Drift Tube Linac Research Articles

Design of the Prototype of the Radio Frequency Quadrupole Sections for the Compact Neutron Source DARIA

The DARIA compact neutron source project based on a linear resonant proton accelerator is aimed at creating a serial facility capable of providing the Russian scientific community with pulsed neutron beams with intensities comparable to those of research nuclear reactors. The absence of fissile material makes it possible to significantly reduce the radiation safety requirements for such installations and, consequently, to place them at the site of leading scientific centers and universities that train specialists in the field of neutron physics. Under a grant from the Ministry of Science and Higher Education, key elements of the facility are being developed. The design and sequence of manufacturing for a full-scale model of an accelerator section with radio frequency quadrupole focusing and a resonator with drift tube linac are presented.

11–20 MeV Alvarez Drift Tube Linac for low energy proton accelerator

Bhabha Atomic Research Centre is developing Low Energy High Intensity Proton Accelerator (LEHIPA) as a pre-injector for proposed Accelerator Driven Sub-Critical Reactor Systems (ADSS). Drift Tube Linac is one of the two major accelerating structures of LEHIPA, the other being the Radio Frequency Quadrupole (RFQ). Like all particle accelerating structures, DTL needs multi-disciplinary expertise for design, production, qualification, and installation. This paper describes the comprehensive design, production, qualification, and installation of the ∼ 11–20 MeV section of DTL. It details the design of all sub-components which form part of the DTL RF cavity. Several novel concepts were implemented in course of this development. Important among them are DT-DTL alignment, referencing methodology, robust vacuum design and hermetically sealed DTs. This paper is a comprehensive coverage of design, development and installation aspects of Alvarez DTL. At present, the DTL is ready for beam commissioning.

Physical design of a compact injector for synchrotron-based proton therapy

A compact room-temperature linear injector has been purposed to accelerate an 18.0 mA proton beam to 7.0 MeV for synchrotron-based proton therapy. The total length is appropriately 5 m. It mainly consists of a 3.01 m radio frequency quadrupole (RFQ) and a 0.82 m compact interdigital H-mode drift tube linac (IH-DTL) structure. Based on a fast-bunching strategy, the RFQ, operated at 325 MHz, accelerates protons to 3.0 MeV. The phase advances have been taken into consideration, and parametric resonance has been carefully avoided by adjusting the vane parameters. After the modulation of the transverse and longitudinal phase advances and a compact external quadrupole triplet, the proton beam is injected into the subsequent IH-DTL. Based on modified Kombinierte Null Grad Strukter (KONUS) beam dynamics, it accelerates protons up to 7.0 MeV, which is composed of a re-bunching section and an accelerating section. The accelerating gradient reaches 4.88 MV/m. The overall dynamic simulation results show that the whole accelerating gradient reaches up to 1.62 MV/m with a transmission efficiency above 95%. The transverse and longitudinal normalized RMS emittances at the exit of the DTL are 0.23 π mm·mrad and 2.216 π keV/u·ns, which meet the synchrotron injection requirements. The details of the specific design of this injector are presented in this paper.

RF measurement and tuning for an Alvarez-type drift tube linear accelerator without post couplers

Alvarez-type Drift Tube Linacs (DTLs) are commonly adopted to accelerate the proton or heavy ion beam with the speed range about 0.04∼0.4 times of the light speed in vacuum. Acquisition of appropriate frequency and distribution of the electromagnetic field, and the field stability inside the Alvarez-type DTL cavity are essential to achieve good beam characteristics. The field requirement can be achieved through tuning by tuners and post couplers when the cavity is off-line. The field stability of cavity is usually measured by the tilt sensitivity and the minimum frequency interval between the operation mode and the nearest neighbor mode. The frequency interval will increase with the reduction of the number of the acceleration units (cells) inside the cavity, which is beneficial to fulfill the tilt sensitivity requirement. If the field stability condition is naturally satisfied, the post couplers inside the DTL cavity are not needed, and the cavity frequency and field distribution can be tuned merely by tuners. In this paper, the tuning method based on the pseudo-inverse matrix is applied to the Alvarez-type DTL cavity without post couplers in Xi’an 200 MeV proton application facility (XiPAF). The method is improved by an expanded matrix algorithm and the tuning process is optimized to expedite the iteration during the tuning. The XiPAF DTL cavity has 23 cells and 5 tuners. By adjusting the insertion depths of the tuners, the tuning was achieved in merely one round of iteration. After tuning, the relative error of the peak field distribution and resonant frequency of the cavity reached to within ±3% and 325 ± 0.007 MHz (25°C, vacuum), respectively. It was verified that the field stability meets the requirement from the physical design both before and after tuning.

