Development of a compact cavity BPM for real-time monitoring of clinical proton beams at HUST-PTF

Jiqin Li¹Yuexin Lu¹Jiapeng Li¹Jian Wang¹Zhengzheng Liu¹Oleg Meshkov²Jinfeng Yang³kuanjun Fan^1*

• ¹State Key Laboratory of Advanced Electromagnetic Technology, Huazhong University of Science and Technology, Wuhan, China
• ²Budker Institute of Nuclear Physics (RAS), Novosibirsk, Novosibirsk Oblast, Russia
• ³Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka, Japan

To ensure the safety and efficacy of precise proton therapy, real-time and non-intrusive monitoring of the clinical beam position is essential. However, in cyclotron-based proton therapy facilities, clinical proton beams with low repetition frequency and exceptionally low intensity due to the Energy Selection Systems (ESS), pose considerable challenges for accurate online beam diagnostics. Conventional non-interceptive beam diagnostic devices lack the sensitivity required to detect such weak beams with sufficient precision. This paper presents an innovative solution to this challenge: an off-center rectangular cavity Beam Position Monitor (BPM) with dielectric loading. This novel design achieves remarkable position sensitivity while maintaining compact transverse dimensions of 500×250×100 mm. A prototype of this cavity has been fabricated and tested offline. Experimental results demonstrate that, within the clinical treatment energy range, the BPM achieves minimum beam position measurement sensitivities of 0.49 nV/mm at 70 MeV and 17.12 nV/mm at 230 MeV. In addition to enabling online beam position monitoring without disturbing the beam path, which ensures real-time beam orbit feedback correction with submillimeter stability (±0.5 mm), the BPMs could allow verification of beam energy through the phase of the BPM signal.