Experimental Research On Beam Emittance In Photocathode RF Guns

Electron beams with low emittance are of great significance in accelerator-based machines such as free-electron lasers,Thomson scattering sources,Me V ultrafast electron diffraction and microscopy,which are always a research hotspot in past decades.This thesis focuses on emittance issues in photocathode rf guns.It mainly studies the new emission mechanism,the emittance measurements,the mechanism of emission growth and emittance correction.We studied the quantum efficiency and emittance of the copper photocathode based on the Tsinghua photoinjector.The quantum efficiency mapping and emittance of copper photocathode were measured.The emittance was optimized by scanning the solenoid,laser phase,and the laser beamsize.The optimal emittance of the 200p C electron bunch is?_x=0.56 mm mrad,?_y=0.66 mm mrad,and the optimal emittance of the 500p C electron bunch is?_x=0.78 mm mrad,?_y=0.92 mm mrad.We studied the laser pulse heating of the copper photocathode in the blowout regime.A modified two-temperature model was developed to analyze the variation of the quantum efficiency and thermal emittance with the change of the laser fluence.The damage threshold of the copper cathode was evaluated to be 40 m J/cm~2.The thermal emittance of the cesium telluride photocathode was measured based on the AWA photoinjector.The thermal emittance of the cesium telluride photocathode is 1.05 mm rad/mm with laser wavelength of 248 nm,which is consistent with the theoretical value given by the three-step model.The thermal emittance mapping of the cesium telluride photocathode was measured with a laser pattern beam produced by Microlens Arrays,and the quantum efficiency mapping was measured simultaneously.The thermal emittance of each beamlet was compared with the quantum efficiency.We found that the beamlet with larger quantum efficiency also has higher thermal emittance.The dependence of the thermal emittance of the cesium telluride cathode on the quantum efficiency is present.The emittance growth due to coupled transverse dynamics aberrations and its cor-rection were studied in this thesis.The sources of the coupled transverse dynamics aberrations in AWA photoinjector were analyzed.It has been found that the transverse coupled motion will lead to an overestimation of the thermal emittance in solenoid scan.The thermal emittance is overestimated to be 35%with a laser rms beamsize of 2.7 mm.The quadrupole in the solenoid was evaluated by flipping the solenoid current direction.A quadrupole corrector with continuously adjustable rotation angle was made to correct the coupled transverse dynamics aberration.The emittance as a function of the rotation angle of the corrector was measured,which is in good agreement with simulation.The corrector parameters were optimized with different laser beamsizes,showing that the cor-rected emittance reaches the thermal emittance with low charge beam.This research result based on the experiments will benefit the study of the high brightness photoinjectors.