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The cryogenic storage ring CSR
The electrostatic cryogenic storage ring CSR, developed for atomic, molecular, and cluster physics experiments in a cryogenic environment, started operation successfully in March 2015. There the CSR provided in a first cryogenic operation period temperatures of below 10 K for almost 4 months. Thus, the aspired design goals of CSR were successfully achieved. In a next step the electron cooler was implemented and from 2017 until now 4 cryogenic beam periods with spans of 9-12 weeks each further experiments with cold atomic, molecular and cluster ions were performed.
The CSR consists of an experimental vacuum system kept below 10 K by integrated pumping units working at 2 K. Two radiation shields at 40 and 80 K house the experimental vacuum chambers. An outer vacuum system acting as a cryostat provides an insulation vacuum of 10-6 mbar. The purely electrostatic storage ring with a circumference of 35 m consist of four 90o-bending corners and four field free straight sections used for beam diagnostics, an electron cooler (ECOOL), a reaction microscope (REMI, still in preparation) and a merged neutral beam setup, see Figure 1.
Already in 2014 the CSR was almost fully constructed. This major milestone allowed for the operation at room temperature. An Ar+-beam could be stored successfully verifying the calculations as well as the stability of electrostatic fields created by the 112 electrodes of the 24 elements. After these tests it took almost one year preparing for cryogenic operation of CSR. This included the assembly of in total 210 m helium tubes of stainless steel and the mounting of 140 m2 of shields and superinsulation. Figure 2 shows the CSR in prior mounting the shields. At the second commissioning phase in 2015 the CSR was cooled down within 18 days to an average temperature of the experimental vacuum chambers of about 6 K. Here, our design goal of a cryogenic environment to investigate ground state properties of cold atoms and rovibrationally cooled molecules was achieved.
To examine the extremely high vacuum the electron capture process Ar+ + e- → Ar was observed continuously during the complete cool-down process. The rate of neutral atoms of Ar vanished at low temperatures implying that the rate of neutrals due to collisions with the decreasing number residual gas particles drops below the dark current of the detector or even lower. The analysis of the measurements considering the geometry of the ring and the experimental detection setup resulted in the estimation of the vacuum background to be less than 10-14 mbar, which corresponds to a density of 140 particles per cm3.
Furthermore, various atomic and molecular species were injected to investigate the storage life times. A dependency between life time and mass could be observed. The silver dimer Ag2- was one of the largest mass injected so far. A life time of 2717 s at 60 keV injection energy was determined (see Figure 3).
So far the stored ion beam energies for singly charged ions, molecules and clusters are in the range from 35 to 280 keV. The following positive and negative ions were stored in the past five years: HD+, H3+, D2+, H2D+, HeH+, C-, CH+, O-, OH+, OH-, F6+, C2-, Si-, Ar+, C4O-, TiO+, Al4-, Ni2-, Co2-, Co3-, Au-, Ag2-, Co4+.
The differently charged ions used in these experiments were produced by well-established ion sources in house: Penning-, MISS- (metal ion sputter source) and ECR-source operated on two different electrostatic platforms with maximum voltages of ±300 kV and ±60 kV, respectively. Typical ion currents ranged from 1 nA to 1 μA.
The injected ion beam was aligned to the storage ring orbit by a beam viewer in front of the CSR and by three further beam viewers that can be moved into the CSR orbit. The beam viewers consist of an aluminium plate on which secondary electrons are produced when hit by the ion beam. Using a grid, the electrons are extracted and accelerated towards an MCP phosphor screen combination with 40 mm diameter. The image of the beam is recorded via a CCD camera.
In the two injection beam lines, a continues ion current from the source is pulsed by switchable electrostatic deflectors (choppers). The first 60 deflector encountered by the ions, after entering into the CSR, serves as the injection element of the ring. Its voltage is switched off at the beginning of an injection cycle. Ions from the injector can then enter the CSR until the deflector voltages raised to their nominal values and thus permits to store an ion beam. The pulse length of the injected ions can be adjusted by setting the opening time of the chopper in the injection line. The time structure (compare Figure 4) impressed by the injection varies during storage and completely vanishes after a time of typically 104 revolutions.
