Superconducting Magnet 4K Shield for Eddy Current Mitigation
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Solution Overview
Problem
Cylindrical superconducting magnets in MRI systems face issues with gradient coil-induced heating and vibrations due to eddy currents in low-cryogen inventory designs, leading to potential magnet quench and noise, as the thermal resistance between the coils and the cooled surface is higher than in liquid cryogen-cooled magnets.
Innovation Solution
A cryogenically cooled, electrically conductive 4K shield is interposed between the magnet coils and the bore tube of the thermal radiation shield, made of materials like aluminum with low emissivity and sufficient thickness to shield magnetic fields and black-body radiation, and designed to have resonant frequencies higher than the operating frequencies of the gradient coils to minimize vibrations and heating.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a thermal radiation shield is positioned between the superconducting magnet and the outer vacuum chamber, then thermal radiation shielding is improved, but eddy current heating and vibrations are worsened due to the conductive bore tube
Solution Approach 1:
A non-conductive radiation shield is introduced as an intermediary layer between the conductive bore tube and the superconducting magnet coils. This non-conductive shield blocks the path for eddy current induction while maintaining thermal radiation shielding, effectively decoupling the harmful electromagnetic interactions from the thermal protection function
Solution Approach 2:
The radiation shielding function is segmented into two distinct components: a conductive bore tube for structural support and thermal radiation blocking, and a separate non-conductive radiation shield for electromagnetic isolation. This segmentation allows each component to optimize its specific function without the adverse effects of combining conductive and insulating properties in a single element
2Object-affected harmful factors
If the thermal radiation shield bore tube is made conductive to shield stray magnetic fields, then magnetic field shielding is improved, but gradient coil-induced heating is worsened due to eddy currents
Solution Approach 1:
A non-conductive radiation shield serves as an intermediary barrier between the gradient coils and the conductive bore tube. This intermediary prevents direct electromagnetic coupling that would induce eddy currents in the conductive tube, while still allowing the tube to provide structural support and thermal radiation shielding
Solution Approach 2:
Instead of making the radiation shield itself non-conductive to prevent eddy currents (which would compromise magnetic field shielding), the invention inverts the approach by using a conductive bore tube for magnetic shielding and adding a separate non-conductive layer for electromagnetic isolation, reversing the conventional assignment of conductive and insulating properties
3Temperature
If liquid cryogen cooling is used to cool the magnet coils, then cooling efficiency is improved, but cryogen inventory is worsened due to high helium consumption
Solution Approach 1:
The invention replaces the liquid cryogen bath cooling system with a gas-phase cooling system. Instead of immersing coils in liquid helium, the coils are cooled by thermal conduction through a cold mass structure that is cooled by circulating cold gas, eliminating the need for large quantities of liquid cryogen while maintaining effective temperature control
Solution Approach 2:
The cooling system transitions from liquid phase to gas phase operation. By changing the physical state of the cryogen from liquid to gas, the system achieves comparable cooling efficiency with dramatically reduced inventory requirements, as gas-phase cooling allows for continuous heat removal without the need for large volumes of cryogenic fluid
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 4K shield effectively reduces gradient coil-induced heating and stray magnetic field interactions, preventing magnet quench and noise, while maintaining efficient cooling and structural integrity, even in dry or minimum-cryogen magnet systems.
Implementation Method 1
The present invention addresses these problems by providing cylindrical superconducting magnets and methods for manufacture thereof as described below. The present invention may be applied to any dry, or minimum-cryogen magnet
Implementation Method 2
the coils are arranged for cooling by thermal conduction through a cooled surface in mechanical contact with the coils
Implementation Method 3
The time-varying magnetic fields generated by the gradient coils induce eddy currents in the material of nearby conductive surfaces, such as bore tubes of the OVC and the thermal radiation shield(s)
Implementation Method 4
These eddy currents, flowing through the resistive material of the bore tubes, cause heating which may risk magnet quench due to a rise in coil temperature
Implementation Method 5
The time-varying currents applied to the gradient coils during an MRI imaging sequence interact with the homogeneous background magnetic field to cause Lorentz forces to act on the gradient coils, resulting in vibration of the gradient coil assembly
Implementation Method 6
The eddy currents produced in the material of the OVC bore tube will help to shield the bore tube of the thermal radiation shield stray magnetic fields from the gradient coils
Data Source
AI summary
A cylindrical superconducting magnet has a number of axially-aligned annular coils of superconducting wire, arranged for cooling by thermal conduction through a cooled surface in mechanical contact with the coils. The coils are provided with a cryogenic radiation shield located between respective radially inner surfaces of the coils and respective axes of the coils. The cryogenic radiation shield is formed of a metal layer in thermal contact with the cooled surface.