Development and beam commissioning of a continuous-wave window-type radio-frequency quadrupole

Abstract

The radio-frequency quadrupole (RFQ) group at Peking University has built a window-type RFQ, operating at 162.5 MHz in continuous-wave (cw) mode. It is designed to accelerate a 50 mA deuteron beam from 50 keV to 1 MeV with a vane length of 1.809 m. The cavity was fabricated in two segments using 100% oxygen-free electronic (OFE) copper. Using an iterative assembly and measurement procedure for the precise alignment of the two segments, we reduced the assembly errors to within ±0.05 mm. The radio frequency (rf) measurements of the whole cavity show excellent rf properties, with the measured intrinsic Q-value of 8962, which equates to 96% of the simulated value for OFE copper. We also investigated field fluctuations caused by misalignment between the two segments, and studied their impact on the beam transmission using beam dynamics simulations. During field tuning, we compiled a set of unique tuning rules for the window-type RFQ. After tuning, the maximal field unflatness of the single quadrant is within ±2%, and the asymmetry of the four quadrants is within ±1%. During rf conditioning, the cw power of the cavity reached 55 kW within 32 hours, and we have recorded nearly seven hours of stable running at a cw power of 50 kW. The measured bremsstrahlung spectrum shows that the accelerator needs 49.9 kW to generate the intervane voltage of 60 kV, with a specific shunt impedance of 130.5 kΩ m. An H_{2}^{+} ion beam extracted from an electron cyclotron resonance ion source was used for the beam commissioning, because deuteron beam acceleration will bring a serious radiation field. We achieved stable and robust acceleration of about 1.5 mA cw H_{2}^{+} beam for one hour.