The Heidelberg CSR: Stored Ion Beams in a Cryogenic Environment

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1 The Heidelberg CSR: Stored Ion Beams in a Cryogenic Environment A. Wolf, R. von Hahn, M. Grieser, D. A. Orlov, H. Fadil, C. P. Welsch, V. Andrianarijaona, A. Diehl, C. D. Schröter, J. R. Crespo López-Urrutia, M. Rappaport, X. Urbain, T. Weber, V. Mallinger, Ch. Haberstroh, H. Quack, D. Schwalm, J. Ullrich and D. Zajfman, Max-Planck Institute for Nuclear Physics, Heidelberg Weizmann Institute of Science, Rehovot, Israel Université Catholique de Louvain, Louvain-La-Neuve, Belgium Technische Universität Dresden, Germany Abstract. A cryogenic electrostatic ion storage ring CSR is under development at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany. Cooling of the ultrahigh vacuum chamber is envisaged to lead to extremely low pressures as demonstrated by cryogenic ion traps. The ring will apply electron cooling with electron beams of a few ev up to 200 ev. Through long storage times of 1000 s as well as through the low wall temperature, internal cooling of infrared-active molecular ions to their rotational ground state will be possible and their collisions with merged collinear beams of electrons and neutral atoms can be detected with high energy resolution. In addition storage of slow highly charged ions is foreseen. Using a fixed in-ring gas target and a reaction microscope, collisions of the stored ions at a spead of the order of the atomic unit can be kinematically reconstructed. The layout and the cryogenic concept are introduced. Keywords: Electrostatic storage ring, molecular beams, cryogenic ring PACS: Dh INTRODUCTION Collision experiments with stored slow ion beams at energies in the range of 100 kev are still relatively rare, although they have a large potential. Thus, a number of experiments were performed with stored, but uncooled heavy ion beams, including clusters and biological species, with heavy ion beams in electrostatic storage devices [1]; however, they mainly concerned unimolecular relaxation and dissociation after their production or radiative excitation. Only a few recent examples exist for studies using electron impact on stored molecular ion beams [2, 3, 4]. Electrostatic storage rings [5] can with a reasonable technological effort be constructed for ion beams below a few hundred kev, using cylinder-capacitor-like deflectors with 10 kv/cm field and a bending radius of the order of 1 m. In addition, fast-beam ion traps have been established as physics instruments [6]. With diameters of 10 m it can be considered to cool electrostatic storage rings and traps entirely to low cryogenic temperatures ( 2 K) sufficient to freeze out all relevant gases and potentially to reach gas pressures of mbar, similar as in penning traps for antiprotons [7]. The development of a fully equipped cooler storage ring for kev ion beams over a large mass range is pursued since 2004 in the Cryogenic Storage Ring (CSR) project at 473

2 FIGURE 1. View of the CSR project, Heidelberg, close to the existing accelerator and the electron beam ion trap (EBIT). Detector regions for the experimental zones are marked with green dots. the Max-Planck Institute for Nuclear Physics, Heidelberg. MOTIVATION Low-energy inelastic electron collisions with MeV molecular ion beams are successfully studied since 1992 at magnetic storage rings focusing in particular on the process of dissociative recombination (DR) with electrons [8, 9]. This process is one of the main loss processes for molecular ions in thin ionized media where also electrons are present. In general, reactions of molecular ions both with electrons and with other heavy species are of utmost importance for the chemical composition of thin media such as interstellar space [10] and their evolution, such as star formation [11]. DR runs exothermically even at mev interaction energies and sensitively depends on the properties of neutral molecular resonance states, produced by binding the electron, in the vicinity of the equilibrium geometry of the molecular ions. While molecular vibrations could be well controlled and the effect of vibrational excitation eliminated in previous experiments [8], a similar research effort now is underway regarding the control of rotational excitation. In contrast to vibration, only few-mev excitation energies are involved and, correspondingly, temperatures of the order of 10 K are required to suppress rotational excitation. Presently, in measurements of the energy dependence of the dissociation cross section, contributions of various excited rotational levels are usually mixed and, in spite of the achieved high energy resolution of 0.5 mev, can hardly be disentangled [12]. 474

