Liquid nitrogen shielded low helium consumption REBCO superconducting nuclear magnetic main magnet structure

Abstract

The application discloses a liquid nitrogen shielding low-helium-consumption REBCO superconducting nuclear magnetic main magnet structure and belongs to the field of superconducting imaging equipment.The structure comprises an REBCO non-insulated coil, a liquid nitrogen-liquid helium composite Dewar, a reverse Brayton recondensation unit, a three-stage heat insulation perfusion pipeline, a sectional isolation superconducting lead and a quench sensing system.A 77K liquid nitrogen layer forms a radiation barrier, is matched with a full-dimension heat break bridge design, is matched with a built-in refrigeration unit and realizes that the annual liquid helium consumption is less than or equal to 10L, and the loss is reduced by 98% compared with a traditional nuclear magnetic loss.The liquid nitrogen layer can provide 24h low-temperature buffering when the refrigeration unit fails, and the coil is prevented from rapidly quenching.The coil adopts gradient turn spacing optimization, the 5T model 45cm imaging spherical field uniformity reaches 0.87ppm, the rated power of the refrigeration unit is only 300W and the design life is 28 years.The structure is modularized and is adapted to 1.5T-7T multi-strength equipment, liquid helium consumables, power consumption and maintenance expenses are greatly reduced, the whole life cycle operation and maintenance cost is obviously reduced, and the imaging needs of clinical diagnosis and frontier scientific research are considered.

Description

Technical Field

[0001] This invention relates to the field of superconducting nuclear magnetic resonance imaging (NMR) equipment technology, specifically to a low-helium-consumption superconducting NMR main magnet structure adapted to REBCO rare-earth barium copper-oxygen high-temperature superconducting tape and employing liquid nitrogen-liquid helium composite thermal insulation shielding and a small liquid helium recondensation system for synergistic cooling. It can be expanded to adapt to multi-field strength models from 1.5T to 7T and is widely used in hospital clinical diagnosis, biomedical high-resolution whole-body imaging, and cutting-edge medical imaging science research. Background Technology

[0002] Currently, commercially available superconducting nuclear magnetic resonance (NMR) equipment is divided into two main categories: traditional liquid helium immersion NMR and REBCO NMR without liquid helium conduction cooling. Both of them have technical drawbacks that cannot simultaneously achieve low helium consumption, low power consumption, and high operational reliability. 1. Traditional liquid helium immersion superconducting NMR radii require 1000-2000L of liquid helium filling, with an annual natural evaporation loss of 300-500L. Liquid helium resources are scarce, with international prices ranging from $30 to $50 per liter. Supply is constrained by geopolitical factors, resulting in high annual liquid helium replenishment and maintenance costs. The equipment relies solely on a single layer of liquid helium insulation, lacking an intermediate temperature buffer structure, leading to continuous heat leakage and further exacerbating helium loss.

[0003] 2. REBCO NMR, which eliminates liquid helium immersion and relies on a 20-30kW high-power refrigerator to directly conduct cooling the superconducting coils, has two major fatal shortcomings: First, the refrigeration unit has to withstand an extremely large temperature gradient of 300K to 20K, which causes severe wear on moving parts. Its design life is only 3 to 5 years, and the cost of equipment maintenance and replacement is extremely high. Secondly, there is no low-temperature buffer medium. Once the power is cut off or the refrigeration unit fails and stops, the superconducting coil will heat up rapidly and lose its superconductivity, forcibly interrupting clinical diagnosis and treatment. The equipment's stability cannot meet the continuous use requirements of medical institutions.

[0004] 3. Current technology gaps: Existing solutions cannot simultaneously achieve the four core performance characteristics of low liquid helium consumption, low refrigeration power, long-term low-temperature buffering during fault shutdown, and high magnetic field uniformity. There is a lack of a composite cold preservation engineering solution that couples a 77K liquid nitrogen intermediate shielding layer with a 4.2K small recondensation system. This invention addresses the above-mentioned industry pain points by achieving structural and process innovation. Summary of the Invention

[0005] To address the shortcomings of existing superconducting nuclear magnetic resonance (NMR) systems, such as high helium consumption, high cooling power, immediate loss of quench power upon system failure, and high life-cycle maintenance costs, this paper proposes a liquid nitrogen-shielded, low-helium-consumption REBCO superconducting NMR main magnet structure. Through the coordinated design of non-insulated precision-wound REBCO coils, a liquid helium-liquid nitrogen double-layer composite Dewar, a three-stage thermal bridge blocking pipeline, segmented isolation superconducting leads, and a small-scale reverse Brayton recondenser unit, this structure achieves stable operation at a high field strength of 5.0T. This significantly reduces liquid helium consumption and cooling power consumption. After the cooling system shuts down, the liquid nitrogen shielding layer maintains a cryogenic buffer for more than 24 hours, significantly reducing equipment purchase and long-term maintenance costs.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A liquid nitrogen-shielded, low-helium-loss REBCO superconducting NMR main magnet structure includes: Main magnet assembly, liquid nitrogen-liquid helium composite Dewar, built-in recondensation assembly, three-stage thermal bridge blocking injection structure, and superconducting lead structure with metal-free continuous thermal conduction path; The main magnet assembly is formed by continuously winding REBCO high-temperature superconducting tape into a disc-shaped stacked coil without insulation, and the entire coil is vacuum epoxy impregnated and cured. The liquid nitrogen-liquid helium composite Dewar consists of a liquid helium liner, a liquid nitrogen intermediate shielding layer, and an outer vacuum cavity from the inside out. The liquid nitrogen intermediate shielding layer forms a 77K constant temperature thermal radiation barrier to enclose the liquid helium liner. The built-in recondensation component cold head couples the liquid helium liner gas phase space for liquefying and evaporating helium gas for reflux, and uses a reverse Brayton cycle to achieve 4.2K low-temperature cooling. The three-stage thermal bridge blocking injection structure connects the liquid helium liner with the external environment, and a triple thermal insulation blocking structure is set along the fluid pipeline, consisting of a low thermal conductivity pipe section, a vacuum multi-layer insulation section, and a liquid nitrogen temperature zone thermal anchor block. In the metal-free continuous thermal conductive path superconducting lead structure, the superconducting positive and negative electrodes are independently led out, and the metal conductive components in each temperature zone are physically disconnected with gaps.

