Interdigital H-mode Drift Tube Linac Research Articles

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.

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.

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.

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.

A compact accelerator-driven deuterium–deuterium neutron source using heavy water jet target

A compact accelerator-driven deuterium–deuterium neutron source using a heavy water jet target is under development at the Institute of Nuclear Physics and Chemistry. The neutrons are generated by bombarding a 0.45 mm-thick heavy water jet target with 11 MeV/0.33 mA deuterons. The angular neutron yield between 10–14 MeV is about 6.5×1011 n/s/sr at 0 degree. The D(d,n)3 reaction is adopted to generate high energy neutrons for its relatively high angular neutron yield, outstanding forward directional characteristics and convenient for obtaining, operating and maintaining. To minimize the length of the neutron source, an IH-DTL is adopted for its high accelerating gradient, low input energy and low power consumption. The design of the deuteron accelerator, consisting of a deuteron injector, a four vane Radio-Frequency Quadrupole (RFQ) and an Interdigital H-mode Drift Tube Linac (IH-DTL), is introduced in this paper. The heavy water jet target, which suffers high deuteron beam power density, is also introduced in this paper.

Development of Inter-Digital H-Mode Drift-Tube Linac Prototype With Alternative Phase Focusing for a Muon Linac in the J-PARC Muon G-2/EDM Experiment

An inter-digital H-mode drift-tube linac (IH-DTL) is developed in a muon linac at the J-PARC E34 experiment. IH-DTL will accelerate muons from 0.34 MeV to 4.5 MeV at a drive frequency of 324 MHz. Since IH-DTL adopts an APF method, with which the beam is focused in the transverse direction using the RF field only, the proper beam matching of the phase-space distribution is required before the injection into the IH-DTL. Thus, an IH-DTL prototype was fabricated to evaluate the performance of the cavity and beam transmission. As a preparation of the high-power test, a test coupler is designed and fabricated. In this paper, the development of the coupler and the result of the low-power measurement will be presented.

RF design and study of a 325 MHz 7 MeV APF IH-DTL for an injector of a proton medical accelerator

A compact interdigital H-mode drift-tube linac (IH-DTL) with the alternating-phase-focusing (APF) method, working at 325 MHz was designed for an injector of a proton medical accelerator. When fed in with a proper RF (radio frequency) power, the DTL cavity could establish the corresponding electromagnetic field to accelerate the “proton bunches” from an input energy of 3 MeV to an output energy of 7 MeV successfully, without any additional radial focusing elements. The gap-voltage distribution which was obtained from the CST® Microwave Studio software simulations of the axial electric field was compared with that from the beam dynamics, and the errors met the requirements within ± 5%. In this paper, the RF design procedure and key results of the APF IH-DTL, which include the main RF characteristics of the cavity, frequency sensitivities of the tuners, and coupling factor of the RF power input coupler are presented.

Beam dynamics design for a compact accelerator-driven neutron source

A compact accelerator-driven neutron source (CANS) has been proposed and designed by the Peking University (PKU) RFQ group. The source is based on a compact deuteron RF accelerator that delivers an average current of a few mA of deuterons at 11[Formula: see text]MeV to the target. The accelerator consists of a short radio frequency quadrupole (RFQ) followed by efficient interdigital H-mode drift tube linac (IH-DTL) structure. The total length of the whole accelerator structure is 4.96[Formula: see text]m, including 1.76[Formula: see text]m for the 4-vane RFQ and 2.70[Formula: see text]m for the IH-DTL. The dynamic simulation results show that the beam has excellent quality, with transmission efficiency higher than 99%. The transverse and longitudinal normalized rms emittance at the DTL exit is 0.29[Formula: see text]mm⋅mrad and 0.12 MeV⋅deg, respectively. Details of the beam dynamics of the RFQ and IH-DTL are presented in this paper.

Beam dynamics and error study of the medium energy beam transport line in the Korea Heavy-Ion Medical Accelerator

The Korea Heavy Ion Medical Accelerator consists of an injector and a synchrotron for an ion medical accelerator that is the first carbon-ion therapy system in Korea. The medium energy beam transport(MEBT) line connects the interdigital H-mode drift tube linac and the synchrotron. We investigated the beam conditions after the charge stripper by using the LISE++ and the SRIM codes. The beam was stripped from C4+ into C6+ by using the charge stripper. We investigated the performance of a de-buncher in optimizing the energy spread and the beam distribution in z-dW/W (direction of beam progress-beam and energy) phase. We obtained the results of the tracking simulation and the error analysis by using the TRACK code. Possible misalignments and rotations of the magnets were considered in the simulations. States of the beam were examined when errors occurred in the magnets by the applying analytic fringe field model in TRACK code. The condition for the beam orbit was optimized by using correctors and profile monitors to correct the orbit. In this paper, we focus on the beam dynamics and the error studies dedicated to the MEBT beam line and show the optimized beam parameters for the MEBT.

