Research On RF Structure Of Window-coupled RFQ

As a kind of standard linear accelerator structure for accelerating ion with low energy, the RFQ accelerator structure is used for acceleration as well for transverse focusing, and the performance of the whole accelerator was affected by the RF structure of RFQ.Four-vane and four-rod structures are the most commonly used RFQ structures. Usually the four-vane structure is preferred at frequencies above about 200 MHz because of its high RF power efficiency, while the four-rod structure is preferred below 200 MHz because of its compact size at low frequencies. Window-coupled RFQ may be configured as a kind of four-vane RFQ with coupled windows between the vanes, or as a four-rod RFQ with individual stems supporting each of the four rods. At the same frequency, the four vane with windows RFQ is more radially compact than the four-vane RFQ, and is less compact than the four-rod RFQ. At a given frequency, the four vane with windows RFQ is less efficient than the four-vane RFQ and is more efficient than the four-rod RFQ.In order to design the RF structure of HIAF(High-Intensity Heavy Ion Accelerator Facility) RFQ, a one meter model cavity working at 81.25 MHz was simulated based on the CST MWS software to discuss the tradition structure including four-vane, four-rod type and simulate the window-coupled structure especially. The optimization on the shape, size, number and symmetric type of the window were performed. The introduction of windows can lower the frequency of the quadrupole operating mode well below that of the dipole modes, reducing the undesirable dipole-mode degeneracy. A one-meter RFQ cavity was fabricated to test the window-coupled RF design. The measured frequency after fabrication was 81.41 MHz, the Q value was 3790 and the nearest mode frequency separation was 12.2 MHz. Bead pull perturbation method was employed to measure the electric field and the field flatness in four quadrant was less than 3‰.The window-coupled structure was comprehensive studied and the cold model test showed that test results were in good agreement with simulation results.