Radio Frequency Quadrupole Research Articles

Design study of compact medical accelerator using superconducting rf quadrupole for boron neutron capture therapy

We investigated the feasibility on the application of a superconducting radio frequency (SRF) niobium cavity to an accelerator-based neutron source for boron neutron capture therapy (BNCT). Neutron source is the key component of BNCT and adopting rf-linac based neutron source realizes a medical care system sufficient to be compact, which can be installed in a hospital and to generate intensive neutron yields that the BNCT requires. However, it is still desirable to improve the efficiency of input power on neutron yields and the achievable accelerate field gradient. SRF accelerator technology potentially allows us to enhance the performance because of its prominent lower ohmic loss and higher sustainable accelerating fields. This paper presents a first feasibility study on the application of a SRF niobium cavity to an accelerator-based neutron source for BNCT, assuming that a superconducting radio frequency quadrupole (SC-RFQ) composed of pure bulk niobium at 4.2 K accelerates the proton/deuteron beams to a beryllium or lithium target for the neutron production of BNCT via $^{7}\mathrm{Li}(\mathrm{p},\mathrm{n})^{7}\mathrm{Be}$, $^{9}\mathrm{Be}(\mathrm{p},\mathrm{n})^{9}\mathrm{B}$, or $^{9}\mathrm{Be}(\mathrm{d},\mathrm{n})^{10}\mathrm{B}$. The following beam parameters were used: beam energy of 2.5 MeV (for Li target)/5 MeV (for Be target), ion source current (50 keV, CW 30 mA), normalized beam emittance of 0.02 cm mrad, and resonance frequency of 325 MHz (for proton)/162.5 MHz (for deuteron). Based on these conditions, we evaluated the feasibility on the following three criteria: comparison of the cooling capacity of the refrigerator to the amount of heat, power consumption of AC, and size of the BNCT system. First, we evaluated the amount of heat generated in a cryomodule by adding the ohmic loss of SC-RFQ ${Q}_{\mathrm{rf}}$, beam losses in SC-RFQ ${Q}_{\mathrm{b}}$, heat penetration into the cryomodule ${Q}_{\mathrm{ext}}$, and beam losses of molecular ion beams and poor quality beams emitted from the ion-source at/near the RFQ entrance ${Q}_{\mathrm{emit}}$. In this study, we typically regarded ${Q}_{\mathrm{ext}}$ as 20 W at 4.2 K and considered a new low-energy beam transport system that can suppress ${Q}_{\mathrm{emit}}$ to 0. In addition, ${Q}_{\mathrm{rf}}$ and ${Q}_{\mathrm{b}}$ were numerically evaluated by beam simulation and electromagnetic calculation. The obtained results revealed that the sum of the heat amounts could be sufficiently suppressed below the typical cooling capacity of a commercially available helium refrigerator. Second, we compared the ac power consumption of BNCT between a conventional and SC-BNCT systems, which indicated that the BNCT system adopting the SRF cavity effectively reduced the ac power consumption of SC-BNCT by almost $1/4$ times. Third, the length of the SC-RFQ could be shortened by adjusting the peak surface E-field, as compared to conventional existing RFQs such as J-PARC, SNS, and IFMIF. Eventually, this study demonstrated that the application of the SRF cavity for the rf-linac-based neutron source of BNCT is feasible, and thus provides a foundation for the future development of design for next-generation BNCT systems.

First Offline Results from the S3 Low-Energy Branch

We present the first results obtained from the S3 Low-Energy Branch, the gas cell setup at SPIRAL2-GANIL, which will be installed behind the S3 spectrometer for atomic and nuclear spectroscopy studies of exotic nuclei. The installation is currently being commissioned offline, with the aim to establish optimum conditions for the operation of the radio frequency quadrupole ion guides, mass separation and ion bunching, providing high-efficiency and low-energy spatial spread for the isotopes of interest. Transmission and mass-resolving power measurements are presented for the different components of the S3-LEB setup. In addition, a single-longitudinal-mode, injection-locked, pumped pulsed-titanium–sapphire laser system has been recently implemented and is used for the first proof-of-principle measurements in an offline laser laboratory. Laser spectroscopy measurements of erbium, which is the commissioning case of the S3 spectrometer, are presented using the 4f126s23H6→4f12(3H)6s6p optical transition.

