RFQ Cooler Research Articles

Towards Tests of an RFQ Cooler and Simulations

The cooling of secondary beams is important for accelerator-based nuclear physics. In the radiofrequency (RF) quadrupole cooler (RFQC), RF fields and ion-gas collisions may give a considerable increase or decrease of the beam transverse emittance and energy spread, depending on a delicate tuning of heating and cooling effects, dominated by the ion beam kinetic energy and the balance of collisions and confinement forces. An extra confinement may be added by a solenoid magnetic field, as in the RFQC prototype installed in the Eltrap machine. This provides a versatile test bench (distinct from a closed accelerator installation) for detailed studies of cooling dynamics and of several RFQC technical optimizations (for gas differential pumping and bias voltages). Modeling concepts and simulation results are summarized. The major RFQC parameters are reviewed, in particular for 133Cs+ collisions against He gas whose pressure pg ranges from 2 to 9 Pa in the reference case, with attention to the extraction, comparing triode/tetrode system, and to the bias voltages. Lower bias voltages request less pg , but provide less cooling of the energy spread.

Targets and ion sources at CERN-ISOLDE — Facilities and developments

At the CERN-ISOLDE radioactive ion beam facility, thick targets are irradiated using a beam of 1.4-GeV protons. One of ISOLDE’s key features is the large choice of ion source types and target materials available, enabling us to select the ideal combination for optimal intensity and purity of the isotopes requested by ISOLDE users. The ever-increasing demands in terms of isotope production yield, beam purity, and overall reliability of the employed systems are driving the continuous development efforts.Over the past few years, CERN has invested heavily in facilities and infrastructure that facilitate ongoing developments required for ISOLDE. A dedicated offline laboratory (Offline 2) has been recently equipped with high repetition rate nanosecond tunable lasers required for scheme development and developments of specialized laser ion source types such as VADLIS, LIST and PI-LIST. Moreover, it hosts a twin setup of the ISOLDE RFQ cooler and buncher (ISCOOL), which is envisaged to be used for studies of molecular beam creation and breakup, as well as the development of improved RFQ services and operational modes. For material development, particularly for nanostructured materials, the new nano laboratory has just been commissioned and will enable the production and development of nano actinide targets for ISOLDE. In this contribution we describe the infrastructure required for target and ion source developments, highlight recent efforts and experimental results on both target material development and ion source development, and provide an outlook on what to expect in the near future.

Recent progress of ISOL facility at RAON

The RAON (Rare Isotope Accelerator complex for ON-line experiments) is currently in the stage of commissioning under the Rare Isotope Science Project (RISP) launched in 2011. RAON will utilize an advanced rare isotope beams produced with a high power target by the Isotope Separation On-Line (ISOL) facility, aiming to deliver high purity and intense, neutron-rich rare isotope beams to the post-accelerator and experimental facilities. The RAON ISOL facility consists of a target/ion source module surrounded by movable shielding blocks in a bunker, remote handling facilities for the target operation, a pre-mass separator, a RFQ cooler buncher, an EBIS charge breeder, and an A/q separator. The installation and alignment of the ISOL facility were completed in June 2021. The target/ion source module allows us to bombard a thick target with a 70 MeV proton beam of RAON, producing a variety of rare isotope beams. Rare isotope beams extracted from the target/ion source can be transported to a pre-mass separator at energies of up to 60 keV, and will be cooled in a RFQ-CB. Cooled ion beams can be sent to two different experimental facilities, such as a Mass Measurement System and a Collinear Laser Spectroscopy in ISOL experimental hall. Alternatively, for post-acceleration of ion beams, the singly charged ion beam of interest can be bunched to 108 ions in RFQ-CB and then delivered to the EBIS charge breeder through the EBIS branch system. The preparation of multi charged ion beams for the post-acceleration using the superconducting LINAC of SCL3 will be carried out through the EBIS charge breeder and A/q separator to match the energy of 10 keV/u with A/q < 6 with the requirement of RAON Injector.The first commissioning experiment of the ISOL system started in March 2021 using the 133Cs and 120Sn stable beams produced from the target container combined with the surface ion source and laser Ion source. The stable beam experiments have demonstrated the overall functioning of the RAON ISOL system, and we are planning to carry out the first RI beam test using the SiC target with 70 MeV proton of cyclotron in the coming months.

Optimization of ion transport in a combined RFQ Cooler with axial magnetic field

The accurate mass spectrometry (with resolution goal 1:20000) of exotic ions requests beams with low energy spread (goal is about 0.5 eVrms or lower) and low transverse emittance, so it is necessary to cool ions produced by a fission source. In a radiofrequency (rf) quadrupole cooler (RFQC), collisions decrease ion kinetic energy, while rf and DC voltages confine and reaccelerate ions towards the extraction, where the cold ion beam is formed. Operation is based on carefully chosen tunings of voltages and of gas pressure p g, which requires an adequate pumping system; efficient simulations of extraction systems, which critically depend on residual speed of ions, are also important. Progresses in the experimental setup are described. Indications from simple ray tracing and ‘ray+collision’ tracing are compared. Results are applied to a simple triode extraction system, and more flexible configurations are considered.

