Studies Of RF Deecting/crab Cavities For Beam Manipulation And Diagnostics

The RF de?ecting cavity provides phase-dependent transverse momentum to par-ticles, and linearly couples the longitudinal and transverse phase space of a particlebunch. It is widely used in the accelerator field, for boosting the luminosity of colliders(also known as"crab cavity") and compressing the X-ray pulses in synchrotron radia-tion facilities. It has become a very popular instrument for beam diagnostics with highlongitudinal resolution. Starting with detailed discussion of the basic physics in RFde?ecting cavities, this dissertation analyzes and solved several important problems oncavity design and the beam dynamics.Unlike regular accelerating cavities, the de?ecting cavities work at the lowestdipole mode, TM110, which has two degenerate modes with di?erent polarizing an-gle. The frequencies of these two modes can be separated by introducing azimuthalasymmetry, for example, using elliptical shape in superconducting cavities or addingholes at specific positions on the cavity wall for normal-conducting cavities.For the superconducting de?ecting cavities used in the storage rings, another prob-lem is the damping of non-working modes, especially the lower order mode (LOM),which has even lower frequency than the working mode. Based on the simulation andthe warm model measurement, we discussed the method of using a rectangular waveg-uide added on the beam pipe to damp the LOM, which has been adopted in the designof the de?ecting cavity for the ALS at LBNL and the APS at ANL.Detailed investigation shows that the"zero-phase"particle would have an o?setwhen it passes a standing-wave de?ecting cavity. We analytically solved this problemby adding end-cells to form a 3-cell structure. Also, by simulation, we designed andfabricated an L-band de?ecting cavity for the emittance exchange experiment on theAWA at ANL.We developed an S-band de?ecting cavity for beam diagnostics at the TsinghuaThomson-Scattering X-ray Source (TTX). At the first phase of this project, we mea- sured the 700-fs bunches with accuracy of 5%. This system also provides 100 fs lon-gitudinal resolution, for the measurement of slice emittance and energy spread. At lowenergy (～2.5MeV), the bunch length was dominated by space charge e?ect even at thecharge of several tens of pC. This was observed by the de?ecting cavity while varyingthe energy of incident laser pulse. Besides the bunch length measurement, we proposeda method for reconstructing the 3-D profile of a bunch using tomography technologyand successfully obtained a 3-D distribution of a real bunch.