Conceptional design of a novel next-generation cryogenic stopping cell for the Low-Energy Branch of the Super-FRS
Introduction
Future radioactive ion beam facilities, such as FAIR, will produce exotic nuclei at unprecedented rates and will give access to more exotic nuclides [1]. To make use of the increased rates detectors and beam handling devices with orders of magnitude higher rate capabilities are indispensable. One type of these devices are gas-filled stopping cells [2], [3], [4], [5], which are used to convert a high-energy beam (>MeV/u) into a low-energy ion beam (∼eV) to enable precision experiments with stored ions, such as mass measurements and decay and laser spectroscopy [6]. At the Low-Energy Branch (LEB) [7], [8] of the Super-FRS [9] such experiments with low-energy exotic nuclei will be performed with MATS and LaSpec [10]. The ions produced by projectile fragmentation and fission at relativistic energies at the Super-FRS must be stopped and thermalized in a gas-filled stopping cell. This is a very challenging task because of the large beam spot size (>200 cm2), range straggling (≳10 mg/cm2 helium), high beam intensities (up to ions per second) and ionization rate ( He+/e− pairs per second). A prototype cryogenic stopping cell (CSC) for the LEB has been developed and tested as part of the FRS Ion Catcher experiment and has reached unprecedented performance, areal density (>6 mg/cm2 helium) and extraction efficiency (close to unity) combined with short extraction times (25 ms) [11]. The experiment consists of the fragment separator (FRS), its mono-energetic degrader system, the CSC, an RFQ beam line and a multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) [12]. The prototype CSC is based on cryogenic operation to achieve high cleanliness of the stopping gas, high DC field for fast extraction and an RF carpet for highest buffer gas density (stopping efficiency) and a lossless transport of the stopped ions into the RFQ beamline and to other experiments downstream [13], [14]. To cope with the challenging conditions at the Super-FRS the conceptual design of a next-generation CSC has been developed. It is based on the existing technology and makes use of several novel concepts to improve the performance by orders of magnitude.
Section snippets
Conceptual design
The conceptual design of the new CSC is shown schematically in Fig. 1 [15]. The CSC consists of two main vacuum chambers. An outer chamber provides the thermal insulation for the inner chamber, which is at cryogenic temperature (∼70 K). The inner chamber is divided and differentially pumped into a high-density stopping region and a low-density extraction region. The ion beam enters the stopping region horizontally through two vacuum windows and is stopped in the helium buffer gas. Using electric
Technical design
In Fig. 3 a digital mockup of the novel CSC can be seen. Included are the RFQ based extraction beam line, the electronics (orange), the cryostat (blue), the purification getter (brown) and the helium recovery unit (HRU, green). The stopping volume has a width of 25 cm, a height of 10 cm and a length of 2 m. This cross-section fits the expected beam diameter at the final focus of the LEB. The HRU is needed to keep the operation cost at a reasonable level, because the helium flow can reach up to 30 m3
Expected performance
The improvements in RF carpet structure size, field arrangement, differential pumping and stopping volume length result in a areal density of 30 mg/cm2, an increase by a factor of 6 from the areal density of the present CSC. In combination with the momentum compression provided by the energy buncher of the Super-FRS, stopping efficiencies close to unity are expected for all but very light nuclei.
The total mean extraction time, i.e. the time from stopping of the relativistic ions to detection
Conclusions and outlook
The conceptual design of the future CSC has been developed. It includes several novel concepts. It is expected to have six times higher areal density, five times faster extraction, four orders of magnitude higher rate capability, improved cleanliness and new diagnostic capabilities. In combination with the momentum compression provided by the energy buncher of the Super-FRS, stopping efficiencies close to unity are expected for all but very light nuclei. Ion survival and extraction efficiencies
Acknowledgements
We would like to thanks V. Varentsov for fruitful discussions and T. Wasem for his help in the technical desgin. This work was supported by the German Federal Ministry for Education and Research (BMBF) under contract No. 05P12RGFN8, by the Hessian Ministry for Science and Art (HMWK) through the LOEWE Center HICforFAIR, by HGS-HIRe, and by Justus-Liebig-Universität Gießen and GSI under the JLU-GSI strategic Helmholtz partnership agreement.
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Masses of exotic nuclei
2021, Progress in Particle and Nuclear PhysicsCitation Excerpt :The masses of extracted ions are measured by the MR-TOF. The system works as a prototype of the future low-energy beam line for the FAIR facility in Darmstadt [560] (see Section 5.1). A similar system was also implemented in the TITAN Penning trap, see Section 3.1.2.