Development of a compact RFQ injector for a high-intensity cyclotron

Abstract

Although RFQ (radio frequency quadrupole)-based axial injection into a high-intensity cyclotron offers several advantages, such as high bunching efficiency and compact design, it requires further research and development (R&D) since it has never been realized before. To support this research, a prototype of a compact RFQ designed for the H₂⁺ beam has been developed. This paper discusses the beam dynamics, RF design, thermal analysis, construction, and RF measurements of the RFQ. The beam dynamics design aims to achieve maximum bunching efficiency at the median plane of the cyclotron. To achieve this, two re-buncher cells are integrated in the exit transition cell of the RFQ. Additionally, the RFQ tank is partially embedded within the cyclotron yoke, allowing it to be positioned as close to the median plane as possible. Considering the operating frequency of 81.25 MHz, a so-called ladder RFQ is proposed. The transverse diameter of the ladder RFQ is less than 300 mm, enabling re-entry into the cyclotron yoke. The ladder RFQ tank has been assembled, and cavity measurements have been completed. The RF measurements show good agreement with the simulations. The frequency matches the design value, and the quality factor Q₀ reaches 92.3 % of the simulated value. The quality of the electric field is evaluated using the perturbation method. Gravity-induced errors of the perturbation bead were corrected by averaging the fields across different quadrants. Consequently, the un-flatness of the electric field in the longitudinal direction is ±3 %. The dipole field component ranges from −3 % to 3 % in the front section and is nearly zero in the latter half of the cavity.