[0007] Furthermore, the design reference field strength of the main magnet assembly is selected from 1.5T, 2T, 3T, 5T, and 7T, and the rated excitation current is selected from 300A, 400A, 500A, and 800A; the REBCO high-temperature superconducting tape specifications can be selected from 4mm, 5mm, 6mm, and 10mm width.

[0008] Furthermore, the parameters of the 5.0T main magnet assembly are as follows: 4.0~6.0mm wide REBCO tape is used; the number of coil layers is 50, the total number of turns is 20,000, and the inner radius of the winding is 500±5mm; the two ends are equipped with reverse balance coils with a bending diameter >50mm and a 1:1 ratio of positive to negative turns; the coil axially is arranged in a three-zone gradient turn spacing, with dense winding in the middle area and sparse winding at both ends; the uninsulated winding tension is 15~25N, the winding speed is 1~5mm / s, and the current equalization and quench energy dispersion between turns are achieved by relying on contact resistance.

[0009] Furthermore, the liquid nitrogen-liquid helium composite Dewar has a liquid helium inner liner volume of 380~420L, is made of S30408 ​​austenitic stainless steel, has a static evaporation rate of ≤2% / day, and an overall helium mass spectrometry leak rate of ≤1×10⁻¹. 0 Pa・m³ / s; the liquid nitrogen intermediate shielding layer has a volume of 2500~3500L, coaxially encased in a liquid helium inner liner, with 15~20 layers of aluminized polyester film filling the interlayer, and the interlayer vacuum degree ≤1×10⁻³Pa; the outer vacuum chamber vacuum degree ≤1×10⁻³Pa; the three-layer structure is isolated by G-10 glass fiber reinforced plastic supports, with no direct metal heat conduction path.

[0010] Furthermore, the built-in recondensation component is a liquid helium circulating refrigerator with a cold head cooling power of 1.5~3.0W@4.2K, a daily helium liquefaction capacity of ≥18L, and a condensation efficiency of ≥90%; when the equipment is running continuously and stably, the liquid helium inner liner achieves zero net evaporation loss.

[0011] Furthermore, the three-stage thermal bridge blocking injection structure comprises: a pipe body made of PEEK or polyimide material with an inner diameter of 10-20 mm and a wall thickness of 2 mm; a first-stage blocking system where the pipe body is connected to the Dewar wall via a ceramic / PEEK solid threaded base, fitted with a perfluoroether cryogenic sealing gasket; a second-stage blocking system where the middle section of the pipe is covered with 20-30 layers of aluminized polyester film vacuum insulation, with a vacuum of ≤1×10⁻³Pa; a third-stage blocking system where an oxygen-free copper thermal anchor block is fixed at the point where the pipe crosses the liquid nitrogen intermediate layer for heat exchange with the liquid nitrogen layer; a cryogenic engineering plastic pressure relief valve is installed at the top of the pipe, with a safe opening pressure of 0.8-1.0 MPa, and a fluororubber flexible piston plug is installed inside the valve; the entire injection structure has a static leakage rate of ≤1×10⁻⁻⁻⁶ across a temperature range of 4.2K-300K. 6 Pa・m³ / s.

[0012] Furthermore, in the superconducting lead structure, the horizontal distance between the positive and negative electrodes is ≥1m, and each lead is independently equipped with a three-stage cross-temperature zone through-lead structure: the first stage 4.2K→77K transition: a narrow REBCO strip is flat-welded to a 10mm wide current-expanding strip, and a copper transition busbar is brazed on the back of the wide strip. This copper busbar has a 10~20mm physical heat insulation gap reserved between it and the next stage metal conductor; the entire strip is coated with low-temperature resistant epoxy resin and passes through an extended alumina ceramic seal; the second stage 77K temperature zone: the wide REBCO strip is integrally brazed to a 10mm×3mm copper busbar, the welding area is covered with epoxy resin for insulation, and it is led out through a secondary extended alumina ceramic seal; the third stage 77K→300K room temperature section: the superconducting strip is connected to a solid copper column with a diameter of 15~25mm, and a copper fin base with wiring holes is welded to the outer end of the copper column; the copper components of each stage are separated and not connected to each other.

[0013] Furthermore, it also includes quench protection and a multi-temperature zone sensor system; at least 25 sensors are arranged at key locations in the main magnetic coil, liquid helium inner liner, liquid nitrogen intermediate shielding layer, and outer vacuum cavity. These sensors include Cernox temperature sensors, silicon diode temperature sensing elements, superconducting level gauges, capacitive level gauges, and cold cathode vacuum gauges; sensor leads are led out through 4-6 temperature zone segmented vacuum terminals, with fine-diameter phosphor bronze wire used for the 4K-77K low-temperature range and copper or constantan wire used for the 77K-300K transition range; the overall withstand voltage of the terminals is ≥10kV, and the helium mass spectrometer leak rate is ≤1×10⁻¹. 0 Pa・m³ / s; Overrun protection logic: When the coil temperature rise rate is >0.5K / s or the coil voltage instantaneously increases by >0.5V, the external energy discharge circuit is automatically triggered.

[0014] Furthermore, under normal operating conditions, the total annual liquid helium loss of the built-in recondenser unit is ≤10L; when the recondenser or external power supply fails and the unit stops, the main magnet superconducting coil can maintain a temperature rise from 4.2K to 30K for ≥24 hours solely by the thermal buffering effect of the 77K liquid nitrogen intermediate shielding layer.

[0015] A shutdown thermal buffering method for liquid nitrogen shielding of a low-helium-consumption REBCO superconducting NMR main magnet structure includes the following steps: S1: During normal operation of the equipment, the liquid nitrogen intermediate shielding layer is maintained at a constant temperature of 77K to absorb the radiant heat flux of the room temperature environment and reduce the heat load of the liquid helium inner liner. S2: After the recondenser unit unexpectedly shuts down, the liquid helium liner is heated and evaporates to produce helium gas, which ensures that the main magnetic coil operating temperature is maintained for a short time. S3: The liquid nitrogen shielding layer continuously blocks room temperature radiation, reducing the radiative heat flux in the magnet area from >10W / m² to ≤2W / m², extending the liquid helium heating cycle, and maintaining the magnet at 4.2K to 30K for ≥24h.

