Study On Crucial Factors To Influence Operation Stability Of MICE Superconducting Coupling Magnet

Muon ionization cooling experiment (MICE) will be a demonstration of muon ionization cooling technology for a neutrino factory. The superconducting coupling solenoid magnet is one of the key components in MICE and will produce a magnetic field 2.6T on the center line to control the muon beam in the RF cavity located inside it. To contain the RF cavities, the inner diameter of the coupling coil is 1500mm. The coupling magnet has a peak induction of 7.3T at the maximum current, and will be cooled by cryocoolers.The high level of stress inside the coil and small mechanical disturbance may cause quench or permanent damage to the magnet due to large scale and high magnetic filed; because the temperature margin of normal operation is only 0.8K, the cooling system should be stable in order to keep the magnet safe operation. This dissertation, studied the crucial factors that affect the operation stability of the MICE superconducting coupling magnet in term of both mechanical stability and cooling stability, optimized the design parameters of the cold mass assembly with slip plane and the cooling system adopting cryocoolers. The research of this dissertation will provide theoretical and practical application guidance for the design of the coupling magnet and similar solenoid magnets.The stress at steady state is the dominant of normal operation. An analytical model to solve the plane stress of the magnet cross section was built. This model considered the effect of the axial stress and the shear stress during cooling and charging. All of stress components after magnet excitation were obtained. The analytical model was verified by an analogous magnet. A finite element model of the coil assembly with slip planes was built and the simulation of normal operation process from winding to charging was performed, and the agreement of the simulation and the analytical results is well. The model was applied to study the effect of steady stress on the normal operation of the magnet.In order to obtain the adequate structure parameters of the cold mass assembly, the finite element model with slip plane was applied to optimize the conductor pre-stress and the structure parameters of banding. The dissertation presents the rational the conductor winding pre-stress. According to the function of banding, the pre- stress, thickness and material of banding were optimized.In order to understand the quench caused by mechanical disturbance and the effect of slip plane on the operation stability of the magnet, study on the mechanical stability at unsteady state was performed. The minimum quench energy of the coupling magnet was calculated by a finite element model, and quantitative analyses of epoxy cracking and conductor motion without slip plane were carried out. The results indicated that the slip planes have great effect on the stability of the magnet. A two dimensional dynamic stress model was built to simulate the dynamic stress during quench. The impact of over- heat and magnetic force of each section of coil was analyzed. Finite element models with and without slip planes for it were developed to simulate the stresses during the process including winding, cooling down and charging. The effect of slip planes on the stress distribution in the coil assembly was investigated. The results show that slip planes with low friction coefficients can improve the stress condition in the coil, and the structure parameters of the slip planes were confirmed.Considering the small temperature margin, the temperature in the magnet must be stable during operation. The key elements in the successful application of cryo cooler to cool magnets are the connection of the cryocooler to the magnet and allowable thermal load. In order to determine the thermosyphon- recondensation cooling scheme, the thermal load of the magnet system at 60K and 4.2K, especially that induced by the resistance of superconducting splices in the coil, were analyzed and calculated in detail. For the purpose of minimizing the ?T between the cooler cold head and the hot point on the magnet and minimizing the thermal load of the magnet, the design of the thermal shields structure was optimized. According to the results, the parameters of cooling system were presented.Because the superconducting splices wiil be wound into the coil, the performance of the splices will affect the stability of magnet. The bonding strength of epoxy used for coil impregnent will directly affect the mechanical stability of the magnet. The resistance of splice was tested by voltammetry method, and the mechanical property was investigated by means of uniaxial tensile test. The results have a good agreement with the calculated values. The bonding strength of epoxy was studied by tensile test. The crack stress was obtained to be used as the base for the magnet design.