Radio Frequency Quadrupole Research Articles

Novel design of a high-frequency radio frequency quadrupole accelerator for medical application

Proton therapy is an advanced particle radiotherapy technology. Lanzhou University recently designed a new linear accelerator system for proton therapy. In the system, a 750[Formula: see text]MHz radio frequency quadrupole (RFQ) accelerator was used to complete the initial acceleration of particles. The RFQ can accelerate a proton beam intensity of 1[Formula: see text]mA from 30[Formula: see text]keV to 3[Formula: see text]MeV kinetic energy within a length of 2[Formula: see text]m. This paper aims to complete the physical design of this high-frequency RFQ, including beam dynamics design, radio frequency (RF) design, and multiphysics analysis. In the dynamics design, a novel design strategy for controlling the longitudinal emittance was adopted to improve the output beam quality of the RFQ. The simulation results showed that the RFQ outlet longitudinal emittance was controlled below 0.1 [Formula: see text] mm mrad. In the RF design, the pi-mold rod structure was applied to RFQ at such high frequencies for the first time. After the whole cavity simulation, it was found that the separation between the operating frequency of the RFQ cavity and its closest dipole mode reached 58.3[Formula: see text]MHz, and its quality factor reached 6352. Finally, a multiphysics analysis was performed. The analysis showed that the maximum temperature rise of the RFQ cavity was less than [Formula: see text]C and the frequency drift due to heating was 18[Formula: see text]kHz. This paper presents the current status of the design of this new linear accelerator. Compared with previous RFQs, the new RFQ has a very high transmission efficiency and higher RF stability.

RF electromagnetic design and multi-physics analysis of CW CH-DTL for AB-BNCT

The accelerator-based boron neutron capture therapy (AB-BNCT) facility is being developed worldwide due to its advantages, including easy maintenance, low construction costs, and high safety. To minimize both the length and cost of the accelerator, we propose a combined acceleration structure that utilizes a radiofrequency quadrupole linac (RFQ) in conjunction with a crossbar H-mode drift tube linac (CH-DTL). The CH-DTL rapidly accelerates the beam to the required energy after the RFQ, with output energy adjustable from 2.2 MeV to 3.2 MeV. We have completed the RF electromagnetic design and optimization of the CH-DTL cavity to fulfill beam dynamics requirements while achieving high effective shunt impedance. Additionally, we designed the cooling channels and conducted multi-physics analyses to evaluate the cavity's performance during continuous wave operation.

Beam dynamics design and optimization of a standalone radio frequency quadrupole accelerator for swift heavy ions

Abstract A standalone high energy, 352 MHz radio frequency quadrupole (RFQ) design is presented, resulting into targeted energy gain of 1.8 MeV/u with a beam current of 1 mA for mass to charge (A/q) ratio ≤ 4. It avoids many separate stages of transverse and longitudinal matching of beams compared to a standard RFQ + DTL accelerator combination due to automatic adiabatic bunching, focusing and acceleration of the beam and thus prevent many beam diagnostic and vacuum components. It is compact (only 4.63 meters) comparatively to other accelerator of equivalent energy gain and operation is quite easy in terms of beam tuning. The beam dynamics and vane designs are presented in details using PARMTEQ and Poisson/Superfish code. Such an accelerator promises to be a good candidate for establishing compact and more efficient nuclear physics research facilities compared to electrostatic Pelletron accelerators.

Beam dynamics studies on a specific radio-frequency quadrupole injector for superconducting linacs

Radio-frequency quadrupole (RFQ) linear accelerators are widely used at the front end of large-scale hadron accelerator facilities owing to their extraordinary performance of progressive bunching, high-efficiency acceleration and low emittance growth. Meanwhile, it is worth noting that RFQ linacs gradually become the only normal conducting accelerators in modern continuous-wave high-power superconducting hadron linacs. As a result, high-quality beams at the exit of RFQ linacs with respect to more strict beam dynamics requirements are demanded, where minimizing the emittance growth is one of the most critical design targets. Not only a small rms emittance is required for an easy rms match to the following superconducting cavities but also a small total emittance is needed to reduce the beam loss in the superconducting cavities. In this paper, the design parameters of some typical RFQ injectors for superconducting linacs worldwide were compared, and the beam dynamics design of a 162.5 MHz, 2.1 MeV, continuous-wave RFQ linac was performed. Guided by the design approach called MEGLET (Minimizing Emittance Growth via Low Emittance Transfer), the beam dynamics design of the example RFQ injector was completed with high transmission efficiency, low emittance growth, and large error tolerance. The design result could be a reference for similar RFQ linacs, and the beam dynamics design approach is useful and applicable for other linacs intended to improve the figures of merit.