[0016] The beneficial effects of this invention are: 1. Composite insulation structure to prevent rapid coil temperature rise and overheating. The system employs a double-layer composite insulation structure consisting of an inner layer of liquid helium immersion and a middle layer of 77K liquid nitrogen constant-temperature shielding. This structure ensures stable operation of the superconducting coil through 4.2K liquid helium while reducing the radiative heat flux from >10W / m² to ≤2W / m² using the liquid nitrogen intermediate shielding layer. When the refrigeration unit or power supply fails to operate, the liquid nitrogen layer can provide a cryogenic buffer time of ≥24 hours, preventing the coil from rapidly heating up and losing its superconductivity. This significantly improves the stability of continuous operation of the equipment and solves the core defect of models without liquid helium conduction cooling that immediately loses its superconductivity upon failure.

[0017] 2. The coil structure and manufacturing process have been optimized, resulting in a novel nuclear magnetic resonance main coil structure. The REBCO high-temperature superconducting tape is used to form a disc-shaped stacked coil without insulation. Combined with the axial three-zone gradient turn spacing arrangement and the reverse balance coil design at both ends, the magnetic field distribution is balanced. Taking the 5.0T specification as an example, the magnetic field uniformity in the 45cm imaging sphere can reach 0.87ppm, which is better than the conventional level of 1~2ppm of existing commercial equipment, and meets the high-precision field strength requirements of whole-body high-resolution medical imaging.

[0018] 3. Low heat leakage design across all dimensions significantly reduces liquid helium loss. A triple-technology approach is used to cut off thermal bridges across temperature zones: a three-stage thermal bridge blocking injection pipeline is equipped with a low thermal conductivity base, a vacuum multi-layer insulation section, and a liquid nitrogen temperature zone thermal anchor; the superconducting lead adopts a segmented isolation design, physically disconnecting each stage of the metal conductor, with only the REBCO tape serving as the sole conductive path; the three-layer Dewar uses G-10 fiberglass support for isolation, with no direct metal thermal conduction path. These multiple designs fundamentally reduce heat leakage through conduction, convection, and radiation. Combined with a recondensation system, this achieves an annual total liquid helium loss of ≤10L, a 98% reduction in loss compared to traditional liquid helium immersion NMR.

[0019] 4. Compact and efficient refrigeration system with significant advantages in power consumption and lifespan. The built-in reverse Brayton cycle re-condenser unit has a rated operating power of only 300W, which is more than 90% lower than the cooling power of traditional liquid helium NMR units (5~10kW) and liquid helium-free units (20~30kW). The unit has a small temperature difference load, which reduces wear on moving parts and extends its design life to 28 years, far exceeding the 3~5 year life of liquid helium-free units, significantly reducing long-term power consumption and equipment maintenance costs.

[0020] 5. Modular and universal design, highly adaptable to various scenarios. The core structure adopts a modular design, which can be flexibly adapted to multiple field strength models such as 1.5T, 3.0T, 5.0T, and 7T by adjusting the strip specifications, coil turns, excitation current, Dewar volume and cooling power, covering diverse scenarios such as clinical diagnosis, small animal imaging and cutting-edge scientific research; the core components such as Dewar, infusion tubing, leads and sensing system have a high degree of commonality, which facilitates serialized production and equipment iteration.

[0021] 6. Significantly improved economic efficiency throughout the entire life cycle. The initial liquid helium filling volume is only 380L, which is more than 80% less than the 1000~2000L filling volume of traditional equipment. Combined with extremely low annual liquid helium loss, low operating power consumption and low liquid nitrogen replenishment cost, the annual operation and maintenance cost of the equipment can be controlled within 50,000 yuan. The 10-year life cycle operation and maintenance cost is more than 75% lower than that of traditional liquid helium NMR, which greatly reduces the equipment purchase and long-term use costs of medical institutions and research units. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the radial cross-sectional structure of the liquid nitrogen shielded low-helium-loss REBCO superconducting nuclear magnetic resonance main magnet of the present invention; Figure 2 This is a schematic diagram of the axial cross-sectional structure of the liquid nitrogen shielded low-helium-consumption REBCO superconducting NMR main magnet of the present invention; Figure 3 This is a schematic diagram of the 5.0T field strength distribution region of the present invention; Figure 4 This is a schematic diagram of the shutdown low-temperature buffering method of the present invention.

[0023] Explanation of reference numerals in the attached figures: 1. Liquid nitrogen zone; 2. Liquid helium zone; 3. Inner and outer vacuum walls of liquid helium Dewar; 4. REBCO main magnetic coil; 5. Main magnetic coil cylinder; 6. Inner and outer vacuum walls of liquid nitrogen Dewar; 7. Liquid helium condenser; 8. Liquid helium condenser system; 9. Wire assembly; 10. Liquid helium cold head; 11. Pressure relief and filling assembly; 12. 5.0T field strength zone. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0025] This invention proposes a liquid nitrogen-shielded, low-helium-consumption REBCO superconducting NMR main magnet structure, which consists of six core modules: a main magnet assembly, a liquid nitrogen-liquid helium composite Dewar, an internal liquid helium recondensation assembly, a three-stage thermal bridge blocking cryogenic injection and depressurization assembly, a segmented isolation superconducting lead assembly, a multi-temperature zone sensor and multi-stage vacuum terminals, and a quench protection system. These modules work together to achieve low heat leakage, low helium consumption, a highly uniform magnetic field, and long-term cryogenic buffering during shutdown. The detailed structure of each module is as follows: Main magnet assembly (5.0T reference field strength) Superconducting tape parameters: A 6.0mm wide REBCO high-temperature superconducting tape with a total thickness of 0.085mm is used. It is composed of a Hastelloy substrate, a buffer layer, a REBCO superconducting layer, a silver protective layer, and a copper stabilizing layer. At 4.2K, the self-field critical current of the 4mm tape is ≥1000A. As shown in Figures 1 and 2, the REBCO main magnetic coil 4 is wound and fixed to the outside of the main magnetic coil cylinder 5, and the entire coil is immersed in the liquid helium zone 2.