Physics design, fabrication, RF measurement and beam commissioning of a dual beam drift tube linac

A novel type dual-beam drift tube linac (DB-DTL) prototype was firstly designed to prove the feasibility of the multi-beam type DTL at the institute of modern physics. The physics design, cavity fabrication, low-power RF measurement, RF conditioning and beam commissioning of the DB-DTL cavity were completed and presented in this paper. Operating at 81.25 MHz, the DB-DTL was designed to accelerate 10 mA proton from 0.56 MeV to 2.5 MeV. The field distribution of the DB-DTL cavity were measured at low RF power level and particle tracing using measured field were done, which both agree well with the physics design. The measured Q 0 value and the shunt impedance are 11606 and 172 MΩ/m, respectively. The RF conditioning and the first beam commissioning of the DB-DTL cavity were completed. The beam experiment platform was built at the Lanzhou university due to the academy research cooperation between the institute of modern physics, Sun Yat-sen university and Lanzhou university. Based on the existing experiment condition, 2.5 MeV proton acceleration and measurement were successfully achieved at 42 kW (0.85*Q 0)) RF power of the DB-DTL cavity, which match the physics design very well.

Beam dynamics and cavity design of an 8 MeV CW IH-DTL for SYSU-PAFA

As a part of the Particle Accelerator Facility at Sun Yat-sen University (SYSU-PAFA), a 200 MHz drift tube linac (DTL), operating under continuous wave (CW) mode, has been designed. PAFA-DTL is expected to accelerate a 10 mA proton beam from 2.5 MeV to 8 MeV. Using the alternative phase focusing (APF) beam dynamics scheme, a compact cell array has been designed and optimized in order to limit the cavity length to 2.4 m with high acceleration efficiency. As a result, PAFA-DTL contains 31 acceleration cells with beam transmission efficiency reaches 100%, and better output beam quality is obtained by optimizing the input beam parameters. Interdigital H-mode (IH) structure is utilized in the PAFA-DTL cavity, which has achieved a high unloaded quality factor of 13987. Additionally, to ensure the stability of PAFA-DTL under CW operation, a low Kilpatrick factor (Kp) of 1.42 is obtained by adjusting the blending radius of the drift tubes (DTs) to reduce the risk of RF breakdown. The gap voltage distribution through RF design is compared with that from the beam dynamics, and the maximum absolute value of deviation is only 0.73%. In this paper, the detailed design and results of PAFA-DTL, including beam dynamics and RF design, are presented.

Non-destructive measurement of the beryllium thickness of the iBNCT neutron-generation target using negative muons

Accelerator-based boron neutron capture therapy (BNCT) has recently been studied by several researchers. In BNCT, a neutron-generation target is an important component because an intense neutron flux of more than 109n/cm2/s is required to complete medical treatments within an acceptable treatment time. In the Ibaraki BNCT (iBNCT) project, a proton linear accelerator (linac) consisting of a 3-MeV radio-frequency quadrupole and an 8-MeV drift-tube linac based on J-PARC linac techniques was used. As for the neutron-generation target, beryllium was adopted with an incident proton energy of 8 MeV. The target has a three-layer structure fabricated using hot-isostatic pressing (HIP). The first layer consists of 0.5 mm-thick beryllium, which is slightly shorter than the 8-MeV proton range in beryllium for neutron generation. The second is a 0.5 mm-thick palladium, which was selected owing to its durability against blistering, and the third is a 10 mm-thick copper for heat removal purposes with cooling water circulation. The thickness of the beryllium is very important because a shortage of thickness reduces the yield of neutrons, while an excess of thickness results in damage by stopped protons owing to a blistering effect. Despite its importance, this thickness has not yet been confirmed because it is not realistic to measure it destructively in terms of the construction cost of the iBNCT target; thus, a non-destructive measurement is desirable. This paper describes a method for measuring the thickness of the beryllium layer of a mockup of an iBNCT neutron target, which was constructed in the same way as the actual iBNCT, in a non-destructive manner using negative muon X-rays. The experiment was performed in the J-PARC MLF muon facility, and the obtained thickness was 0.48 ± 0.02 mm, which corresponds to the designed value of 0.50 ± 0.05 mm.