High resolution mass separator dipole design studies for SPES project

The purposes of the SPES (Selective Production of Exotic Species) project at INFN laboratory in Legnaro (Italy) is to study nuclei close to the drip lines. Therefore, a High-Resolution Mass Separator (HRMS) must provide full separation of the ions with a resolution 1/20000, to be sensible to the proton-neutron mass difference in the fission products. SPES HRMS consists of: two 90° magnet dipoles, one electrostatic multipole in between them, six electrostatic quadrupoles, two electrostatic hexapoles and two electrostatic triplets before and after the slits on the object and image point. All these components will be installed on a high voltage platform with a maximum operating voltage of -240 kV. Before entering the HRMS, a 40 keV energy beam go through an RFQ Cooler, designed to have an output energy spread of 1 eV. Mass separation within target resolution is the most critical part: dipoles must provide a magnetic field homogeneity of 4 10−5 throughout beam occupancy (half magnet pole surface), at a field intensity of 0.562 T for the reference ion 132 Sn. Therefore, a very accurate dipole design is mandatory. This contribute will show the studies which lead to a possible dipole design.

Status of the CANREB high-resolution separator at TRIUMF

A new ISOL rare isotope beam production facility, ARIEL, is under construction at TRIUMF. ARIEL aims at increasing the delivery of radioactive beams three fold with respect to the present capability of the ISAC facility. Part of ARIEL is the new CANREB equipment that can be described by the two main functionalities: a charge breeding system that includes RFQ cooler, EBIS and Nier separator, and a high-resolution mass separator system. The latter is designed to achieve a resolving power of twenty thousand for a transmitted emittance of three micrometer. The separator optics has been designed with symmetry in order to minimize high-order aberrations. The dispersion of the system is created by two identical ninety degree magnetic dipoles with a field flatness of one part in one hundred thousand. The dipoles are tested and the magnetic field characterized before being installed on line for operation. High-order aberrations can also be corrected by an electrostatic multipole; this features a novel design as well as a new tuning technique. In this paper we will present the latest results from the field characterization and discuss the high level application to tune the multipole.

A compact RFQ cooler buncher for CRIS experiments

A compact radio frequency cooler buncher (RFQCB) is currently in development between The University of Manchester, KU Leuven, and CERN. The device will be installed as part of the Collinear Resonance Ionisation Spectroscopy (CRIS) experiment at the Isotope separator On-line device (ISOLDE) at CERN. The purpose of developing a dedicated RFQCB for the CRIS experiment is to increase data collection efficiency, and simplify the process of obtaining reference measurements with stable isotopes. The CRIS technique is outlined in addition to an overview of the proposed RFQCB, and its potential compatibility for implementation at ISOLDE.

Time-of-Flight study of molecular beams extracted from the ISOLDE RFQ cooler and buncher

Molecular beams injected into the ISOLDE Radio-Frequency Quadrupole cooler and buncher (RFQcb), ISCOOL, have been studied under varying conditions using a new Time-of-Flight (ToF) detector. When a beam of molecules is injected into the RFQcb and interacts with the buffer gas, collisional dissociation processes may occur. In this study, two different beams of molecules, CO+ and N2+, were separately injected into ISCOOL, and two different buffer gases (pure helium or a 90:10 mixture of helium and neon) were used. The radio-frequency of the RFQcb was varied as the molecules, along with the fragments from the dissociation processes, were extracted from ISCOOL and studied using the new Time-of-Flight detector. The main finding of this work is that the rates for molecular dissociation within ISCOOL were very small for both CO+ and N2+, with the largest rates found for CO+.

