A superconducting double pancake coil composite former structure

Abstract

The application discloses a superconducting double-deck coil composite framework structure and belongs to the technical field of superconducting power, which comprises an inner framework and an outer framework outside the inner framework, and transition turns of a double-deck coil are tightly wound on the outer circumferential surface of the outer framework; the inner framework is made of a metal material with high Young's modulus, and the outer framework is made of a metal material with high electrical conductivity and high thermal conductivity; the double-deck coil comprises a first coil section and a second coil section which are stacked along the axial direction, the first coil section and the second coil section are connected through the transition turns, and both of them are coaxially wound and fixed on the outer circumferential surface of the framework with the central axis of the framework as the axis; the outer cylindrical surface of the framework serves as a winding mold, the inner diameter of the double-deck coil matches the outer diameter of the framework, and both of them form a tight surface contact. According to the double-layer composite framework design, the transition turns are directly wound on the outer framework, the deformation space is reserved for the transition turns, the stress is flexibly released, and the long-term operation reliability of the coil is improved.

Description

Technical Field

[0001] This invention belongs to the field of superconducting power technology, specifically relating to a superconducting double-pancake coil composite skeleton structure. Background Technology

[0002] Uninsulated high-temperature superconducting double-coil magnets improve electro-thermal stability due to their natural inter-turn shunt capability, and their outer leads facilitate multi-coil series connection, making them the mainstream technology for high-field superconducting magnets. However, as... Figure 1 As shown, the innermost transition turn 3, which connects the upper coil 1 to the lower coil 2, is typically a single-turn structure. Under high magnetic field and high current conditions, the double-coil system experiences enormous Lorentz forces, and the thermal stress introduced by repeated heating and cooling cycles makes this single-turn transition turn 3 highly susceptible to damage. Once the transition turn 3 degrades or breaks, the lack of adjacent turns to provide an alternative shunt path causes the shunt protection mechanism relied upon by the uninsulated coil to completely fail at this point, directly resulting in an open circuit between the upper and lower coils and irreversible damage to the coil.

[0003] Regarding the reliability issues of the transition turns, existing improvement solutions each have their shortcomings:

[0004] 1. Parallel winding reinforcement: Using two superconducting strips wound in parallel or using high elastic modulus metal strips wound in parallel (such as MIT's H835 coil) can enhance mechanical strength, but it significantly increases the winding complexity under the condition of limited inner diameter, and the thermal shrinkage matching and mechanical coupling between multiple strips need to be precisely controlled, which makes the process more difficult.

[0005] 2. Wide strip welding bridging: Welding wide superconducting strips between the upper and lower discs to achieve electrical connection simplifies the structure, but inevitably introduces relatively high welding resistance and increases the cryogenic cooling load; at the same time, the welding process requires strict temperature control to prevent degradation of the strip's current carrying capacity, and there is a risk of joint resistance deterioration under long-term thermal cycling.

[0006] 3. Rigid Fixing with Welded Frame: The transition turns are directly soldered to the inner circular frame to share the electromagnetic force, but this makes the transition turns rigid, losing their ability to release stress through minute deformations. Rigid binding easily leads to localized stress concentration, which actually increases the probability of damage.

[0007] In summary, the aforementioned solutions are either complex in terms of process, introduce additional resistance at the expense of electrical performance, or exacerbate local stress due to rigid constraints, making it difficult to simultaneously achieve mechanical reliability, electrical performance, and process feasibility. Therefore, there is an urgent need for a novel composite skeleton structure that can provide effective protection for the transition turns and endow them with the necessary flexible stress relief mechanism without introducing additional resistance or significantly increasing process complexity. Summary of the Invention