[0026] Winding process: Uninsulated continuous winding stacked disc coil, with a total of 50 layers and 20,000 turns, rated operating superconducting current of 300A; 1,600-turn reverse balancing coils are configured at both ends of the coil to counteract edge magnetic field distortion; winding tension is 20N±0.5N, winding speed is 3mm / s, and the inner radius of the coil is 500mm to match the clinical imaging aperture.

[0027] Axial partition gradient turn spacing optimization: The coil is divided into three sections along its axis: central region, transition region, and end region. Differentiated turn spacing is set to balance the magnetic field distribution, forming a 5.0T field strength region 12 as shown in Figure 3. The total axial length is 3600mm, and the effective radial range covers ±400mm: Central region: Within ±0.15m of the coil's axial midpoint, the turn spacing is 0.07~0.08mm, with dense arrangement to concentrate magnetic flux and enhance the central field strength; End region: Within 0.10m at each end of the coil, the turn spacing is 0.20~0.25mm, widening the turn spacing to compensate for edge magnetic field attenuation; Transition region: Within 0.08~0.12m between the two regions, the turn spacing is linearly increased to achieve a smooth magnetic field transition.

[0028] Magnetic field performance: After optimization by finite element simulation, the magnetic field uniformity of the 45cm imaging sphere (DSV) is 0.87ppm, the magnetic field at the center point is 5.002T, and the maximum magnetic field deviation is only +0.0042T, which meets the industry standard of ≤1ppm for whole-body high-resolution imaging.

[0029] Base and vacuum epoxy encapsulation: The main magnetic coil tube 5 is made of GFRP epoxy resin fiberglass tubing with an inner diameter of 1000mm and an outer diameter of 1140mm. The flanges at both ends are reserved with fixing holes and winding grooves. The coil is vacuum epoxy impregnated and cured, in accordance with the fiberglass strength standards such as GB / T 1447 and GB / T 1449. The circumferential and bending stresses are far below the material's ultimate strength, and the safety factor is >1.7.

[0030] Liquid nitrogen-liquid helium composite Dewar The structure consists of three coaxial layers from the inside out: a liquid helium inner liner, a liquid nitrogen intermediate shielding layer, and an outer vacuum cavity. The radial cross-sectional structure is shown in Figure 1, and the axial cross-sectional structure is shown in Figure 2. Liquid helium liner: The main magnet is completely immersed in a 4.2K liquid helium zone 2. The liquid helium liner is formed by inner and outer vacuum walls 3 of a liquid helium Dewar, with a volume of 400L and a standard filling of 380L of liquid helium. The liner is made of seamless S304 austenitic stainless steel, conforming to the GB / T 25198 pressure vessel standard, with a design pressure twice the working pressure and an ultimate temperature resistance of -269℃. The welds of the liner undergo 100% RT radiographic testing (Level II) and 100% PT penetrant testing (Level I). The interlayer features high-vacuum multi-layer insulation with an ultimate vacuum ≤5×10⁻⁻⁻⁶. 4 Pa, helium mass spectrometry leak rate ≤1×10⁻¹ 0 Pa・m³ / s; sandwiched molecular sieve adsorbent, static evaporation rate ≤2% / day.

[0031] Liquid nitrogen intermediate shielding layer: A ring-shaped liquid nitrogen Dewar surrounds the outer side of the liquid helium inner liner. Liquid nitrogen zone 1 is formed by the inner and outer vacuum walls 6 of the liquid nitrogen Dewar, with an effective volume of 3000L, forming a complete 77K radiation shielding layer to block radiative heat leakage from room temperature to the liquid helium inner liner. Both the inner and outer liners are made of S30408 ​​stainless steel and have passed a -196℃ low-temperature impact test with an impact energy ≥34J. The interlayer support uses G-10 fiberglass plastic to cut off metal thermal bridges. The interlayer is filled with 15~20 layers of aluminum foil fiberglass composite insulation material, with an interlayer vacuum ≤1×10⁻³Pa. All welds undergo non-destructive testing, and the helium mass spectrometry leak rate is ≤1×10⁻⁻⁻⁻⁻⁻⁻⁶. 9 Pa・m³ / s, static pressure rise rate ≤0.05Pa / day. Equipped with independent liquid nitrogen filling and pressure relief pipelines, the pipelines are made of 304 stainless steel with an outer layer of epoxy resin insulation, and the valve body is made of low thermal conductivity PEEK material; equipped with a ±2% accuracy liquid level sensor and a ±0.1K accuracy temperature sensor, a single 2500L liquid nitrogen filling can sustain natural evaporation for 3~6 months; when shutting down, the liquid nitrogen layer can reduce the radiant heat flux from >10W / m² to ≤2W / m², ensuring 24-hour cryogenic buffer.

[0032] Outer vacuum chamber: vacuum degree ≤1×10⁻³Pa, isolating ambient air convection heat leakage. The three-layer structure is physically connected only by G-10 glass fiber reinforced plastic support components, with no direct metal heat conduction path.

[0033] Built-in liquid helium recondensation unit The self-developed reverse Brayton cycle cryogenic refrigeration unit, namely the liquid helium condenser system 8, integrates the refrigeration unit and helium circulation pump, using high-purity helium as the working fluid. Its core is a cylindrical reverse heat exchanger with 32 heat pipes and 24 cold pipes. High-pressure 77K pre-cooled helium flows in internally, while 4.2K low-temperature helium flows out externally in the reverse direction, maximizing the recovery of cooling capacity. The unit's liquid helium cold head 10 has a power of 1.5~3.0W@4.2K, a daily liquefaction capacity ≥18L, a condensation efficiency ≥90%, and a rated operating power of only 300W. The liquid helium condenser 7 is located in the gas phase zone of the liquid helium liner, with the cold head directly coupled to the gas phase space. Evaporated helium is liquefied and refluxed in real time, achieving zero net evaporation of liquid helium under normal operating conditions, with a total annual liquid helium loss ≤10L.

