Driver Linac Research Articles

Beam physics design of a superconducting linac

The China initiative Accelerator-Driven subcritical System (CiADS) project aims to design and build the world’s first accelerator-driven subcritical system demonstration facility. The facility will attain 2.5 MW thermal power in 2026 to realize accelerator-target-reactor coupling experiments. As the first approved accelerator-driven subcritical system facility, the CiADS driver linac will adopt cutting-edge accelerator technologies to achieve the challenging goals of high beam power and high reliability. The accelerator will operate in cw mode to produce a 500 MeV, 5 mA proton beam. To meet the extremely high demands on beam availability, the linac design lays strong emphasis on beam loss control and increased fault tolerance via a novel fault-compensation method. The accelerator complex consists of a room temperature front-end section, a main linac that employs superconducting accelerator technologies, and a high-energy beam transport line. The beam-loss-oriented beam dynamics design of the driver linac complex is presented in the paper. Published by the American Physical Society 2024

RF-Acceleration Studies for the HBS-Linac Applying Alternating Phase Focusing Concepts

The recent layout of the Jülich High Brilliance Neutron Source (HBS) driver linac is based on short crossbar H-mode (CH) cavities operated at a fixed synchronous phase. In the last decades the computing power for the development of linacs, available to physicists and engineers, has been increased drastically. This also enabled the accelerator community to finally carry out the required R&D to generate further the idea of drift tube linacs with alternating phase focusing (APF) beam dynamics, originally proposed in the 1950s. This focusing method uses the electric fields in between the drift tubes (i.e., gaps) to provide subsequent transverse and longitudinal focusing to the beam along multiple gaps. The beam focusing properties within each gap are adjusted individually by means of the synchronous phase. As a result of the alternating phase focusing method, these linacs can operate completely without internal magnetic lenses. The R&D-program for the high brilliance neutron source HBS offered the opportunity to investigate the APF concept further in order to open this advanced concept for high duty-factor, high intensity hadron beam acceleration. Besides, a prototype APF-interdigital H-mode (IH)-cavity has been designed and is going to be build and tested in the next future.

Beam Dynamics Studies for the Target Beamlines of the High Brilliance Neutron Source

The High Brilliance Neutron Source (HBS) belongs to the class of High Current Accelerator based Neutron Sources (Hi-CANS). The driver Linac for the HBS must be able to reliably accelerate a 100 mA proton beam to an energy of 70 MeV [2, 1, 3]. The beam is sent pulse-by-pulse to three different targets via a multiplexer in the High Energy Beam Transfer (HEBT) section. Each individual proton pulse behind the multiplexer has a specific time structure in order to optimally serve the different instruments grouped around one target station. The main development steps of the HEBT are the development of a three-field septum magnet, which is an essential part of the multiplexer magnet system, the beam dynamics integration of the multiplexer magnet system into the beamline, and the ion-optical layout of the individual target beamlines.

Certification testing of production superconducting quarter-wave and half-wave resonators for FRIB

The Facility for Rare Isotope Beams (FRIB) is designed to accelerate ion beams to ≥ 200 MeV per nucleon with 46 superconducting radio-frequency cryomodules. The FRIB driver linac uses quarter-wave resonators and half-wave resonators; with a total of 324 resonators, the linac is currently the largest heavy ion accelerator in the world. Production of jacketed resonators began in January 2014 and was completed in December 2019. Dewar certification testing of all the cavities was done before they were installed into cryomodules. The experience and lessons from more than 5 years of FRIB resonator production and certification testing are analyzed and summarized in this paper.

Design and Test of an 8T Focusing Superconducting Solenoid of FRIB Driver Linac

The Facility for Rare Isotope Beams (FRIB) was funded by the U.S. Department of Energy, which is the world’s most powerful isotope beam device. We have developed an 8T superconducting solenoid magnet which is used to focus and steer the heavy ion beams for FRIB Driver Linac. The solenoid consists of a main focusing coil, two shielding coils and two pairs of corrector dipoles, fabricated by NbTi wire only. A helium vessel was designed and fabricated to accommodate the coils with an entire length of 652 mm with end flange. The magnet in an operating current of 89.6 A achieved the maximum magnetic field of 8T in 40-mm cold bore without quench. In this paper, the overall design of the whole magnet was introduced, including the magnetic design, mechanical design, quench simulation and the protection circuit. Moreover, the winding process and the test results were showed.

