Research And Development Of Septum Magnet For The Beam Distribution In Free Electron Laser Facility

As the most advanced fourth generation light source,free electron laser(FEL)has more excellent characteristics such as ultra short pulse,ultra high brightness,full spatial coherence and so on,which brings new development opportunities for the fields of life,medicine,chemistry,materials and energy etc,and will promote the great development of these fields.In the future,FEL will develop towards the directions of high repetition rate,short wavelength and controllable pulse.The FEL facility will also operate in the mode of multiple undulator lines at the same time to produce FEL with various modes and performance parameters,so as to meet the needs of more extensive users.Different from the synchrotron radiation source,FEL uses the linear accelerator to produce high quality electron beam,which limits its user capacity to a great degree.In order to improve the operation efficiency of FEL facility,it is necessary to research and design the beam distribution system.At present,most of the FEL facilities in the world have designed their own unique beam distribution systems according to their own device parameters and experimental requirements.The Shanghai high reparation rate XFEL and extreme light facility(SHINE)is now under construction,which has its own beam distribution system designed and will operate in the mode of multiple beam lines in the future.In the beam distribution system of SHINE,the combination of kicker and septum is selected.The kicker magnet deflects the electron beam vertically at a small angle,and then the septum magnet provides a large horizontal angle to achieve beam separation,and finally different electron beams are transferred into corresponding undulator lines.In this paper,the design of Lambertson septum magnet used in the beam distribution system of SHINE is introduced.According to the requirements of the beam distribution section of SHINE,the overall design index of Lambertson magnet is proposed,and then the magnet core material is selected.The size parameters of the Lambertson magnet and the current in the excitation coil are also designed andoptimized based on the actual magnet design and engineering application.In order to improve the magnetic field uniformity in the good field region of Lambertson magnet,the method of adding shims on the pole edge is adopted.Finally,the magnetic field uniformity in the good field area of Lambertson magnet is greatly improved and meet the whole requirements by optimizing the size of added shims,which is demonstrated through model simulation.At the same time,the upper pole is stretched and shielding plates are designed at two ends of the magnet to reduce the leakage field in the field-free region of Lambertson magnet.Through the high precious simulation of three-dimensional model,it is found that the method of lengthening the pole and taking shielding measures at two ends of the magnet can effectively reduce the integral leakage field in the field-free area of Lambertson magnet.At the same time,on the basis of complete physical design of the Lambertson magnet,the other supporting systems of the magnet are designed,and the processing and assembly of the magnet components are completed.At the same time,an experimental system is built to measure the parameters of the magnet.Finally,the complete design parameters of the Lambertson magnet are given,which lays the foundation for the application of Lambertson magnet in SHINE.Besides,this paper also puts forward innovative solutions to the difficulties in the design of Lambertson magnet.In order to improve the uniformity of magnetic field in the good field region of Lambertson magnet,a new method of adding holes in the opposite pole of field-free region is introduced by adopting the idea of symmetrical compensation.Through high precious simulation,it is concluded that the magnetic field uniformity in the good field region of Lambertson magnet can be effectively improved when the size,distance between the two added holes and the depth of added holes to the good field region meet certain parameter relationship.This is the first proposed way to improve the magnetic field uniformity of Lambertson magnet,which supplies determined parameter relationship.Aiming to reduce the leakage field in the field-free region of Lambertson magnet,which is the most challenging,we creatively combine the design of Lambertson magnet withquadrupole magnet and put forward the concept of ‘magnetic field reshaping' for the first time.Is is demonstrated that by adding coils in the field-free area of Lambertson magnet,the leakage field is ‘reshaped' to be a quadrupole field.So the original leakage field at the magnetic center of quadrupole field is reduced to zero,and the zero field area can also be moved by adjusting the current in the added coils,which promises the flexibility of the proposed way.This is also the first time to reduce the leakage field of Lambertson magnet to zero and the additional cost is very small,which guarantees the great application value of the new way.