[0034] Three-stage thermal bridge blocking cold liquid injection pressure relief assembly The liquid helium filling pipeline, also known as the pressure relief filling assembly 11, employs a three-stage progressive insulation structure to completely cut off heat conduction leakage across temperature zones: First-stage blocking: The main body of the pipeline is made of low thermal conductivity polyimide / PEEK tubing, with an inner diameter of 18mm and a wall thickness of 3mm. At 4K, its thermal conductivity is only 1 / 1600 that of oxygen-free copper. The tubing and Dewar liner use a ceramic-polyimide threaded sealing base, sealed with a perfluoroether cryogenic gasket. Second-stage blocking: The pipeline in the liquid nitrogen temperature zone is equipped with a vacuum jacket, covered with 20-30 layers of aluminized polyester film insulation, with a jacket vacuum ≤1×10⁻³Pa, eliminating convective and radiative heat leakage. Third-stage blocking: An oxygen-free copper thermal anchor is installed where the pipeline crosses the liquid nitrogen layer, allowing for sufficient heat exchange with the 77K liquid nitrogen layer and intercepting heat conducted from room temperature. The pipeline is equipped with a low-temperature resistant engineering plastic pressure relief valve at the top, with an opening pressure of 0.8~1.0MPa. It features a built-in fluororubber flexible piston plug that adapts to cold shrinkage deformation; the static leakage rate of the entire pipeline is ≤1×10⁻ 6 Pa・m³ / s, pressure resistance ≥1.6MPa, suitable for extremely low temperature conditions of -196℃.

[0035] Segmented isolated superconducting lead assembly The conductor assembly (9) features superconducting positive and negative electrodes positioned separately on the left and right sides, with a horizontal spacing of 1m. There is no crossing or parallel operation throughout the entire path, mitigating the risk of creepage and short circuits. Each lead is independently routed through three temperature zones, with each level of copper metal component physically disconnected by 10-20mm. Only REBCO tape serves as the sole continuous conductive path, cutting off large thermal bridges in the metal: 4.2K→77K First-stage routing: 6mm narrow REBCO tape is flat-welded to a 10mm wide current-expanding tape, with a copper transition busbar brazed on the back. A thermal insulation gap is left between the copper busbar and the next metal component. The entire structure is covered with low-temperature resistant epoxy resin, passing through an extended alumina ceramic seal to increase the surface insulation distance. 77K Second-stage routing: Wide REBCO tape is fully brazed to the copper busbar, with overall epoxy resin insulation coverage. An extended ceramic seal provides independent isolation for the second-stage routing. 77K→300K Three-stage room temperature lead-out: superconducting tape connected to solid copper pillar, copper fin base with wiring hole welded to the outer end, and external low-voltage power supply cable connected.

[0036] Sensors and multi-stage vacuum terminals and quench protection system A total of 30 sensors are arranged in the main coil, liquid helium Dewar, liquid nitrogen shielding layer, and vacuum chamber, including Cernox temperature probes, silicon diode temperature sensing elements, superconducting / capacitive level gauges, and cold cathode vacuum gauges. 60-core sensing leads are led out through 4-6 temperature-division segmented vacuum terminals, with thin-diameter phosphor bronze wire used in the low-temperature section to reduce thermal conductivity. The terminals adopt a ceramic-metal sealing structure, with a withstand voltage ≥10kV and a helium leakage rate ≤1×10⁻¹. 0 Pa・m³ / s; Matching overrun protection logic: When the coil temperature rise rate is >0.5K / s or the voltage surge is >0.5V, the external energy discharge circuit will be automatically activated.

[0037] Supporting operating methods (1) Zero Evaporation Stable Operation Method: The main magnet assembly is completely immersed in a 4.2K liquid helium liner. The heat load of the equipment generates evaporated helium. The gaseous helium liquefies upon contact with the 4.2K cold head of the recondenser. The liquid helium flows back to the liquid helium pool by gravity, forming a closed loop of "evaporation-condensation-recirculation". The PLC collects liquid level and temperature signals in real time, automatically adjusts the output power of the refrigeration unit to match the real-time heat leakage, stabilizes the gas-liquid interface balance, and achieves zero net loss of liquid helium during long-term continuous operation.

[0038] (2) Long-term low-temperature buffering method for refrigeration shutdown: During normal operation, the 77K liquid nitrogen shielding layer continuously absorbs room temperature radiation heat, reducing the heat load of the liquid helium inner liner; after the condenser is shut down, the liquid nitrogen layer continuously blocks room temperature radiation, reducing the radiative heat flux of the magnet area from >10W / m² to ≤2W / m², and the buffering time for the magnet to rise from 4.2K to 30K is ≥24h, reserving sufficient time for equipment repair and avoiding instantaneous overload shutdown.

[0039] Comparison of core performance indicators sheet

[0040] Economic advantages (1) Purchase cost: The market price of a domestic 5.0T NMR system is about RMB 46 million. The estimated manufacturing cost of the system of this invention is RMB 10 million, which greatly reduces the purchase cost of high-end high-field NMR. (2) 10-year life cycle operation and maintenance cost: The total operation and maintenance cost of traditional liquid helium NMR is RMB 2 million over 10 years, and that of NMR without liquid helium conduction cooling is RMB 1.34 million, while that of this invention is only RMB 500,000. Compared with traditional equipment, the operation and maintenance cost can be saved by RMB 8 million over 10 years. The cost savings come from three major areas: liquid helium procurement, high electricity costs, and frequent replacement and maintenance of the refrigeration unit. (3) Consumable cost: A single liquid nitrogen refill lasts for 3 to 6 months, and the annual liquid nitrogen replenishment cost is only RMB 0.1 million, with extremely low consumable expenditure.

[0041] Example 1: 5.0T Standard Clinical-Grade Liquid Nitrogen-Shielded Low-Helium-Consumption REBCO Superconducting MRI Main Magnet (Optimal Example) (a) Machining and assembly of main magnet components Raw material selection: 6.0mm wide REBCO high-temperature superconducting tape was selected, with a total thickness of 0.085mm. At 4.2K, the critical current of the 4mm tape specification is 1050A. The main magnetic coil tube 5 is made of GFRP epoxy fiberglass tubing, with an inner diameter of 1000mm, an outer diameter of 1140mm, and a wall thickness of 30mm. Both ends have pre-fabricated 50mm thick flanges, and circumferentially machined with 10mm wide and 10mm deep winding grooves. Winding process: A CNC precision winding machine was used for continuous winding without insulation. The winding tension was constant at 20N, and the wire feed speed was 3mm / s. A total of 50 layers were stacked, with a total of 20,000 turns, forming the REBCO main magnetic coil 4. 800 turns of reverse balancing coil were wound at each end of the coil, totaling 1600 turns, to cancel out stray magnetic fields at the edges. The coil axis was strictly divided, resulting in a 5.0T field strength region 12 as shown in Figure 3. Central zone: Axial midpoint ±0.15m, turn spacing fixed at 0.075mm; End regions: 0.10m interval each, turn spacing 0.22mm; Transition region 0.10m, turn spacing linearly increases from 0.075mm to 0.22mm.

