Medium Energy Beam Transport Research Articles

Design and commissioning of a two-harmonic pre-buncher for extended bunch spacing at LEAF

The Low Energy Heavy Ion Accelerator Facility (LEAF) is a low-energy, high-intensity heavy-ion linear accelerator facility at Institute of Modern Physics (IMP) for multidiscipline research. The beam bunch repetition rate coincides with the LINAC frequency of 81.25 MHz, resulting in a 12.3 ns period. However, in the context of certain nuclear physics experiments, a longer bunch spacing is deemed essential for the precise detection of experimental products. To address this need, a strategy to extend the bunch spacing to 98 ns has been proposed by integrating a pre-buncher operating at a fundamental frequency of 10.156 MHz before the radio-frequency quadrupole (RFQ) in conjunction with a fast chopper in the medium energy beam transport (MEBT) system. The pre-buncher is a pivotal component in enabling a greater beam current at the target, surpassing the capability of employing a chopper alone. To enhance the bunching efficiency, the pre-buncher operates with two frequencies: the fundamental frequency of 10.156 MHz and its first sub-harmonic of 20.3125 MHz, each of which is produced by a coaxial resonator. This paper describes the design, manufacture, off-line testing, and beam commissioning of the pre-buncher.

SPIRAL2 commissioning and operations

The SPIRAL2 linac is now successfully commissioned; H+, 4He2+, D+ and 18O6+ have been accelerated up to nominal parameters and 18O7+ and 40Ar14+ beams have been also accelerated up to 7 MeV/A. The main steps with 5 mA H+, D+ beams and with 0.6 mA 18O6+ are described. The general results of the commissioning of the RF, cryogenic and diagnostics systems, as well as the preliminary results of the first experiments on NFS are presented. In addition of an improvement of the matching to the linac, the tuning procedures of the 3 Medium Energy Beam Transport (MEBT) rebunchers and 26 linac SC cavities were progressively improved to reach the nominal parameters in operation, starting from the classical “signature matching method”. The different cavity tuning methods developed to take into account our particular situation (very low energy and large phase extension) are described. The tools developed for an efficient linac tuning in operation, e.g. beam energy and intensity changes are also discussed.

Design and beam dynamics simulation of an 8 MeV compact accelerator-driven neutron source

A compact accelerator-driven neutron source is proposed at Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-Sen University, called Sun Yat-Sen University Proton Accelerator Facility (SYSU-PAFA). The proton accelerator is composed of a proton electron cyclotron resonance source, a four-vane radio frequency quadrupole (RFQ), and an alternative phase focusing drift tube linac (APF-DTL). It can accelerate 10 mA proton beam to 8 MeV. Due to the high current, beam matching is particularly important. In order to achieve beam matching between various components, beam transport sections are needed. The beam transport line is divided into three segments. The Low Energy Beam Transport (LEBT) ensures that the beam parameters are matched before entering the RFQ. The Medium Energy Beam Transport (MEBT) segment efficiently transfers the beam between the RFQ and DTL. The High Energy Beam Transport (HEBT) focuses on transporting the beam to the targets. The design goal of beam transport line is as short as possible while ensuring high efficiency of beam transportation. SYSU-PAFA has an overall transmission efficiency of 99%, with optimal transverse matching conditions between beam transport and RFQ or DTL accelerators. The efficient use of solenoids and magnets allows for a compact transmission section, resulting in a total length of 13.6 meters, shorter than most accelerators at the same beam energy. This paper will provide the detailed beam dynamics of the compact accelerator.

