Francesco Grespan

An α-drift tube linac (DTL) is a high-performance linac conceived for radioisotope production. The use of alpha particle beams may make yielding radionuclides hard to obtain with traditional nuclear reactors or by proton accelerators, by exploiting new reaction routes. This approach may lead to better radionuclide impurity profiles, simplifying the radiochemical separation and purification process. The key idea of the α-DTL is to use a high duty cycle linac composed of an electron cyclotron resonance ion source, a radio frequency quadrupole, and a DTL, able to accelerate an average current of 0.5 mA alpha beam from few MeV to 40 MeV, to cover the cross sections of many interesting reactions for radionuclides. The innovative feature of this linac is the capability of regulating energy at the exit of the DTL, by a particular use of the stabilization system (post couplers) of the DTL cavity.

Machine Learning (ML) techniques are increasingly being adopted in particle accelerator operations to enable efficient control of complex systems. At INFN–LNL, we investigated both offline and real-time ML-driven approaches to enhance beam quality, reduce setup time, and improve reliability across different accelerator facilities. As part of this effort, we developed Adaptive Region Bayesian Optimization (ARBO), a custom Bayesian Optimization algorithm that dynamically expands its search domain when the predicted optimum approaches a boundary. Offline studies applied ARBO to the design optimization of the medium-energy beam transport line of the ANTHEM BNCT facility. Real-time online tests demonstrated the effectiveness of ARBO. At PIAVE–ALPI, the combined transmission improved from 44.2% to 52.6%, corresponding to an ALPI-only increase from approximately 69% to 82%, approaching the theoretical maximum of 93%. At the ESS normal-conducting linac, ARBO enabled the simultaneous tuning of more than 50 control elements while improving transmission and maintaining stable trajectory correction. These results indicate that adaptive optimization strategies can substantially improve accelerator performance and support future advances in ML-assisted accelerator operations.

On-line monitoring of beam phase and energy gain in the cavity is of great importance for the stable operation of accelerators, especially in high-power scenarios. As cavities are typically calibrated with a low-current, short-pulse beam for safety purposes, the beam behavior may deviate from the calibrated case when running high-current beams, due to the strong space charge effect and other possible factors. Motivated by the high-power operation demand at European Spallation Source and the further understanding of beam-cavity interaction, methods for beam phase and energy gain on-line measurement have been developed. The beam phase was measured based on the steady-state cavity model, as demonstrated in the half wave resonator cavity at CAFE2. In this paper, this method was not only applied to a single-cell buncher cavity but also, for the first time, extended to the more complex low-energy multicell RFQ cavity for verification. Additionally, an energy-conservation-based method for measuring beam energy gain in a cavity was proposed and validated.

The performance and stability of radio-frequency quadrupoles (RFQs) depend critically on the structural integrity of their electrodes (vanes or rods), which are subjected to high-intensity ion beam exposure during accelerator operation. This study investigates the sputtering induced vane erosion on the European Spallation Source (ESS) RFQ, a critical component of the ESS linear accelerator. Particle tracking simulations for the determination of RFQ beam loss profiles were coupled to rim software (static-dynamic transport of ions in matter sequential-parallel processing) modeling to estimate the sputtering yield distribution and the resulting erosion rates under nominal operating conditions. Methodology validation and benchmarking was performed using quartz crystal microbalance (QCM) measurements employing proton (2, 6 keV) and argon ( Ar + 2 keV, Ar 6 + 36 keV) ion beams with incident angles of 0°–70°. The QCM experiments were used to benchmark rim software simulation parameters for the calculation of sputtering yields and total electrode erosion rates across the energy and angular ranges relevant to the ESS RFQ. The results and conclusions are extended to RFQs employed for heavy-ion acceleration providing insights into long-term stability and performance degradation. The results indicate that heavy-ion irradiation leads to significantly higher sputtering yields and erosion rates compared to protons. It is estimated that the increased erosion rates result in critical frequency perturbations in RFQ cavities especially in high current or cw configurations that need to be effectively compensated.

The ANTHEM (Advanced Technologies for Human-centered Medicine) Radio-Frequency Quadrupole (RFQ) will employ eight coaxial power couplers, which will be magnetically coupled to the device through a loop antenna. The coupler design can support up to 140 kW in continuous wave operation. This paper presents the design of the cavity used for high-power testing, with the primary objectives of both optimizing the coupling between the couplers and ensuring operations at the designated operating frequency. Furthermore, the paper encompasses thermal and structural assessments conducted through numerical simulations.