[0008] To address the technical challenges of low mechanical reliability of the innermost transition turn in existing non-insulated high-temperature superconducting double-pane coils, lack of shunt protection after a fault leading to open circuits between the upper and lower pane coils, and to overcome the shortcomings of existing reinforcement methods such as complex parallel winding processes, resistance introduced by welding, and stress concentration caused by rigid fixing, this invention provides a composite skeleton structure for superconducting double-pane coils.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A superconducting double-pane coil composite skeleton structure includes an inner skeleton and an outer skeleton coaxially nested outside the inner skeleton. The transition turns of the double-pane coil are tightly wound around the outer circumferential surface of the outer skeleton. The inner skeleton is made of a metal material with high Young's modulus, and the outer skeleton is made of a metal material with high electrical conductivity and high thermal conductivity. The double-pane coil includes a first coil segment and a second coil segment stacked along the axial direction. The first coil segment and the second coil segment are connected by transition turns, and both are coaxially wound and fixed to the outer circumferential surface of the skeleton with the central axis as the center. The outer cylindrical surface of the skeleton serves as a winding mold, and the inner diameter of the double-pane coil matches the outer diameter of the skeleton, forming a tight surface contact.

[0011] The present invention has the following beneficial effects:

[0012] 1. This invention employs a dissimilar metal double-layer composite skeleton design of stainless steel and oxygen-free copper, directly winding the transition turns onto the outer oxygen-free copper skeleton. The high conductivity of oxygen-free copper provides electrical backup and an unconditional shunt path for the transition turns, while its high thermal conductivity enhances cooling. Its large cold shrinkage rate achieves low-temperature self-tightening, effectively solving a series of problems such as the mechanical fragility of the transition turns, lack of shunt protection, complex winding process, introduction of welding resistance, and stress concentration due to rigid fixation. The structure is simple and compact, without adding extra winding space or process complexity, and it retains a certain amount of space for minor deformation of the transition turns, flexibly releasing stress and significantly improving the long-term operational reliability of the non-insulated high-temperature superconducting double-pancake coil.

[0013] 2. This invention provides a transition turn electrical backup, eliminating the risk of single-point open-circuit failure: the transition turn is tightly wound around an oxygen-free copper frame, forming a parallel backup conductive path. When the transition turn is damaged, fails, or breaks due to electromagnetic force or thermal shock, the current is automatically diverted to the oxygen-free copper frame, maintaining the electrical connection between the upper and lower coils. This fundamentally solves the problem of the lack of shunt protection after the failure of a single-turn transition turn, which leads to an overall coil open circuit.

[0014] 3. This invention enhances the cooling of the transition turns and suppresses the formation of local hot spots: The oxygen-free copper skeleton, with its high thermal conductivity, rapidly conducts the Joule heat of the transition turns to the stainless steel skeleton and cooling medium, effectively reducing its temperature rise rate and steady-state temperature. Compared with existing parallel winding or welding solutions, this invention significantly improves the thermal stability of the transition turns without excessively increasing the coil's thermal resistance.

[0015] 4. This invention employs low-temperature self-tightening to ensure reliable interface contact without the need for additional welding processes: Utilizing the characteristic that oxygen-free copper has a greater cold shrinkage rate than stainless steel, at superconducting operating temperatures, the two-layer skeleton generates radial compressive stress due to the difference in shrinkage, resulting in spontaneous and tight clamping. A stable structure can be automatically established at low temperatures without the need for additional fasteners or welding.

[0016] 5. The present invention has a simple and compact structure and high process feasibility: The present invention only requires nesting an oxygen-free copper sleeve outside a stainless steel frame and directly winding the transition turns on it, without increasing the space occupied by the inner diameter of the coil. The winding process is basically the same as that of a conventional double-panel coil. Compared with the parallel winding scheme, the process complexity is significantly reduced; compared with the welding bridging scheme, it does not involve high-temperature operation of the superconducting tape, avoiding the risk of current carrying capacity degradation.

[0017] 6. This invention retains the flexibility and stress release capability of the transition turn: the transition turn is only wrapped around the outer periphery of the oxygen-free copper skeleton and is not rigidly fixed by means of soldering or other methods. Under the action of electromagnetic force and thermal stress, local stress can be released through micro-slippage, avoiding stress concentration and strip damage caused by the completely rigid constraint of existing welding and fixing schemes. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of a high-temperature superconducting double-pane coil in the prior art;

[0019] Figure 2 This is a schematic diagram of a superconducting double-pancake coil composite skeleton structure according to the present invention;

[0020] Figure 3 This is a schematic diagram of the composite skeleton structure and the assembly of the double-pancake coil of the present invention;

[0021] Figure 4 A schematic diagram of slotting on the outer periphery of the outer skeleton;

[0022] Figure 5 A schematic diagram showing multiple outer skeletons nested axially within an inner skeleton.

