Study On The Microbunching Instability In Electron Linear Accelerator

The performance of x-ray free electron laser(FEL)facilities critically depends on the electron beam quality out of linear accelerators,i.e.linacs.However,in the linear accelerator,due to the presence of collective effects and multiple stage bunch length compression process,the resulted microbunching instability(MBI)could significantly amplify the rf-gun generated modulations or even the shot-noise fluctuations of the elec-tron beam.This could cause significant degradation of the electron beam quality at the entrance of undulator for x-ray radiation.A conventional method to control the insta-bility is to use a laser heater to increase the electron beam uncorrelated energy spread before bunch compression to damp the instability.This is typically tolerable for the operation of self-amplified spontaneous emission(SASE)FELs.However,for seeded FELs it could limit the achievable shortest radiation wavelength,i.e.the higher har-monic radiation.The MBI,which is generated by a mechanism similar with the bunching process in the undulator of FELs but at much longer wavelengths,may lead to deterioration of the electron beam properties.It is a challenging issue to be controlled in FELs.In this dissertation,we mainly focus on the suppression of microbunching instability without sacrifice of the beam quality,such as slice energy spread.And we also did some research about the application of bunching effects for generating coherent THz radiation,which is introduced in the last section of this thesis.For linac based x-ray FELs,the longitudinal space charge(LSC)impedance is the dominated one driving MBI among those collective effects,such as coherent syn-chrotron radiation(CSR)and linac structure wakefield.During the analytical derivation of the MBI gain driven by LSC impedance for multi-stage bunch length compression FELs,we found an instability cancellation phenomenon due to the opposite sign be-tween different amplification stages.A case study of using a nonisochronous dogleg section in a double compression scheme to mitigate the current modulation at the exit of dogleg is illustrated.Simulations show that the nonisochronous dogleg can suppress the density modulations,but not valid for energy modulations.We then propose a scheme using a quadrupole inserted chicane to suppress the MBI by introducing the transverse-to-longitudinal coupling effects,which will smear out the already induced current and energy modulations.After the long distance linear acceler-ator transportation,we finally use a dogleg section to eliminate the coupling terms for restoring achromaticity and suppressing the dispersion.The whole beam line constructs a reversible system.Detailed studies show that this scheme can destroy the density and energy modulations at the exit of dogleg.However,the electron beam quality,such as slice emittance and slice energy spread,is not reversible.Although the previous two schemes can not completely solve the issues induced by the microbunching instability in FELs,but the detailed theoretical and simulation studies can help us have a deeper understanding of the instability.By careful analysis we found that if we use a quadrupole inserted chicane associated with an energy chirped electron beam,the upgraded previous scheme could effectively suppress the MBI and successfully preserve the beam quality simultaneously.This is because that with the help of energy chirp and transverse-to-longitudinal coupling effects,the initial accumu-lated energy and density modulations are smeared out.More importantly,the electron beam slice energy spread is enlarged in this scheme,which could efficiently suppress the bunching process during the transportation of linacs.Finally,at the upstream of undula-tor,a dogleg section is used to eliminate the transverse-to-longitudinal coupling terms,thus restoring the enlarged slice energy spread.The final uncorrelated energy spread increase roughly by the current compression factor,which is quite helpful for achieving high harmonic short-wave length x-ray radiation in seeded FEL schemes.Multiparticle simulations show that the transverse space charge,structure wakefield,and the coherent synchrotron radiation effects will have a relatively small impact on this scheme.In the last section of this dissertation,we propose a scheme about the generation of THz radiation based on pre-bunched electron beam and dielectric loaded waveguide(DLW).The cylindrical dielectric loaded waveguide supports a discrete set of modes,which can be excited by electron beam passing through the structure,i.e.Cherenkov wakefield radiation.The high-order modes can be the harmonics of the fundamental one by properly choosing the parameters of the DLW.By using a train of electron bunches repeated at the fundamental frequency of the DLW,as the driving source,coherent and simultaneous excitation of multi-modes can be expected.With this proposed scheme,multi-color narrow-band THz radiation with high pulse power and high frequency can be obtained simultaneously.