Application of autoresonance in rapid beam extraction of synchrotrons

Abstract

In recent years, ultra-high dose rate (FLASH) radiotherapy has become a novel cancer treatment technique because of its similar tumor-killing efficacy as conventional particle therapy while significantly protecting normal tissues. However, due to the limitation of particle number, achieving FLASH condition in a compact heavy-ion synchrotron requires a short extraction time of tens of milliseconds, which is challenging for the conventional RF-KO method. To tackle this challenge, we introduce autoresonance into the third-order resonant extraction for the first time, offering an alternative to the conventional approach of merely increasing the excitation strength. By leveraging a strong detuning effect, a frequency sweeping excitation with small amplitude can drive the entire beam into the autoresonant state, thus enabling rapid beam extraction within a single sweeping period. Compared with the conventional method, this innovative method requires only the addition of an octupole magnet. At the same time, it shows that the conventional RF-KO method has a high autoresonance threshold, so that only a small number of particles that meet the threshold can be excited to large amplitude and be extracted in each sweeping period. In this paper, the autoresonance threshold of a particle in the presence of sextupole and octupole magnetic fields is analyzed, and the single particle simulation shows good agreement with the theoretical formula. Furthermore, the autoresonance based rapid extraction process is simulated and studied, revealing the possibility of millisecond scale beam extraction.