Technical Research On Septum Magnet For Injection Of Diffraction Limited Storage Ring

With development of modern accelerator technology,the higher brightness and quality of synchrotron radiation light has become a new pursuit for scientific experiments.Diffraction Limited Storage Ring(DLSR)light sources,which turn into an important direction for the development of next-generation light sources,are characterized by their beam emittance as low as to achieve the DLSR,excellent transverse coherence,and brightness about two orders of magnitude higher than that of third-generation light sources.It brings new development opportunities for bioscience,material & energy,pharmaceutical chemistry and many other fields.In the lattice design of the DLSR light sources,the Multi-Bend Achromat(MBA)magnetic focusing structure is usually adopted and the strength of sextupole magnets are enhanced,so as to increase the synchrotron radiation damping and reduce the beam emittance.On the other hand,strong focusing leads to the need of beam chromaticity compensation,and the enhanced sexupole for compensating chromaticity will introduce strong nonlinearity,which leads to the Dynamic Aperture(DA)of storage ring light source is usually only 2～5 mm,which is about one order of magnitude smaller than the third-generation synchrotron radiation light sources.The sharp decline of DA leads to the failure of many mature technologies using in the third-generation light sources,in which beam injection could be realized by using local bump.However,in the DLSR of small DA,beam injection is achieved by on-axis injection(i.e.,on-axis swap-out injection and on-axis longitudinal injection)or off-axis injection by multi-pole pulsed magnets.A fast kicker(such as strip-line kicker)combined with thin septum magnets are used to solve the problem that the beam cannot be injected by traditional local bump method in DLSR.The research focuses on the development of thin septum magnet.In this paper,relevant parameters about thin septum and shielding theory of the eddy current field have been calculated and analyzed,and then finite element simulation analysis has been carried out for septa less than 1 millimeter.The magnetic field analysis software modules Opera 2D transient analysis(TR)and 3D ELEKTRA(TR)have been used to dynamically analyze the evolution of the main & leakage field,and eddy current field varying with time.Focus on the changes of leakage field under different septa thickness,excitation waveform and combinations of material thickness ratio,so as to obtain the comprehensive optimal solution.Additionally,the static temperature distribution of thin septum has been simulated briefly.In terms of the structure and process designning of the thin septum,the paper aims at the optimization of the septa,especially the wire-electrode cutting processing technology of thin septa.Besides,the fitting of copper with high permeability shielding material also deserves more attention.Furthermore,integrated oxygen-free copper coil and non-oriented silicon steel core are also discussed.After the final assembly of the septum,a dynamic pulsed magnetic field measurement system has been set up,including a 20-turn coil with diameter of 4 mm,a single turn coil of 900 millimeter long,a half-sine impulsed power supply and a three-dimensional electric motion platform.In order to achieve more accurately measure of the integral magnetic field,the voltages induced by the point and the long coil have been de-noised to improve the detection ability.The uniformity of the main field,the magnet flux density distribution of the end field and the decay characteristics of the eddy current field have been tested comprehensively,comparing with the theoretical calculation results at the same time.Three-dimensional dynamic calculation and field measurements show that the main field uniformity,the main field integral along the beam direction and the leakage field at 3 millimeters outside of the septa(better than 0.1%)all could meet the requirement of the physical parameters of DLSR.Finally,further optimization measures are put forward on designing,integration and testing.The research had laid a foundation for the future injection design of advanced light sources.