| Magnetic resonance imaging(MRI)has become a very important medical detection technology,which is widely used in clinical diagnosis and treatment of various diseases.Ultra-high magnetic field is the main development direction in the field of MRI.The ultra-high field can significantly increase the signal-to-noise ratio and improve the image resolution,which can allow medical staff to observe finer structures;the ultra-high field can increase the imaging speed,and also can be used to detect small amounts of elements,such as carbon,phosphorus,and calcium;in addition,brain neurons can be observed under ultra-high field,thereby providing a way to explore human brain consciousness,Alzheimer’s and other cranial nerve diseases.The development of superconducting technology has promoted the development of MRI magnet.For MRIs with magnetic fields of 1.5 T,3 T and even higher,the main magnets are all superconducting magne.Obviously,superconducting magnet technology is needed for ultra-high field MRI.At present,7 T,9.4 T,10.5 T MRI have been developed abroad,and the higher magnetic field 11.75 T MRI is undergoing final commissioning and installation.However,China is still blank in development in the field of ultra-high field MRI,and is far behind foreign countries.Therefore,it is needed to carry out the research and development of ultra-high-field MRI in a timely manner.The research and development of the whole body large-aperture 14 T MRI is proposed.Superconducting magnet is the key system of MNI.The 14 T MRI superconducting magnet has a higher magnetic field strength,large volume and higher system requirements,which poses certain challenges to its development.The current-carrying unit is the key of the superconducting magnet system.For large superconducting magnets,superconducting cables are usually used to increase the operating current and reduce the number of turns and inductance.Rutherford Cable has strong current-carrying capacity and high mechanical strength,which is mainly used in accelerator superconducting magnets,as well as solenoid magnets.Rutherford cable is a cable with a rectangular cross-section formed by twisting multiple superconducting strands.Its size is uniform and controllable.Therefore,the use of this type of cable to wind the magnet can improve the uniformity of the magnetic field.For 14 T MRI,the magnetic field strength of the main coil is slightly higher than 14 T.NbTi type Rutherford cable cannot meet its current carrying requirements.Only Nb3Sn type Rutherford cable or high temperature superconducting Rutherford cable can meet the requirement.In comparison,Nb3Sn Rutherford cable is more suitable for 14 T MRI main coil.Nb3Sn Rutherford cable and cabling technology as the research objective in this paper,and carried out research work.Firstly,based on the 14 T MRI coil design at present,this article first proposes a Rutherford cable as the basic unit,and accurately analyzes the magnetic distribution of the coil that can provide relevant information for the cable design.And then,the cable’s parameter is discussed through the calculation of the sharing temperature.Then,a quench propagation model of Rutherford cable composite conductor is established,and the model is solved by numerical method.The minimum quench energy(MQE)and quench propagation velocity under different conditions are obtained through the model,and the hot spot temperature under different discharge time are also analyzed that can provide relevant information for magnet quench protection design.Secondly,the cabling process of Rutherford cable is introduced,the Rutherford cable’s parameters are described,the geometry of the cable is established,and the cabling coefficient is analyzed.The mandrel tool is the key equipment in the cabling process.The design principle of the mandrel,three types of the mandrel are designed that is suitable for different specification of the cable,and cabling experiments are carried out.Tension control is an important factor which guaranting the cabling process and the final quality of the cable.Experimental study under different tension control method is carried out.The source of the cabling tension is analyzed,the feedback control method by servo motor and tension sensor is proposed to control the cabling tension,the tension feedback control system and the simulation model is established.The dynamic response process is analyzed based on the model.Experimental platform is finally built and tension response under the feedback control is measured.Thirdly,experiment have observed the effects of different compaction on the sub-element of Nb3Sn superconducting strand,and 2D finite element model(FEM)of strand is established.The simulated results were compared with the experiment results and the critical failure criteria of the sub-element is determined.And then a 2D Rutherford cable compaction is established.The critical compaction ratio of the cable is analyzed according to the critical failure criteria,which can provide information for cable design.The influence of the pitch length on the cable is discussed,and the bending model and tensile model are established to study the mechanical behavior of the cable.Fourthly,the critical performance of Nb3Sn strand under different compaction ratio is measured that can provide relevant information for cable design.Four types of composite Rutherford cables(4Sc+6Cu)with different widths are cabling.Two measured methods are proposed and adopted to evaluate the performance of the.The first method is to wind a single strand on the ITER-barrel for testing,and the second method is to measure the entire cable.The critical performance of the strands and the four types of composite Rutherford cables are measured,and the measured results are discussed and analyzed.The residual resistivity ratio at the corners after cabling is measured.Finally,to measure the critical performance of all-superconducting Nb3Sn Rutherford cable,a small interpolated coil is designed and manufactured. |
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