Study of Beam Loss by Radial Oscillation in Compact High-current Cyclotron

High-current cyclotrons are widely utilized because of their high intensity and power. China Institute of Atomic Energy (CIAE) has designed and constructed an 18 MeV/1 mA high-current proton cyclotron, which has been used in fields such as boron neutron capture therapy (BNCT) and neutron imaging. Beam loss is one of the major concerns for high-current cyclotrons. It represents a critical challenge in the physical design of high-current cyclotrons, as it leads to vacuum deterioration, elevated residual activity levels, and increased maintenance difficulty. In high-current cyclotrons, the main mechanisms of beam loss include beam dynamics problems (e.g., coupling resonance crossing), residual gas stripping, electromagnetic (Lorentz) stripping, and intra-beam stripping. In this analysis, only beam dynamics issues, particularly the Walkinshaw resonance, are considered. In the cyclotron, the beam center undergoes radial oscillation around the equilibrium orbit. The amplitude of radial oscillation is one of the significant factors leading to beam loss. To investigate the impact of radial oscillation on beam loss and beam quality in the high-current cyclotron, beam dynamics simulations were conducted using non-ideal bunches extracted from the central region with varying amplitudes of radial oscillation. The beam loss with different radial oscillation amplitudes was compared by numerical simulations in the Walkinshaw resonance region and the extraction region. In a cyclotron, particles undergo axial oscillation vertically about the median plane. When the beam passes through the Walkinshaw resonance, radial oscillations couple into the axial motion, leading to an increase in axial emittance. The axial oscillation amplitude increases with the radial oscillation amplitude of the bunch. z_rms is the root-mean-square axial oscillation. For example, when the radial oscillation is 2 mm, z_rms lies in the interval 1.5-1.8 mm; and at 10 mm radial oscillation, z_rms reaches 1.0-4.2 mm. The larger the radial oscillation, the stronger the axial oscillation. Most stripping points for particles within the bunch are distributed within a radial range of 5 mm and an axial range of ±8 mm. As the amplitude of radial oscillation increases, the spatial distribution of stripping points broadens, leading to a higher proportion of stray particles and an increase in the energy spread of beam. A higher amplitude of radial oscillation leads to an increased beam loss rate. The case of a 10 mm radial oscillation was analyzed to illustrate this effect. Pronounced beam losses are identified in specific regions: the fringe fields of the magnet poles, the accelerating cavities, and the vacuum chamber. For a beam with 10 mm radial oscillation, total beam loss is 3.57%, with 1.77% lost in the Walkinshaw resonance region. The exit point is designed based on the radius of 1 000 mm. Beam emittance increases with larger radial oscillation amplitudes. For bunches with radial oscillations of 2 mm and 10 mm, the normalized emittance was 1.02 mm·mrad and 13.34 mm·mrad in the x, and 0.61 mm·mrad and 1.54 mm·mrad in the z. Based on the simulation results, the maximum allowable radial oscillation for the 18 MeV cyclotron is determined to be 8 mm. Exceeding this limit leads to degraded extracted beam quality and a significant increase in the proportion of beam loss. For a beam bunch undergoing radial oscillations with an amplitude of 10 mm, the total beam loss is 3.57%, with 1.77% occurring axially within the Walkinshaw resonance region.