CW RFQ design and investigation for multi-charge-state acceleration of radioactive beams from BISOL

A continuous-wave (CW) heavy-ion radio frequency quadrupole (RFQ) has been designed to accelerate radioactive beams from the Beijing Isotope Separation On-Line (BISOL) facility. This RFQ will accelerate high-charge-state ions such as Sn-132(21+) from 3 keV/u to 300 keV/u with a vane length of 3.77 m at a frequency of 81.25 MHz. The transmission efficiency reached 98.1% in simulation. The output longitudinal normalized rms emittance is 0.31 keV/u.ns. Tolerance analysis shows that this RFQ can handle a wide range of non-ideal beams while retaining a relatively high transmission efficiency and low longitudinal emittance. In order to increase the intensity of the radioactive ion beams (RIB), this RFQ will accelerate multi-charge-state beams simultaneously. In the case of the tin beam (Sn), we have studied the transmission of adjacent charge state beams and multi-charge-state beams in this RFQ. The transmission efficiency and output transverse beam quality of the multi-charge-state beams are close to the results of the single-charge-state beam, but the output longitudinal emittance is larger. For the radio frequency (RF) structural design, we have compared different types of RFQ, including an DI RFQ, a 4-rod RFQ and a 4-vane RFQ. We have selected a 4-vane RFQ design with dipole stablizer rods, due to the considerations of power consumption, the flatness of electric field, cooling convenience and machining efficiency. We have performed full 3D simulations with multi-physics analysis.