[0023] Figure 6 This is a schematic diagram showing the upper and lower coils wound sequentially on multiple outer skeleton layers.

[0024] The attached diagram is labeled as follows: upper coil 1, lower coil 2, transition turn 3, inner skeleton 4, outer skeleton 5, and slot 6. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0026] like Figure 2 As shown, this invention provides a superconducting double-pancake coil composite frame structure, including an inner frame 4 and an outer frame 5. The outer frame 5 is coaxially sleeved and tightly nested outside the inner frame 4, and the transition turns 3 of the double-pancake coil are tightly wound around the outer peripheral surface of the outer frame 5. Figure 3 As shown, the double-coil includes a first coil segment (i.e., upper coil 1) and a second coil segment (i.e., lower coil 2) stacked along the axial direction. The first coil segment and the second coil segment are connected by a transition turn 3, and both are coaxially wound and fixed on the outer circumferential surface of the skeleton with the central axis of the skeleton as the axis. The outer cylindrical surface of the skeleton serves as a winding mold, and the inner diameter of the double-coil matches the outer diameter of the skeleton, forming a tight surface contact between the two.

[0027] Preferably, the inner skeleton 4 is made of a high Young's modulus metal structural material and is cylindrical in shape. It provides the main mechanical support for the entire coil, bears electromagnetic force and external load, and ensures the overall rigidity and dimensional stability of the skeleton. The high Young's modulus is preferably above 100 GPa, and the metal structural material is preferably stainless steel.

[0028] Preferably, the outer skeleton 5 is made of a metal material with high electrical and thermal conductivity, and is in the shape of a sleeve, with its inner surface tightly attached to the outer surface of the inner skeleton 4. The transition turn 3 is directly wound on the outer skeleton 5, forming good electrical and thermal contact between the two. The high electrical and thermal conductivity is preferably 30 MS / m or higher, and the thermal conductivity is 200 W / (m·K or higher).

[0029] The outer skeleton 5 simultaneously performs the following functions:

[0030] (1) Conductive backup and shunting function: The high conductivity of oxygen-free copper makes it form a conductive path in parallel with the transition turn 3. When the transition turn 3 is partially degraded, over-current exceeds or completely breaks due to electromagnetic or thermal shock, the current can be automatically shunted to the outer frame 5 to maintain the electrical connection between the upper and lower pancake coils, so that the double pancake coils can avoid overall failure due to the break of the transition turn and can achieve degraded operation;

[0031] (2) Enhanced cooling effect: The high thermal conductivity of oxygen-free copper can quickly conduct the Joule heat generated by the transition turn to the stainless steel skeleton and the surrounding cooling medium, effectively reducing the temperature rise of the transition turn and inhibiting the formation of hot spots.

[0032] (3) Low temperature self-tightening effect: The cold shrinkage rate of oxygen-free copper is greater than that of stainless steel. During the process of cooling from room temperature to superconducting operating temperature, the inner diameter shrinkage of the outer skeleton 5 is greater than that of the outer diameter shrinkage of the inner skeleton 4, thereby generating radial compressive stress, which causes the outer skeleton 5 to automatically tighten onto the inner skeleton 4, ensuring a stable mechanical bond between the two, and at the same time forming uniform radial support for the transition turn 3 wrapped around its outer periphery.

[0033] The working process of this invention is as follows: During normal operation, the current in the double-pane coil flows from the upper pane coil 1 through the transition turn 3 in series to the lower pane coil 2. The transition turn 3 is a superconducting tape that carries the main current. The outer skeleton 5 is in close contact with the transition turn 3, serving as a parallel backup path. When the transition turn 3 experiences increased resistance or current overload due to local damage, the outer skeleton 5 naturally diverts some of the current due to good contact, sharing the heat load of the transition turn and inhibiting its further degradation. Even if the transition turn 3 completely breaks, the current can still flow between the upper and lower pane coils through the outer skeleton 5, and the double-pane coil will not experience an open circuit accident. At the same time, the high thermal conductivity of the outer skeleton 5 effectively transfers heat away, and the self-fastening effect at low temperatures automatically establishes and maintains the thermal, electrical, and mechanical connections of each contact interface from the beginning of assembly cooling.

