The Study Of Laser Performance In Oscillator Free Electron Laser

FEL is now widely accepted by researchers in various scientific fields because of its merits,such as high power,coherent,adjustable wavelength.The common mode of FEL in the Infrared region is FEL oscillator built around the world in great demands.Now the first IR FEL user facility of China,FELiCHEM,is built and commissioning in Hefei National Synchrotron Radiation Lab.The FEL output laser power draws most users attention and will be our main target in this thesis.We have studied the saturation mechanism of oscillator FEL.Tapering undulator after optical field saturation is an effective method to increase the electron energy extraction efficiency in high gain regime.But it is hard to adjust the undualtor taper in oscillator FEL.Thus we analyze the application of small taper undulator in oscillator FEL.We derived the interaction equation between electron and light in the low taper regime with linear approximation.Based on this we got the gain variation with different undulator taper compared to that in normal case.The study shows that the output energy could be enhanced with appropriate undulator taper.Three dimension simulation result in FELiCHEM shows higher output macropulse energy compared to normal case.However,large undulator taper leads to the decrease of gain and limits the increase of laser power.Then another common method,ramping electron energy post-saturation,is studied.The particle tracking in ponderomotive bucket and theory result show high agreement in the optimum electron energy.Therefore we acquired the maximum extraction efficiency and intra-cavity power theoretically in the oscillator FEL.This agrees well with 3D simulation result.Then we discussed the usage of this method to FELiCHEM facility.Finally,we put forward a proposal to combine these two methods together.The study shows that we can increase the oscillator FEL laser output further with energy ramping post-saturation in small taper undulator case.In the long range wavelength region such as Far Infrared and THz radiation,the diffraction effect increase the loss so much that the intra-cavity light hardly reach saturation.Thus the vacuum chamber is designed as a waveguide to limit this effect.However,this would arise the spectral gap phenomenon which destroy the continuity of FEL radiation.The common FEL simulation code Genesis cannot be used in waveguide,then our research group develop wGenesis to cover the waveguide situation.Then we combine wGeneis and OPC code to give 3D simulation result of waveguide FEL which fits well with the CLIO experiment.Our simulation method is proved to be reliable in waveguide FEL.The study shows that the higher order mode in x direction is not negligible.So we modify the spectral gap calculation theory and it agrees well with simulation result.The spectral gap in FELiCHEM waveguide situation is also same in simulation and experiment,therefore as guidance of FELiCHEM commissioning.We also give the spectral power with different waveguide size to offer a reference for further FEL design.Cavity detuning is an important parameter in short electron bunch oscillator FEL.At large detuning situation optical light increase fast with low saturation power while at small detuning vice versa.Through Dynamic Cavity Desynchronization(DCD)technique,the FEL could make use of both merits and avoid drawbacks.The optimum position to change the detuning value in DCD technique is given which lies at the gain decrease to its half.Meanwhile the highest saturation power is achieved at the same detuning value with the constant detuning situation.We discussed the possibility of DCD application in FELiCHEM facility.Through changing the micro-pulse repetition frequency we get equilibrium cavity detuning and can be put into use.The three-dimension simulation result shows that higher macro-pulse energy could be achieved with DCD technique.Finally we give a brief presentation of FEL experiment done at FHI FEL facility.By using the measurement of energy spread at insert target,we accomplished electron bunch length measurement experiment.Then we get high peak current,short length micro pulse electron bunch through magnetic compression method.Afterwards the bunch is sent to the oscillator to generate high output laser power.The short bunch lasing is firstly observed in the FHI FEL facility.We also verify the existence of second harmonic and the affection factor on it.