Physical design and tuning of the long radio frequency quadrupole for accelerator-based boron neutron capture therapy

A continuous-wave radio-frequency quadrupole (RFQ) accelerator for boron neutron capture therapy (BNCT) has been designed, tuned, and experimentally validated. The RFQ delivers a beam energy of 2.8 MeV with a design input current of 25 mA. Optimized beam dynamics resulted in a transmission efficiency above 99.66% and a transverse emittance growth below 5.5%. The RF structure employs optimized cross-section and undercut geometries, achieving a cavity quality factor of 14, 286. π-mode stabilizing loops were introduced to enhance mode separation, and 100 tuners were implemented for field and frequency control. Multiphysics simulations predict a maximum temperature rise of 32.5 °C and a frequency shift of −91 kHz under CW operation. During low-power measurements and full-cavity tuning, a sag compensation method based on catenary fitting was developed to correct gravity-induced field distortions. After tuning, the operating frequency was set to 165.118 MHz, with dipole and quadrupole perturbative components controlled within ±1% and ±1.2%, respectively, and the measured Q value reaching 87% of the simulated value. Error analysis indicates that, within tuning tolerances, the transmission efficiency remains above 99.5%, the beam centroid shift is below 0.05 mm, and the output energy variation is less than 0.01 MeV. Beam commissioning demonstrated a output peak current of 22.1 mA and a transmission efficiency exceeding 94% at 125 Hz repetition rate and pulse width of 900 μs. Time-of-flight measurements yielded a proton energy of 2.812 ± 0.002 MeV, confirming the reliability of the RFQ design and tuning.