[0042] Vacuum epoxy curing: After winding, the whole thing is sent into a vacuum impregnation tank with a vacuum degree of 0.1 Pa. Low-temperature radiation-resistant epoxy resin is poured in and cured at 80℃ for 12 hours. The overall dimensions after molding are 720 mm outer diameter and 1800 mm axial length. The strength test shows a circumferential stress of 25 MPa, a bending stress of 15 MPa, and a safety factor of 1.8, which meets the usage requirements.

[0043] Magnetic field calibration: With a rated DC current of 300A applied to the coil, the magnetic field uniformity of the 45cm DSV sphere was measured to be 0.87ppm using a gaussmeter, the central magnetic field was 5.002T, and the maximum deviation was 0.0042T, which meets the high-resolution imaging specifications.

[0044] (II) Processing and assembly of liquid nitrogen-liquid helium composite Dewar Liquid helium inner liner fabrication: S30408 ​​seamless stainless steel roll welding forms the inner and outer vacuum walls 3 of the liquid helium Dewar, with an inner liquid helium zone 2 and a volume of 400L. Working filling volume is 380L of liquid helium. All welds undergo 100% RT and PT non-destructive testing. The interlayer is lined with 18 layers of aluminum foil and fiberglass insulation. Ultimate vacuum is 4×10⁻⁻⁻⁶. 4 Pa, helium leak rate 8×10⁻¹¹ Pa・m³ / s, static evaporation rate 1.6% / day.

[0045] Liquid nitrogen shielding layer fabrication: A coaxial annular S30408 ​​stainless steel Dewar forms the inner and outer vacuum walls of the liquid nitrogen Dewar (6 layers), with an internal liquid nitrogen zone (1 layer) having an effective volume of 3000L. The interlayer is filled with 16 layers of aluminized polyester film, and the interlayer vacuum is 8×10⁻⁻⁶. 4 Pa; all support components are made of G-10 fiberglass plastic, eliminating metal heat conduction channels; liquid nitrogen pipeline uses 2mm thick 304 stainless steel pipe, valve body uses PEEK insulation material, equipped with capacitive level gauge (accuracy ±2%) and Pt100 temperature sensor (accuracy ±0.1K). Single filling capacity is 2500L liquid nitrogen, with a natural evaporation maintenance cycle of 4 months. Outer vacuum chamber: double-layer stainless steel shell, interlayer vacuum 9×10⁻ 4 Pa, the three-layer Dewar is assembled coaxially, with a coaxiality error of ≤0.3mm.

[0046] (III) Installation and commissioning of the built-in liquid helium recondenser unit The system utilizes a self-developed reverse Brayton cycle refrigeration unit, specifically a liquid helium condenser system 8, with a rated input power of 300W. The liquid helium cold head 10 has a cooling capacity of 2.2W at 4.2K, liquefying 20L of helium per day, and achieving a condensation efficiency of 92%. The cylindrical heat exchanger incorporates 32 heat pipes and 24 cold pipes, enabling reverse heat exchange between the hot and cold media. The liquid helium condenser 7 is flange-sealed and fixed to the top of the liquid helium liner in the gas phase space, with vacuum-insulated piping connections. After commissioning and operation, under stable conditions, there is no net evaporation of liquid helium, with an annual comprehensive loss of 8L.

[0047] (iv) Assembly of three-stage thermal bridge blocking injection pressure relief pipeline The pressure relief injection assembly 11 uses polyimide tubing with an inner diameter of 18mm and a wall thickness of 3mm; the first-level blocking system is a ceramic threaded base sealed with a perfluoroether cryogenic gasket; the second-level blocking system is a liquid nitrogen section covered with a vacuum interlayer of 25 layers of aluminized polyester film; the third-level blocking system is an oxygen-free copper thermal anchor immersed in the liquid nitrogen layer.

[0048] The top is fitted with an engineering plastic pressure relief valve with an opening pressure of 0.9 MPa and an internal fluororubber piston stopper; the entire pipeline has a helium mass spectrometry leak detection rate of 5×10⁻⁻. 7 Pa・m³ / s, pressure resistance 1.6MPa, leak-free after 1000 cycles at -196℃.

[0049] (v) Assembly of three-stage segmented isolation superconducting leads Conductor assembly 9: Positive and negative leads are placed separately on the left and right sides, with a horizontal spacing of 1000mm, and independent three-stage lead-in. 4.2K temperature range: 6mm narrow REBCO flat welding 10mm wide strip, back brazed copper transition busbar, 15mm heat insulation gap reserved between the copper busbar and the next metal part, overall coated with low temperature epoxy, passing through extended alumina ceramic seal; 77K temperature range: Wide REBCO and 10×3mm copper busbar are fully brazed, epoxy-coated insulation, and secondary ceramic sealed lead-out; Room temperature range: The superconducting strip connects to a φ20mm solid copper column, and the end is welded to a 50×50×50mm copper finned terminal base; the copper components at each level do not contact each other, and only the REBCO strip runs through the entire temperature range.

[0050] (vi) Sensor and multi-stage vacuum terminal block system A total of 30 sensors are deployed at key locations: 10 Cernox CX-1030 temperature sensors, 8 silicon diodes DT-670, 6 superconducting level gauges, 4 capacitive level gauges, and 2 cold cathode vacuum gauges. The 60-core sensor leads use five-level temperature-divided segmented ceramic-metal sealed terminals, with AWG36 phosphor bronze wire used for the 4K~77K segment and constantan wire used for the 77K~300K segment. The terminals have a withstand voltage of 12kV and a leakage rate of 7×10⁻¹¹ Pa・m³ / s. The quench protection threshold is set to automatically discharge energy when the temperature rise rate is >0.5K / s or the coil voltage surge is >0.5V.