Quasi-Alvarez drift-tube linac structures for heavy ion therapy accelerator facilities

Next generation heavy ion therapy and research facilities require efficient accelerating structures. Particularly, at low beam energies, right after the standard scheme of the ion source, low-energy beam transfer, and radio-frequency quadrupole (RFQ), several options for accelerating structures are available including the classic drift-tube linac (DTL), the interdigital H-mode DTL (IH-DTL), and superconducting quarter-wave resonators. These structures need to integrate the beam acceleration with the focusing channel, nowadays typically provided by permanent-magnet quadrupoles (PMQs). The frequency of operation needs to be in line with that of the RFQ structure, and it has been chosen at 750 MHz for practical considerations for the Next Ion Medical Machine Study (NIMMS) that is the application focus of this manuscript. While classic DTL structures at low ion beam energies do not provide enough space for PMQs at that frequency within a single $\ensuremath{\beta}\ensuremath{\lambda}$ period, IH-DTL structures do not provide the regular focusing channel with consequences on the beam quality. For these reasons, quasi-Alvarez drift-tube linac (QA-DTL) structures are reevaluated in this manuscript as they might fill this gap. They have not received much attention in the literature so far and therefore their design is described in detail. The design procedure presented here may serve as a blueprint for DTL design in general. In addition to the overall rf design, axial field stabilization with a new technique and multiphysics studies of the rf structure are described. A cost estimation completes the NIMMS QA-DTL study.

Measurement of H0 particles generated by residual gas stripping in the Japan Proton Accelerator Research Complex linac

The Japan Proton Accelerator Research Complex (J-PARC) linac is a high-intensity accelerator in which beam loss is a critical issue. In the J-PARC linac, H− beams are accelerated to 191 MeV by a separated drift tube linac (SDTL) and subsequently to 400 MeV by an annular-ring coupled structure (ACS). Because there are more beam loss mechanisms in H− linacs than in proton linacs, it is imperative to investigate the beam loss circumstances for beam loss mitigation. Electron-stripping phenomena, which generate uncontrollable H0 particles, are characteristic beam loss factors of H− linacs. To clarify the beam loss causes in the J-PARC linac, a new diagnostic line was installed in the beam transport between the SDTL and ACS. In this diagnostic line, H0 particles were separated from the H− beam, and the intensity profiles of the H0 particles were successfully measured by horizontally scanning a graphite plate in the range where H0 particles were distributed. By examining the intensity variation of the H0 particles with different residual pressure levels, we proved that half of the H0 particles in the SDTL section are generated by the residual gas stripping in the nominal beam operation of the J-PARC linac.

The parameters of radio frequency quadrupole linear accelerator for DARIA project

The new proton linear accelerator (linac) with output energy 13 MeV and 100 mA current is under development at NRC “Kurchatov Institute” - ITEP for DARIA project. The linac consists of Radio-Frequency Quadrupole (RFQ) and Drift Tube Linac (DTL) with operating frequency 162.5 MHz. The RFQ is based on 4-vanes structure with shifted coupling windows. DTL has a modular structure and consists of separated individually phased cavities with focusing magnetic quadrupoles located between the cavities. The DTL is based on interdigital H-mode (IHDTL) 5-gaps cavities. The 6D beam matching between RFQ and DTL is provided by magnetic quadrupole lenses and RF-bunchers. The paper presents results of the radio-frequency (RF) design, thermal analyses and RF tuning of RFQ linac accelerating structure.

The 300 MeV proton and heavy ion accelerator complex for SESRI project in China

The 300 MeV proton and heavy ion accelerator complex, which is the key part of SESRI (Space Environment Simulation and Research Infrastructure) project, is currently under construction in China. It comprises an 18 GHz high charge-state ion source, a 108.48 MHz cascaded linac injector consisting of a four-rod RFQ and three interdigital H-mode drift tube linacs, a compact synchrotron, and three irradiation terminals. The extensive ion species from proton to 209Bi32+ will be injected into the synchrotron using multi-turn injection method. The third-order resonance and RF-knockout extraction scheme is adopted to provide quasi-continuous spill for irradiation experiments. The general design of the accelerator complex and simulation results are discussed in this paper.

