Design, RF measurement, tuning, and high-power test of an X-band deflector for Soft X-ray Free Electron Lasers (SXFEL) at SINAP

摘要

An X-band transverse-deflecting structure is a crucial RF unit for bunch-length measurement conducted at the Shanghai Soft-X-ray Free Electron Laser Facility (SXFEL) at the Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences. The target of this prototype deflector is to operate at 30 MW to have a deflection of 10 MV/structure to realize 20-fs resolution in a full-size structure (0.6 meters long, used on user facility of SXFEL), which satisfies the requirement of the linac beam size measurement at 840 MeV. The program was started in 2010 focusing on X-band deflectors, including design, fabrication, and RF measurement. The electrical tuning of the deflector was successfully done using a unique "cage"-type bead. Recently, a high-power test was conducted at KEK on the first prototype deflector. The high-gradient features at the nominal gradient was proven, but the further increase of power and pulse width was limited at 45 MW with a 173-ns pulse width due to approaching the feasible limit of the modified pointing vector value 5 MW/mm(2). In this final stage, frequent breakdowns near the badly brazed point occurred and further conditioning had to be suspended owing to limited available time.