Cooled Ion Beams Research Articles

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.

Compact Electron Gun for High-Voltage Electron Cooling Systems

One way to increase the efficiency of electron cooling is to increase the electron density in the beam reference frame. As a rule, with an increase in the energy of the cooled ion beam, they reduce its size; therefore, the electron current density can be increased by reducing the diameter of the electron beam and preserving its total current. This paper describes two versions of an electron gun for high-voltage electron cooling systems with flat and convex cathodes, in each of which the electron gun is capable of producing a beam with a diameter of up to 1 cm and a current about 1 A.

Space Charge studies on LEIR

The performance of the CERN Low Energy Ion Ring with electron cooled ion beams is presently limited by losses occurring once the beam has been captured in the RF buckets. An intense machine study program was started by the end of 2015 to mitigate the losses and improve the performance of the accelerator. The measurements pointed to the interplay of direct space charge forces and excited betatron resonances as the most plausible driving mechanism of these losses. In this paper, we present the systematic space-charge measurements performed in 2017 and compare them to space-charge tracking simulations based on an adaptive frozen potential.

Electron cooling of a bunched ion beam in a storage ring

A combination of electron cooling and rf system is an effective method to compress the beam bunch length in storage rings. A simulation code based on multiparticle tracking was developed to calculate the bunched ion beam cooling process, in which the electron cooling, intrabeam scattering (IBS), ion beam space-charge field, transverse and synchrotron motion are considered. Meanwhile, bunched ion beam cooling experiments have been carried out in the main cooling storage ring (CSRm) of the Heavy Ion Research Facility in Lanzhou, to investigate the minimum bunch length obtained by the cooling method, and study the dependence of the minimum bunch length on beam and machine parameters. The experiments show comparable results to those from simulation. Based on these simulations and experiments, we established an analytical model to describe the limitation of the bunch length of the cooled ion beam. It is observed that the IBS effect is dominant for low intensity beams, and the space-charge effect is much more important for high intensity beams. Moreover, the particles will not be bunched for much higher intensity beam. The experimental results in CSRm show a good agreement with the analytical model in the IBS dominated regime. The simulation work offers us comparable results to those from the analytical model both in IBS dominated and space-charge dominated regimes.

The design of the Spectrometer Ring at the HIAF

The Spectrometer Ring (SRing) is an essential part of the High Intensity heavy-ion Accelerator Facility project (HIAF) in China. It is designed as a multi-functional experimental storage ring, which will be able to operate in three ion optical operation modes. The SRing will be used as a time-of-flight mass spectrometer for short-lived, especially neutron-rich nuclei. It will also be used to collect and cool Rare Isotope Beams (RIBs) or highly-charged stable ion beams for nuclear and atomic physics experiments. The design magnetic rigidity is in the range 1.5 to 15 Tm. The beam cooling system consists of stochastic cooling and electron cooling devices. With a help of an electron cooler, stored ions will be decelerated to a minimum energy of 30 MeV/u by RF cavities. The extraction system of the SRing will allow cooled ion beams to be extracted to an external target for further ion manipulations or reaction experiments. The general ion optics design and technical requirements of SRing subsystems are presented and discussed in this paper.

Status and outlook of the CRYRING@ESR project

Once operational, CRYRING@ESR will store and decelerate ions delivered by the experimental storage ring ESR at energies well below those of ESR. In addition to that, CRYRING@ESR has an electron cooler operating with an ultracold electron beam, allowing to provide cooled ion beams for precision experiments. These ions will be delivered to a broad range of experiments presently in preparation; either in-ring or extracted to a dedicated beamline for experiments. An overview and status report of the installation and commissioning of the CRYRING-@ESR storage ring for highly charged ions at the GSI Helmholtzzentrum fur Schwerionenforschung is presented. The installation of this storage ring started in 2014 and was completing end of 2016, when this publication was written.

Injection and stability of high-intensity ion beams in synchrotrons with electron cooling

The electron cooling-stacking injection is one of the most widely used injection methods applied in forming high-intensity cooled ion beams in the synchrotrons. The maximum available ion intensity in the synchrotron is determined by the lifetime of ions and the cooling-stacking efficiency. The formation of cooled high-intensity beams is constrained by instability, leading to a strong reduction in the ion lifetime. The results of experimental investigations and simulations of the cooling-stacking injection and stability of high-intensity cooled ion beams in synchrotrons with electron cooling are discussed.

Longitudinal dynamics of RF-bunched and electron-cooled ion beam at the CSRe

The longitudinal dynamics of RF-bunched and electron cooled ion beams have been studied at the experimental cooler storage ring (CSRe), at IMP Lanzhou. By RF-bunching the ion beam at the 50th and 100th harmonic of the revolution frequency, the longitudinal momentum spread and the bunch length of the 22Ne10+ ion beam with an energy of 70MeV/u were measured by the new resonant Schottky pick-up and the capacitive pick-up, respectively. A minimum momentum spread of Δp/p=1.6×10−5 has been reached with less than 107 ions stored in the ring. By using the harmonic potential extracted from the Taylor expansion and the real sinusoidal potential of the bucket, the trend of momentum spread and synchrotron frequency as well as the bunch length as a function of beam current can be interpreted very well. According to this experiment, the RF-buncher is suitable for upcoming experiments on laser cooling of relativistic heavy ion beams at the CSRe.