Design of an alternating phase focusing Interdigital H-mode Drift-Tube-Linac with low injection energy

An Inter-digital H-mode (IH) Drift Tube Linac (DTL) was designed to accelerate a proton beam in the low energy region with high RF efficiency and high gradient. The IH DTL is 1.078m long and operates at 200MHz. Protons could be accelerated from 0.04MeV to 2.4MeV (the β range is from 0.0092 to 0.0714). The method of alternating phase focusing (APF) was applied for beam focusing. The simulation results show that the transmission is 38% and the longitudinal acceptance is approximately 140°. The shunt impedance of the entire cavity is 365MΩ/m. Adjustments of the electric-field were performed, and the beam dynamics design was described. The beam loss and voltage sensitivity were calculated.

Optimization of the three-dimensional electric field in IH-DTL

The tuning process of the three-dimensional electric field near the beam axis is very important in the optimization of the Interdigital H-mode Drift Tube Linac (IH-DTL). The tuning of the longitudinal field distribution, the Kilpatrik (Kp) factor, and the transverse dipole field have been discussed in detail, combined with the radio-frequency tuning process of the 53.667 MHz short IH-DTL cavity, which was designed to accelerate 238U34+ from 0.143 MeV/u to 0.289 MeV/u in the SSC-Linac injector project at the Institute of Modern Physics. The flatness criterion and the tube tuning method are discussed in order to meet the beam dynamics requirements. In the tube tuning process, the energy gain error in the cells should be reduced to less than ±2%, and the Kp factor should be reduced to 1.6. The transverse dipole field and the method that uses a “plunger" to dismiss this dipole field are evaluated. The experience gained from the first cavity optimization benefits the tuning process of the three remaining IH-DTL cavities in the SSC-Linac project.

Effects of voltage errors caused by gap-voltage and automatic-frequency tuning in an alternating-phase-focused linac

A compact injector for a heavy-ion medical-accelerator complex was developed. It consists of an electron-cyclotron-resonance ion-source (ECRIS) and two linacs, which are a radio-frequency-quadrupole (RFQ) linac and an Interdigital H-mode drift-tube-linac (IH-DTL). Beam acceleration tests of the compact injector were performed, and the designed beam quality was verified by the measured results, as reported earlier. Because the method of alternating-phase-focusing (APF) was used for beam focusing of the IH-DTL, the motion of beam ions would be sensitive to gap-voltage errors, caused during tuning of the gap-voltage distribution and by automatic-frequency tuning in actual operation. To study the effects of voltage errors to beam quality, further measurements were performed during acceleration tests. In this report, the effects of voltage errors for the APF IH-DTL are discussed.

Performance of a compact injector for heavy-ion medical accelerators

A compact injector, designed for a heavy-ion medical accelerator complex, was constructed. It consists of an Electron-Cyclotron-Resonance Ion-Source (ECRIS) and two linacs, which are a Radio-Frequency-Quadrupole linac and an Interdigital H-mode Drift-Tube-Linac (IH-DTL) having the same operating frequency of 200 MHz. For beam focusing of the IH-DTL, the method of Alternating-Phase-Focusing (APF) was employed. The compact injector can accelerate heavy ions having a charge-to-mass ratio of q m = 1 3 up to 4.0 MeV / u . Use of the APF IH-DTL and operating frequency of 200 MHz allowed us to design compact linacs; the total length of the two linacs is less than 6 m. Beam-acceleration tests of the compact injector system were performed. The measured intensity of accelerated C 4 + 12 beams with the compact injector was 380 e μ A . Beam transmission of the APF IH-DTL was estimated to be as high as 96%, which is comparable to the value calculated by a simulation code. Transverse phase-space and energy distributions of accelerated beams were measured and compared with those calculated by the simulation code, and we found that they were agreed well with each other.

Model cavity of an alternating-phase-focused IH-DTL

A compact injector for medical accelerators was designed. The injector consists of a compact Electron-Cyclotron-Resonance Ion Source (ECRIS) and two linacs, which are a Radio-Frequency-Quadrupole (RFQ) linac and an Interdigital H-mode Drift-Tube-Linac (IH-DTL) having the same operating frequency of 200 MHz. For beam focusing of the IH-DTL, the method of Alternating-Phase-Focusing (APF) was applied. Before constructing the compact injector, a model cavity of the IH-DTL was manufactured, and measurements of the electric-field distributions were performed. The gap-voltage distributions, obtained by integrating the measured electric field, were compared with those calculated using a three-dimensional field solver. The accuracy of the calculated field distributions is discussed. Finally, fine adjustments of the gap voltages using inductive tuners are described.