3 FIGURE2. Detectorarrangementforneutralrecombinationproducts. Performed with stored ion beams at cryogenic temperatures (<10 K), electron impact measurements of this type would assume the character of pure state-to-state experiments. It should be noted that the use of energetic particle beams together with suitable multihit imaging detectors makes it possible to detect and reconstruct the full dissociation geometry ot the interacting molecule, including both charged and neutral fragments [12]. The performance of molecular collision experiments with high energy resolution and including the analysis of the dissociation channels is one of the main motivations of the Heidelberg CSR. Owing to the moderate beam energy of <300 kev, it is planned to use also stationary gas targets and a merged atomic beam. LAYOUT The CSR is a fourfold-symmetry electrostatic storage ring for ion beams of kev, consisting of four pairs of separated 39 deflectors and four pairs of small 6 deflectors at the start and end of each bend. It has four straight sections with a length of 4 m, each of which includes also a pair of electrostatic focusing doublets. The total circumference is 34 m; further accelator properties are discussed by Fadil et al. [14]. Two straight sections will be equipped with detectors for neutral and charged fragments and used for collision experiments; in one of them an electron cooler and target will be installed, whereas the other section will house a fixed target and an associated reaction microscope as well as the merged atomic beam. The planned emplacement and the basic experimental arrangement are shown in Fig. 1. The electron cooling system uses the cryogenic photocathode system run already at the TSR [12]. At low beam energies in the range <10 ev this system is expected to provide large advantages regarding the obtainable electron density at a transverse temperature below 1 mev; thus, the emission current limit of presently 400 ma of the photocathode becomes irrelevant as the space-charge limited currents for, e.g., 3 mperv are clearly lower than this limit (< 100 ma) [15]. Moreover, the small (<10 475

4 mev) initial energy spread of the photoelectrons makes it possible to reduce the beam expansion required for a given transverse temperature by a factor of 10 and also yields lower longitudinal energy spread at low energy [15]. The reaction microscope [16] will use a well collimated gas jet (primarily He) of a thickness of <10 11 cm 2 and kinematically reconstruct collisions with the circulating kev ions (in particular also highly charged ions) by completely detecting all released collision products (ions, electrons and the deflected projectile). In the same ring section, it is also planned to produce a neutral atomic beam (e.g., H, D, O ) of >10 3 cm 3 velocity-matched to and collinearly overlapped with the circulating molecular ion beam. The beam will be produced by photodetaching a negative ion beam (see Fig. 1). Heavy particle reactions such as deuteration (the exchange of a hydrogen atom against deuterium) should become observable at an estimated energy resolution of a few mev. From the zero-energy rate coefficient of 10 9 cm 3 s 1 reaction rates, observable essentially background-free, of 10 s 1 are estimated for 1 ma of ion current (e.g., CH + ). All experiments will use the detector regions set up in two regions of the ring (see Fig. 1). Molecular fragmentation leads to quite large transverse fragment momenta, so that a free drift has to be ensured for neutral fragments with an opening angle of 1 over several meters. This is accomplished by the deflector layout and positioning as shown in Fig. 2. In addition to the neutral fragment detectors, several positions are foreseen for charged atomic and molecular fragments having experienced up- or downcharging as well as pick-up and loss of heavy particles. The operation of the detectors at cryogenic temperatures is under study. CRYOGENIC CONCEPT AND PROTOTYPE Starting with a room-temperature ultrahigh vacuum, the cooling of the storage ring to cryogenic temperatures, down to 2 K, can be expected to reduce the residual gas density to extremely low values ( 10 3 cm 3 ), corresponding at this temperature to pressures of the order of a few mbar). To our knowledge, the CSR will be the first large-scale machine exploring such extreme vacua. In addition, the thermal blackbody radiation field will be virtually eliminated, offering the experimental possibilities discussed above. The beam pipe and the ion-optical components will be situated in an inner, stainlesssteel vacuum chamber designed to reach a base pressure below mbar at room temperature. This chamber will will be pumped mainly by volume getters (NEG) after bakeout to 300 C. During the subsequent cooling down to cryogenic temperatures, most of the remaining gas components will be cryo-sorbed on the chamber walls; in contrast to practically all other gases, cryo-sorption of hydrogen requires very low temperatures [17] and the attainment of wall temperatures near or slightly below 2 K can be considered the safest prerequisite to ensure the stable adsorption even of this gas component. Liquid helium will be evaporated under low pressure inside cooling units mounted at the inner vacuum chamber and interconnected by a pipe system. The cooling lines, the suspension, and two thermal screens will be located in the insulation vacuum ( 10 6 mbar) of the outer chamber, acting as a cryostat interconnected by 600 mm diam. flanges between machine modules. As a critical elememt the suspension of the inner vacuum chamber, which has to ensure stable positioning of the ion optical elements, will be carefully designed and tested. The specifications of the cryogenic 476