Breit interaction in dielectronic recombination of hydrogenlike uranium

Absolute rate coefficients for dielectronic recombination (DR) of H-like ${\mathrm{U}}^{91+}$ ions have been measured. The electron-ion merged-beam technique at a heavy-ion storage ring was employed using a stochastically cooled ion beam. Thereby, the previously accessible electron-ion collision energies could be greatly extended to the range 63--90 keV. High-resolution DR spectra were measured covering all $\mathit{KLL}$ and $\mathit{KLM}$ resonances. For the resonance strengths, excellent agreement between relativistic theory and experiment is found only if the Breit contribution to the electron-electron interaction is included in the calculations. For the $\mathit{KL}$${}_{1/2}$$L$${}_{1/2}$ and $\mathit{KL}$${}_{1/2}$$M$${}_{1/2}$ groups the Breit contribution amounts to 44% of their total resonance strengths.

Calculation of the variable-profile electron beam for electron coolers

Interaction of an electron beam with a cooled ion beam makes it possible to reduce its phase volume, perform accumulation of particles, and suppress various “heating” effects. The electron beam can also be used as a target for an electron-ion recombination reaction, which offers a chance to carry out atomic physics experiments and ensure slow uniform extraction of the ion beam from the storage ring. A high-perveance electron beam with a variable profile is required for effective cooling, while a high current density and a low energy of transverse motion of electrons in the beam is needed for extraction by means of recombination. It is shown that a convex cathode placed in a magnetic field can be used to form such a beam. A high current density can be attained with this shape of the cathode, but additional efforts must be focused on optimizing the gun’s optics in order to obtain a low energy of transverse motion of particles. Since ions repeatedly pass through the cooling section during their lifetime at different values of the betatron oscillation phase, the rates of recombination and cooling are dependent on the rms electron velocity averaged over the volume in which the beam interaction occurs. The proposed design of the gun with a convex cathode 10.2 mm in diameter ensures formation of a variable-profile electron beam with a nominal current of 1 A and a current density of 1.2 A/cm2. The rms energy of Larmor gyration of electrons at the exit from the gun, averaged over the beam cross section (the “transverse” temperature) is 1 eV. A focusing electrode that forms the Pierce optics near the edge of the cathode, an electrode controlling the beam profile, and an anode are included in the optics of the electron gun.

Atomic physics with ions stored in the round

The talk contained examples of recent atomic physics experiments with stored and cooled ion beams from different ion storage-ring facilities. Here, we first introduce the principles of storage rings and electron cooling. A whole class of experiments exploits the cold electron beams available in the electron coolers and electron targets of storage rings. The recombination experiments have applications in fusion and astrophysical plasmas. Dielectronic resonances at meV to eV energies are measured with a resolution and absolute accuracy to much below a meV. The measurements of these resonances provide a serious challenge to theories for describing correlation, relativistic, quantum electrodynamical effects, and isotope shifts in highly ionized ions. Experiments with internal targets in storage rings use the high luminosity of cooled MeV ions for collisions. First measurements demonstrate the resolution with a He RIMS apparatus (He gas-jet Target for Recoil Ion Momentum Spectroscopy) in Thomas-like electron-transfer processes by protons. An outlook into the future with the new Facility for Anti-proton and Ion Research (FAIR) and the Stored Particle Atomic Research Collaboration (SPARC) is given.

Photorecombination studies of highly charged ions at the storage ring ESR: a progress report

In this contribution, we report on photorecombination measurements of highly charged ions (HCI) that were performed using the electron cooler of the heavy-ion storage ring ESR of the Gesellschaft für Schwerionenforschung (GSI) as a target of free electrons. The experimental technique of resonance reaction spectroscopy by means of dielectronic recombination (DR) was employed in two new ways:The determination of isotope shifts in the DR resonance spectrum of few-electron heavy ions is a novel method with unique properties for investigations at the interface between electrons and the atomic nucleus, e.g., charge radii and hyperfine effects. First results from a pilot experiment with the two isotopes A = 142 and A = 150 of Li-like neodymium ANd57+ are discussed.Until very recently, electron-ion collision experiments at high relative energies of more than a few keV were not accessible at the electron coolers of storage rings. Such high energies are needed to access DR processes that involve the K-shell electrons of very heavy ions. The novel approach of DR measurements with a stochastically (pre-)cooled ion beam resolves this problem. Results for the photorecombination of hydrogen-like U91+ in the energy range (63 keV - 74 keV) of the KLL-DR process are presented.