LSTAR — An isobar separator for expanding radioactive ion beam production at the Cyclotron Institute, Texas A&M University

A new isobar separator, LSTAR (Light-ion guide Separator for Texas A&M’s Radioactive ion beams), has been designed to purify the exotic beams produced by a He-driven light-ion guide (He-LIG) system at Texas A&M University, primarily for the TAMUTRAP experiment. The main purpose of TAMUTRAP is to probe for physics beyond the standard model by searching for possible scalar or tensor currents in the weak interaction using nuclear β decay. The proton-rich isotopes of interest for this program will be produced at low yields, requiring efficient reduction of contaminant species to avoid overloading of TAMUTRAP’s radiofrequency quadrupole cooler-buncher. The layout, ion-optics, and specifications of LSTAR will be presented.

Beam dynamics design and error analysis of a high frequency RFQ for medical application

Proton therapy based on the linear accelerator is an advanced radiation treatment technology. Lanzhou University has proposed a linac scheme for proton therapy, which features a 750 MHz Radio Frequency Quadrupole (RFQ) as its first accelerator. The RFQ is capable of accelerating a 1 mA proton beam from 30 keV to 3 MeV. The article systematically introduces the beam dynamics design and error analysis of the RFQ. In the design part, a new strategy that can reduce the outlet longitudinal emittance of the RFQ by adjusting its modulation and synchronous phase was proposed. Subsequently, multi-particle simulations based on different codes consistently demonstrated that the dynamics design achieved a good output beam quality. The longitudinal emittance of the RFQ output beam was successfully controlled to below 0.1 π mm·mrad. In the error analysis part, firstly, all types of errors were analyzed separately to determine their impact patterns and to give preliminary tolerances. Then, the combined errors which considered all types of errors together were analyzed. Finally, the tolerances for input beam errors, cavity voltage errors, and mechanical errors were obtained.

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.

Development of a new tuning method for an radio frequency quadrupole accelerator

Tuning is a critical step in the low-power test of the radio frequency quadrupole (RFQ) accelerator. A 163 MHz RFQ for boron neutron capture therapy (BNCT) at Lanzhou University has been fully assembled. Its designed output energy is 2.6 MeV, and the designed output current in continuous wave mode is 30 mA. Before the feeding of high power, a low-power tuning was performed. First, based on the analysis of the tuning algorithm, a new tuning code capable of tuning the RFQ longitudinal field distribution to any desired target field distribution has been developed using Matlab. The tuning code mainly consisted of two parts: the data processing module and the tuning module, including sub-functions such as smoothing processing function and Fourier expansion function. Among them, the smoothing function based on the Gaussian distribution can effectively eliminate random errors in the measured voltage distribution, thereby optimizing the tuning procedure. Then, a new bead-pull system which can directly track the cavity phase shift was built. The bead-pull system was mainly composed of a vector network analyzer, a stepper motor, etc. Finally, the field unflatness of the RFQ was controlled below 1% after 4 times of tuning. The experiment also measured the radio-frequency performance of the RFQ cavity after installing the fixed copper tuners. The results showed that the RFQ operation frequency was 162.937 MHz, the unflatness of the quadrupole voltage was ±0.74%, and the asymmetry of the dipole voltage was ±1.19%, all of which met the requirements of beam dynamics.

Design of a Cavity for the High-Power Radio-Frequency Quadrupole Coupler Test for the ANTHEM Project.

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.

Injection and capture of antiprotons in a Penning–Malmberg trap using a drift tube accelerator and degrader foil

The Antiproton Decelerator (AD) at CERN provides antiproton bunches with a kinetic energy of 5.3 MeV. The Extra-Low ENergy Antiproton ring at CERN, commissioned at the AD in 2018, now supplies a bunch of electron-cooled antiprotons at a fixed energy of 100 keV. The MUSASHI antiproton trap was upgraded by replacing the radio-frequency quadrupole decelerator with a pulsed drift tube to re-accelerate antiprotons and optimize the injection energy into the degrader foils. By increasing the beam energy to 119 keV, a cooled antiproton accumulation efficiency of (26±6)% was achieved.

Development of a muon linac for the J-PARC Muon g − 2/EDM experiment

At Japan Proton Accelerator Research Complex (J-PARC), a muon linac is being developed for future muon g−2/Electric Dipole Moment (EDM) experiments. The muon linac starts with an ultra-slow muon (USM) source that generates muons with an extremely small momentum of 3 keV/c (kinetic energy W=25 meV) by laser ionization of thermal muonium. The generated USMs are accelerated to 5.6 keV by an electrostatic field and injected into a radio frequency quadrupole (RFQ). The injected muons are accelerated to 0.34 MeV by the 324-MHz RFQ. Then, the energy of the muon beam is boosted to 4.5 MeV with a 324-MHz interdigital H-type drift tube linac (IH-DTL). Following the IH-DTL, 1296-MHz disk-and-washer (DAW) structures accelerate the muon up to 40 MeV. Finally, the muons are accelerated from 40 MeV to 212 MeV using a 2592-MHz disk-loaded traveling wave structure (DLS). In this paper, details of the linac design and the recent progress toward the realization of the world's first muon linac will be discussed.

