Cold Head Thermal Decoupling Using Pressure-Activated Gas Gap
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Solution Overview
Problem
Cryostats face significant thermal load issues when the cooling machine fails, leading to premature quenching of superconducting magnets due to the cold head acting as a thermal bridge, especially in systems with minimal or no liquid helium, where existing solutions require operator intervention or are not fully automatic.
Innovation Solution
The hollow volume between the cooling arm and the object to be cooled is filled with a gas or gas mixture that expands upon heating, allowing the cooling arm to move and create a thermal gap, reducing heat input by increasing the gap between thermal contact surfaces, which can be up to 10 mm, and using a pretensioning device to ensure decoupling upon threshold pressure exceedance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the cooling machine fails, then the cold head continues to provide thermal contact to the object, but this causes excessive heat input leading to premature quenching of the superconducting magnet
Solution Approach 1:
The cooling arm is designed to be movable rather than fixed, allowing it to dynamically adjust its position in response to changing pressure conditions. When the cooling machine fails and gas pressure increases, the cooling arm automatically moves away from the object to be cooled, reducing thermal contact and heat input to extend the time until quenching occurs.
Solution Approach 2:
The system utilizes changes in gas pressure as a parameter to control thermal contact. When the cooling machine operates normally, gas pressure remains low and thermal contact is maintained. Upon failure, gas pressure increases, triggering the cooling arm to move and reduce thermal contact, thereby changing the heat transfer parameter to extend operational time.
2Reliability
If the cooling arm is fixed in position to ensure stable thermal contact, then good heat transfer is maintained, but automatic thermal decoupling upon cooling machine failure cannot be achieved
Solution Approach 1:
The cooling arm system is designed to automatically respond to cooling machine failure without external intervention. The increase in gas pressure upon failure directly acts on the cooling arm to move it away from the object, creating self-service thermal decoupling that eliminates the need for complex control systems or operator intervention.
Solution Approach 2:
The system uses gas pressure (pneumatics) as the actuating force to move the cooling arm. The gas contained in the cryostat expands when the cooling machine fails, and this pressure change directly drives the cooling arm to change position, providing a simple and reliable mechanism for automatic thermal decoupling.
3Ease of operation
If gas pressure is used to move the cooling arm away from the object, then thermal contact is reduced, but the cooling arm may not return to contact position when cooling is restored
Solution Approach 1:
The cooling arm mechanism incorporates a counterbalancing force (such as a spring) that opposes the gas pressure force. When gas pressure increases upon cooling machine failure, it overcomes the counterbalancing force to move the cooling arm away. When gas pressure decreases upon restoration of cooling, the counterbalancing force automatically returns the cooling arm to its contact position, ensuring reliable re-contact without complex control systems.
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 configuration significantly reduces heat input to the superconducting magnet, extending the time before quenching by minimizing thermal conduction and convection, allowing for automatic operation without operator intervention.
Implementation Method 1
filled with a gas or gas mixture with positive thermal expansion coefficient
Implementation Method 2
the internal pressure of the gas or gas mixture pressurizes part of the cooling arm
Implementation Method 3
the thermal contact surfaces no longer contact each other in this position but are thermally separated from each other by a gap filled with gas or a gas mixture
Data Source
AI summary
A cryostat has a cooling arm with a first thermal contact surface which can be brought into thermal contact with a second thermal contact surface on an object to be cooled. A hollow volume (2) between the inner side of the neck tube, the cooling arm, and the object is filled with gas and the cooling arm is pressurized by the inner pressure of the gas and also by atmospheric pressure. A contact device brings the first and the second contact surfaces into thermal contact below a threshold gas pressure and moves them away from each other when the threshold pressure has been exceeded such that a gap (13) filled with gas thermally separates the first and second contact surfaces. Operationally safe and fully automatic reduction of the thermal load acting on the object to be cooled is thereby obtained in case the cooling machine fails.