Development of a radio-frequency quadrupole cooler for high beam currents

The development of the new RFQ Cooler, called SHIRaC, was carried out. As a part of SPIRAL 2 facility, SHIRaC aims to handle and cool typical SPIRAL 2 beams with large emittances (up to 80 pi.mm.mrad) and high currents (up to 1 uA). Its purposes are to enhance as much as possible the beam quality (transverse geometric emittance of less than 3 pi.mm.mrad and longitudinal energy spread close to 1 eV) and to transmit more than 60 % of ions. Numerical simulations and experimental studies have shown that the required beam quality can be reached only in term of the emittance. The energy spread is very far from expected values. It is sensitive to the space charge and the buffer gas diffusion and more importantly to the RF field derivative effect. The latter arises at the RFQ exit and increases with the RF parameters (the frequency and the amplitude of the RF voltage). Studies allowing to enhance the cooled beam quality, mainly the energy spread reduction, are presented and discussed along this paper. They consist in implementing a miniature RFQ at the RFQ exit. Using this method, it becomes possible to improve the cooled beam quality and to reach 1 eV of longitudinal energy spread and around 1.75 {\pi}.mm.mrad of transverse geometric emittance for beam currents going up to 1 uA. The transport of the cooled beam from SHIRaC towards a HRS has been done with an electrostatic quadrupole triplet. Simulations and first experimental tests showed that more than 95 % of cooled beams can reach the HRS. Finally, developments related to the nuclearization protection methods aiming to avoid the escape of any nuclear matter from the SHIRaC beamline are studied.

The ILIAS project for selective isobar suppression by laser photodetachment

Laser photodetachment is the process when the extra electron of a negative ion is removed by means of laser radiation. This can happen only if the photon energy is larger than the electron affinity of the ion. The process can be used in mass spectrometry to selectively suppress unwanted isobars, provided that the electron affinity of the unwanted isobar is lower than that of the isobar under investigation.At the Ion Laser InterAction Setup (ILIAS) at the University of Vienna laser photodetachment of negative atomic and molecular ions is studied and its applicability for selective isobar suppression in accelerator mass spectrometry (AMS) is evaluated. The setup provides mass separated beams of negative ions with energies up to 30keV. Negative ions are produced in a Middleton type cesium sputter ion source, mass selected and overlapped with a strong continuous wave laser beam. In order to extend the interaction time of ions and laser, the ion beam is decelerated to thermal energies in a gas-filled radio frequency quadrupole cooler. For an appropriate choice of the photon energy, unwanted isobars are neutralized while the isobar of interest is unaffected and remains negatively charged.A description of the ILIAS setup and results from the commissioning phase of the RFQ cooler are presented. Up to 8% ion beam transmission could be achieved after a recent redesign of the extraction system. Furthermore first results of photodetachment experiments of 63Cu− within the RFQ cooler are presented.

Energy Spread and Emittance Simulation for RISP RFQ Cooler

RFQ cooler of RAON facility is a device to enhance the quality of ion beam. It is to secure the resolving power of high resolution mass separator by reducing the energy spread and the emittance of ion beam. Key components of this device are deceleration optics for ions with kinetic energy of up to 40 keV, segmented RFQ electrodes for trapping and cooling, and extraction optics for reacceleration. Using SIMION simulation code, we could test the device and characterize the output beam quality. Output emittance of less than 3  mm mrad and energy spread of less than 1 eV were obtained in this simulation.

Status of Lanzhou Penning Trap for accurate mass measurements

The Lanzhou Penning Trap (LPT) is an ion trap facility that aims at accurate mass measurements on fusion-evaporation residues possibly including heavy isotopes and is presently under construction at the Institute of Modern Physics, Chinese Academy of Sciences. The progress and status of the LPT system, including the RFQ cooler and buncher RFQ1L, the Penning trap system, and the 7T home-made superconducting magnet, are reported.

RF power supplies for the RFQ cooler and buncher RFQ1L

In order to match directly the beams from the gas-filled recoil separator, a RFQ cooler and buncher RFQ1L has been designed to have a large radius (r0=60 mm), thus the RF power supply becomes not a trivial problem. We have successfully solved this problem by two different methods, and the RF power supplies specific to the RFQ1L have been developed and tested. Technical descriptions and test results are presented in this paper. It shows that the power supply with series resonance circuits should be used for the operation of the RFQ1L.

DESIR high resolution separator at GANIL, France

A high-resolution separator for the SPIRAL2/DESIR project at GANIL has been designed. The extracted isotopes from SPIRAL2 will be transported to and cooled in a RFQ cooler yielding beams with very low transverse emittance and energy spread. These beams will then be accelerated to 60 keV and sent to a high-resolution mass separator where a specific isotope will be selected. The good beam properties extracted from the RFQ cooler will allow one to obtain a mass resolution of c26000 with the high-resolution mass separator.