[0051] (vii) Overall debugging and performance testing Pre-cooling process: First, add liquid nitrogen to the liquid nitrogen jacket to pre-cool to 77K and keep it at that temperature for 24 hours. Then, add 380L of liquid helium to the liquid helium inner liner and let it stand for 12 hours to stabilize the temperature. When the recondenser unit is started, the power is stable at 300W. After running continuously for 30 days, the liquid helium level does not drop significantly, and the annual loss is estimated to be 8L. Shutdown buffer test: With the refrigerator shut down and external power supply cut off, relying solely on the 77K liquid nitrogen shielding layer, continuous monitoring for 26 hours showed that the coil temperature rose to a maximum of 28.6K without quenching, meeting the ≥24h buffer requirement; The estimated annual maintenance cost of the entire unit is 46,000 yuan, saving 150,000 yuan / year compared to the traditional 5.0T liquid helium NMR.

[0052] Example 2: 3.0T Adaptation and Modification Example The main magnet coil uses 4mm wide REBCO tape with a total of 120,000 turns and 40 layers, and a rated excitation current of 200A; the liquid helium inner liner has a volume of 300L and is filled with 260L of liquid helium; the liquid nitrogen shielding layer has a volume of 2200L; the recondenser cold head power is 1.5W@4.2K, and the annual liquid helium loss of the whole machine is ≤6L; the magnetic field uniformity in the 45cm sphere area is 0.92ppm. The remaining Dewar, three-stage heat insulation piping, lead wire structure, and sensing system are completely interchangeable with those in Example 1, with only minor adjustments to the coil winding parameters and the chiller power, making it suitable for small and medium-sized clinical MRI equipment.

[0053] Example 3: 1.5T Scientific Miniaturization Example REBCO features a 5mm wide strip, 65,000 total coil turns, and an inner radius of 400mm; a 180L liquid helium inner liner and a 1500L liquid nitrogen shielding layer; a 200W refrigerator; an annual liquid helium loss of ≤4L; and a 26-hour low-temperature buffer time during shutdown. It is suitable for small animal biomedical imaging and low-field research scenarios in laboratories.

[0054] Example 4: 7T High-Field Scientific Magnet Modification Example The tape uses a 6mm high-critical-current REBCO coil with 65 layers, a total of 280,000 turns, and an excitation current of 400A; the liquid helium inner liner has a volume of 500L, and the liquid nitrogen shielding layer has a volume of 3500L; the recondenser cold head has a power of 3.0W@4.2K and a rated input power of 400W; the 45cm spherical uniformity is 0.95ppm; the downtime buffer is ≥24h, making it suitable for cutting-edge biomolecular high-resolution imaging research equipment.

[0055] Implementation instructions Zero-evaporation operation process: When the equipment is working normally, the coil heat load causes a small amount of liquid helium to evaporate. The helium rises to the gas phase zone at the top of the liquid helium tank, and then the 4.2K cold head of the condenser liquefies the gaseous helium. The liquid helium flows back to the liquid helium pool by gravity. The PLC collects liquid level and temperature signals in real time and automatically adjusts the output power of the refrigeration unit to match the real-time heat leakage, so as to achieve zero net consumption of liquid helium during long-term continuous operation.

[0056] Shutdown thermal buffering implementation process: After a sudden power outage / refrigeration unit failure, the liquid nitrogen shielding layer maintains a constant temperature of 77K, significantly reducing the radiant heat flux; the evaporated helium gas circulates naturally through convection, slowly carrying away the heat from the magnet. The coil temperature does not exceed 30K within 24 hours, allowing sufficient time to complete equipment maintenance and backup power switching, avoiding shutdown losses caused by emergency overload.

[0057] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications or equivalent substitutions based on the technical solutions of the present invention without departing from the core ideas of the present invention shall fall within the protection scope of the present invention.

Claims

1. A liquid nitrogen-shielded, low-helium-loss REBCO superconducting nuclear magnetic resonance (NMR) main magnet structure, characterized in that, include: Main magnet assembly, liquid nitrogen-liquid helium composite Dewar, built-in recondensation assembly, three-stage thermal bridge blocking injection structure, and superconducting lead structure with metal-free continuous thermal conduction path; The main magnet assembly is formed by continuously winding REBCO high-temperature superconducting tape into a disc-shaped stacked coil without insulation, and the entire coil is vacuum epoxy impregnated and cured. The liquid nitrogen-liquid helium composite Dewar consists of a liquid helium liner, a liquid nitrogen intermediate shielding layer, and an outer vacuum cavity from the inside out. The liquid nitrogen intermediate shielding layer forms a 77K constant temperature thermal radiation barrier to enclose the liquid helium liner. The built-in recondensation component cold head couples the liquid helium liner gas phase space for liquefying and evaporating helium gas for reflux, and uses a reverse Brayton cycle to achieve 4.2K low-temperature cooling. The three-stage thermal bridge blocking injection structure connects the liquid helium liner with the external environment, and a triple thermal insulation blocking structure is set along the fluid pipeline, consisting of a low thermal conductivity pipe section, a vacuum multi-layer insulation section, and a liquid nitrogen temperature zone thermal anchor block. In the metal-free continuous thermal conductive path superconducting lead structure, the superconducting positive and negative electrodes are independently led out, and the metal conductive components in each temperature zone are physically disconnected with gaps.

2. The liquid nitrogen-shielded, low-helium-loss REBCO superconducting nuclear magnetic resonance main magnet structure according to claim 1, characterized in that, The design reference field strength of the main magnet assembly is selected from 1.5T, 2T, 3T, 5T, and 7T, and the rated excitation current is selected from 300A, 400A, 500A, and 800A; the REBCO high-temperature superconducting tape specifications can be selected from 4mm, 5mm, 6mm, and 10mm width.

3. The liquid nitrogen-shielded, low-helium-loss REBCO superconducting nuclear magnetic resonance main magnet structure according to claim 1, characterized in that, The parameters of the 5.0T main magnet assembly are as follows: 4.0~6.0mm wide REBCO tape is used; the number of coil layers is 50, the total number of turns is 20,000, and the inner radius of the winding is 500±5mm; the two ends are equipped with reverse balance coils with a bending diameter >50mm and a 1:1 ratio of positive to negative turns; the coil axially is arranged in a three-zone gradient turn spacing, with dense winding in the middle area and sparse winding at both ends; the uninsulated winding tension is 15~25N, the winding speed is 1~5mm / s, and the current equalization and quench energy dispersion between turns are achieved by relying on contact resistance.

