A linear radiofrequency ion trap for accumulation, bunching, and emittance improvement of radioactive ion beams

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Abstract

An ion beam cooler and buncher has been developed for the manipulation of radioactive ion beams. The gas-filled linear radiofrequency ion trap system is installed at the Penning trap mass spectrometer ISOLTRAP at ISOLDE/CERN. Its purpose is to accumulate the 60-keV continuous ISOLDE ion beam with high efficiency and to convert it into low-energy low-emittance ion pulses. The efficiency was found to exceed 10% in agreement with simulations. A more than 10-fold reduction of the ISOLDE beam emittance can be achieved. The system has been used successfully for first on-line experiments. Its principle, setup and performance will be discussed.

Introduction

The development of new techniques for the manipulation of radioactive ion beams is actively pursued by several groups worldwide. One of the main objectives is a better matching of the properties of the radioactive ion beams to the specific requirements of the experiments. Here ion trap techniques have started to play an increasingly important role, in particular for the accumulation, cooling, and bunching of these beams. Both, Penning traps [1] and radiofrequency multipole ion traps or guides [2] can fulfill this task. In addition, Penning traps offer high-resolution mass separation and can be used for beam purification [3]. Radiofrequency multipole ion guides have been employed in ion chemistry and molecular physics for many years [4], [5]. Now they have gained increasing importance in the field of nuclear physics, where they are used for guiding charged nuclear reaction products from high-pressure gas cells into high-vacuum regions in order to form low-energy radioactive ion beams [6], [7], [8], [9] or for enabling ultra-sensitive laser experiments on radioactive ions [10].

A rather new application is the use of radiofrequency multipole systems for the manipulation and improvement of radioactive ion beams as they are available from on-line mass separators. At the ISOLDE facility [11] at CERN, a beam accumulator, cooler, buncher, and emittance improver based on a linear radiofrequency quadrupole (RFQ) ion trap has been realized and used for on-line physics experiments. The system is installed at the Penning trap mass spectrometer ISOLTRAP [12]. Its task is the conversion of the continuous 60-keV ISOLDE ion beam into low-energy, low-emittance ion bunches that can be transferred with high efficiency into the mass spectrometer.

Section snippets

The ISOLTRAP facility

Fig. 1 shows an overview of the layout of the ISOLTRAP Penning trap mass spectrometer together with the radiofrequency quadrupole ion beam buncher. The ISOLTRAP spectrometer itself consists of two Penning traps and has been described in detail in earlier publications [12], [3]. Here only a short description of the tasks of the Penning traps and of the basic operation will be given.

Low-energy ion pulses delivered by the RFQ ion beam buncher are captured in the first Penning. The main task of

Principle of the ISOLTRAP ion beam cooler and buncher

The principle of the ISOLTRAP ion beam cooler and buncher is illustrated in Fig. 2. The 60-keV ISOLDE ion beam is electrostatically retarded to an energy of a few eV and injected into a linear radiofrequency quadrupole ion trap, which is filled with a buffer gas. The trap system consists of four segmented rods to which radiofrequency voltages are applied so as to obtain a transversely focusing force. The segmentation of the rods allows the creation of a DC electric field along the axis of the

The experimental setup

Fig. 8 shows the setup of the ISOLTRAP ion beam cooler and buncher. On one side the system is connected to the ISOLDE beam line system and on the other side to the ion beam transport system of ISOLTRAP. A 60-keV test ion source and a beam switchyard (not shown in the figure) are installed upstream in the ISOLDE beam line in order to allow test measurements without the ISOLDE ion beam. Beam intensities and profiles can be measured with a needle beam scanner and a Faraday cup in front of the ion

Operation

Table 2 gives a typical set of operating parameters for ions with mass number close to A=39 and 133. The ions enter the linear trap with an energy of 20eV after retardation. The value given for the buffer gas pressure value is the (helium-corrected) vacuum gauge reading. As discussed above the pressure inside the linear trap is estimated to be a factor of 10 higher. Depending on the delivered ion beam current, the ions are accumulated for a period Taccu=0.01–1000ms. Then the ions are allowed to

Performance of the ISOLTRAP ion beam buncher

A number of systematic investigations have been performed with the buncher in order to characterize its performance and to compare the results with simulations. For these measurements, both ions from ISOLDE and from the test ion source have been used.

Future developments

A new type of radiofrequency quadrupole ion beam cooler and buncher has been investigated recently. It will be briefly discussed here while a comprehensive study including simulations will be presented in Ref. [25].

Fig. 25 shows the principal layout of the system, which is based on a segmented cylinder. Four segments of constant width are used to create the radiofrequency quadrupole field by applying RF voltages URF with appropriate phases. An axial trapping potential Φ(z) is produced by

Conclusions and outlook

In the work presented here, we have demonstrated for the first time the accumulation, bunching, cooling and emittance improvement of radioactive ion beams from an on-line mass separator by means of a linear radiofrequency ion trap. The efficiency of the system and the properties of the energy-variable ion bunches are in agreement with the theoretical expectations.

Compared to the previously used Paul trap system [18], the efficiency of ISOLTRAP has been increased by three orders of magnitude.

Acknowledgements

The authors wish to thank J. Bernard, W. Hornung, G. Marx and W. Quint at GSI Darmstadt, F. Ames and P. Schmidt of the REXTRAP group and the CERN summer students S. Lindner, S. Harto, and C. Richter for their valuable help during the development and commissioning of the system. This work was partly carried out within the EXOTRAPS project in the EU LSF-RTD program and financially supported under contract number ERBFMGECT980099. It was also supported by NSERC of Canada.

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