Research On Short-Period High-field Undulators

As the 4th-generation light source,free electron laser has been widely concerned by the scientific community at home and abroad since the 1970s with the characteristics of continuous wavelength adjustment,high brightness,excellent time structure and excellent coherence.It can operate in wavelengths that are not possible with conventional lasers,including the very attractive X-ray range.As one of the core devices in the research of free electron lasers,the research of undulators has also received extensive attention.The short-period and high-field undulator facilitates the generation of shorter wavelength free-electron lasers and also facilitates the miniaturization of free-electron laser devices.In this context,this paper focuses on the study of short-period and high-field undulators.According to actual needs,this paper studies the staggered array undulator and high-field permanent magnet undulator.This paper also studies a variable elliptical polarization undulators,in the end,this paper studies other special structures of undulator.The staggered array undulator is studied.A typical staggered array undulator consists of periodically arranged poles in the longitudinal direction.When a longitudinal magnetic field enters the undulator,a periodic transverse magnetic field is generated.This paper proposes a scheme of adding vertically magnetized magnets between the poles.Since the magnets themselves will generate a periodic transverse magnetic field,it will further enhance the peak field strength of the undulator and also reduce the magnetic field requirements of the solenoid.The results show that the addition of magnets can increase the peak field strength of the staggered array undulator,but the enhancement of the peak field strength is not that ideal.In this paper,the causes of this phenomenon are analyzed,and the scheme of adjusting the magnetization direction of the magnets is proposed.The results show that the scheme can reduce the peak position difference of the transverse magnetic field generated by the solenoid and the magnets,and can alleviate the problem of excessive saturation of the magnetic pole to a certain extent,thus obtaining a higher peak field strength of the undulator.For the existing scheme of horizontal magnetization of the magnets,the demagnetization of the magnetic block in the reverse magnetic field is analyzed.We believe that the magnetic field of the undulator may be greatly affected in consideration of the demagnetization of the magnets.This paper also analyzes the influence of the shape of the magnetic pole and magnets of the undulator on the magnetic field,and proposes a series of optimization schemes to improve the peak field strength of the staggered array undulator.This paper also takes the staggered array undulator as an example to illustrate the application of short period high field strength undulator in the miniaturization of free electron laser.Research on high field strength pure permanent magnet undulators.Conventional pure permanent magnet undulators are periodically arranged by magnets with permanent magnet characteristics.In this paper,a new pure permanent magnet undulator pole structure is researched,which divides the pole into two parts,each of which is tilt magnetized,so that the magnetic lines of the poles converge more on the central axis of the undulator,improving the the peak field strength.The results show that the scheme can increase the peak field strength of the undulator under all the gaps.When the gap is small,the undulator field strength is increased to the intensity equivalent to the hybrid undulator.At large gap,the peak field strength is stronger than the hybrid undulator.This paper analyzes the problem of the narrowed good field region of the undulator encountered by using this scheme,and proposes a scheme to improve the good field region.By changing the shape of the pole face of the undulator or dividing the magnetic block into three parts,the purpose of ensuring the width of the good field region is achieved.The results show that changing the shape of the magnetic pole surface not only ensures a certain good field width,but also can appropriately increase the peak field strength.This paper also analyzes the harmonic characteristics of the undulator.The variable elliptical polarization undulator is studied.Conventional variable-polarization undulators,such as cross-type undulators,Apple II undulators,and Delta undulators,require two sets of four undulator permanent magnet arrays,which are relatively complex in structure and increase the difficulty of adjustment.In this paper,a novel variable elliptical polarization undulator is researched.The magnetization direction of the undulator is tilted in the transverse direction.Each undulator period consists of only two upper and lower magnets.By adjusting the displacement of the upper and lower magnet arrays,the polarization characteristics of the undulator magnetic field can be adjusted,and various polarization states such as linear polarization in horizontal and vertical directions,elliptical polarization and pure circular polarization are generated.In this paper,one such undulator is designed and its parameters are optimized by program simulation.The results show that this type of undulator can obtain various polarization states by adjusting the displacement of the upper and lower parts of the undulator.Since the magnets of the undulator are tilt magnetized,the peak field strength on the central axis is low.The results of the simulation calculation also show that the undulator magnetic field has a certain good field width in the vertical direction,but its width in the horizontal direction is narrow,which needs further optimization.Special structures of poles and magnets have been studied.By adjusting the structure of poles and magnets and the magnetization direction,it is possible to enhance the field strength of the undulator.In this paper,we study the transverse split pure permanent magnet undulator,the longitudinal tilt magnetization pure permanent magnet undulator and the longitudinal triangle pure permanent magnet and hybrid undulator.Practice has shown that the transverse split pure permanent magnet undulator can play the role of enhancing the field strength of the undulator when the gap period of the undulator is relatively small.The problem of the narrowed good field region can also be improved by further structural optimization.The enhancement of field strength of the longitudinal tilt magnetization pure permanent magnet undulator is relatively small compared to the standard pure permanent magnet undulator.Both the pure permanent magnet and the hybrid undulator of the longitudinal triangle cannot increase the peak field strength of the undulator,and the field strength of the two is weaker than that of the standard pure permanent magnet undulator.