Design and cold measurements of a coupled RFQ-SFRFQ cavity

Based on the previous research on 26 MHz coupled RFQ-SFRFQ (Radio Frequency Quadrupole-Separated-function Radio Frequency Quadrupole) cavity, we propose a higher frequency coupled RFQ-SFRFQ structure to accelerate proton beam from 35 keV to 2.5 MeV in continuous wave (CW) mode. RFQ and SFRFQ are placed in a single cavity, which greatly reduces the size of the accelerator. Instead of two RF amplifiers, only one RF amplifier is needed with this structure, as a result, this structure becomes more compact. The beam dynamic design and electromagnetic simulation have been accomplished. A 104 MHz coupled RFQ-SFRFQ aluminum model cavity has been constructed and tested at low power successfully, and the results show good agreement with the simulations.

Beam-dynamics design of a 2 MeV/u heavy-ion IH-DTL with electromagnetic quadrupole structure

The interdigital H-mode (IH) drift tube linac (DTL) with an electromagnetic quadrupole (EMQ) structure combines an IH-DTL acceleration structure with a quadrupole focusing–defocusing (FD) lattice and can achieve a high acceptance when high shunt impedance is ensured. Between the radio frequency quadrupole (RFQ) and IH-DTL, the IH-EMQ dynamics allows only one quadrupole magnet to match the two structures directly, which is different from KONUS (Kombinierte Null Grad Struktur, combined zero-degree structure) and alternating-phase focusing (APF) dynamics. To match KONUS or APF DTL with the RFQ, at least three quadrupole magnets are required. This paper describes the design process of the IH-EMQ beam dynamics and applies and verifies the method by applying it to design a 2MeV/u heavy-ion IH-DTL with the help of detailed simulation and analysis. With the input energy of 400keV/u, the DTL accelerates particles with a charge-to-mass ratio as low as 1/6.5. The normalized root mean square emittance increases by 7%. The IH-DTL with the IH-EMQ dynamics design offers a normalized transverse acceptance (99% particles) of 3.89(x)/3.56(y)πmm⋅mrad while preserving an effective shunt impedance of 87MΩ/m.

RF design and tuning of a deuteron RFQ for detecting impurity deposition on a tokamak

Peking University and Southwestern Institute of Physics are jointly developing a new deuteron Radio Frequency Quadrupole (RFQ) accelerator to study the migration and deposition of impurities on the first wall of a tokamak facility. The RFQ operates at 162.5 MHz with the duty factor of 1%, which can accelerate 10 mA deuteron beam from 40 keV up to 1.5 MeV within 2.2 m length. Such four-vane RFQ is divided into two segments and equipped with 40 tuners in total. In the electromagnetic (EM) design, the dipole stabilizer rods (DSRs) were optimized to obtain a mode separation of 3.1 MHz. The tuners and undercuts were also designed and optimized to fulfill the requirements of the resonant frequency, flat longitudinal field distribution and field tuning. Following the EM design, multi-physics analysis and structure error analysis was carried out. The cavity was fabricated with 99.9% oxygen-free electronic (OFE) copper. The assembly and braze of each RFQ module have been completed, the results of metrological measurements via coordinate measuring machine (CMM) indicate that there was no unexpected deformation. The intrinsic Q-value of the whole cavity is 12834 in the low level radio frequency (RF) measurements, which is 87% of the theoretical simulation with electrical conductivity of 5.8 × 107 S/m. After tuning, the quadrupole perturbative component decreased from 2.6% to 0.6%, and the dipole perturbative components decreased from 6.1% to 1%.

Beam dynamics of the project DARIA RFQ

The project DARIA (neutron source dedicated to applied research and industrial applications) directed to the development of compact neutron generators for universities, scientific centers and industry. In framework of the DARIA project, the 3 MeV, 162.5 MHz and 100mA pulse proton Radio Frequency Quadrupole (RFQ) is under development at National Research Centre "Kurchatov Institute". In this article the main aspects of the beam dynamics design of this RFQ are described, namely the choice of RFQ parameters, the optimization of the transport and the transmission. Results of beam dynamic simulation in RFQ are presented.

