For high-intensity linear accelerators, space-charge halo mechanisms are largely classified into two families: particle resonances and parametric instabilities. The dominance between the fourth-order particle resonance and the envelope instability has been argued and studied. Our studies and previous literature indicate the dominance of particle resonances over parametric instabilities in high-intensity linear accelerators. Any counter evidence has not been found yet. Furthermore studies indicate that parametric instabilities except the envelope instability are unlikely to be observed in actual linear accelerators unless waterbag or KV distributions are generated. We propose a way to overcome the previous design rule to avoid the zero-current phase advance > 90° for the high-intensity linac. The interplay is presented of the envelope instability and the fourth-order parametric instability.

The linear accelerator RAON consists of an injector and a superconducting linac. The injector contains two ECR ion sources and an RFQ. These ion sources produce various ions from protons (A/Q=1) to uranium (A/Q=7.2), with an energy of 10 keV/u. The RFQ accelerates these ions to an energy of 500 keV/u. The superconducting accelerator consists of two types of superconducting cavities (QWR and HWR). The linac is designed to accelerate uranium beams to 18.5 MeV/u. The beam commissioning of the injector system started in August 2021 with various ions (argon, oxygen, neon, helium, proton). The beam commissioning of the superconducting linac started in October 2022 with argon beams. This work summarizes the current status of the beam commissioning of the RAON linac.