Drift Tube Linac Research Articles

Simple Design Criterion for High-Intensity Hadron Linacs

Abstract A simple procedure is established to optimize fundamental design parameters of a high-intensity hadron linac where interparticle Coulomb interaction plays a crucial role. Based on the recently proposed semi-empirical resonance condition, a stability map is constructed which reveals potentially dangerous operating regions in tune space. The map is shown to be consistent with numerical data obtained from more complicated approaches. The effectiveness of the new design scheme is demonstrated through systematic particle-in-cell simulations assuming the most typical structure of an Alvarez-type drift tube linac. The present results suggest that the so-called equipartitioning condition, which has often been taken very seriously in high-intensity linac designs, does not need to be met to guarantee the best machine performance. The basic design concept described here can be applied not only to linacs but also to circular machines.

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.

Operational Status and Future Plan of Proton Linear Accelerator at KOMAC

Since the second half of 2013, Korea Multi-purpose Accelerator Complex (KOMAC) has been supporting user beam service by using a 100-MeV proton linac and several beam line facilities. Proton beam service fields include various applications and research programs, such as material science, bio-medical science, semiconductor applications as well as nuclear physics and basic science. The 100-MeV linac is composed of an injector based on a 50-keV microwave ion source and two-solenoid LEBT (low energy beam transport), a 3-MeV RFQ with four-vane structure and conventional drift tube linac (DTL), where proton beam is accelerated from 3 MeV to 100 MeV. As the operation period of the proton accelerator exceeds 10 years and the cumulative operating time surpasses 30,000 hours, we judge that it is an opportune time to establish a long-term plan to prepare for the aging of the accelerator. To replace the currently operating RFQ, which shows degradation in performance (especially the beam transmission), we design a new RFQ with some modifications. For DTL, we are supposed to change the quadrupole magnet inside each drift tube from electromagnet type to permanent magnet type because the electromagnet has shown some trouble in view of long term stability. To leap forward in device and utilization technologies, we complete the construction of new facility of a Beam Test Stand (BTS) with a structure similar to the beam injection system (ion source, LEBT, RFQ) of the 100 MeV linear accelerator. We plan to actively conduct beam physics and accelerator research using this facility. Moreover, we are supposed to upgrade the proton linac to 1 GeV for spallation neutron source applications, which will be a long-term plan. In short term, we take phased approach with focusing the first phase at 200 MeV energy upgrade.

Table-top ion-trap experiment on the stability of intense short bunches in linear hadron accelerators

The novel experimental system “S-POD” (Simulator of Particle Orbit Dynamics) is employed to explore the stability of short hadron bunches in high-intensity linacs. In a previous study with the S-POD [M. Goto et al., ], a static potential was used to focus the bunch in the longitudinal direction. We here make a step forward to include the possibility of pure synchrotron resonance, introducing periodic modulation to the longitudinal potential well. The modulation period was taken a half of the transverse alternating-gradient focusing period, which reflects the most typical lattice condition of a drift-tube linac. Detailed stability maps are constructed to reveal dangerous parameter regions where serious beam loss may occur due to resonance. We reconfirm the existence of various betatron and synchrobetatron resonance stop bands whose widths and locations change in tune space depending on the bunch intensity. It turns out that the periodicity of the longitudinal focusing potential brings about no pronounced effect on the resonance feature; the result is very similar to what we obtained in the previous study with a static longitudinal potential. As long as the lattice periodicity mentioned above is maintained, no serious noncoupling synchrotron resonance appears even with a high synchrotron phase advance above 90° per unit alternating-gradient cell. Severe envelope instability may, however, be excited in the longitudinal direction if the axial focusing force includes error components that affect the original lattice periodicity. The experimental observations can be explained with the free from the concept of incoherent tune spread. Published by the American Physical Society 2024

Proposal and design of 81.25 MHz heavy ion drift tube linacs for BISOL

Based on the design requirements proposed by the Beijing On-Line Isotope Separation project (BISOL), four Sn22+-based, 81.25 MHz continuous wave (CW) drift tube linac (DTL) cavities have been designed. These DTLs are capable of accelerating Sn22+ of 0.1 pmA from 0.5 MeV/u to 1.8 MeV/u over a length of 7 m, with an output longitudinal normalized RMS emittance of 0.35π·mm·mrad, and transmission efficiency higher than 95%. The dynamics design adopted the KONUS (Kombinierte Null Grad Struktur Combined 0° Structure) scheme. Comprehensive error study implies that these DTLs can accommodate a wide range of non-ideal beams and cavity alignment errors while maintaining high transmission efficiency. The electromagnetic design employed a Cross-bar H-mode (CH) structure for superior water-cooling characteristics, and a detailed tuning analysis was conducted to derive an optimal tuning scheme. The results of the multiple-physics analysis indicate that the frequency shift of each cavity is within an acceptable range. Comparing the dynamics requirements with the RF design results, similar particle output phase distribution, equivalent energy gain and consistent emittance growth are observed. Detailed designs will be presented in this manuscript.