[0034] Example 1:

[0035] This embodiment provides a superconducting coil composite framework structure, the fabrication and assembly process of which is as follows:

[0036] First, the cylindrical inner skeleton 4 is machined. The inner skeleton 4 is made of a high Young's modulus metallic structural material, preferably stainless steel, but Hastelloy or titanium alloys can also be used. The high Young's modulus of stainless steel gives the inner skeleton 4 excellent stiffness and load-bearing capacity, enabling it to withstand the enormous electromagnetic forces generated by the superconducting coil in a strong magnetic field environment, providing the main mechanical support for the entire coil. Hastelloy has excellent mechanical properties and resistance to hydrogen embrittlement at extremely low temperatures, while titanium alloys combine high strength and low density, making them suitable for weight-sensitive applications. The above materials can be selected based on the specific operating conditions and cost requirements of the superconducting coil. During machining, it is necessary to ensure that the outer surface of the inner skeleton 4 has sufficient cylindricity and surface finish to form good initial contact with the outer skeleton 5.

[0037] Next, the outer skeleton 5 is formed. The outer skeleton 5 is made of a metal material with high electrical and thermal conductivity, preferably oxygen-free copper, pure aluminum, or solder, and is made into a sleeve shape. Its inner diameter is slightly smaller than the outer diameter of the inner skeleton 4 to form an interference fit, and its axial length matches the length of the load-bearing section of the inner skeleton 4. The inner surface of the outer skeleton 5 is precision machined to ensure a tight fit with the outer surface of the inner skeleton 4.

[0038] As an alternative, such as Figure 5 , Figure 6 As shown, multiple outer skeletons 5 can be axially nested on the inner skeleton 4 to form a long solenoid structure. The multiple outer skeletons 5 are made of different materials and have different connection methods (such as nesting assembly or electroplating). The upper coil 1 and the lower coil 2 are wound sequentially on the multiple outer skeletons 5.

[0039] As an alternative, the outer skeleton 5 can also be implemented without using a separate oxygen-free copper sleeve, but in the following form:

[0040] (1) A thick copper plating layer is formed on the outer surface of the inner skeleton 4 by electroplating, chemical plating or thermal spraying. This method can save the precision machining and assembly process of the sleeve, and the plating layer is tightly bonded to the substrate, with low interface thermal resistance and electrical contact resistance.

[0041] (2) A superconducting tape segment is tightly attached to the outer surface of the inner skeleton 4, for example, several short superconducting tape segments are laid along the circumferential direction and fixed to form a conductive path in parallel with the transition turn 3. This method can make full use of the zero resistance characteristic of the superconducting tape at low temperature and hardly generate additional Joule heat during current shunting. All of the above alternatives can achieve the function of providing a parallel conductive path for the transition turn 3.

[0042] When using independent oxygen-free copper sleeves, the outer skeleton 5 and inner skeleton 4 can be fitted together using either a hot fitting method or a cold fitting method. The hot fitting method involves heating the outer oxygen-free copper sleeve to increase its inner diameter through thermal expansion, then quickly fitting it into the inner skeleton 4. After cooling and shrinking, the two will form a tight nest. The cold fitting method involves placing the inner skeleton 4 in a low-temperature medium to cool it, causing its outer diameter to shrink through cold contraction, then quickly fitting it into the outer oxygen-free copper sleeve. After returning to room temperature, the two will form a tight nest. Both assembly methods can achieve a reliable interference fit connection; the specific choice depends on equipment conditions and workpiece dimensions.