Two-beam instability in electron cooling

The drift motion of cooling electrons makes them able to respond to transverse perturbations of a cooled ion beam. This response may lead to dipole or quadrupole transverse instabilities at specific longitudinal wave numbers. While the dipole instabilities can be suppressed by a combination of the Landau damping, machine impedance, and the active damper, the quadrupole and higher order modes can lead to either emittance growth, or a lifetime degradation, or both. The growth rates of these instabilities are strongly determined by the machine x-y coupling. Thus, tuning out of the coupling resonance and/or reduction of the machine coupling can be an efficient remedy for these instabilities.

Atomic physics experiments with cooled stored ions

This presentation contains examples of recent atomic physics experiments with stored and cooled ion beams from the CRYRING facility in Stockholm. One of these experiments uses the high luminosity of a cooled MeV proton beam in a He COLTRIMS apparatus (COLd supersonic He gas-jet Target for Recoil Ion Momentum Spectroscopy) for measuring correlation effects in transfer ionization. Another class of experiments exploits the cold electron beam available in the CRYRING electron cooler and cooled heavy-ion beams for recombination experiments. A section concerns the still rather open question of the puzzling recombination enhancement over the radiative recombination theory. Dielectronic resonances at meV–eV energy are measured with a resolution in the order of 10−3–10−2eV with highly charged ions stored at several hundreds of MeV kinetic energy in the ring. These resonances provide a serious challenge to theories for describing correlation, relativistic, QED effects, and isotope shifts in highly ionized ions. Applications of recombination rates with complex highly charged ions for fusion and astrophysical plasmas are shown.

Laser spectroscopy of radioactive Ti, Zr and Hf isotopes and isomers at the JYFL laser-IGISOL facility

The recent progress at the laser-ion guide isotope separator on-line facility, JYFL, is presented. At the facility new techniques for studying short-lived radioisotopes by laser spectroscopy have been developed and applied to the study of isotopes in refractory metal elements. In particular, recent results on the spectroscopy of cooled ion beams of radioactive Ti, Zr and Hf isotopes are discussed.

Beam accumulation with the SIS electron cooler

An electron cooling system has started operation in the heavy ion synchrotron SIS which is used to increase the intensity for highly charged ions. Fast transverse cooling of the hot ion beam after horizontal multiturn injection allows beam accumulation at the injection energy. After optimization of the accumulation process an intensity increase in a synchrotron pulse by more than one order of magnitude has been achieved. For highly charged ions the maximum number of particles has been increased from 1×10 8 to 1×10 9. For lighter ions intensity limitations have been encountered which are caused by the high phase space density of the cooled ion beam. Momentum spreads in the 10 −4 range and emittances well below 10 π mm mrad have been demonstrated. Recombination losses both in the residual gas and with the free cooler electrons determine the maximum intensity for highly charged ions. Systematic measurements of the recombination rates have been performed providing data for an optimum choice of the charge state. Strong enhancement of the recombination rate with free electrons compared to theoretical calculations of radiative electron capture have been observed.

Test of special relativity in an ion storage ring

An accurate measurement of the Doppler effect in collinear laser spectroscopy has been performed at the TSR storage ring with electron cooled7Li+ ions atΒ=0.064. This experiment is a sensitive test of theγ=(1−Β2)−1/2 factor(Β=v/c) in the special theory of relativity. The Doppler shifted frequencies of the moving7Li+ ions are compared with calibrated molecular lines at rest. The frequencies at rest for the7Li+ ions are known from independent measurements. The Doppler shifted frequencies in the collinear experiment have been measured with a precision ofδv/v=6×10−9, mainly limited by the signal width of the resonance. A corresponding upper limit of 8×10−7 is deduced for any deviation of the time dilatation factorγ in special theory of relativity. A still higher accuracy is expected with a laser cooled ion beam. If such a beam is simultaneously subjected to RF bunching, the particle velocity and phase are exactly known. With an additional high resolution laser frequency measurement an improvement of at least one order of magnitude may be achieved.

Experiments on cooled molecular ions

Ion storage ring technology allows molecular ions to be accelerated to MeV energies and stored for tens of seconds in an ultra-high vacuum ring system. By merging the ion beam with a cold electron beam in one section of the ring, a phase-space cooled ion beam with low divergence and low momentum spread is obtained. Infrared active molecular ions are also vibrationally cooled by spontaneous emission of radiation. A stored ion beam of this quality is ideal for studies of molecular processes such as dissociative recombination.

Storage rings for low-energy experiments

In this article an introduction and overview of heavy ion storage rings is given. The different cooling schemes for ion beams, i.e. electron, laser, and stochastic cooling as well as beam heating mechanisms are described. Connected to this are collective effects in cold stored ion beams which lead to a form of crystallization. The potential of cooler rings in atomic and molecular physics experiments with stored and cooled ion beams is discussed. This will be exemplified with results from atomic lifetime measurements and the interaction of atomic and molecular ions with electrons and photons utilizing the electron cooler.

New physics with cooled ion beams

We review new possibilities for heavy ion cooler rings in different fields of physics and describe present experiments at the Heidelberg Test Storage Ring TSR.