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.

Commissioning and first operation of East Japan Heavy Ion Center at Yamagata University

A world smallest carbon ion radiotherapy facility, East Japan Heavy Ion Center, Yamagata University, started treatment operations in February 2021. The treatment system consists of an ECR ion source, an RFQ and IH-type drift tube linac, a 430 MeV/u synchrotron and a spot-scanning irradiation system. It has two treatment rooms, one is a fixed horizontal beam port and the other is a rotating gantry beam port with superconducting magnets. The ECR ion source is the Kei2-type permanent magnet 10 GHz ECR ion source with a maximum field of 0.8 T. All the improvements of the previous facilities are applied to the ion source, such as helium gas-assisted operation and changes in the shape of an anode electrode and an extraction electrode to reduce the discharge. Owing to these improvements, the ion source has been operated stably during the commissioning and clinical operation. The irradiation system of Yamagata University eliminated the plastic block range shifter to realize a compact gantry, and has 600 energy levels to control the beam range in step of 0.5 mm. In order to optimize the beam transport parameters quickly, automation tools for orbit correction were developed. Machine commissioning and clinical commissioning were carried out in parallel to allow treatment to begin earlier. First, the horizontal fixed beam port was commissioned by verifying the dose distribution of the treatment planning system. Then, the gantry beam port was commissioned by full beam distribution measurement for representing beam angle of 90 degree, and other angles were sequentially released after the compatibility measurement of beam position, beam size, and two-dimensional uniformity. In September 2022, we were able to accept all treatment sites. By August 2023, 1151 cancer patients had been treated. Further improvements to increase available gantry angle and to increase beam efficiency are ongoing.

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.

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 %.

Field flattening concepts for linac cavities of the cross-bar H type

The development of cross-bar H-type cavities (CH-DTL, H21 mode) is ongoing to improve the technology of this attractive type of drift tube linac (DTL). In comparison with the conventional DTL, the H-type cavities can reach a higher effective field gradient and are competitive in shunt impedance at an energy range up to 100 A MeV. H-mode DTL’s profit in shunt impedance additionally when applying the KONUS beam dynamics. They are in use at research laboratories as well as at hospitals. This paper describes a new concept to approach the zero mode in CH-type cavities, by extending the cavity diameter at the tank ends in combination with tilted drift tube stems. RFQs of the four-vane type are operated as well in the H210 mode and the strategy for voltage flattening can be partly applied there too if conventional vane undercuts are causing problems. Up to around 35 MeV, it is attractive to integrate one or more triplet lenses into each cavity, as one KONUS section is relatively short and would not exploit the full rf power of 3 MW klystrons which are available above 300 MHz. Such a cavity is denoted as a coupled CH-cavity CCH. Three possible arrangements of those internal triplet lenses are discussed and are compared to each other. The operating modes as well as higher modes are then compared with a lens-free CH cavity, where the lens position was filled by ordinary drift tubes. As a result, it is shown that the rf behavior and resonance frequency for the higher harmonics of the frequency band are very similar for all four investigated arrays. This means, that the tuning behavior of the CCH cavity can be simply deduced from the lens-free CH cavity by replacing an even number of ordinary drift tubes (nβλ) with a lens-containing drift tube with adequate length and large outer diameter. This large drift tube itself oscillates like an Alvarez-type drift tube. rf simulations on a 30-gap CH cavity show that the reduction in shunt impedance is about 10% when installing a lens with length 2 beta lambda. Published by the American Physical Society 2024

Alternating phase focusing beam dynamics for drift tube linacs

In contrast to conventional E-mode resonance accelerators, H-mode DTLs provide for compact linac sections and have been established as highly efficient resonators during the last decades. Thus, H-mode structures are widely applied for heavy-ion acceleration with medium beam energies because of their outstanding capability to provide high acceleration gradients with relatively low energy consumption. To build upon those advantages, an alternating phase focusing beam dynamics layout has been applied to provide for a resonance accelerator design without internal lenses, which allows for eased commissioning, routine operation, maintenance, and potential future upgrades. The features of such a channel are going to be demonstrated on the example of two interdigital H-mode cavities, separated by an external quadrupole triplet. This setup provides for heavy ion (mass-to-charge ratio A/z≤6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$A/z\\le 6$\\end{document}) acceleration from 300 keV/u to 1400 keV/u and is used as an injector part of the superconducting continuous wave accelerator HELIAC. Hence, this promising approach generally enables effective and compact routine operation for various applications, such as super heavy ion research, material science, and radio biological applications such as heavy-ion tumor therapy.

Tuning methods for multigap drift tube linacs.

Multigap cavities are used extensively in linear accelerators to achieve velocities up to a few percent of the speed of light, driving nuclear physics research around the world. Unlike for single-gap structures, there is no closed-form expression to calculate the output beam parameters from the cavity voltage and phase. To overcome this, we propose to use a method based on the integration of the first and second moments of the beam distribution through the axially symmetric time-dependent fields of the cavity. A beam-based calibration between the model's electric field scaling and the machine's rf amplitudes is presented, yielding a fast online energy change method, returning cavity amplitude and phase necessary for a desired output beam energy and energy spread. The method is validated with 23Na6+ beam energy measurements.