Quadrupole multiplet for medium energy beam transport for proton improvement plan −2

The Proton Improvement Plan-II (PIP-II) is a proposed upgrade for the accelerator complex at Fermi National Accelerator Laboratory, Batavia, USA. The injector experiment called the PIP-2 Injector Test (P2IT) is a 20 MeV front end comprising a warm front end and superconducting cryomodules. Quadrupole Multiplets for MEBT of P2IT are designed, developed, and qualified by Bhabha Atomic Research Centre (BARC) which is the lead laboratory for the design, development, and qualification of all warm magnets and superconducting solenoid magnets of Proton Improvement Plan – 2 under Indian Institute and Fermilab Collaboration (IIFC). Quadrupole doublets and triplets for MEBT of P2IT are beam commissioned at FNAL and went through a series of beam trials and experiments. This paper describes the design, development, qualifications, and beam experiment results of MEBT Quadrupole multiplets.

Design of the gradient dipole magnet for LLICTF

The Lanzhou Light Ion Cancer Therapy Facility (LLICTF) is a compact medical accelerator currently under construction. It is designed to treat cancer using a 230 MeV, 30 mA H+ beam and a 85 MeV/u, 1 mA 3He2+ beam. The facility comprises two ion sources, a low-energy beam-transport (LEBT), a Radio Frequency Quadrupole (RFQ), a medium-energy beam-transport (MEBT), and the main ring accelerating structure. Due to the presence of two ion sources, it is necessary to introduce a dipole magnet which is symmetrically focused as much as possible to meet the focusing requirements of the LEBT beam. Therefore, a gradient dipole magnet has been designed to achieve this symmetrical focusing. This paper discusses the theoretical and simulated symmetric focusing of the gradient dipole magnet. It also analyzes the effect of fringe fields and space charge. Additionally, this paper presents the results of the model design with CST and the multi-particle simulation results with TraceWin.

Measurement and Analysis of the Intensity-dependent Effects on the CSNS Medium Energy Beam Transport line

A power upgrade project (CSNS-II) has been approved in 2021 to increase the beam power to 500 kW for the China Spallation Neutron Source (CSNS), for which the Linac energy will reach 300 MeV and the beam intensity is expected to be 50 mA. In this study, the beam measurement results given by the wire scanners at various current intensities, and the numerical modeling and fitting methods to obtain the evolution of the beam envelope and emittance along the CSNS medium energy beam transfer (MEBT) are given and discussed. Finally, new matching results have been given to control the emittance blowup.

Upgrade of the medium energy dump geometry for the SPIRAL2 single bunch selector

The medium energy beam transport of the SPIRAL2 superconducting linac contains a single bunch selection system equipped with a 7.5 kW beam dump. This device, originally designed with a long flat slope to decrease the power density so that the maximum operating temperature was 170 °C, was impacted by Coulomb scattering generating two side effects: heating of the downstream beam transport components and degrading of the beam current measurement uncertainty. The paper relates the way these two problems were solved.

Beam commissioning of the SARAF Injector and MEBT (protons)

The SARAF Phase II cryogenic linear accelerator (linac) is currently under construction and will be commissioned through collaborative efforts between SNRC and CEA. The linac will accelerate a 5 mA pulsed/continuous wave (CW) proton and deuteron beam, reaching energies of up to 35 MeV and 40 MeV, respectively. The linac injector consists of Phase I components, including an ECR ion source, a Low-Energy Beam Transport (LEBT) line, and a 4-rod Radio Frequency Quadruple (RFQ). Additionally, a new Medium Energy Beam Transport (MEBT) line has been installed and integrated into the injector infrastructure. The MEBT includes three rebuncher resonators, magneto-optical elements, and beam diagnostics. The MEBT beam diagnostics, as well as the Phase I D-plate installed downstream, were used for beam characterization. The downstream-installed Temporary Beam Line (TBL) and a prototype of the Gallium Indium Liquid Target (GALIT) were used for commissioning, operating at high beam duty cycles, including CW mode. The report describes the proton beam commissioning process of the injector prior to the delivery of the cryomodules, scheduled for the second half of 2023.

Design, fabrication, and characterization of a drift tube linac type double gap re-buncher cavity.