Design options for a superconducting ion linac

Following a review of the design options for a superconducting ion linac, we present an alternative design for the pre-stripper section of the superconducting driver linac for the rare isotope science project (RISP) of the Institute for Basic Science in Korea. The proposed alternative design takes advantage of the recent accelerator developments at Argonne, namely for the recent ATLAS intensity and efficiency upgrade and the Fermilab proton improvement plan project (PIP-II). In particular, the state-of-the-art performance of quarter-wave (QWRs) and half-wave resonators (HWRs), the integrated steering correctors and cold beam position monitors (BPMs) for a compact cryomodule design. In order to simplify the design and avoid frequency transitions, we used two types of QWRs at 81.25 MHz while the baseline design has QWRs at 81.25 MHz and HWRs at 162.5 MHz. The new QWR types were optimized for β∼0.05 and ∼0.11, respectively, and corrected for beam steering effects. Nine cryomodules are required to reach the stripping energy of 18.5 MeV/u for uranium beam. An alternative radio-frequency quadrupole (RFQ) design was also developed. It is optimized with the multi-harmonic buncher (MHB) to produce a much smaller longitudinal emittance, offering much needed flexibility for beam tuning and more tolerance to errors. Following the lattice design optimization, end-to-end beam dynamics simulations including most important sources of machine error were performed. The results showed that the design is robust and tolerant to errors with no beam loss observed for typical machine errors.

Positron production using a 9 MeV electron linac for the GBAR experiment

For the GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at CERN’s Antiproton Decelerator (AD) facility we have constructed a source of slow positrons, which uses a low-energy electron linear accelerator (linac). The driver linac produces electrons of 9 MeV kinetic energy that create positrons from bremsstrahlung-induced pair production. Staying below 10 MeV ensures no persistent radioactive activation in the target zone and that the radiation level outside the biological shield is safe for public access. An annealed tungsten-mesh assembly placed directly behind the target acts as a positron moderator. The system produces 5×107 slow positrons per second, a performance demonstrating that a low-energy electron linac is a superior choice over positron-emitting radioactive sources for high positron flux.

Beam dynamics design and error study of BISOL deuteron driver linac

The high intensity deuteron linac, one of the drivers of BISOL, aims to accelerate a continuous wave (cw) deuteron beam up to 40 MeV with a beam current of 20 mA, which can also be operated independently as the Multi-Aimed Intense Neutron Source (MAINS). As the high beam power and beam current, high reliability and strict beam loss are demand for the deuteron driver linac. The beam dynamics design of the driver linac, including the radio frequency quadrupole (RFQ), medium energy beam transport line (MEBT) and superconducting rf (SRF) linac, has been designed and optimized to meet the required requirements. In order to prevent halo formation and achieve strict beam losses, beam halo matching is implemented in our design. The simulation results predict a transmission efficiency of above 99% and the beam power losses meet requirements of less than l W/m in the SRF linac. Detailed error studies are performed to evaluate the sensitivity of the bean dynamics design at last. The general design considerations and the overall beam dynamics of the driver linac are presented in this paper.

Design and study of a continuous wave RFQ for the BISOL high intensity deuteron driver linac

A high intensity four-vane continue wave (cw) RFQ for the Beijing Isotope Separation On-Line (BISOL) high intensity deuteron driver linac has been designed. This RFQ will accelerate the 25 mA cw deuteron beam the 3.0 MeV with an operating frequency of 162.5 MHz. Three beam dynamics design schemes based on different strategies have been proposed and detailed comparisons, including tolerance study and error analysis, are performed to evaluate them. The final design of RFQ can achieve the requirements with a length of 5.29 m and the transmission of 99.1%. The inter-vane voltage is ramped from 60 kV to 75 kV and the maximum Kilpatrick factor is 1.42, which is very modest and beneficial to cw operation mode. The RFQ electromagnetic design is carried out based on the full-length 3D model, including vane modulation, undercuts, π-mode stabilization rods (PSLs) and tuners. The requirements of correct work frequency and voltage ramp are fulfilled by varying vane width and adjusting undercuts. In addition, the π-mode stabilization rods (PSLs) are inserted and optimized to increase mode separation and enhance voltage stabilization. The rf power loss and effective shunt impendence are 118 kW and 210 kΩ⋅m, respectively. In addition, multi-physics analysis of the cavity, based on the electromagnetic codes CST MWS, HFSS and ANSYS, has been carried out to check the cooling system, the temperature rise and deformation of the cavity, as well as the frequency shift in the cw operation. Finally, the frequency sensitivities to cooling water temperature are studied to obtain water temperature tuning coefficients, which will be used for fine tuning of the RFQ cavity during operation.

