Study On Beam Physics And Free-electron Laser Physics Based On Transverse Gradient Undulator

In recent years,free-electron lasers(FELs)driven by a conventional large-scale radio-frequency accelerator become increasingly mature,especially with the great success of several operational facilities around the world,such as LCLS,SACLA and European XFEL.Comparing to the 3rd-generation synchrotron radiation light source,FEL radiation has more excellent features such as short wavelength,short pulse duration,high brightness,coherence,quasi-monochromaticity and continuously tunable frequency,so that FELs are widely used in the fields of biology,chemistry,materials science and so on.With the requirements of the light source from users,FELs are developing towards ultrahigh brightness,ultrashort wavelength,ultrashort pulse,high repetition rates,etc.However,these facilities based on RF accelerators usually have large scales,which increase the expense significantly.Hence,it is very desirable to develop compact x-ray FELs that are similar in characteristics but much smaller in footprint.As a new accelerator,laser plasma accelerator can generate high energy electron beams within an ultrashort distance,which holds great promise to realize a compact FEL.But the major problems of the LPA beam for FEL radiation are the large initial divergence and the rela-tively large energy spread which drastically increase the difficulty of transporting the LPA beam from accelerator to radiator and cause FEL radiation gain degradation.In the FEL community,there are several methods put forward in order to overcome the shortcoming of the large energy spread.Typically,a transverse gradient undulator(TGU)was proposed to minimize the energy spread effect leading to an improvement of FEL gain and radiation power substantially.Cur-rently,Shanghai Institute of Optics and Fine Mechanics(SIOM)is working on the related ex-periment.Besides,as a novel magnet element,TGU also has some particular characteristics of beam dynamics that enables the accurate beam manipulation and the transverse-to-longitudinal coupling.Consequently,it is worthy of further theoretical and experimental research on TGU.Under such background,this dissertation will focus on TGUs to carry out the theoretical research on beam physics and FEL physics,where the content mainly includes the following aspects:On one hand,we study on the beam dynamics of a TGU starting from the fundamen-tal Maxwell's field equations,and consider the feasible applications such as suppressing mi-crobunching instability.As the result,a TGU module can be treated as a basic magnet module and applied to beam optics,e.g.suppressing the microbunching instability,since a TGU will induce a transverse dispersion and lead to a transverse-longitudinal coupling.We proposed a reversible electron beam heater using two TGUs to suppress the microbunching instabil-ity.This scheme introduces both an energy spread increase and a transverse-to-longitudinal phase space coupling,which suppress the microbunching instabilities driven by both longitudi-nal space charge and coherent synchrotron radiation before and within the system.Finally the induced energy spread increase and emittance growth are reversed.Theoretical analysis and numerical simulations are presented to verify the feasibility of the scheme and indicate the ca-pability to improve the seeded FEL radiation performance.Besides,we also analyze the FODO lattice combined with TGUs,by which we can solve the evolutions of the beam size and trans-verse dispersion along TGUs.This work provides the way to FEL radiation in TGUs with a long transport distance.On the other hand,study on the realization of the LPA based FEL.A multi-GeV LPA beam can be generated in several-centimeter acceleration distance,with a high peak current and a low transverse emittance,which will considerably benefit a compact FEL design.How-ever,the large initial angular divergence and energy spread make it challenging to transport the beam and realize FEL radiation.In this dissertation,a novel design of beam transport system is proposed to maintain the superior features of the LPA beam and a TGU is also adopted as an effective energy spread compensator to generate high-brilliance FEL radiation.Theoretical analysis and numerical simulations are presented based on a demonstration experiment with an electron energy of 380 MeV and a radiation wavelength of 30 nm.This work is based on the LPA facility in SIOM,so that we presented a systematic and complete scheme design.The ultra-high magnetic gradient quadrupoles and sextupoles are induced to focus the beam and cancel chromaticity,a simgle dipole is adopted to induce dispersion and simultaneously avoid a larger decompression.The FEL simulation shows a significant FEL gain within a short TGU undulator is achievable,instead of within a planar undulator.Such a beam transport combining with the TGU undulator is being tested at the LPA-FEL facility in SIOM experimentally.In addition,we also consider some other way to realize the LPA based FEL.Although some special methods were proposed for the LPA beam,the relatively large jitters including transverse jitter and energy jitter still limit the advance of these experiments.Hence,we also proposed another simple and practical method to realize coherent harmonic generation(CHG)based on LPA beam.A strong power laser is splitted from the driven laser,a short modulator and a short radiator are adopted for realization of the CHG within a short distance,which has a large tolerance for these typical jitters.Numerical simulations are presented to verify the feasibility of the scheme and indicate the capability to realize LPA-FEL radiation.This work is also considered to be demonstrated at the LPA-FEL facility in SIOM experimentally,which might be a great push significance and an important reference for the development of the LPA based FEL.