Radio Frequency Quadrupole Linear Accelerator Research Articles

Fabrication and RF test of the 500 MHz-RFQ linear accelerator for a transportable neutron source RANS-III

At RIKEN, a transportable accelerator-driven compact neutron source (RANS-III) is under development for an on-site nondestructive inspection of the degradation of old concrete and reinforcing steel. RANS-III consists of an ion source, a low-energy beam transport, a radio frequency quadrupole linear accelerator (RFQ linac), a radio frequency (RF) system, a high-energy beam transport, a target station and a neutron measurement system. Because the inner diameter of the RFQ linac is inversely proportional to the resonance frequency, the resonance frequency of the RANS-III RFQ linac in this study was chosen to be 500 MHz, which is 2.5 times that of the RANS-II RFQ linac. Therefore, the inner diameter and weight of the RANS-III RFQ linac were reduced to approximately half and one third, respectively, of those of the RANS-II RFQ linac. The RANS-III RFQ linac was designed to accelerate a proton beam with a 10 mA peak current and 100 μA average beam current from 30 keV to 2.49 MeV (Journal of Disaster Research 12(3) (2017) 585–592). Based on the evaluations, an RFQ linac for RANS-III was fabricated, and the RF characteristics of the cavity, such as the resonant frequency and electric-field distribution, were measured using a low-power test and tuned using fixed tuners. In addition, RF couplers and RF systems were constructed to inject RF power into the RANS-III RFQ linac, and RF input tests were performed.

Demonstration of an intense lithium beam for forward-directed pulsed neutron generation

As an alternative to research nuclear reactors, a compact accelerator-driven neutron generator that uses a lithium beam driver could be a promising candidate since it produces almost no undesired radiation. However, providing an intense lithium-ion beam has been difficult, and it has been thought that the practical application of such a device would be impossible. The most critical problem of insufficient ion fluxes has been solved by applying a direct plasma injection scheme. In this scheme, a pulsed high-density plasma from a metallic lithium foil generated by laser ablation is efficiently injected and accelerated by a radio-frequency quadrupole linear accelerator (RFQ linac). We have obtained a peak beam current of 35 mA accelerated to 1.43 MeV, which is two orders of magnitude higher than a conventional injector and accelerator system can deliver.

J-PARC H- ion source and space-charge neutralized LEBT for 100-mA high energy and high duty factor LINACs.

In July 2019, a design beam of 8.3 × 1013 protons/pulse, whose 25 Hz operation produced the design beam power of 1 MW, was accelerated for 10.5 h by using the J-PARC 3 GeV rapid cycling synchrotron. During the operation, a J-PARC H- ion source and a 400 MeV linear accelerator (LINAC) were operated with the design beam duty factor (BDF) of 1.25% (0.5 ms × 25 Hz) and ejected beam intensities of 58 mA and 50 mA, respectively. Furthermore, as expected from the transverse emittances measured in an H- ion source test-stand, they were successfully operated with the BDF of 1.5% (0.6 ms × 25 Hz) and ejected beam intensities of 72 mA and 60 mA, respectively. In the operation, the beam pulse accelerated by the J-PARC radio-frequency quadrupole LINAC (RFQ) had a rapid rise-time of about 10 µs and an excellent flatness with a slow variation of ±0.2%. A flat beam pulse was produced by the predicted 100% space-charge neutralization (SCN) in the upstream half of the J-PARC low-energy beam transport (LEBT) with a vacuum pressure of 3.6 × 10-4 Pa and the unpredicted high and almost constant SCN of about 69% in the downstream half of the LEBT with a vacuum pressure of 2.8 × 10-5 Pa. The rapid rise-time was produced by the preformed 100% SCN with a preceding 100 µs 40 keV beam, which was not accelerated by the RFQ due to the low energy outside the RFQ acceptance. In the test-stand operated with the terminal voltage of 62 kV and BDF of 5% (1 ms × 50 Hz), the transverse emittances of a 100-mA beam were also measured to be suitable for the RFQs of high energy LINACs. A 100-mA high energy and high duty factor LINAC could be realized with this source since the transverse emittances were further improved by about 8% with the shortest beam extractor in the test-stand.

Neutron generator based on intense lithium beam driver.

We are proposing a compact neutron generator based on a Li beam driver. The proposed neutron generator comprises a laser ion source, a radio-frequency quadrupole linear accelerator (RFQ linac), a drift tube linac, and a target containing protons. In the generator, the lithium ion is used as a projectile instead of protons to utilize the kinematic focusing technique. The technique enables us to enhance the neutron flux without increasing the beam energy, which is important to develop a clean compact neutron generator. Moreover, the combination of a laser ion source and a RFQ linac with the direct plasma injection scheme will provide several tens of mA of a fully ionized lithium beam, which is much higher than that of conventional heavy ion sources comparable with proton drivers. Neutrons are generated by the nuclear reaction of the lithium ions and protons in the beam target. In this paper, we reported the current status of the development. For RFQ, we designed the RFQ rods to accelerate 40 mA of 7Li3+. We fabricated and installed the rods into a cavity, and, as a first test, accelerated 10 mA of C6+ successfully.