Two capacitive pick-ups are used in the diagnostic section to detect the time structure of the circulating ion beam. These pick-ups with 100 mm inner diameters differ in their length. The short pick-up (current pick-up) with a length of 30 mm is used to determine the influenced voltage of the circulated ion beam. The long pick-up (Schottky pick-up, l = 340 mm) allows to measure the Schotty noise of the circulation ion beam. With the current pick-up, in particular, the ion injection and the first few revolutions of the ions van be directly monitored. The voltage at this pick-up as a function of time is shown in Figure 4. From the influenced voltage the number of injected particles can be determined.
The time structure imprinted by the injection process onto the stored ions vanished after less then 1 s of storage time. The Schottky noise signal of the stored ion beam can be observed up to very long storage times of about 1000 s. The power spectra of the amplified Schottky-noise signal for 60 keV Co2- ions (A= 118, f0=8.915 kHz) measured at the harmonic number h=20 are shown in Figure 5.
After losing the time structure of an injected ion bunch due to momentum spread the Schottky pickup is employed to measure the intensity of a coasting beam. It detects the beam by measuring the Schottky noise of the stored ions. Figure 5 shows the measured Schottky power of a stored Co2- ion beam at different times after injection. The Schottky spectra were recorded at the 20th harmonic of the revolution frequency for various storage times. With the current pick-up about 108 injected ions were detected. The Schottky signal remained visible until reaching an ion number of about 2·107 at 1200 s after injection.
A much more sensitive method to detect weak stored ion beams is to bunch the stored ion beam using a radio-frequency system. The rf system consist of a 340 mm long drift tube that is powered by an rf signal with an amplitude of 1-20 V and a frequency in the range of 100-500 kHz. Ion beam bunches measured for an Al4- ion beam are shown in Figure 6. In this experiment about 5·106 Al4- ions with an energy of 275 keV were injected. The rf frequency (frf=159.73 kHz) was set to 8 times of the revolution frequency (f0=1/T0, T0=50.1 μs). By filling about half of the ring with a single turn injection bunches are visible only in half of the revolution period, seen in Figure 6.
The closed orbit of a bunched ion beam in the CSR can be measured with six horizontal and six vertical beam position monitors (BPMs) distributed along the ring. A single BPM consist of a 100 mm inner diameter and a 60 mm long cylindrical capacitive pickup, split along their diagonal into a pair of separate electrodes. For the position measurements one electrode of a pair is connected to a single amplification chain, using a cryogenic relay, while grounding the other. The measurement is then repeated with interchanged connections. The centre of mass of the ion beam in the BPM is derived from the difference of the signal amplitudes in the two measurements, normalized to their sum. Beam position measurements were made to measure the closed orbit of the stored ion beam as well as to determine the dispersion of the storage ring at the pick-up positions. In these measurements, an Ar+ ion beam was injected into the CSR and the closed orbit was then changed by varying all electrostatic potentials by the same relative amount ΔU/U. This leads to a horizontal shift Δx of the center of mass position of the stored ion beam, which is related to the dispersion Dx at the pickup position by Dx=-2Δx/(ΔU/U). Since all BPMs are installed at equivalent positions within the ion ion-optical lattice of the storage ring, the dispersion at the six horizontal pick-up positions should be identical. The measurements yielded an average dispersion of Dx=2.2 m for the standard mode, while the Dx values measured at the individual BPM varied in the range of 2.08 m-2.28 m. Calculations using the G4beamline code, where ions were tracked through the realistically modelled fields of the storage ring, yield for the used working point a dispersion of Dx=2.14 m, in reasonable agreement with the measurements.
The CSR is also equipped with an electron cooler which has to serve as an electron target for high resolution recombination experiments. Commonly used thermionic electron sources with typically energy spreads in the order of 100 meV are not ideally suited for ion beam cooling at low velocities. To obtain much lower energy spreads a photocathode irradiated by a near-infrared laser is used in the electron target. The electron beam is guided, like in a conventional cooler, by solenoid and toroid fields. The merging scheme of the CSR electron cooler is, however, unconventional. It deviates from the standard scheme, where toroids are used to merge the electron beam with the ion beam. At the low ion velocities in the CSR, the homogenous toroidal field especially for lighter ions causes distortions of the ion trajectories which cannot be corrected by simple dipole compensation coils. For this reason, a toroidal-free merging scheme is used in the electron target (compare Figure 7).
With two sets of corrections coils in front and after the cooler section the ion trajectories are corrected. The magnetic field of the guiding solenoids also allows the magnetic expansion of the electron beam to lower the transverse temperature. The typically magnetic field in the main solenoid is about 100 Gauss.
