Radio Frequency Quadrupole Linac Research Articles

Field tuning study and RF electromagnetic design of a continuous-wave RFQ with ramped inter-vane voltage for AB-BNCT

A compact radio frequency quadrupole (RFQ) linac for accelerator-based boron neutron capture therapy is being developed at Xi'an Jiaotong University. By adopting a ramped inter-vane voltage in the beam dynamics design, the RFQ could accelerate a continuous wave proton beam to 2.6 MeV over a length of 4 m, effectively decreasing costs and saving space. To meet the beam-dynamics requirements, both the vane width and undercut were optimized during the electromagnetic design. In addition, π-mode stabilization loops were included to increase the mode separation, and fixed tuners with varying diameters were installed to obtain close frequency-tuning sensitivity. It was established that the field unflatness can be controlled within ±1.5% when all tuners remain at their nominal insertion depth. Optimized cooling-channel design was completed, and multi-physics analysis was conducted. The water tuning coefficients of the vane and wall channels were analyzed to establish the ability to fine tune the RFQ during operation.

Cavity tuning and beam commissioning of the radio-frequency quadrupole linac for a transportable neutron source

A transportable, compact, accelerator-based neutron source is under development at Xi’an Jiaotong University. At the heart of the linear accelerator is a radio-frequency quadrupole (RFQ) unit that has been designed to accelerate a proton beam from 30 keV to 2.5 MeV with a peak current of 12 mA and a duty factor of 3%. Recently, the manufacturing of the RFQ cavity has been completed. Cavity tuning, including field unevenness, resonant frequency, and rf coupling coefficient, was carried out after finishing brazing and assembly. The quadrupole perturbative component was controlled within ±2.0%, and the two dipole components were about ±2.5%. The value of the intrinsic Q was 9609, 89% of that of the theoretical simulation. We have completed the installation of the RFQ experimental setup and performed beam commissioning experiments. The output beam current reached 10.6 mA with a transmission efficiency of 93.8%.

Rf and thermal design studies of a 704.4 MHz, β≈0.2 , continuous-wave rf quadrupole accelerator for frequency jump

The radio-frequency quadrupole (RFQ) linac is often used as the first radio-frequency (rf) structure for both bunching and preaccelerating proton or heavy ion beams directly behind an ion source or a low-energy beam transport section. Typically, RFQ linacs operate at frequencies below 450 MHz and beam velocity ranges from β=0.01–0.08. Taking the MYRRHA project as an example, a 704.4 MHz continuous-wave (cw) RFQ linac has been recently proposed to realize an efficient fourfold frequency jump at β≈0.2. Due to its high frequency, small dimensions, and cw operation mode, careful design studies for the RFQ cavity structure including the water-cooling channels have been further performed, integrating the idea of using additive manufacturing technology for future construction. Systematic rf and thermal simulations and analysis demonstrated the reliability of the 704.4 MHz RFQ linac at cw operation and the frequency tuning ability via water-speed adjustments. This investigation shows that it could be a highly promising solution to perform an efficient frequency jump with a high-frequency RFQ accelerator at β≈0.2 for large-scale hadron linacs, which could potentially shorten the low- and medium-β parts of the whole facility. Published by the American Physical Society 2024

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.

Beam dynamic design and study of a linac with variable output energy for an AB-BNCT facility

The Accelerator-based Boron Neutron Capture Therapy (AB-BNCT) is being established worldwide as a future modality to start an era of in-hospital facilities. The most popular reaction for AB-BNCT is 7Li(p, n)7Be and high-flux neutron beams can be produced by bombarding lithium targets with low-energy proton beams. We chose the combined acceleration structure of Radio Frequency Quadrupole linac (RFQ) and Cross-bar H-mode DTL (CH-DTL) considering the compactness of the structure and the adjustment of output energy. The CW proton beam could be accelerated to 1.8 MeV by one RFQ cavity, and then to the final energy by one CH-DTL cavity. The output beam energy can be adjusted in the range of 2.2 MeV to 3.0 MeV with small beam loss and high beam quality, which could be achieved by controlling the feed power and RF phase of the CH-DTL. The variation of beam energy can meet requirements of the BNCT in treating tumors of different depths without adjusting structure of beam shaping assembly (BSA). The beam dynamics of the RFQ and DTL were completed to meet all requirements and the energy stability and adjustment method of output beam were investigated. In addition, we also performed the start-to-end beam tracking and error sensitivity analysis at last.