Efficient continuous wave accelerating structure for ion beams

The Facility for Rare Isotope Beams driver linac was designed for acceleration of multiple-charge-state beams after the stripping at ion beam energies of 17 to $20\text{ }\text{ }\mathrm{MeV}\mathrm{/u}$ depending on the ion species. The linac includes a 180\ifmmode^\circ\else\textdegree\fi{} achromatic bend to select multiple charge states for further acceleration and to dump unwanted charge states after the first 45\ifmmode^\circ\else\textdegree\fi{} magnet. Two rf rebunchers between the stripper and the linac segment 2 (LS2) are required to minimize the effective emittance growth of the multiple-charge-state beams and provide matching to the LS2. Recent studies have shown that the best choice for these two rebunchers is a room-temperature (RT) accelerating structure capable of providing robust operation in the presence of the stripped heavy ion beam contaminants. The latter can result in uncontrolled losses of contaminant ions after the passage off the stripper. Therefore, using a superconducting (SC) rebuncher after the stripper is not rational due to possible contamination and performance degradation of SC cavities over a long period of operation. For the beam bunching in the energy range from 13 to $22\text{ }\text{ }\mathrm{MeV}\mathrm{/u}$, two 161 MHz rebunchers based on interdigital H-type (IH) structure were developed, built, installed and commissioned with beam at the FRIB linac. This is the first experimental demonstration for application of a cw RT drift tube accelerating structure in this energy range.

Cryogenic test of the FRIB superconducting magnets at IMP

The superconducting magnets of the Facility of Rare Isotope Beams (FRIB) are used to focus and steer the heavy ion beams of the driver linac. All the magnets are designed as a solenoid with bucking coils to suppress the stray field, and all the magnets have superconducting dipole correctors to steer both horizontal and vertical field. Two types of magnets have been manufactured in China, and some of them have been tested at Institute of Modern Physic (IMP). This paper describes the test system and magnetic axis measurement method. We also present a summary of the measurement process and test results of the magnetic performance for the magnets.

Design study of a radio-frequency quadrupole for high-intensity beams**Supported by Korea University Future Research Grant

The Rare isotope Accelerator Of Newness (RAON) heavy-ion accelerator has been designed for the Rare Isotope Science Project (RISP) in Korea. The RAON will produce heavy-ion beams from 660-MeV-proton to 200-MeV/u-uranium with continuous wave (CW) power of 400 kW to support research in various scientific fields. Its system consists of an ECR ion source, LEBTs with 10 keV/u, CW RFQ accelerator with 81.25 MHz and 500 keV/u, a MEBT system, and a SC linac. In detail, the driver linac system consists of a Quarter Wave Resonator (QWR) section with 81.25 MHz and a Half Wave Resonator (HWR) section with 162.5 MHz, Linac-1, and a Spoke Cavity section with 325 MHz, Linac-2. These linacs have been designed to optimize the beam parameters to meet the required design goals. At the same time, a light-heavy ion accelerator with high-intensity beam, such as proton, deuteron, and helium beams, is required for experiments. In this paper, we present the design study of the high intensity RFQ for a deuteron beam with energies from 30 keV/u to 1.5 MeV/u and currents in the mA range. This system is composed of an Penning Ionization Gauge ion source, short LEBT with a RF deflector, and shared SC Linac. In order to increase acceleration efficiency in a short length with low cost, the 2nd harmonic of 162.5 MHz is applied as the operation frequency in the D+ RFQ design. The D+ RFQ is designed with 4.97 m, 1.52 bravery factor. Since it operates with 2nd harmonic frequency, the beam should be 50% of the duty factor while the cavity should be operated in CW mode, to protect the downstream linac system. We focus on avoiding emittance growth by the space-charge effect and optimizing the RFQ to achieve a high transmission and low emittance growth. Both the RFQ beam dynamics study and RFQ cavity design study for two and three dimensions will be discussed.

Simulation Study on the Beam Loss Mitigation in the 1st Arc Section of FRIB Driver Linac

Simulation Study on the Beam Loss Mitigation in the 1st Arc Section of FRIB Driver Linac

Minimization of three-dimensional beam emittance growth in rare-isotope accelerator

In this paper, we describe a research to minimize the three-dimensional (3D) emittance growth (EG) in the RAON accelerator, a heavy ion accelerator currently being developed in Korea to produce various rare isotopes. The emittance minimization is performed using the multi-objective genetic algorithm and the simplex method. We use them to analyze the driver linac for the in-flight fragmentation separator of the RAON facility and show that redesign of the 90-degree bending section of the RAON accelerator together with adjustment of optics in the upstream and downstream superconducting linacs can limit the 3D EG to ~20 % in the entire region of the driver linac. Effects of various magnet and rf accelerating cavity errors on the beam-EG are also discussed.