[0043] Next, transition turns 3 of the double-pane coil are wound on the outer circumferential surface of the double-layer composite skeleton. Transition turns 3 are made of high-temperature superconducting tape and are tightly wound around the outer circumferential surface of the outer skeleton 5. The number of turns is determined according to design requirements, typically one to several turns. One end of the transition turns 3 continues to be wound to form the upper pane coil 1, and the other end forms the lower pane coil 2, achieving a series electrical transition between the upper and lower pane coils. Because the transition turns 3 are directly wound on the surface of the outer skeleton 5, good electrical and thermal contact is formed between them. The winding tension is determined according to the specifications of the superconducting tape to ensure tight winding without damaging the tape.

[0044] As a further optimization, a groove 6 can be formed on the outer circumferential surface of the sleeve-shaped outer skeleton 5. The groove 6 can be configured as a spiral groove or an annular groove, such as... Figure 4 As shown, the width and depth of the spiral or annular groove match the cross-sectional dimensions of the superconducting tape used for the transition turn 3. This spiral or annular groove must not disrupt the circumferential continuity of the outer skeleton 5. During winding, the transition turn 3 is placed within the groove 6. The wall of the groove 6 provides axial positioning and support for the transition turn 3, ensuring uniform spacing and accurate orientation between turns. Simultaneously, it reduces axial slippage and micro-vibration of the transition turn 3 under electromagnetic force, further enhancing the overall reliability of the structure.

[0045] The assembled composite skeleton structure is as follows Figure 2 As shown. The working process and principle of this embodiment are as follows: During normal power-on operation, the superconducting tape is in a superconducting state, the transition turn 3 carries almost all the current, and the outer skeleton 5, due to the superconducting tape with its own resistance greater than zero resistance, only carries a very small parallel current, and is in standby state as an electrical backup path.

[0046] When transition turn 3 suffers localized damage due to electromagnetic impact, thermal cycling fatigue, or other reasons, leading to increased resistance or even localized quenching at that location, the resistance of transition turn 3 rises significantly. At this time, the current flowing through the outer frame 5, which is in close contact with and connected in parallel to transition turn 3, automatically increases, naturally diverting some of the current to share the thermal load of transition turn 3 and suppress its temperature rise and degradation propagation. Simultaneously, the outer frame 5, with its high thermal conductivity, rapidly conducts the Joule heat generated by transition turn 3 radially to the inner frame 4 and the surrounding cooling medium, effectively suppressing the formation of localized hot spots and preventing thermal runaway of transition turn 3.

[0047] When transition turn 3 completely breaks due to severe damage, almost all the current is transferred to the outer frame 5, which maintains the electrical connection between the upper and lower coils. Although the double coil enters a degraded operating state due to the presence of parallel resistance, it will not experience sudden open-circuit failure between the upper and lower coils and can still maintain a certain current carrying capacity, thus achieving fault-tolerant operation for transition turn faults.

[0048] During the cooling process from room temperature to the cryogenic superconducting operating temperature, when the outer skeleton 5 is made of oxygen-free copper and the inner skeleton 4 is made of stainless steel, the cold shrinkage rate of the outer oxygen-free copper is greater than that of the inner stainless steel. This means the inner diameter shrinkage of the outer skeleton 5 is greater than the outer diameter shrinkage of the inner skeleton 4, resulting in radial compressive stress. This radial compressive stress causes the outer skeleton 5 to automatically tighten onto the inner skeleton 4, ensuring a stable mechanical bond between the two layers without the need for additional fasteners. When the outer skeleton 5 uses a thick copper plating layer, the interface can maintain a reliable connection even at low temperatures because the copper and stainless steel substrate have already formed a metallurgical bond during the plating process.

[0049] When subjected to electromagnetic and thermal stress, the transition turn 3 is connected to the outer skeleton 5 only through winding contact, without the use of rigid fixing methods such as soldering. Therefore, the transition turn 3 can undergo microscopic slippage under stress, thereby releasing local stress and avoiding stress concentration and strip damage caused by completely rigid constraints. When grooves are made on the outer circumferential surface, the groove walls can provide additional lateral constraints to the transition turn 3, improving axial positional stability, but the transition turn 3 can still undergo a certain degree of microslippage in the radial direction, thus preserving the flexible stress release mechanism.