Beam Commissioning of the Heavy-ion Accelerator RAON

RAON (Rare-isotope Accelerator complex for ON-line experiments) is the superconducting linac (SCL3 and SCL2) facility in Korea. It consists of an injector, a superconducting linac, and user facilities. It is designed to accelerate various ion beams from proton (A/Q=1) to uranium (A/Q=7.2). The ECR ion source in the injector system generates ion beams with the energy of 10 keV/u. They are accelerated to the energy of 500 keV/u by the RFQ (Radio-Frequency Quadrupole) in the injector. The superconducting linac (SCL3) includes 22 QWR (Quater-Wave Resonator) cavities and 102 HWR (Half-Wave Resonator) cavities. The energy becomes 18.5 MeV/u for uranium beams after the linac. The beam commissioning of the injector started from August 2021. The first beam commissioning of the superconducting linac was performed from October 2022 to June 2, 2023. Research and development of the cavities for the high-energy part (SCL2) of the superconducting linac is in progress. This work explains some characteristic features of the beam dynamics for RAON linac and some beam commissioning results.

Field tuning study and RF electromagnetic design of a continuous-wave RFQ with ramped inter-vane voltage for AB-BNCT

A compact radio frequency quadrupole (RFQ) linac for accelerator-based boron neutron capture therapy is being developed at Xi'an Jiaotong University. By adopting a ramped inter-vane voltage in the beam dynamics design, the RFQ could accelerate a continuous wave proton beam to 2.6 MeV over a length of 4 m, effectively decreasing costs and saving space. To meet the beam-dynamics requirements, both the vane width and undercut were optimized during the electromagnetic design. In addition, π-mode stabilization loops were included to increase the mode separation, and fixed tuners with varying diameters were installed to obtain close frequency-tuning sensitivity. It was established that the field unflatness can be controlled within ±1.5% when all tuners remain at their nominal insertion depth. Optimized cooling-channel design was completed, and multi-physics analysis was conducted. The water tuning coefficients of the vane and wall channels were analyzed to establish the ability to fine tune the RFQ during operation.

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.

Extraction of ion beam from laser ion source for direct plasma injection scheme

Laser ion sources are expected to be used in various applications of heavy ion beam technology. The plasma direct injection scheme (DPIS) is a method in which ion beams extracted from a laser ion source are directly injected into a radio frequency quadrupole (RFQ) linear accelerator. In this study, a new shape of the plasma electrode with a concave surface for the DPIS was proposed to inject a converging beam to a cavity of RFQ accelerator. This approach allows the use of a large-diameter extraction electrode, which is not limited by the aperture of the RFQ electrode rods. The DPIS, using the concave surface electrode, was employed to accelerate C6+ ion beams. The results indicated that both the beam current and the number of ions increased nearly twice with the proposed electrode shape compared to values obtained with the conventional electrode. This enhancement corresponded to the increased extraction area of the beam.

Beam dynamics design and error study of the coupled structure with ladder RFQ and IH-DTL

The coupled structure with ladder radio-frequency quadrupole (RFQ) and interdigital H-type drift tube linac (IH DTL) has been proposed to accelerate the proton beam to several MeV with high acceleration gradient and one RF feed-in system. A ladder RFQ-IH DTL coupled structure was designed to accelerate a proton beam 2.5 MeV with a peak current of 15 mA. Detailed dynamics optimization and error study were performed to achieve high transmission efficiency and small emittance growth, including ladder RFQ, coupling section and IH DTL section. The Kombinierte Null Grad Struktur (KONUS) dynamics scheme with two quadrupole doublets (QDs) was adopted in the IH-DTL section. Start-to-end beam tracking results showed that the proton beam can be accelerated to the final energy with a length of 2.11 m and a transmission efficiency above 98.5%. In addition, we performed error sensitivity analysis and the combined error study to evaluate the error tolerance limits of the ladder RFQ-IH DTL coupled structure.

Towards Tests of an RFQ Cooler and Simulations

The cooling of secondary beams is important for accelerator-based nuclear physics. In the radiofrequency (RF) quadrupole cooler (RFQC), RF fields and ion-gas collisions may give a considerable increase or decrease of the beam transverse emittance and energy spread, depending on a delicate tuning of heating and cooling effects, dominated by the ion beam kinetic energy and the balance of collisions and confinement forces. An extra confinement may be added by a solenoid magnetic field, as in the RFQC prototype installed in the Eltrap machine. This provides a versatile test bench (distinct from a closed accelerator installation) for detailed studies of cooling dynamics and of several RFQC technical optimizations (for gas differential pumping and bias voltages). Modeling concepts and simulation results are summarized. The major RFQC parameters are reviewed, in particular for 133Cs+ collisions against He gas whose pressure pg ranges from 2 to 9 Pa in the reference case, with attention to the extraction, comparing triode/tetrode system, and to the bias voltages. Lower bias voltages request less pg , but provide less cooling of the energy spread.