High-power test of an interdigital H -mode drift tube linac for the J-PARC muon g−2 and electric dipole moment experiment

We conducted a high-power test of a prototype cavity of a 324-MHz interdigital H-mode drift tube linac (IH-DTL) for the precise measurement of the muon anomalous magnetic moment ($g\ensuremath{-}2$) and electric dipole moment (EDM). This prototype cavity (short IH) was developed to verify the fabrication methodology for the IH-DTL cavity with a monolithic drift tube structure. The electromagnetic field distribution was measured and compared with the finite element method simulation results, and the fabrication accuracy of the monolithic drift tube was confirmed to satisfy the requirements. After 40 h of conditioning, the short IH was stably operated with an rf power of 88 kW, which corresponds to a 10% higher accelerating field than the design field (${E}_{0}$) of $3.0\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. In addition, the thermal characteristics and frequency response were measured, verifying that the experimental data were consistent with the three-dimensional model. In this paper, the design, fabrication, and low-power and high-power tests of this IH-DTL for muon acceleration are described.

Compact and efficient radio frequency digital feedback control system for accelerator applications

The digital feedback (DFB) setup employed in a low-level radio frequency (LLRF) control system is crucial to ensure RF field stability in accelerating cavities, thus guaranteeing successful operation. To this end, a novel in-house and multi-purpose prototype DFB setup was developed for multiple applications at the Japan Proton Accelerator Research Complex (J-PARC). Being efficient, low-cost, and compact is key to achieving a cost-effective system that fulfills the performance specifications. It also reduces the production and maintenance costs of future DFB setups. A field-programmable gate array (FPGA)-based design with a digital signal processing (DSP) function uses an analog in-phase/quadrature-phase (I/Q) demodulator for RF-to-baseband signal conversion and an I/Q modulator to generate RF power. This arrangement compensates for RF phase fluctuations and amplitude modulations by employing a proportional and integral (PI) feedback controller. In addition, systematic errors are minimized by applying a hardware-error-compensation process. The system was utilized to conduct high-power tests on the interdigital H-mode drift-tube linac (IH-DTL) cavity of a muon linac for short RF pulses. The setup was also tested on a buncher cavity (Buncher-2) in the J-PARC linac, achieving efficient performance for longer RF pulses. The stability of the RF accelerating field in the IH-DTL was achieved at ±0.25% peak-to-peak (pp) in amplitude and ±0.36 degree pp in phase. For the Buncher-2, the amplitude stability of ±0.18% pp and phase stability of ±0.13 degree pp were obtained. Therefore, this DFB setup can be used to conduct high-power tests of RF cavities and klystrons. This study discusses the design aspects of a cost-effective DFB system and reports high-power measurements.

Thermomechanical responses of the drift tube to beam loss at the China spallation neutron source.

The China Spallation Neutron Source drift tube linac (DTL) is a high current intensity linear accelerator operated in pulsed mode. Beam loss is the most important cause of DTL performance degradation. Therefore, control of beam loss is very crucial to the linac's commission and routine operation. The beam loss in beam pulse width (∼several hundreds of microseconds) can be regarded as an instantaneous process, which may cause the local temperature of the drift tubes (DTs) to exceed the melting point or the thermal stress to exceed the material yield strength, resulting in permanent damage to the DTs. RF heating and beam loss are two major heat sources of the DTs during operation. First, temperature rise due to RF power dissipation is evaluated in this paper. Then, the thermomechanical response due to beam loss at different energies is investigated. It is found that the 3-MeV beam loss has the greatest stopping power and causes the largest temperature rise to the DTs. The maximum beam current loss for different working conditions is analyzed by both analytical method and ANSYS simulations, and the details will also be discussed in this paper.

A System for Magnetic Measurement of the Triplet Quadrupoles for DTL

The Institute of Modern Physics (IMP) has developed a magnetic measurement system of the triplet quadrupoles in drift tube linac (DTL), based on the rotating coil method. The most important measurement requirement of the triplet in DTL is the deviation between the machinery axis of the cavity and the magnetic axis of the triplet quadrupoles, which is less than 0.1 mm, the angle deviation is less than 1 mrad. The magnetic field quality is also measured. There are two aspects to be considered when designing the system, one is the mechanical adjustment mechanisms for the three quadrupoles respectively, another is the specialized compensated coil consisting of two sets of windings which can be connected to measure the magnetic center and the high order harmonic errors. Simultaneously, optimizing the testing process is also crucial. Based on the results obtained on the two triplets in two tanks for a Heavy Ion Medical Machine with Linear Accelerator (HIMM-LINAC), the magnetic measurement system could meet the requirements

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.