Alvarez Drift Tube Linac for Medical Applications in the Framework of Hitriplus Project

A first beam dynamics and RF design of an Alvarez-type drift tube linac (DTL) has been defined in the framework of the EU project, HITRIplus. It is meant primarily as a carbon (12C4+) and helium (4He2+) ion injector of a compact synchrotron for patient treatment. As a second implementation, helium particle acceleration with a higher duty cycle (10%) enables radioisotope production. The 352.2 MHz structure efficiently accelerates ion species with A/q=3 and 2, in the energy range from 1 to 5 MeV/u and for a beam current up to ∼0.5 mA. The design extends to a full length of ∼6.4 meters. Permanent magnet quadrupoles are utilized all along the DTL for focusing both ion beams. This paper presents a first-phase analysis towards a realistic DTL design capable of providing full beam transmission and minimum overall emittance increase for both A/q values.

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.

Comparison of 352 MHz linac structures for injection into an ion therapy accelerator

In the frame of ongoing initiatives for the design of a new generation of synchrotron-based accelerators for cancer therapy with ion beams, an analysis of linac designs has been started, to address a critical element with a strong impact on the performance and cost of the accelerator. The goal is to identify alternatives at lower cost and a similar or possibly smaller footprint than the standard 217 MHz injector presently used in all carbon therapy facilities in Europe. As an additional feature, a new linac design can be tailored to produce radioisotopes for treatment and diagnostics in parallel with operation as a synchrotron injector. In this paper, the attractive option of moving to 352 MHz frequency is analyzed, to a profit of reliable mechanical designs already developed for protons and of the cost savings that can be obtained using Radiofrequency (RF) power sources klystrons with a much lower cost per Watt than tubes or solid-state units. The paper is presenting a Quasi-Alvarez Drift Tube Linac (QA-DTL) version of an injector linac for carbon ions at q/A=1/3 and compare it with Inter-Digital H-Mode DTL (IH-DTL) designs. The option of a Separated-tank IH-DTL structure (S-IH-DTL) is also discussed, along with a standard IH-DTL, both at 352 MHz. Finally, a DTL design at 352 MHz for the injection of fully stripped helium ions into the synchrotron is presented.

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.

Frequency detuning analysis of drift tube linac for CSNS

The China Spallation Neutron Source (CSNS) has been operational for five years. As the beam power increased from 10 kW to 140 kW, the radio frequency (RF) pulse width also increased, leading to an increased cavity frequency detuning along with the beam pulse width. Simulations were conducted to analyze the frequency shift of the drift tube linac (DTL) cavities under a pulse width of 650 μs and a repetition frequency of 25 Hz. These simulations investigated the contributions of the cavities' components to the frequency offset, and the results were consistent with the operating data. To evaluate the frequency detuning of the cavities in CSNS-Ⅱ with an extended pulse width, the sensitivity of frequency detuning to different pulse widths and inlet water temperature was assessed. The results indicate that the cavity detuning caused by an extended pulse width can be compensated by reducing the inlet water temperature or adjust the insertion depth of movable tuners in CSNS-Ⅱ.

Study and design of the coupled cavity with ladder RFQ and IH DTL structure for transportable neutron source

A transportable accelerator-driven neutron source is under development in Xi'an Jiaotong University (X-TANS), aiming at realizing various outdoor applications, such as on-site degradation diagnosis of concrete infrastructures and analysis of raw materials. To downsize the accelerator cavity for X-TANS, we coupled the Ladder RFQ and an Interdigital H-type Drift tube linac (IH-DTL) into a single cavity to combine the strength of the RFQ's efficient bunching efficiency and DTL's high accelerating gradient and to distantly reduce the weight and size of the linac. To achieve electromagnetic resonance between two structures and a high coupling factor, a novel coupled structure was adopted by adjusting the last supporting plate at the exit of RFQ. The mode separation between 0-mode and π-mode was expanded to 7.96 MHz. Consequently, the 15 mA proton beam can be accelerated to 2.5 MeV with a length of 1.73 m and the effective transmission of above 90%. The dissipated cavity power was 96.87 kW with an operation frequency of 200 MHz and a duty factor of 3%. The design concept of the coupled structure and the results of the beam dynamics design, electromagnetic simulation and multi-physics will be comprehensively presented in this paper.

High-Energy Part of the Accelerator for the DARIA Compact Neutron Source: Drift Tube Linac

High-Energy Part of the Accelerator for the DARIA Compact Neutron Source: Drift Tube Linac

High-Energy Part of the Accelerator for the Compact Neutron Source DARIA: Drift Tube Linac

High-Energy Part of the Accelerator for the Compact Neutron Source DARIA: Drift Tube Linac