CARIBU: a new facility for the study of neutron-rich isotopes

The Californium Rare Ion Breeder Upgrade (CARIBU) to the ATLAS superconducting linac facility is currently being commissioned. It provides low-energy and re-accelerated beams of neutron-rich isotopes obtained from 252Cf fission. The fission products from a 252Cf source are stopped in a large high-intensity gas catcher, thermalized and extracted through an RFQ cooler, accelerated to 50 kV and mass separated in a high-resolution separator before being sent to either an ECR charge breeder for post-acceleration through the ATLAS linac or to a low-energy experimental area. This approach gives access to beams of very neutron-rich isotopes, many of which have not been available at low or Coulomb barrier energies previously. These beams provide unique opportunities for measurements along the r-process path. To take advantage of these unique possibility, the reaccelerated beams from CARIBU will be made available at the experimental stations of ATLAS to serve equipment such as Gammasphere, HELIOS and the reaction spectrometers. In addition, the Canadian Penning Trap (CPT) mass spectrometer has been moved to the CARIBU low-energy experimental area and a new injection line has been built. The new injection line consists of a RFQ buncher sitting on a 50 kV high-voltage platform that will accumulate the mass separated 50 kV radioactive beams, cool and extract them as a pulsed beam of 3 keV. This beam can be sent either to a tape station for diagnostics and tuning, or a cryogenic linear trap for preparation before transfer to the high-precision Penning trap where the mass measurements will take place. Initial CARIBU commissioning is proceeding with a 2 mCi source that will be replaced by a 100 mCi source as the commissioning proceeds. Final operation will use a 1 Ci source and attain yield in excess of 107 ions/sec for the most intense beams at low energy, an order of magnitude less for reaccelerated beams.

The DESIR facility at SPIRAL2

The DESIR Collaboration proposes the construction of an experimental facility to exploit the low-energy beams from SPIRAL1, SPIRAL2 and S3. The high degree of purity required to push experiments towards the limits of stability will be achieved by the implementation in the SPIRAL2 production building of a high-efficiency RFQ cooler coupled to a high-resolution mass separator. Beams from the low-energy branch of the separator spectrometer S3 and from SPIRAL1 will allow complementary studies of refractory elements produced by means of fusion reactions as well as of light and intense exotic beams, respectively.

HIE-ISOLDE

The HIE-ISOLDE project proposal is a major staged upgrade to the existing REX accelerator facility at ISOLDE with the objective to provide radioactive beams up to 6MeV/u with a future option of going to 10MeV/u. The ambition is to make all isotopes produced at ISOLDE (>800) available as post accelerated beams. The beam quality will be much improved for ISOLDE users with the installation of a RFQ cooler, a new resonant laser ionization system and a renovated high resolution mass separator. New isotopes will be made available through target and ion source development. The driver intensity will be increased (5×), with a first increase coming from a faster cycling of the PS Booster and a later increase coming from the new Linac-4. We will in this contribution review the project and report on the present status of the staged proposal.

Status of the LPCTrap facility at GANIL

The LPCTrap is a device whose central element is a transparent Paul trap for the efficient trapping of low energy radioactive ions produced by the SPIRAL facility at GANIL. The installation comprises a linear RFQ cooler and buncher for the beam preparation, and a decay chamber where the Paul trap is located. The system has been tested with ions having mass-to-charge ratios ranging from 4 to 40. The first run with radioactive 6He+ ions has demonstrated the overall performance of the system. In this contribution we describe the current status of the device, we present in particular the results of a numerical study carried out to determine the properties of the ion cloud inside the trap and provide updated values for the overall efficiency of the system.

A low energy storage ring for partly stripped radioactive ions

Storage rings are well established for the study of stable nuclei with many facilities operating world wide. Radioactive nuclei are being studied in storage rings at GSI and major upgrades are planned to this facility including two new storage rings. The production method at GSI, in-flight, results in fully stripped ions with large transverse and longitudinal emittance. We have studied the possibility of injecting ISOL beams into a storage ring. While the two production methods are considered to be complementary considering the elements that can be produced, the ISOL method yields higher intensities and cooler beams. Furthermore, as the ISOL beam is produced with low kinetic beam energies the beam can be further cooled with e.g. an RFQ cooler. Cool beams will simplify the injection process and result in very bright beams ideally suited for reaction studies and for the more exotic production of e.g. anti-protonic atoms through the so called merging beam method.

MAFF — The Munich accelerator for fission fragments

At the new high flux reactor FRM-II in Munich the accelerator MAFF (Munich accelerator for fission fragments) is under design. In the high neutron flux of 1014 n/cm2 s up to 1014 neutron-rich fission fragments per second are produced in the 1 g U-235 target. Ions with an energy of 30 keV are extracted from the ion source. In the mass separator two isotopes can be selected. One of the beams is used for low energy experiments, the other one is injected into an ECRIS (or EBIS) for charge breeding to a q/A≥0.16. A gas filled RFQ cooler is used for emittance improvement. The subsequent LINAC delivers beams with an energy ranging from 3.7 MeV/u to 5.9 MeV/u. New IH structures are being developed at the Munich tandem laboratory. A small storage ring is planned in a further stage to recycle the fission fragments. A thin target foil can be placed into this ring, e.g., for synthesis of super-heavy elements. The through-going beam tube has been installed in the heavy water tank of the reactor. Tests of the target ion source in a special oven to test long term stability and safety tests were in progress.