Cryostat Recondensing Chamber Pressure Control for Stable Cooling
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Solution Overview
Problem
Conventional MRI magnet systems face challenges in maintaining stable, homogeneous magnetic fields while avoiding over-cooling of cryogen vessels, which can lead to undesirably low pressures and increased contamination risks due to the inability to decouple the refrigeration stages effectively.
Innovation Solution
The solution involves controlling the thermal coupling between the second cooling stage of the refrigerator and the cryogen vessel by varying the pressure in the secondary recondensing chamber, allowing for full-power operation of the first stage to cool thermal radiation shields while preventing over-cooling of the cryogen vessel, thereby maintaining a stable pressure and reducing contamination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the refrigerator operates at full power to cool thermal radiation shields, then the cooling effectiveness is improved, but the cryogen vessel becomes over-cooled leading to low pressure
Solution Approach 1:
The refrigerator is divided into two independent stages: first stage for cooling thermal radiation shields and second stage for cooling cryogen vessels. This segmentation allows each stage to be controlled independently, resolving the contradiction by enabling full-power operation of the first stage without over-cooling the cryogen vessel through the second stage's regulated operation.
Solution Approach 2:
The second stage of the refrigerator is made dynamically controllable through variable speed drive (VSD) technology, allowing its operating speed to be adjusted based on actual cooling needs. This dynamic control prevents over-cooling and maintains appropriate pressure in the cryogen vessel while the first stage operates at full power for optimal shield cooling.
2Productivity
If the refrigerator operates at full power, then the cooling performance is improved, but contamination risk increases due to low pressure
Solution Approach 1:
By segmenting the refrigeration system into two independently controllable stages, the first stage can operate at full power for optimal cooling performance while the second stage maintains appropriate pressure levels to prevent contamination, thus resolving the contradiction between productivity and harmful factors.
Solution Approach 2:
The system incorporates feedback control where the operating speed of the second stage is continuously adjusted based on temperature and pressure measurements from the cryogen vessel. This feedback mechanism ensures that cooling performance is maintained while preventing pressure from dropping to levels that would increase contamination risk.
3Device complexity
If the refrigeration stages are coupled, then the system structure is simplified, but the ability to independently control cooling power is reduced
Solution Approach 1:
The refrigeration system is segmented into two distinct stages with separate control systems. While the physical structure remains relatively simple with sequential arrangement, the independent control capability is achieved through separate temperature sensors, control logic, and variable speed drive for the second stage, resolving the contradiction between structural simplicity and control versatility.
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
This approach ensures effective cooling of thermal radiation shields while preventing over-cooling of the cryogen vessel, maintaining a stable pressure and reducing contamination risks, thereby optimizing the operation of MRI magnet systems.
Implementation Method 1
First stage heat exchanger 12a of the refrigerator 4 is in thermal contact with shield 2
Implementation Method 2
The second stage 7 of the refrigerator 4 is situated in the lower part 8 of refrigerator interface sleeve 5. Second stage 7 terminates in cooling stage 9 which is cooled by the refrigerator to a low temperature
Implementation Method 3
provides a heat transfer medium for transferring heat from gaseous cryogen in recondensing chamber 11, via recondenser 11a and base 10 to secondary recondenser 9, by boiling at base 10 and recondensation at cooling stage 9
Implementation Method 4
Cooling stage 9 liquefies the gas within the sleeve 5 and more particularly within the secondary recondensing chamber 8
Implementation Method 5
The operating speed of the second stage of the refrigerator may be varied by a variable speed drive 116 to vary the cooling power delivered into the cryogen vessel 1
Data Source
AI summary
The present invention provides a cryostat comprising a cryogen vessel (1), a thermal radiation shield (2), and a sleeve (5) for accommodating a cryogenic refrigerator. Also provided is a first thermal contact for thermally and mechanically connecting a first stage of a cryogenic refrigerator to the radiation shield for cooling thereof. A secondary recondensing chamber is provided (8) for accommodating a second stage of a cryogenic refrigerator, and means (10; 24) are provided for thermally connecting the secondary recondensing chamber to a recondensing surface (11a; 44) exposed to the interior of the cryogen vessel. The cryostat further comprises a pressure control arrangement (100) for controlling the pressure of a gas within the secondary recondensing chamber.