5 supply system have been considered iteratively also under the aspect of economical optimization. A number of technological issues, such as the indispensable connections to roomtemperature equipment (at, e.g., the ion injection lines), which risk to be the main critical load of external gas and blackbody radiation input, will be investigated experimentally at a test setup presently under construction. This prototype contains an electrostatic ion beam trap [6] designed for electrode voltages of 30 kev, corresponding to the deflector voltage of the CSR, with cryogenic sections for detectors and for observations on the stored ions. At the date of the Conference, this system is being manufactured, while the detailed design of the CSR machine elements is being started on the basis of the completed prototype design. REFERENCES 1. L. H. Andersen, O. Heber and D. Zajfman, J. Phys. B 37 R57 R88 (2004) 2. T. Tanabe, K. Noda, M. Saito, S. Lee, Y. Ito, and H. Takagi, Phys. Rev. Lett. 90, (2003) 3. A. Diner, Y. Toker, D. Strasser, O. Heber, I. Ben-Itzhak, P. D. Witte, A. Wolf, D. Schwalm, M. L. Rappaport, K. G. Bhushan, and D. Zajfman, Phys. Rev. Lett. 93, (2004); O. Heber, P. D. Witte, A. Diner, K. G. Bhushan, D. Strasser, Y. Toker, M. L. Rappaport, I. Ben-Itzhak, N. Altstein, D. Schwalm, A. Wolf, and D. Zajfman, Rev. Sci. Instrum. 76, (2005) 4. M. O. A. El Ghazaly, A. Svendsen, H. Bluhme, A. B. Nielsen, S. B. Nielsen, and L. H. Andersen, Phys. Rev. Lett. 93, (2004) 5. S. P. Møller, Nucl. Instrum. Methods A 394, 281 (1997) 6. M. Dahan, R. Fishman, O. Heber, M. Rappaport, N. Altstein, D. Zajfman, and W. J. van der Zande, Rev. Sci. Instrum. 69, 76 (1998) 7. G. Gabrielse et al., Phys. Rev. Lett. 65, 1317 (1990); ibid. 74, 3544 (1995) 8. M. Larsson, Annu. Rev. Phys. Chem. 48, 151 (1997) 9. A. Wolf, S. Krohn, H. Kreckel, L. Lammich, M. Lange, D. Strasser, M. Grieser, D. Schwalm, D. Zajfman, Nucl. Instrum. Methods A 532, (2004) 10. E. Herbst and W. Klemperer, Astrophys. J. 185, (1973) 11. R. B. Larson, Rep. Prog. Phys. 66, (2003) 12. D. A. Orlov, F. Sprenger, M. Lestinsky, U. Weigel, A. S. Terekhov, D. Schwalm, and A. Wolf, J. Phys.: Conf. Ser. 4, (2005) 13. A. Wolf, D. Schwalm, and D. Zajfman, Chapter 26 of Many-Particle Quantum Dynamics in Atomic and Molecular Fragmentation, ed. by J. Ullrich and V. P. Shevelko (Springer, Berlin, 2003) 14. H. Fadil et al., these proceedings 15. D. Orlov et al., these proceedings 16. J. Ullrich, R. Moshammer, A. Dorn, R. Dörner, L. Ph. H. Schmidt, and H. Schmidt-Böcking, Rep. Prog. Phys. 66, 1463 (2003) 17. C. Benvenuti, R. S. Calder, and G. Passardi, J. Vac. Sci. Technol. 13, 1172 (1976) 477