4. The liquid nitrogen-shielded, low-helium-loss REBCO superconducting nuclear magnetic resonance main magnet structure according to claim 1, characterized in that, The liquid nitrogen-liquid helium composite Dewar features a liquid helium inner liner with a volume of 380-420L, made of S30408 ​​austenitic stainless steel, a static evaporation rate of ≤2% / day, and an overall helium mass spectrometry leak rate of ≤1×10⁻¹. 0 Pa・m³ / s; the liquid nitrogen intermediate shielding layer has a volume of 2500~3500L, coaxially encased in a liquid helium inner liner, with 15~20 layers of aluminized polyester film filling the interlayer, and the interlayer vacuum degree ≤1×10⁻³Pa; the outer vacuum chamber vacuum degree ≤1×10⁻³Pa; the three-layer structure is isolated by G-10 glass fiber reinforced plastic supports, with no direct metal heat conduction path.

5. The liquid nitrogen-shielded, low-helium-loss REBCO superconducting nuclear magnetic resonance main magnet structure according to claim 1, characterized in that, The built-in recondensation component is a liquid helium circulating refrigerator with a cold head cooling power of 1.5~3.0W@4.2K, a daily helium liquefaction capacity of ≥18L, and a condensation efficiency of ≥90%. When the equipment is running continuously and stably, the liquid helium inner liner achieves zero net evaporation loss.

6. The liquid nitrogen-shielded, low-helium-loss REBCO superconducting nuclear magnetic resonance main magnet structure according to claim 1, characterized in that, The three-stage thermal bridge blocking injection structure consists of: a pipe body made of PEEK or polyimide tubing with an inner diameter of 10-20 mm and a wall thickness of 2 mm; a first-stage blocking structure where the pipe body is connected to the Dewar wall via a ceramic / PEEK solid threaded base, fitted with a perfluoroether cryogenic sealing gasket; a second-stage blocking structure where the middle section of the pipe is covered with 20-30 layers of aluminized polyester film vacuum insulation, with a vacuum of ≤1×10⁻³Pa; and a third-stage blocking structure where an oxygen-free copper thermal anchor block is fixed at the point where the pipe passes through the liquid nitrogen intermediate layer for heat exchange with the liquid nitrogen layer. The pipeline is equipped with a low-temperature resistant engineering plastic pressure relief valve at the top, with a safe opening pressure of 0.8~1.0MPa. The valve is fitted with a fluororubber flexible piston plug. The entire filling structure has a static leakage rate of ≤1×10⁻ in a temperature range of 4.2K~300K. 6 Pa・m³ / s.

7. The liquid nitrogen-shielded, low-helium-loss REBCO superconducting nuclear magnetic resonance main magnet structure according to claim 1, characterized in that, In the superconducting lead structure, the horizontal distance between the positive and negative electrodes is ≥1m. Each lead independently has a three-stage cross-temperature zone through-lead structure: The first stage, 4.2K→77K transition: a narrow REBCO strip is flat-welded to a 10mm wide current-expanding strip, and a copper transition busbar is brazed on the back of the wide strip. This copper busbar has a 10~20mm physical thermal insulation gap with the next stage metal conductor. The entire strip is coated with low-temperature resistant epoxy resin and passes through an extended alumina ceramic seal. The second stage, 77K temperature zone: the wide REBCO strip is brazed to a 10mm×3mm copper busbar. The welding area is insulated with epoxy resin and led out through a secondary extended alumina ceramic seal. The third stage, 77K→300K room temperature zone: the superconducting strip is connected to a 15~25mm diameter solid copper column, and a copper fin base with wiring holes is welded to the outer end of the copper column. The copper components at each stage are separate and not interconnected.

8. The liquid nitrogen-shielded, low-helium-loss REBCO superconducting NMR main magnet structure according to claim 1, characterized in that, It also includes quench protection and a multi-temperature zone sensor system; no fewer than 25 sensors are arranged at key locations in the main magnetic coil, liquid helium inner liner, liquid nitrogen intermediate shielding layer, and outer vacuum cavity. These sensors include Cernox temperature sensors, silicon diode temperature sensing elements, superconducting level gauges, capacitive level gauges, and cold cathode vacuum gauges; sensor leads are led out through 4-6 temperature zone segmented vacuum terminals, with fine-diameter phosphor bronze wire used for the 4K-77K low-temperature range and copper or constantan wire used for the 77K-300K transition range; the overall withstand voltage of the terminals is ≥10kV, and the helium mass spectrometer leak rate is ≤1×10⁻¹. 0 Pa・m³ / s; Overrun protection logic: When the coil temperature rise rate is >0.5K / s or the coil voltage instantaneously increases by >0.5V, the external energy discharge circuit is automatically triggered.

9. The liquid nitrogen-shielded low-helium-loss REBCO superconducting nuclear magnetic resonance main magnet structure according to any one of claims 1 to 8, characterized in that, Under normal operating conditions, the total annual liquid helium loss of the built-in recondenser unit is ≤10L. When the recondenser or external power supply fails and the unit stops, the main magnet superconducting coil can maintain a temperature rise from 4.2K to 30K for ≥24 hours by relying solely on the 77K thermal buffer effect of the liquid nitrogen intermediate shielding layer.

10. A shutdown thermal buffering method based on the liquid nitrogen shielded low-helium-consumption REBCO superconducting nuclear magnetic resonance main magnet structure according to any one of claims 1 to 9, characterized in that, Includes the following steps: S1: During normal operation of the equipment, the liquid nitrogen intermediate shielding layer is maintained at a constant temperature of 77K to absorb the radiant heat flux of the room temperature environment and reduce the heat load of the liquid helium inner liner. S2: After the recondenser unit unexpectedly shuts down, the liquid helium liner is heated and evaporates to produce helium gas, which ensures that the main magnetic coil operating temperature is maintained for a short time. S3: The liquid nitrogen shielding layer continuously blocks room temperature radiation, reducing the radiative heat flux in the magnet area from >10W / m² to ≤2W / m², extending the liquid helium heating cycle, and maintaining the magnet at 4.2K to 30K for ≥24h.

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