RF power coupling of RAON RFQ

A radio frequency quadrupole (RFQ) has been constructed as a front accelerator for a rare isotope accelerator facility. The RFQ has two power couplers that are operated simultaneously for high power operation to accelerate from proton to uranium. Coaxial transmission lines have been adopted to connect each power coupler to the 80 kW solid state power amplifier(SSPA) which is consisted with combiners, circulators and 5 kW unit amplifiers. In this study, the coupling to the cavity with two power couplers was investigated for the failure of RF power units. Moreover, the operation scenario of the RFQ linac was also presented in considering the availability.

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.

Design of a compact RFQ linac for the transportable neutron source

A transportable accelerator-driven neutron source based on the Radio Frequency Quadrupole (RFQ) linac is developing at Xi’an Jiaotong University (X-TANS). It will be a promising tool for the non-destructive inspection of immovable objects. The neutron yield above 10 11n/s can be obtained via 7Li(p,n)7Be reaction with a proton beam energy of 2.5 MeV. To exactly guarantee the transportability of the neutron source, it is essential to achieve the light, compact and low-power consumption of RFQ linac. The operation frequency of 325 MHz is chosen considering the balance among cavity size, cavity consumption, and electrode aperture. In addition, compact dynamics scheme is adopted to achieve short RFQ by sacrificing a beam bunching efficiency. The beam dynamics design and optimization have been performed based on Parmteqm. Beam tracking results show that RFQ can accelerate the proton beam to 2.5 MeV with an electrode length of 2.6 m and an effective transmission efficiency of 93.8%. To enlarge the redundancy of the input beam mismatch and make the beam match easier from LEBT to RFQ, the electrode aperture is designed to shrink gradually in the RFQ entrance. In addition, we also performed the RF electromagnetic design and optimization based on the full-length 3D RFQ model. Multiphysics simulations of the cavity in different working conditions are investigated at last.

Theoretical and experimental studies of multi-port RF power feeding in rod-type radio frequency quadrupole (RFQ) linac

An analytical and experimental study of multi-port RF power feeding in a four rod-type radio frequency quadrupole (RFQ) has been discussed. First, we have derived generalized relations for multi-coupler power feeding based on S-parameters for n balanced couplers. Using simulation and measurements on a prototype, the optimum position and orientation of two and four couplers in an 80 MHz extended vane rod type RFQ have been determined. A low power RF test has been carried out to show the phase and power balancing using S parameter analysis for two and four mock-up couplers for the RFQ. As a corroborative study, the vane tip voltage measured with a vector voltmeter showed that voltage corresponding to power fed through a single coupler (β = 1) agrees with the value when the same power is fed using two or four couplers.

Soundness evaluation of J-PARC RF ion source after 5-month continuous operation

In the J-PARC user operation from Nov. 2020 – Apr. 2021, continuous operation of J-PARC Radio Frequency (RF) negative hydrogen ion (H-) source up to 3,651 hours (5 months) has been achieved. The ion source was operated with the output H- current of 60 mA, the duty factor (for plasma generation) 2% and the input RF power up to 30 kW. After the operation, phase space diagrams at the Radio Frequency Quadrupole linac (RFQ) entrance were measured by the emittance monitor at the ion source test stand (IS-TS) under the same operation condition as in the J-PARC Linac. Comparison of the phase spaces and the beam emittances between the ion sources in the present and the previous operations shows slight difference. From the direct observation of the antenna coil, no exhaustion or the decrease in the thickness of the enamel coating of the coil have been confirmed. The results indicate the possibility of the next goal of the long-run up to 7 months, which is the same as the full duration of the J-PARC user operation in 1 year.

Schemes for producing multi-energy fast neutrons using a low-energy ion accelerator

In this article we explore neutron producing schemes using a compact accelerator for producing fast neutrons for nuclear data measurements relevant to GEN-IV reactors and Accelerator Driven Systems and also for other applications such as radioisotopes production. The compact accelerator which consists of an Electron Cyclotron Resonance ion source along with a Radio Frequency Quadrupole and Drift Tube Linac is expected to provide p,d and 7Li beams of energies in the range of 0.5–3 MeV/u. Two different neutron producing schemes are examined for their suitability for the above mentioned applications. In the first scheme, using the 7Li(d,n)2 4He reaction, a variety of neutron energies ranging from 1 MeV to 16 MeV with reasonable flux and energy-time characteristics can be generated making it suitable for studies in (n,cp), (n,f) and (n,Xn) reactions important for nuclear fusion technology, security applications and radioisotope production. In the second scheme, the p(7Li,n)7Be reaction can be used to produce quasi mono-energetic neutrons in inverse kinematics mode with very little background making it suitable for high precision nuclear data measurements relevant to GEN-IV reactors and Accelerator Driven Systems.