Minimization of transverse beam-emittance growth in the 90-degree bending section of the RAON rare-isotope accelerator

The major contribution of the transverse beam emittance growth (EG) in a RAON heavy-ion accelerator comes from the bending section, which consists of a charge-stripping section, a matching section, and a charge-selection section in sequence. In this paper, we describe our research to minimize the two-dimensional EG in the 90-degree bending section of the RAON currently being developed in Korea. The EG minimization was achieved with the help of multi-objective genetic algorithms and the simplex method. We utilized those algorithms to analyze the 90-degree bending section in a driver linac for the in-flight fragmentation system. Horizontal and vertical EGs were limited to below 10 % in the bending section by adjustment of the transverse beam optics upstream from the charge-stripping section, redesign of the charge-selection section, and optimization of the vertical beam optics at the entrance of a charge-selection section.

High-Brightness High-Energy Electron Beams from a Laser Wakefield Accelerator via Energy Chirp Control.

By designing a structured gas density profile between the dual-stage gas jets to manipulate electron seeding and energy chirp reversal for compressing the energy spread, we have experimentally produced high-brightness high-energy electron beams from a cascaded laser wakefield accelerator with peak energies in the range of 200-600MeV, 0.4%-1.2%rms energy spread, 10-80pC charge, and ∼0.2 mrad rms divergence. The maximum six-dimensional brightness B_{6D,n} is estimated as ∼6.5×10^{15} A/m^{2}/0.1%, which is very close to the typical brightness of e beams from state-of-the-art linac drivers. These high-brightness high-energy e beams may lead to the realization of compact monoenergetic gamma-ray and intense coherent x-ray radiation sources.

Error analysis in post linac to driver linac transport beam line of RAON

We investigated the effects of magnet errors in the beam transport line connecting the post linac to the driver linac (P2DT) in the Rare Isotope Accelerator in Korea (RAON). The P2DT beam line is bent by 180-degree to send the radioactive Isotope Separation On-line (ISOL) beams accelerated in Linac-3 to Linac-2. This beam line transports beams with multi-charge state 132Sn45,46,47. The P2DT beam line includes 42 quadrupole, 4 dipole and 10 sextupole magnets. We evaluate the effects of errors on the trajectory of the beam by using the TRACK code, which includes the translational and the rotational errors of the quadrupole, dipole and sextupole magnets in the beam line. The purpose of this error analysis is to reduce the rate of beam loss in the P2DT beam line. The distorted beam trajectories can be corrected by using six correctors and seven monitors.

Beam experiments with a non-intercepting beam induced fluorescence profile monitor for the ADS LINAC

An accelerator-driven subcritical system (ADS) project was launched in China in 2011, aiming to design and build an ADS demonstration facility with the capability of more than 1000 MW thermal power. The driver linac is defined to be a 10 mA current of high energy protons at 1.5 GeV in continuous wave operation mode. To meet the extremely high power and intense beam accelerator requirements, non-interceptive monitors for the beam transverse profile are required for this proton linac. Taking advantage of the residual gas as active material, the Beam Induced Fluorescence (BIF) monitor exploits gas-excited fluorescence in the visible spectrum region for transverse profile measurements. The advantages of this non-intercepting method are that nothing is installed in the vacuum pipe, component design is compact and there is no need for expensive signal processing electronics. Beam experiments have been performed under constant beam conditions. The helium spectrum has been verified with different optical filters, showing that a proper optical band-pass filter covering 400–500 nm is necessary for fluorescence experiments with helium. By changing gas pressure, it is shown that gas pressure is proportional to the signal amplitude but has no influence on detected profile width. Finally, a comparison experiment between the BIF monitor and a wire scanner shows that the detected profile width results of both methods agree well.

Designing a test facility LEBT for RISP

Raon, the rare isotope accelerator of the Rare Isotope Science Project (RISP) in Daejeon, South Korea, is being designed to accelerate multiple-charge-state beams simultaneously. Using an Electron Cyclotron Resonance (ECR) Ion Source to produce the ions, Raon will transport the beam through two 90-degree bending magnets and a Low Energy Beam Transport (LEBT) system to a Radio Frequency Quadrupole (RFQ). A test facility is under development to test the components of the injector and LEBT system. A new LEBT, based upon the LEBT of the main driver linac, is being designed to fit within the test facility’s restrictive space requirements. This work will briefly review the main driver linac LEBT design, and then discuss the current status of the test facility LEBT design.

Recent R&D progress of the RAON injector

The injector for the main driver linac of the Rare Isotope Science Project in Korea has been developed to supply heavy ions up to uranium which will be used for the inflight fragmentation system. The injector has two critical components, superconducting electron cyclotron resonance (ECR) ion sources and a radio frequency quadrupole (RFQ). The physical and engineering designs of these critical components were performed. Prototypes of the 28 GHz ECR ion source and the vane-type RFQ have been fabricated and tested to confirm the design of the major components.