The Low Energy High Intensity Proton Accelerator (LEHIPA) at Bhabha Atomic Research Centre, India, has recently been commissioned to the target energy of 20MeV by beam acceleration through a 352MHz radio frequency quadrupole (RFQ) and drift tube linac (DTL). The medium energy beam transport (MEBT) line of LEHIPA matches the 3MeV beam from RFQ to the DTL using four electromagnetic quadrupoles and a re-buncher cavity. In the beam transport lines of high frequency linacs, TM010 mode single gap buncher cavities are conventionally used, while a double gap re-buncher cavity was chosen for the short length LEHIPA MEBT based on beam dynamics simulations. The 352MHz double gap re-buncher cavity has been designed in a DTL type geometry with a cell length of βλ. This double gap cavity is found to be more power efficient with a higher shunt impedance and transit time factor than the conventional single gap buncher cavity. Electromagnetic design simulations, fabrication details, low power and high-power RF test results, and beam test results of the re-buncher cavity are presented in this paper.

Results of on-line orbit correction at the medium energy beam transport section of the RAON accelerator

Results of on-line orbit correction at the medium energy beam transport section of the RAON accelerator

Clustering fusion algorithm for selection of historical beam steady-state transmission data in CAFe facility

Accelerator-driven systems (ADS) are promising technologies for nuclear waste transmutation and energy production. The China ADS Front-end Superconducting Demo Linac (CAFe) is a prototype of the China Initiative Accelerator Driven System (CiADS), which aims to verify the feasibility of key technologies of CiADS. In this article, a novel method for historical data screening of the beam transport in the medium energy beam transport (MEBT) section of CAFe is presented. A clustering fusion algorithm based on unsupervised learning and beam transmission characteristics is designed to extract a large number of samples with the beam in steady-state transmission from historical beam data. A deep neural network model was constructed to fit the beam transport characteristics and verify the reliability of the screened data samples. The method can improve the efficiency and accuracy of data analysis and provide valuable insights for the optimization and control of beam transport in CAFe.

Surrogate model of particle accelerators using encoder–decoder neural networks with physical regularization

Accelerator engineering could benefit from faster and higher-quality physics simulations. Machine learning has emerged as a promising tool for developing accelerator simulation programs that are both fast and accurate. In this study, we propose a surrogate model based on encoder–decoder neural networks. We incorporate physical regularization into the loss function, which allows us to integrate prior physical knowledge into the deep learning network. The advantage of this regularization is that it ensures the results are more consistent with the underlying physical laws. The method was applied to beam dynamics modeling of the medium energy beam transport (MEBT) section in the China Accelerator Facility for Superheavy Elements (CAFe II). The final results indicate that after training, the network maintains a mismatch, emittance difference, and transmission efficiency of approximately 0.01. Our scheme has been demonstrated to be effective in the simulation of the accelerator’s beam dynamics.

Improvement of multi-harmonic buncher’s quality factor for RAON

The high-intensity rare-isotope accelerator RAON, a component of the Rare Isotope Science Project, was developed to accelerate heavy ions from elements ranging from proton to uranium. The injector line consists of an Electron Cyclotron Resonance Ion Source, a Low Energy Beam Transport line, a Radio Frequency Quadrupole (RFQ), and a Medium Energy Beam Transport line. To improve beam quality, a multi-harmonic buncher (MHB) was proposed for inclusion before the RFQ. The MHB is needed to efficiently utilize two beams with different mass-to-charge (A/q) ratios. The fundamental frequency of the MHB is half the frequency of the RFQ; as such, a harmonic synthesis is effectively bunching a beam. Electric field simulations of the MHB were conducted using the CST Microwave Studio (MWS). The electrodes were designed as cone- type electrodes so that the electric field can be concentrated between the electrodes through which the beam passes. The electric field was optimized to account for structural changes such as gap length between electrodes, resonator position and other design details. The model optimized in the simulation tool was engineered through the 3D Inventor program.