Innovative Cesiation Deriving Incredible 145 mA Beam from J-PARC Cesiated RF-Driven H− Ion Source

In NIBS2022, the stable 8 hours operation of the J-PARC cesiated RF-driven H− ion source (IS) in a test-stand with a 69.9 keV 120 mA beam and a beam duty factor of 4 % (1 ms x 40 Hz) was reported. However, the Cesiation condition was produced after many times and rather large amount of Cs and H2Os injections. The fluctuation of the H− ion beam intensity (IH −) in a pulse and the transverse emittances were rather larger than those for the previous 65 keV 110 mA operation. The plasma electrode temperature (TPE) of 254 °C much higher than that not only for the usual J-PARC IS (about 70 °C) but also for the standard cesiated H− ion sources (180 ∼ 200 °C) suggested the existence of novel H2O mediated cesiation surviving against the high TPE. In this paper, the J-PARC IS improvements based upon a hypothesis of the best cesiation constituted of sub monolayer H2O (chemically bound with Mo) mediated cesiation and Cs half monolayer on remaining surface are presented. The innovative cesiation derived a 76.5 keV 145 mA beam stably with the small beam fluctuation and transverse emittances suitable for Radio Frequency Quadrupole LINACs of high energy LINACs. The measured results of the 145 mA / 83 mA beam, extraction electrode current and RF waveforms, parameter trends of an 8 hours 145 mA operation and the transverse emittances are presented. The optimum TPE was increased from 70 °C to 230 °C. Furthermore, the beam intensity for the J-PARC IS operation energy of 52.5 keV was increased from 72 mA to 83 mA, which was consistent with the 1.5 power perveance law on the beam energy compared with 145 mA for 76.5 keV.

The study and design of a deuteron drift tube linear accelerator for middle energy neutron source

The paper concerns a room-temperature cross-bar H-mode (CH) drift tube linac (DTL) with KONUS (Kombinierte Null Grad Struktur) [1,2] beam dynamics. To make the acceleration in DTL cell more efficient, we studied the correlation between transit time factor (TTF) and structural coefficients, first. Furthermore, we developed a new code with Python to demonstrate the longitudinal dynamics more clearly. The code computationally generates clusters, bunch centers, and emittance growth in a single figure. Thus, the stabilization region and cluster evolution at various negative phases can be studied. Based on the above studies, we designed a 162.5 MHz CH-DTL to accelerate 10 mA D+ from 2.11 MeV to 3.25 MeV in continuous-wave (CW) mode. The proposed CH-DTL is a part of the Middle Energy Neutron Source (MENS). The dynamics and RF design were iterated to make the gap voltage error lower than 1 %. The initial beam is assumed to come from a Radio Frequency Quadrupole accelerator (RFQ). The geometries of the CH-DTL are optimized by using CST. Multiparticle tracking from LEBT to RFQ is performed with TraceWin and the transmission efficiency in the CH-DTL is 100 %.

Neural networks as effective surrogate models of radio-frequency quadrupole particle accelerator simulations

Radio-frequency quadrupoles (RFQs) are multi-purpose linear particle accelerators that simultaneously bunch and accelerate charged particle beams. They are ubiquitous in accelerator physics, especially as injectors to higher-energy machines, owing to their impressive efficiency. The design and optimization of these devices can be lengthy due to the need to repeatedly perform high-fidelity simulations. Several recent papers have demonstrated that machine learning can be used to build surrogate models (fast-executing replacements of computationally costly beam simulations) for order-of-magnitude computing time speedups. However, while these pilot studies are encouraging, there is room to improve their predictive accuracy. Particularly, beam summary statistics such as emittances (an important figure of merit in particle accelerator physics) have historically been challenging to predict. For the first time, we present a surrogate model trained on 200 000 samples that yields < 6% mean average percent error for the predictions of all relevant beam output parameters from defining RFQ design parameters, solving the problem of poor emittance predictions by identifying and including hidden variables which were not accounted for previously. These surrogate models were made possible by using the Julia language and GPU computing; we briefly discuss both. We demonstrate the utility of surrogate modeling by performing a multi-objective optimization using our best model as a callback in the objective function to select an optimal RFQ design. We consider trade-offs in RFQ performance for various choices of Pareto-optimal design variables—common issues for any multi-objective optimization scheme. Lastly, we make recommendations for input data preparation, selection, and neural network architectures that pave the way for future development of production-capable surrogate models for RFQs and other particle accelerators.