| In 2017,the China Accelerator Driven Subcritical-system front-end demo facility(CAFe)was successfully commissioned at the Institute of Modern Physics,Chinese Academy of Sciences.25 Me V/0.17 m A continuous proton beam and 26.1 Me V/12.6m A pulsed proton beam were achieved.In an ADS system,beam stability and availability are the uttermost requirements for the Linac.So far,they have become the hot topic and urgent task for the high-power accelerator community.One of the most cases of beam trips in the CAFe is the abnormal pickup signal of the SRF cavity caused by field emission,which occurs as frequent as once every 20 minutes.This was also reported in other facilities.Therefore,this dissertation thesis will focus on the study about the physical mechanism behind the abnormal pickup signal and find the solution for it.The abnormal pickup signals can result from various reasons.By analyzing the waveform of pickup abnormal signal,the control system characteristics,and the traces on ceramic window of the pickup coupler,a conclusion could be drawn that it was the surface flashover at the pickup coupler ceramic window.During the flashover process,the electromagnetic field in the cavity remains constant.The false signals lead the LLRF system act in the wrong direction and eventually caused beam trips.To understand how the flashover generates,online monitoring experiments were performed on CAFe.The abnormal pickup signals were observed and recorded by LLRF system at different fileds.The time of sparking and the waveform were recorded.These experimental data indicates that it is positive correlation between the frequency of the abnormal signals and the field emission current.Meanwhile,the particle tracking simulation method was used to carry out the movement of the field emission electrons in the cavity at the working field.Combined with the XPS characterization of the surface of the ceramic windows,the accumulated field emission electrons on the ceramic window can be blamed for the surface flashover.Then,the intrinsic physical process of how field emission electrons developing into electric discharge was discussed.The boundary conditions in the cavity and on the Pt coupler were analyzed,including residual pressure,structure,displacement between electrodes,and the distribution profile of electromagnetic field.It was found that there is no high-voltage that can stimulate electrons emission or initiate a discharge.The SEY coefficient of ceramics window material was measured.And the test result showed that the SEY factor was far less than 1when the incident electron energy was among a few hundred ke V,which did not meet the condition for the reproduction of secondary electrons on the window.The FLUKA software developed by CERN was employed to simulate the interaction between incident electrons and ceramic window,including particle collide,energy transfer,and electron implantation.The charge amount was analyzed by measuring the field emission current,while the discharge amount was analyzed by taking abnormal Pt signal’s integral.Through the comparison,the mechanism of electron-induced discharge was studied.This dissertation found that the phenomenon on CAFe cannot be explained by the hypothesis raised by other accelerator researchers.Instead of SEEA(secondary electron emission avalanche),ETPR(electron triggered polarization relaxation)can better explain the discharge phenomenon.A simple but effective solution was proposed and applied based on the analysis results above.The structure of the antenna in pickup coupler was optimized to protect the coupler port from the field emission electrons.The following vertical RF tests verified that the optimized antenna functioned well without affecting the performance of the superconducting cavity and control system.The modified antenna design was installed on CAFe with a few replacement cryomodule.According to the operation experience,this kind of abnormal signal problem disappeared,and the operation stability of the accelerator has been improved. |
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