Commissioning of the 7 MeV H[formula omitted] linac injector for a 200 MeV proton synchrotron

This paper describes the beam commissioning progress of the 7 MeV H− linac injector for the proton synchrotron of Xi’an 200 MeV Proton Application Facility (XiPAF). To fulfill a flux of 105∼108p/cm2/s with a uniformity better than 90% on a 3 cm × 3 cm sample size at the experimental station, the number of the accumulated protons in each cycle in the synchrotron is designed to be larger than 2 × 1011. The protons are acquired from the H− beam stripped by the carbon foil, which enables transverse space painting during injection. The maximum pulse length of the injected beam is 40μs. With an estimation of 40 for the intensity gain factor, the injected beam is required to have a peak current of higher than 1 mA, with the normalized transverse emittance of 90% particles less than 6 and 4 πmm mrad for the horizontal and vertical planes, respectively, and the momentum spread of 90% particles within ±0.45%. The injector mainly consists of an H− ion source, a low energy beam transport line (LEBT), a 3 MeV radio frequency quadrupole (RFQ) accelerator, a 7 MeV interdigital H-mode drift tube linac (IH-DTL), and an injection beam transport line. After several stages of beam commissioning, the linac injector has reached a pulsed peak current of 2.5 mA, with normalized emittance (90% particles) less than 4.4 and 2.2 πmm mrad, 90% momentum spread less than ±0.55%, which lays a good foundation for the beam commissioning of the synchrotron.

Development of a beam energy adjustment system after a Radio-Frequency-Quadrupole

The Low Energy heavy-ion Accelerator Facility (LEAF) is a low-energy, high-intensity, heavy ion accelerator for multidisciplinary research developed at the Institute of Modern Physics (IMP). Beam commissioning of the facility began in 2018, but until recently, only a fixed beam energy of 0.5 MeV/u was available because the main accelerator structure is a Radio-Frequency Quadrupole (RFQ), which hinders LEAF to meet the requirements of the experimental terminals, for example, 12C+12C fusion reaction investigation at astrophysical energies requires beam energies adjustable from 0.3 MeV/u to 0.7 MeV/u. To expand the beam energy range, a “constant-beta” structure Drift Tube Linac (DTL) was developed providing an adjustable beam energy from 0.3 to 0.7 MeV/u. To control the beam quality, specifically the energy spread, two re-bunchers are employed, one upstream and one downstream of the DTL. Here we present the development of the key device: an Interdigital H-mode DTL (IH-DTL). First, we discuss the beam dynamics design for the beam energy adjustment system, then we present details of the test bench, and finally, we report measurements performed with the finished hardware. The measurements of energy adjustment and energy spread agree well with the simulations, demonstrating the feasibility of this design.

Superconducting Linac Booster for Super-Heavy Element Experiments at RIKEN Radioactive Isotope Beam Factory

The Rikagaku Kenkyūjo (RIKEN) heavy-ion linac was upgraded by introducing a new super-conducting linac-booster to advance the super-heavy elements synthesis program beyond nihonium at the RIKEN Radioactive Isotope Beam Factory (RIBF). The total acceleration voltage was upgraded from 25 MV with 12 room temperature drift-tube-linacs (DTLs) to 39 MV by introducing a superconducting linac booster, SRILAC. The upgrade of the beam intensity is realized by a newly constructed superconducting electron-cyclotron resonance ion source (SC-ECRIS). The construction of SRILAC and the SC-ECRIS started in FY2017. After the hardware installation and commissioning, the first beam acceleration test was successfully conducted at the end of FY2019, and user beam service was started in FY2020. This article provides an overview of the upgrade and its present performance.

A 3D printed pure copper drift tube linac prototype.

Radio frequency cavities are among the most challenging and costly components of an accelerator facility. They are usually manufactured in individual parts, which are then joined by complex processes, e.g., several brazing steps. 3D printing has become an alternative to these conventional manufacturing methods due to higher cost efficiency, freedom in design, and recent achievement of high print quality for pure copper. A fully functional 3GHz drift tube linac (DTL) prototype was 3D printed in one piece, made from pure copper by selective laser melting (SLM). To achieve a higher surface quality, the DTL geometry was optimized for the SLM process. The DTL design is related to the design of the DTL part of the side-coupled DTL modules used in linac-based proton therapy facilities. The quality factor (8750) and the shunt impedance per unit length (102mΩm) of the printed prototype are already comparable to traditionally manufactured DTL structures and can be further enhanced by surface treatments.