NG-TRAP: Measuring neutron capture cross-sections of short-lived fission fragments

We lack significant nuclear physics input to understand the rapid-neutron capture (r-)process fully. The r-process is the source of half the elements heavier than iron and the only way to produce the long-lived actinides we find on earth. This process’s key nuclear physics inputs are nuclear masses, cross-sections of (n,γ) and (γ,n), and decay half-lives and branching ratios of neutron-rich isotopes. However, there is currently no method to directly measure neutron-induced reaction rates on short-lived nuclides, so there is no experimental data for the primary nuclear reaction that drives the r-process. We show here a conceptual design of a novel approach to access this information experimentally. The idea is to form a target of short-lived isotopes by confining them as ions in a radio-frequency (RF) trap. Next, they are irradiated with an intense neutron flux, and the reaction products are identified by mass spectrometry. The chosen method is a two-stage process in the presence of high neutron fluxes. The first process is neutron-induced fission in a thin actinide foil to create fission fragments. These fragments are slowed down in a cryogenic stopping cell before being filtered through a radio frequency quadrupole (RFQ) system. The RFQ system selects fission fragments of a specific atomic mass number A and confines them to a small volume in an RF trap, where they are irradiated for a second time in a controlled manner. The resultant A+1 isotopes are mass-selectively transported to a multiple-reflection time-of-flight mass spectrometer, where the reaction products are identified and counted.

High-current H2 + beams from a filament-driven multicusp ion source.

We report the results from a new multicusp ion source (MIST-1) that produces record steady-state currents of H2 + (1 mA) from this type of ion source with high purity (80% H2 +). We built MIST-1 to fulfill the stringent beam purity and beam quality requirements for IsoDAR, a proposed discovery-level neutrino experiment, requiring a 10 mA, 60 MeV/amu continuous wave (cw) proton beam on the target. IsoDAR will use a cyclotron accelerating H2 + ions and using a novel radio frequency quadrupole (RFQ) direct injection method. Systematic measurements, varying discharge voltage, discharge current, and gas pressure, indicate that the ideal operating regime is at low pressure, high discharge current, and high discharge voltage. We have measured the combined species emittance after the source extraction to be <0.05 π-mm-mrad (rms, normalized) for a 0.95 mA beam. Beyond showing high currents and high H2 + fraction, our measurements agree well with high fidelity simulations. These results show the feasibility of using a multicusp ion source for IsoDAR and the RFQ direct injection prototype and paves the way to record breaking cw beam currents of 5 mA H2 + (equivalent to 10 mA protons) from compact cyclotrons, ideal for underground installation.

Formation of Deuterium-Substituted Ions from a Solution Introduced into a Radio-Frequency Quadrupole under the Action of a Gas Jet Passed through an Electron Ionization Ion Source

Formation of Deuterium-Substituted Ions from a Solution Introduced into a Radio-Frequency Quadrupole under the Action of a Gas Jet Passed through an Electron Ionization Ion Source

First Proof-of-Concept Prototype of an Additive Manufactured Radio Frequency Quadrupole

Continuous developments in additive manufacturing (AM) technology are opening up opportunities in novel machining, and improving design alternatives for modern particle accelerator components. One of the most critical, complex, and delicate accelerator elements to manufacture and assemble is the radio frequency quadrupole (RFQ) linear accelerator, which is used as an injector for all large modern proton and ion accelerator systems. For this reason, the RFQ has been selected by a wide European collaboration participating in the AM developments of the I.FAST (Innovation Fostering in Accelerator Science and Technology) Horizon 2020 project. The RFQ is as an excellent candidate to show how sophisticated pure copper accelerator components can be manufactured by AM and how their functionalities can be boosted by this evolving technology. To show the feasibility of the AM process, a prototype RFQ section has been designed, corresponding to one-quarter of a 750 MHz 4-vane RFQ, which was optimised for production with state-of-the-art laser powder bed fusion (L-PBF) technology, and then manufactured in pure copper. To the best of the authors’ knowledge, this is the first RFQ section manufactured in the world by AM. Subsequently, geometrical precision and surface roughness of the prototype were measured. The results obtained are encouraging and confirm the feasibility of AM manufactured high-tech accelerator components. It has been also confirmed that the RFQ geometry, particularly the critical electrode modulation and the complex cooling channels, can be successfully realised thanks to the opportunities provided by the AM technology. Further prototypes will aim to improve surface roughness and to test vacuum properties. In parallel, laboratory measurements will start to test and improve the voltage holding properties of AM manufactured electrode samples.