Physics design of a coupled RFQ-DTL cavity for a compact neutron source

Neutrons have been widely used, such as neutron imaging, transmutation of nuclear waste, Neutron Capture Therapy (NCT), et al. Neutrons are generally produced by nuclear reactors or accelerator-driven neutron sources. Comparing with nuclear reactors, accelerator-driven neutron sources have many advantages, such as compact and cheap. We proposed a coupled RFQ-DTL cavity as the main component of a miniaturized neutron source. A high accelerating efficiency was achieved by placing Radio Frequency Quadrupole (RFQ) and Drift Tube Linac (DTL) in one cavity. Since the fringe field will increase the energy spread, we propose a method using the fringe field as a buncher to reduce the negative effect, and it will eliminate the use of external bunchers. The coupled RFQ-DTL cavity will accelerator a 25 mA proton beam from 35 keV to 2.5 MeV within 2.23 m. RFQ section and DTL section are 1.55 m and 0.68 m, respectively. Compared with a conventional RFQ, the length of coupled RFQ-DTL is shorten by 53%. The total transmission efficiency is 98.55%. A variable energy design for the beam output energy can be achieved by changing the insertion depth of the DTL tuner and controlling the feeding power of the cavity.

Efficacy of radial matching section in enhancing RFQ cooler acceptance and optimization of its injection optics

Radio Frequency Quadrupole (RFQ) cooler is a device used for reducing transverse and longitudinal emittance of ion beams. The cooler employs a time varying field and as a result of which the acceptance ellipse, as seen by the beam while entering the cooler, is dependent on the RF phase. Acceptance ellipses having different orientations corresponding to the changing RF phase have a much smaller phase independent central overlapping area, so the real transmission for an injected DC beam into the RFQ cooler gets reduced. In RFQ linacs radial matching section has long been used for matching “time independent” DC input beam to the “time dependent” RF structure. Our simulation study shows that a similar radial matching section at the entrance of RFQ cooler can enhance the transmission for a DC beam into the RFQ cooler. Optimization of length of the radial matching section and the corresponding deceleration and injection optics shows an enhancement of more than two times in the area of time independent acceptance ellipse for ions over a mass range of 0A=4 to 240. Such innovative design could play a significant role in experiments with rare ion beams which are produced in miniscule quantities.

Integration of 175-MHz LIPAc RF System and RFQ Linac for Beam Commissioning

The eight-chain RF system and radio frequency quadrupole (RFQ) linac were successfully integrated at the facility of the Linear IFMIF Prototype Accelerator (LIPAc). We achieved an unprecedented high current deuteron beam up to 55 mA in pulse operation ( $300~\mu \text{s}$ , 1 Hz). The keys of the success were (1) understanding reflected power from RFQ to RF system, (2) tuning each RF chain individually, (3) intensive RF conditioning for holding the stable RF power in the RFQ, and (4) optimizing the beam loading compensation control. Interestingly, the reflection at the breakdown in RF conditioning was higher than an estimation and reached up to 600 kW so far. In order to avoid damage on tetrodes by the high reflection, the impedance of each circulator was accurately adjusted.

Measurement of quadrupolar asymmetry in prototype rod-type radio frequency quadrupole linac for protons.

An analytical and cold model study of a prototype extended vane rod-type RFQ (radio frequency quadrupole linac) is presented. The bead-pull measurement allowed us to experimentally determine the effect of shaping of the vane-supporting posts and, most interestingly, the size of the vacuum enclosure in the dipolar and quadrupolar field asymmetry. Disk shaped beads of BaTiO3 have been tailor made for this purpose. The special shape of the beads has allowed precise measurements of frequency shifts all along the length of the RFQ. The measured quadrupolar asymmetry and dipole component at a radial distance of 7 mm from the beam axis are within ±1% for the optimized structure. Particle tracking of a 10 mA proton beam with a simulated 3D field for the optimized full scale RFQ with modulated vanes has been carried out. Simulation shows vertical and horizontal shifts of only 0.01 mm and 0.03 mm, respectively, for the accelerated beam at the exit of RFQ. The percentage of accelerated particles lying within an energy width of ±40 keV about 800 keV is increased from 86% for the original enclosure to 88% for the reduced enclosure, whereas the transmission efficiency is 100% for both the cases.

Optimization of laser-target parameters for the production of stable lithium beam.