After the implementation of the electron-ion merged beam setup into the CSR cryostat and successful tests of the electron beam transport through the electron cooler section, low-energy electron cooling experiments at the CSR were performed. An example of longitudinal electron cooling of a bunched HeH+ ion beam is shown in Figure 8. The ion beam energy in this example was 250 keV which corresponds to a revolution frequency of 88.251 kHz. Bunching was done at a harmonic number of 4. Because only 75 % of the ring circumference was filled with single turn injection only three bunches were visible after injection. The time range T0=11.3 μs shown in Figure 8 corresponds to one revolution. The electron energy used in this experiment was 27.32 eV. For the expansion factor a value of α=21.59 was chosen resulting in an electron density of 6.55·105 cm-3.
By analysing the development of the bunches, shown in Figure 8, a longitudinal cooling time of 1.4 s could be determined for the bunched HeH+ ion beam.
Also, transverse electron cooling experiments of coasting ion beams were carried out at ion energies of 250 keV for HeH+, HD+ and for O+ ions. In these experiments the transverse ion beam profile during electron cooling is measured with neutral fragment imaging, where the dissociative recombination (DR) process between the singly charged molecules and free electrons of the electron cooler is measured. The neutral fragments from this process are detected with a multi-channel detector with a large sensitive diameter of 120 mm, located behind the electron cooler. The profile of the stored ion beam is calculated from the distribution of the centre of mass positions of the neutral fragments created in the DR process. For a HeH+-beam at an electron density of 6.6·105 cm-3 a transverse cooling time of 1.5 s, similar to the longitudinal one of a bunched ion beam, was measured. Due to the smaller mass of the HD+-molecule a lower transverse cooling time of about 1 s was determined for HD+ ions at an electron density of 4.3·105 cm-3.
Reference
Focus: The Coolest Molecular Ion Beams 
Selected list of publications
2017
| 3. | Radiative Rotational Lifetimes and State-Resolved Relative Detachment Cross Sections from Photodetachment Thermometry
of Molecular Anions in a Cryogenic Storage Ring C. Meyer, A. Becker, K. Blaum, C. Breitenfeldt, S. George, J. Göck, M. Grieser, F. Grussie, E. A. Guerin, R. von Hahn, P. Herwig, C. Krantz, H. Kreckel, J. Lion, S. Lohmann, P. M. Mishra, O. Novotný, A. P. O’Connor, R. Repnow, S. Saurabh, D. Schwalm, L. Schweikhard, K. Spruck, S. Sunil Kumar, S. Vogel, and A. Wolf Phys. Rev. Lett. 119, 023202 (2017)
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2016
| 2. | Photodissociation of an Internally Cold Beam of CH+ Ions in a Cryogenic Storage Ring A. P. O’Connor, A. Becker, K. Blaum, C. Breitenfeldt, S. George, J. Göck, M. Grieser, F. Grussie, E. A. Guerin, R. von Hahn, U. Hechtfischer, P. Herwig, J. Karthein, C. Krantz, H. Kreckel, S. Lohmann, C. Meyer, P. M. Mishra, O. Novotný, R. Repnow, S. Saurabh, D. Schwalm, K. Spruck, S. Sunil Kumar, S. Vogel, and A. Wolf Phys. Rev. Lett. 116, 113002 (2016)
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| 1. | The cryogenic storage ring CSR R. von Hahn, A. Becker, F. Berg, K. Blaum, C. Breitenfeldt, H. Fadil, F. Fellenberger, M. Froese, S. George, J. Göck, M. Grieser, F. Grussie, E. A. Guerin, O. Heber, P. Herwig, J. Karthein, C. Krantz, H. Kreckel, M. Lange, F. Laux, S. Lohmann, S. Menk, C. Meyer, P. M. Mishra, O. Novotný, A. P. O’Connor, D. A. Orlov, M. L. Rappaport, R. Repnow, S. Saurabh, S. Schippers, C. D. Schröter, D. Schwalm, L. Schweikhard, T. Sieber, A. Shornikov, K. Spruck, S. Sunil Kumar, J. Ullrich, X. Urbain, S. Vogel, P. Wilhelm, A. Wolf, and D. Zajfman Rev. Sci. Instrum. 87, 063115 (2016)
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