[0050] Example 2:

[0051] The difference between this embodiment and Embodiment 1 is that the inner skeleton 4 is made of Hastelloy to improve its mechanical properties in extremely low temperature environments; the outer skeleton 5 is made of oxygen-free copper sleeve with an appropriately increased wall thickness to further improve electrical backup capability and thermal conductivity, making it suitable for high-field superconducting magnet scenarios with larger current carrying capacity and more stringent operating conditions. The remaining structure, assembly method, and operating principle are the same as in Embodiment 1.

[0052] Example 3:

[0053] The difference between this embodiment and Embodiment 1 is that the inner skeleton 4 is made of titanium alloy to reduce the overall weight of the magnet; the outer skeleton 5 is formed by electroplating a thick copper layer on the outer surface of the inner skeleton 4, eliminating the need for precision machining and assembly of the independent sleeve. After copper plating, a spiral groove is machined on the outer surface. The size of the groove matches the cross-section of the strip used for the transition turn 3, guiding the winding direction of the transition turn 3 and providing lateral support. The remaining structure, assembly method, and operating principle are the same as in Embodiment 1.

[0054] The above description is merely a preferred embodiment of the present invention and does not limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the concept and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A superconducting double-pancake coil composite frame structure, characterized in that, The device includes an inner skeleton and an outer skeleton coaxially nested outside the inner skeleton. The transition turns of the double-pane coil are tightly wound around the outer circumferential surface of the outer skeleton. The inner skeleton is made of a metal material with high Young's modulus, and the outer skeleton is made of a metal material with high electrical conductivity and high thermal conductivity. The double-pane coil includes a first coil segment and a second coil segment stacked along the axial direction. The first coil segment and the second coil segment are connected by transition turns, and both are coaxially wound and fixed to the outer circumferential surface of the skeleton with the central axis as the center. The outer cylindrical surface of the skeleton serves as a winding mold, and the inner diameter of the double-pane coil matches the outer diameter of the skeleton, forming a tight surface contact. The high Young's modulus is ≥100 GPa, the high electrical conductivity is ≥30 MS / m, and the high thermal conductivity is ≥200 W / (m·K).

2. The superconducting double-pancake coil composite frame structure according to claim 1, characterized in that, The inner skeleton is a stainless steel skeleton, a Hastelloy skeleton, or a titanium alloy skeleton, and the outer skeleton is an oxygen-free copper skeleton, a pure aluminum skeleton, or a solder skeleton.

3. The superconducting double-pancake coil composite frame structure according to claim 2, characterized in that, The outer skeleton is sleeve-shaped, and its inner surface is tightly fitted with the outer surface of the inner skeleton. A groove for accommodating the transition turn is formed on the outer peripheral surface of the outer skeleton.

4. The superconducting double-pancake coil composite frame structure according to claim 2, characterized in that, The outer skeleton is a conductive layer formed on the outer surface of the inner skeleton by hot-dip plating, electroplating, chemical plating or thermal spraying.

5. The superconducting double-pancake coil composite frame structure according to claim 4, characterized in that, A groove is formed on the outer peripheral surface of the conductive layer to accommodate the transition turn; the groove does not interrupt the circumferential continuity of the conductive layer.

6. The superconducting double-pancake coil composite frame structure according to claim 2 or 5, characterized in that, The groove has a spiral groove or annular groove structure.

7. The superconducting double-pancake coil composite frame structure according to claim 3, characterized in that, The shrinkage of the inner diameter of the outer skeleton is greater than that of the outer diameter of the inner skeleton, generating radial compressive stress, which causes the outer skeleton to spontaneously tighten onto the inner skeleton.

8. The superconducting double-pancake coil composite frame structure according to claim 1, characterized in that, During normal operation, the current in the double-pancake coil flows from the first coil segment through the transition turns in series to the second coil segment. The transition turns carry the current, and the outer frame is in close contact with the transition turns, serving as a parallel backup path.

9. The superconducting double-pancake coil composite frame structure according to claim 2, characterized in that, Multiple outer skeletons are arranged axially upwards from the inner skeleton to form a long solenoid structure, and the multiple outer skeletons are made of different materials and in a nesting or plating combination manner.

10. The superconducting double-pancake coil composite frame structure according to claim 1, characterized in that, The transition turn is made of superconducting tape.

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