Advanced Beam Measurements Of Emittance, Bunch Length And Beam Size For High-brightness Electron Beam

Advanced beam diagnostics plays a crucial role in the path to the realization of the short wavelength free electron laser, Thomson scattering based X-ray source, International Llinear Collider, etc. The successful construction and operation of the high-brightness beam driven facility requires the beam quality to be maintained during the long transportation from the electron source to the final target. The high quality accelerator is characterized by the low emittance, short bunch length and small beam size, which also challenges the beam diagnostic methods. Our dissertation is devoted to extensively investigating advanced beam diagnostics both theoretically and experimentally for the above mentioned accelerators.We first performed systematical investigations on low emittance measurement with the photocathode RF gun developed at Accelerator Lab of Tsinghua University. The thermal emittance was measured with solenoid scan method under the condition of very low charge for which the space charge induced emittance growth is negligible. A multislit system was designed and fabricated to measure the emittance of the space charge dominated beam at the gun exit. We also for the first time applied the computerized tomography (CT) technique to reconstruct the phase space details of the beam at the photocathode RF gun exit with a solenoid, which may further enable nonlinear emittance compensation that leads to breakthrough in photoinjector performance. Finally, with the CT technique, we observed the stochastic distribution of phase space dominated by thermal emittance. To the best of our knowledge, this is the first time that the thermal emittance was reconstructed from CT technique and visualized with such fruitful details.We then devoted much efforts to developing a cost-effective, simple and non-interceptive technique to measure electron bunch length with sub-ps temporal resolution. We first used the random walk model to find the theoretical basis for bunch length measurement in frequency domain. Then a virtual photon diffraction model was applied to calculate the single electron diffraction radiation spectrum with finite size target effect and near field effect taken into account. Our model and results were recently verified by other researchers with vector electromagnetic theory. We then proceed to develop a home-made Martin-Puplett interferometer with which the diffraction radiation spectrum of a whole bunch was measured and further used to find the bunch form factor. Finally we reconstructed the longitudinal bunch profile with Kramers-Kronig relation. We also proposed a novel method to measure ultrashort bunch length with diffraction radiation deflector. The method has wide applicability, high temporal resolution and great simplicity.Attention was also paid to measurement of micron size beam profile. We calculated the point spread function of optical transition radiation and optical diffraction radiation in beam imaging. A deconvolution method was proposed to restore micron size beam profile from the blurred image. A new method suitable for small beam size determination with diffraction radiation from rectangular slit by scanning the slit in transverse direction is proposed and analyzed. The theories for imaging of high energy beam with optical diffraction radiation were developed and found to be in reasonable agreement with the recently reported experimental results.