On the Feasibility of Rovibrational Laser Cooling of Radioactive RaF+ and RaH+ Cations

Polar radioactive molecules have been suggested to be exceptionally sensitive systems in the search for signatures of symmetry-violating effects in their structure. Radium monofluoride (RaF) possesses an especially attractive electronic structure for such searches, as the diagonality of its Franck-Condon matrix enables the implementation of direct laser cooling for precision experiments. To maximize the sensitivity of experiments with short-lived RaF isotopologues, the molecular beam needs to be cooled to the rovibrational ground state. Due to the high kinetic energies and internal temperature of extracted beams at radioactive ion beam (RIB) facilities, in-flight rovibrational cooling would be restricted by a limited interaction timescale. Instead, cooling techniques implemented on ions trapped within a radiofrequency quadrupole cooler-buncher can be highly efficient due to the much longer interaction times (up to seconds). In this work, the feasibility of rovibrationally cooling trapped RaF+ and RaH+ cations with repeated laser excitation is investigated. Due to the highly diagonal nature between the ionic ground state and states in the neutral system, any reduction of the internal temperature of the molecular ions would largely persist through charge-exchange without requiring the use of cryogenic buffer gas cooling. Quasirelativistic X2C and scalar-relativistic ECP calculations were performed to calculate the transition energies to excited electronic states and to study the nature of chemical bonding for both RaF+ and RaH+. The results indicate that optical manipulation of the rovibrational distribution of trapped RaF+ and RaH+ is unfeasible due to the high electronic transition energies, which lie beyond the capabilities of modern laser technology. However, more detailed calculations of the structure of RaH+ might reveal possible laser-cooling pathways.

Experimental study of pulsed ion beam generated by different pulsing topology in 2.45 GHz ECR proton source

Electron Cyclotron Resonance Ion Source (ECRIS) is used as a front end for Low Energy High Intensity Proton Accelerator (LEHIPA). A 50 keV ECRIS is designed and developed for the proton ion beam at BARC. The key requirements of the ion source are stability of ion beam, repeatability, reliability with beam emittance of 0.2 π mm mrad, and proton fraction of 90% or more. Presently LEHIPA accelerated proton beam to 3 MeV from Radio Frequency Quadrupole (RFQ) and further beam experiments are in progress. The beam acceleration from RFQ is in pulsed mode with 250 μs on time at a repetition rate of 2 Hz. The incoming beam from the ion source is of 50 keV, 4 to 5 mA at the entrance of RFQ with a beam on time of 1 ms. In this work, two different beam pulsing methods for ECRIS are experimentally investigated. One is by pulsing the extractor High Voltage (HV) with continuous plasma operation and the other is by pulsing the plasma with continuous extractor HV. The details of the experimental setup, plasma tuning, optimization, extracted beam current results, and comparison of both methods are discussed. It is concluded that for obtaining a stable pulsed ion beam, pulsing the extractor HV is a better option than pulsing the plasma.

Functional Overview of the RF Power System for the LIPAc RFQ

Design, development, manufacturing, and test activities of the RF power system (RFPS) for Linear IFMIF Prototype Accelerator (LIPAc) were completed in Europe. Installation and commissioning activities were carried out at the International Fusion Materials Irradiation Facility-Engineering Validation and Engineering Design Activities (IFMIF/EVEDA) site in Rokkasho, Japan. Challenging IFMIF requirements led to a number of innovations during design and development. Commissioning required a major effort on calibration and fine setting and led to development of new functionalities. The RFPS was validated under full power and 125-mA deuteron beam loading conditions, in the radio frequency quadrupole (RFQ), demonstrating good performance. This article is an overview about how technical challenges impacted the prototype RFPS design and its functional evolution during commissioning, cavity conditioning, and beam operation of the RFQ.

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.