A laser ion source coupled with a radio frequency quadrupole linac accelerator is being proposed as a suitable system for the production of a low energy, high-current stable lithium beam. In order to maximize the lithium yield, plasmas generated by laser ablation of different materials based on lithium (Li, LiOH, and LiNbO3) have been characterized by using a Faraday cup and an electrostatic ion analyzer in the time of flight configuration. A wide range of laser power density has been investigated (109-1012 W/cm2) using two Nd:YAG lasers operating at different wavelengths (1064 nm and 532 nm), pulse durations (6 ns and 17 ns), and maximum energies (1400 mJ and 210 mJ). This paper outlines the pros and cons of the investigated materials by studying how the ion energy, yields, and charge state distributions are modified when the laser power density is changed. Considerable attention has been paid to the higher charge states of oxygen, which may occur with the same mass-to-charge ratio of Li3+. The analysis has evidenced that LiNbO3 represents a valid target since it allows minimizing the O6+/7Li3+ ratio down to 2.5% by using a laser power density of 1.8 × 1010 W/cm2. For such a condition, a Li3+ current of 1.4 mA/cm2 has been measured.

Development of a radio frequency quadrupole linac implemented with the equipartitioning beam dynamics scheme

Radio frequency quadrupole linac (RFQ) is the key component which realized modern high-current proton linacs, however, many RFQs are designed based on very conventional design schemes. We developed 3-MeV 50-mA ${\mathrm{H}}^{\ensuremath{-}}$ (negative hydrogen) RFQ based on a beam space-charge physics concept. The equipartitioning scheme, which is widely used in the high intensity linac design, is implemented into the RFQ design. Design performances of 99.1% transmission, $0.24\ensuremath{\pi}\text{ }\text{ }\mathrm{mm}\text{ }\mathrm{mrad}$ transverse normalized rms emittance, and $0.11\ensuremath{\pi}\text{ }\text{ }\mathrm{MeV}\text{ }\mathrm{deg}$ for longitudinal direction are achieved with a vane length of 3 m and the final energy of 3 MeV. An RFQ employing this design concept was fabricated, and the design performance was confirmed by beam commissioning. In this paper, the design and result of the beam commissioning of this fully equipartitioned RFQ are described.

Development of four-beam IH-RFQ linear accelerator

It is difficult to accelerate a high intensity heavy ion beam because the upper limit of the beam current in an RFQ (Radio Frequency Quadrupole) linac depends on the applied voltage at the RFQ electrode and the strength of the space charge effect. A multi-beam RFQ linac, with multiple beam-channels accelerating several beams in parallel in one cavity, and utilizing multiple beams to decrease the space charge effect can be used to achieve a higher beam current. In order to demonstrate that a four-beam IH-RFQ (Interdigital H-type RFQ) linear accelerator is suitable for high-intensity, heavy ion beam acceleration, we have developed a four-beam prototype.A four-beam IH-RFQ cavity was fabricated according to design parameters obtained from our previous detailed simulations. Low-power RF property measurements of the fabricated four-beam IH-RFQ cavity were carried out with a network analyzer. After the low-power RF property measurements, we performed a high-power test and an acceleration test.

The Proton Linac for Compact Neutron Source Daria

We develop a concept of the CW proton linac with the parameters of 13 MeV 162.5 MHz 5 mA. This linac designed for compact neutron source DARIA is based on the linac designed for the project BELA. The different linac layouts were considered. The most perspective linac layout includes Radio Frequency Quadrupole (RFQ) and Drift Tube Linac (DTL) sections with 6D beam matching between them. The drift tube linac section has a modular structure and consists of the separated individually phased IH-cavities with beam focusing by quadrupole magnets between the cavities. This DTL structure provides linac compactness, tuning and commissioning “cavity by cavity”. Results of the beam dynamic simulation and radio frequency parameters of the linac cavities are presented.

Longitudinal Measurements and Beam Tuning in the J-PARC Linac MEBT1

The Japan Proton Accelerator Research Complex (J-PARC) linac is operated with design peak current of 50 mA from 2018. For operation with such a high beam current, it is important to understand transverse and longitudinal beam properties especially in low-velocity region. A medium energy beam transport (MEBT1) line between the 3-MeV radio-frequency quadrupole linac (RFQ) and the 50-MeV drift-tube linac (DTL) is a 3-m-long transport line to match the beam to the DTL and produces a macro pulse configuration for a 3-GeV rapid-cycling synchrotron (RCS). In this paper, recent measurements and beam tuning results in MEBT1 will be presented.