Orbit correction based on improved reinforcement learning algorithm

Recently, reinforcement learning (RL) algorithms have been applied to a wide range of control problems in accelerator commissioning. In order to achieve efficient and fast control, these algorithms need to be highly efficient, so as to minimize the online training time. In this paper, we incorporated the beam position monitor trend into the observation space of the twin delayed deep deterministic policy gradient (TD3) algorithm and trained two different structure agents, one based on physical prior knowledge and the other using the original TD3 network architecture. Both of the agents exhibit strong robustness in the simulated environment. The effectiveness of the agent based on physical prior knowledge has been validated in a real accelerator. Results show that the agent can overcome the difference between simulated and real accelerator environments. Once the training is completed in the simulated environment, the agent can be directly applied to the real accelerator without any online training process. The RL agent is deployed to the medium energy beam transport section of China Accelerator Facility for Superheavy Elements. Fast and automatic orbit correction is being tested with up to ten degrees of freedom. The experimental results show that the agents can correct the orbit to within 1 mm. Moreover, due to the strong robustness of the agent, when a trained agent is applied to different lattices of different particles, the orbit correction can still be completed. Since there are no online data collection and training processes, all online corrections are done within 30 s. This paper shows that, as long as the robustness of the RL algorithm is sufficient, the offline learning agents can be directly applied to online correction, which will greatly improve the efficiency of orbit correction. Such an approach to RL may find promising applications in other areas of accelerator commissioning.

Online beam orbit correction of MEBT in CiADS based on multi-agent reinforcement learning algorithm

The proton linac accelerator has been the mostly appropriate equipment applied to radioactive waste disposal and cancer treatment. However, beam offset is a particularly significant problem during proton acceleration, which not only produces a halo and causes orbital distortion of the beam to make the beam injection inefficient. In exceptionally complex accelerator systems, accurate correction of beam offset is very difficult. The traditional method is based on manual experience and manual adjustment to achieve beam trajectory correction, the automation level is low and the adjustment speed is slow. Therefore, this paper proposes a new method of beam orbit correction based on Multi-Agent Reinforcement Learning. In order to verify the feasibility of the algorithm in beam orbit correction, this method takes the MEBT section of the virtual accelerator as the control object, and uses three agents to control the beam orbit of Medium Energy Beam Transport (MEBT) in sections. Each agent is designed based on the Reinforcement Learning of the Deep Deterministic Policy Gradient (DDPG) algorithm, the deterministic strategy is used to explore the large state and action space to realize discretization, and the training model convergence is successfully realized. The experimental results show that the final calibrated average beam offset of this method is about 0.39 mm, which achieves the expected experimental goal. It can control the beam orbit of MEBT, and has application value and competitiveness in the beam orbit correction of linac.

Design of a CW BISOL RFQ for Three Kinds of High-Charge-State Ions Simultaneous Acceleration

Based on the latest design requirements proposed by the Beijing On-Line Isotope Separation (BISOL) project, a new Sn22+-based, 81.25 MHz CW radio frequency quadrupole (RFQ) with external bunching has been designed. This RFQ can accelerate Sn22+ to 0.5 MeV/u with an output longitudinal-normalized rms emittance of 0.20 keV/u·ns over a length of 5.6 m. The tolerance and error analysis results indicate that this RFQ can handle a wide range of non-ideal beams while maintaining relatively lower longitudinal emittance growth and higher transmission efficiency. To maintain the beam intensity, the RFQ will simultaneously accelerate three kinds of high-charge-state mixed ions (132Sn21+, 132Sn22+ and 132Sn23+), the simulation results given by Impact-T show that the RFQ can achieve high transmission of the mixed beam. Compared with the previous Sn21+-based internal bunching RFQ scheme, this RFQ has a shorter length and smaller output emittance, which is beneficial to the designs of subsequent Medium-energy Beam Transport (MEBT) and Drift Tube Linac (DTL). In electromagnetic design, a four-vane structure with 48 tuners and 16 π-mode stabilizers (PSLs) were chosen. The results of the multi-physics analysis show that the maximum temperature rise and the maximum deformation of the cavity are 13.6 K and 40.3 µm, respectively. The results simulated with CST Microwave Studio (CST) and HFSS software were consistent.