Development of a downsized proton accelerator system for compact neutron sources

In order to facilitate the widespread use of neutrons in industrial applications, it is important to provide a compact, semi-ready-made neutron source that a company and a factory can easily install. We are therefore developing a compact neutron source system utilizing a downsized proton accelerator as an entry level model for industrial use and materials development, which consists of a proton source, a four-vane radio frequency quadrupole linear accelerator (RFQ) and a lithium target for neutron production by the Li(p,n)37Be47 reaction. An RFQ is suitable to accelerate protons in the low energy region because it accepts DC beams extracted from the proton source and accelerates effectively to the design energy with bunching and focusing using the electric fields. Furthermore, it is possible to design the RFQ with a higher frequency than conventionally used owing to technological advances in high power solid-state RF amplifiers. We investigated the specifications for 500 MHz and 650 MHz through beam trajectory and electromagnetic simulations and fabricated a 2.5 MeV prototype of the 500 MHz type with a 2.45 GHz ECR proton source.

Tuning an 81.25 MHz four-vane RFQ with a ramped field profile at RISP

A radio-frequency quadrupole linear accelerator (RFQ) was developed and tuned for the heavy ion accelerator facility at Rare Isotope Science Project (RISP). The RISP RFQ has an 81.25 MHz operational frequency and a four-vane structure for a continuous-wave mode operation despite the fabrication difficulties of the huge cavity generated due to the brazing technology. The cavity is inherently insensitive to perturbations due to low frequency and a short cavity length. The linearly increasing profile of inter-vane voltage was tuned for all quadrants through not only movable slug tuners but also modification of the end-plate. In this study, a low-frequency RFQ with a novel, ramped field profile was tuned and the commissioning tests were conducted with a new tuning method compatible with the modification of the end-region geometry.

Laser ion source with solenoid field

Pulse length extension of highly charged ion beam generated from a laser ion source is experimentally demonstrated. The laser ion source (LIS) has been recognized as one of the most powerful heavy ion source. However, it was difficult to provide long pulse beams. By applying a solenoid field (90 mT, 1 m) at plasma drifting section, a pulse length of carbon ion beam reached 3.2 μs which was 4.4 times longer than the width from a conventional LIS. The particle number of carbon ions accelerated by a radio frequency quadrupole linear accelerator was 1.2 × 1011, which was provided by a single 1 J Nd-YAG laser shot. A laser ion source with solenoid field could be used in a next generation heavy ion accelerator.

Conceptual design of an 88-MHz KBSI radio-frequency quadrupole linear accelerator

In order to accelerate ions, a radio-frequency quadrupole (RFQ) linear accelerator is being developed at the Korea Basic Science Institute (KBSI). The KBSI RFQ is designed to accelerate 1-pmA of lithium trivalent ions (A/Q = 7/3) by using a four-vane structure. We chose 88-MHz for the RF operating frequency. The energy of the beam injected into the RFQ is 12 keV/u whereas the energy of the downstream output beam is 300 keV/u. The beam dynamics inside RFQ linear accelerator was studied using the PARMTEQM code. The length of the RFQ was approximately 2 m with a transmission rate of 97.4% for lithium trivalent ions. For the structural design, a 2-dimensional geometry structure was studied using the SUPERFISH code near the resonance frequency of the quadrupole and the dipole modes. The 3-dimensional field distributions inside the RFQ cavity were investigated using CST Microwave Studio. The RFQ’s conceptual design and preliminary results for this model are reported.

Superconducting RFQ linear accelerator

We are investigating the performance capabilities of a niobium superconducting radiofrequency quadrupole (RFQ) accelerator for high-field continuous-wave operation, to provide greater acceleration and stronger focusing of low-velocity ion beams. We present an example of an RFQ to produce radioisotopes for positron-emission tomography (PET), and we report on the mechanical design aspects of a short, superconducting RFQ four-vane prototype structure that will be tested at high fields during the coming year.

Mechanical Features of the ATS RFQ Linear Accelerator

A radio-frequency quadrupole (RFQ) linear accelerator has been constructed and placed in operation on the Los Alamos National Laboratory accelerator test stand (ATS). This accelerator uses an evacuated rf manifold to distribute rf excitation from the 425-MHz rf power supply to the slot-coupled, RFQ vane-cavity, resonator assembly. The RFQ vanes are supported on commercially available copper-plated, linear, resilient "C-seals" to provide a high-conductivity rf contact that permits aligning and positioning the vanes during tuning, and demounting the vanes for evaluation and modification as necessary. All rf structures are fabricated from stress-relieved, bright-acid copperplated carbon steel. Measurements made on the accelerator as assembled have demonstrated >8000 vane-cavity Q at the quadrupole's ~423.400-MHz accelerating-mode frequency. Operating manifold vacuum of 3-6 × 10-8 torr has been observed after rf conditioning; conditioning required 150 h for stable high-power rf operation. Experience to date has indicated the desirability of modifying the vane rf-contact seat configuration to improve assembly and alignment procedures, improving vane-machining processes to increase vane straightness, installing periodic vane-shorting rings to minimize the effect of dipole modes in the quadrupole accelerating structure, and modifying the waveguide-coupling slot in the manifold to improve forward rf power flow.

Performance of Neutron Radiography Systems Using Particle Accelerators

The most crucial element of building a useful neutron radiography system is an intense source of neutrons. Among nuclear reactions employing accelerators, the 9Be(d,n)10B reaction is probably the best method for generating neutrons for radiography. This is due to its superior yield of neutrons that can be efficiently moderated and very low gamma-ray contamination. This has been demonstrated using a KN-4000 Van de Graaff accelerator coupled to an IRT Model ANR-100 moderator-collimator assembly, and excellent quality radiographs have been obtained. The Radio Frequency Quadrupole Linear Accelerator is found to be capable of providing an intense source of neutrons using the same reaction and is recommended for neutron radiography applications.