Design and optimization of the RF input-coupler for a low-frequency, split-coaxial RFQ

The Isotope Decay-At-Rest experiment (IsoDAR) is a proposed underground experiment which is expected to be a definitive search for sterile neutrinos. In order to be decisive within 5 years, high rates of neutrinos must be produced, byimpinging a 10 mA continuous wave proton beam at 60 MeV on a high power target.Due to space restrictions, a compact cyclotron was chosen as an accelerator to producethis driver beam.To overcome space charge limitations during injection, H2 + ions are accelerated andlater stripped into protons by means of a carbon foil.IsoDAR uses an especially designed low-frequency (32.8 MHz) split-coaxial Radio-FrequencyQuadrupole (RFQ) to effectively bunch H2 + ions before injecting them into this cyclotron.The RFQ will be embedded vertically in the cyclotron yoke,facilitating a very compact design. This puts stringent limits on RFQ size, type, andaccessibility.Here, we present the design and optimization of the low-frequency (32.8 MHz) RF input-couplerfor the IsoDAR RFQ.The design is challenging due to the necessarily small diameter of the RFQ (28 cm)and the split-coaxial type, as well as limited access to the RFQ.We have determined the optimal position and shape for the coupler, leading to a low powerconsumption of < 6 kW for an inter-vane voltage of 22 kV. The highest calculated fieldsare safely below the Kilpatrick limit for this structure.

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.

An ultralow-noise superconducting radio-frequency ion trap for frequency metrology with highly charged ions.

We present a novel ultrastable superconducting radio-frequency (RF) ion trap realized as a combination of an RF cavity and a linear Paul trap. Its RF quadrupole mode at 34.52MHz reaches a quality factor of Q ≈ 2.3 × 105 at a temperature of 4.1K and is used to radially confine ions in an ultralow-noise pseudopotential. This concept is expected to strongly suppress motional heating rates and related frequency shifts that limit the ultimate accuracy achieved in advanced ion traps for frequency metrology. Running with its low-vibration cryogenic cooling system, electron-beam ion trap, and deceleration beamline supplying highly charged ions (HCIs), the superconducting trap offers ideal conditions for optical frequency metrology with ionic species. We report its proof-of-principle operation as a quadrupole-mass filter with HCIs and trapping of Doppler-cooled 9Be+ Coulomb crystals.

Radio-frequency waveform investigation for ion transport within the RFQcb at ISOLDE's Offline 2 facility

A theoretical and practical investigation into the relation of applied waveform, voltage, and frequency of the quadrupole potential applied to the Radio-Frequency Quadrupole cooler and buncher (RFQcb) used at the ISOLDE facility and the Offline 2 facility. Non-radioactive ion beams were transported through the RFQcb over a range of quadrupole parameters to quantify beam transport efficiency via pre and post beam current measurements. Maximum beam transport was directly compared to the theoretical model of ion stability within an oscillatory quadrupolar field via the solutions of the Hill's equation (shown within this work) for each waveform applied the RFQcb and over several masses 20Ne, 40Ar, 84Kr and 131Xe. The result of this work has led to interesting findings on the beam transport dependency with the waveform, the potential to reduce the overall costs of future RFQ's, and possible molecular capture break-up options inside RFQcb structure.

Rf measurements and tuning of the 1-m-long 750 MHz radio-frequency quadrupole for artwork analysis

The 750 MHz PIXE-RFQ (radio-frequency quadrupole), developed and built at CERN, provides 2 MeV protons over a length of one meter for proton-induced X-ray emission analysis (PIXE) of cultural heritage artwork. In this paper, we report low-power rf measurements and tuning of the PIXE-RFQ, which have been completed mid-2020. Using a novel algorithm based on direct measurements of the response matrix, field and frequency could be tuned at the same time in only two steps to satisfying accuracy. Additionally, we report measurements of single modules, quality factors, and coupling strength. In all cases, very good agreement between rf measurement and design values was observed.

Manufacturing, assembling and plasma cleaning of new SANAEM RFQ

A new 1.2 m radio frequency quadrupole (RFQ) accelerator cavity, made of oxygen-free copper (OFC) and capable of accelerating protons up to 1.3 MeV energy, is recently manufactured for the Turkish Atomic Energy Authority (TAEA), based on the motivation of establishing a proton beamline with indigenous capabilities. In the present article, the manufacturing and assembling procedures related to the new RFQ copper vanes, especially the novelties applied to a vane type RFQ accelerator cavity, namely plasma cleaning of vane surfaces and utilization of indium wire in vacuum sealing, will be described. The preliminary results, the vacuum level and quality factor, suggest that these applications indicate promising options for future accelerator cavity production.