Neutron production measurement in the 125 mA 5 MeV deuteron beam commissioning of Linear IFMIF Prototype Accelerator (LIPAc) RFQ

The validation of the Linear IFMIF Prototype Accelerator (LIPAc) is being conducted in Rokkasho Fusion Institute of QST, Aomori, Japan. The beam commissioning of the novel LIPAc RFQ has been progressing significantly, and the acceleration of the world highest current deuteron beam of 125 mA to 5 MeV (1 ms, 1 Hz pulsed beam) was achieved for the 1st time in July 2019. This study is devoted to evaluating the amount of beam loss during the high current acceleration phase by measuring the neutron produced by the interaction of the beam with accelerator components so as to verify the accelerator design. The foil activation method and the 3He neutron probes were utilized. It turned out that there was no significant trace of unexpected beam loss at the high energy section of the RFQ and the medium energy beam transport (MEBT) section. The measurement using the 3He neutron probes provided an evidence that the beam loss was well controlled in the RFQ cavity as designed. This study concludes that the LIPAc RFQ as well as MEBT is working well as predicted by the design calculation, and its concept is successful.

Magnetic analysis and cross-talk fields for the ESS MEBT quadrupole magnet

The focusing lattice of the Medium Energy Beam Transport (MEBT) section of the European Spallation Source (ESS) linac is formed of 11 identical quadrupole magnets, the design of which was determined by beam requirements, spatial limitations and nearby instrumentation. Each magnet is only 89 mm long, produces quadrupole fields above 2.74T, and includes embedded steerer coil systems that excite dipole fields of up to 20G m. In this work, a comprehensive magnetic analysis of the ESS MEBT magnet will be presented through a combination of simulations and measurements. A magnetic model was updated and benchmarked with measurements. An integrated steerer system with coils wound on its return-arms was used to simplify mechanical design and improve magnetic field characteristics in comparison to others with coils integrated into their poletips. Steering field strength and harmonic content were studied versus quadrupole field excitation. Cross-talk fields were investigated for an MEBT doublet magnet system with magnetically shielded diagnostics positioned between its magnets. Due to this magnetic shielding, integrated quadrupole field was decreased by 3% and steering fields by 20% with respect to values calculated using a single magnet model.

RF design studies on a 325 MHz rebuncher cavity for high intensity linear accelerators producing neutrons

Two identical Coupled Cavity Linac type (CCL) rebuncher cavities to be integrated in the Medium Energy Beam Transport line (MEBT) of a high intensity linac Accelerator have been studied for matching a 40-mA proton beam accelerated by a 3 MeV Radio-Frequency Quadrupole (RFQ) to a 13 MeV Drift Tube Linac (DTL). The rebuncher operates at 325 MHz, and is designed for β = 0.080. The rebuncher aperture radius is 20 mm required by transverse beam dynamics. It is a single gap structure operating in the TM_010 mode for providing longitudinal beam focusing. The unloaded quality Q0 and the required input power to achieve an effective voltage of 150 kV are 27253 and 20.5 kW, respectively. In this work, we present the RF design studies for the high power rebuncher resonator, as well as the considerations of the tuning scheme and the coupler design.

Study and test of the rebuncher for SARAF-LINAC phase II

In order to match the beam exiting the RFQ to the superconducting cavities and to minimize the longitudinal beam extension, three rebunchers will be installed in the Medium Energy Beam Transport (MEBT) of SARAF-LINAC phase II. CEA is in charge of the design and fabrication of the copper-plated stainless steel, 3-gap rebunchers. Beam dynamic simulation leads to the global MEBT design. The rebuncher was successfully conditioned to the RF power of 4.4 kW required for CW operation, with sufficient power margin. A solid state power amplifier of 10 kW developed by SNRC is used during the RF test. The rebuncher shows a good performance in terms of the dissipated power, peak temperatures and vacuum level.