for preliminary cooling to intermediate temperatures (4-100 K), and a second two-stage Gifford-McMahon cooler for final cooling to below 4 K. This segmentation allows each stage to operate optimally within its temperature range, achieving both low temperature and fast cooling speed.
Solution Approach 2:
A heat exchanger is introduced as an intermediary component between the two cooler stages. The heat exchanger efficiently transfers cold from the first cooler to the inlet of the second cooler, enabling the second stage to reach ultra-low temperatures rapidly without being constrained by the limited cooling capacity at intermediate temperatures.
2Use of energy by moving object
If the specimen is placed in direct contact with the cold head, then the cooling efficiency is improved, but the convenience and adaptability deteriorate
Solution Approach 1:
The specimen is extracted from direct contact with the cold head and placed in a separate measurement chamber. The first cooler's cold head is positioned near the chamber to provide cooling via thermal radiation and conduction through the chamber wall, while the second cooler's cold head remains isolated. This allows specimens to be measured conveniently without direct contact requirements.
Solution Approach 2:
The cooling system provides different cooling mechanisms to different locations: the first cooler's cold head provides intensive cooling to the measurement chamber region, while the second cooler's cold head provides intensive cooling to the liquid helium vessel region. This local differentiation optimizes both cooling efficiency and measurement convenience.
3Loss of energy
If the enclosure is evacuated to suppress convection, then the heat transfer by convection is reduced, but the significance of radiant heat transfer increases
Solution Approach 1:
The enclosure is evacuated to eliminate convective heat transfer, and the previously harmful radiant heat transfer is converted into a beneficial cooling mechanism. The cold head of the first cooler is positioned to radiate cold directly to the measurement chamber, making radiant heat transfer the primary cooling pathway and achieving efficient cooling without convection.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The apparatus achieves rapid cooling to temperatures below 1 K with continuous cooling power, effectively suppressing vibrations and radiant heat transfer, enabling precise low-temperature measurements.
Implementation Method 1
A number of different thermo-mechanical devices are known for achieving such low temperatures, for example using pressure cycling of helium gas... In the case of the Gifford-McMahon cooler, high-pressure helium at a pressure typically between 10 and 30 bar is used as the working fluid... Gas expansion takes in heat from the environment at one end of the cylinder, so one end of the cylinder may be referred to as a cold head
Implementation Method 2
a first heat exchanger within the gas flow duct... a first duct to carry cold helium gas from a fluid outlet of the first thermo-mechanical cooler and through the first heat exchanger to the fluid inlet of the second stage of the second thermo-mechanical cooler
Implementation Method 3
During use the enclosure would be evacuated, so heat transfer by convection is suppressed; under the circumstances radiation is a significant cause of heat transfer... the apparatus may also include a heat shield at the intermediate temperature, the heat shield being in thermal contact with the second thermo-mechanical cooler at a position having the intermediate temperature
Data Source
AI summary
A cryogenic apparatus (10) includes an enclosure (12), a first thermo-mechanical cooler (20) and a second thermo-mechanical cooler (22) which project into the enclosure (12), at least the second thermo-mechanical cooler (22) being a two-stage cooler, and each cooler (20, 22) having a fluid inlet and a fluid outlet for each stage, and a helium gas extraction flow duct (40) which extends into the enclosure (12) and which communicates with a vessel (42) to contain liquid helium within the enclosure (12). There is a first heat exchanger (62) within the gas flow duct (40). A first duct (74) carries cold helium gas from a fluid outlet (73) of the first thermo-mechanical cooler (20) and through the first heat exchanger (62) to the fluid inlet (75) of the second stage of the second thermo-mechanical cooler (22).