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GSI-UPGRADE-ACC-09 GSI SCIENTIFIC REPORT 2009 154 GSITemplate2007 Status of the SIS 18 Vacuum Upgrade* M.C.Bellachioma, M.Bender, J. Cavaco, H. Kollmus, A. Kraemer, J. Kurdal, and H. Reich-Sprenger GSI, Darmstadt, Germany. In the context of the technical developments for the construction of the Facility for Antiproton and Ion Research at GSI, an intensive programme for the vacuum upgrade of the existing heavy ion synchrotron (SIS 18) was started in 2005, with the aim to improve the beam lifetime and the beam intensity. To reach these purposes the installation of NEG coated dipole and quadrupole chambers was foreseen, and additionally to overcome the dynamic vacuum instability a collimation system equipped with thin film coated absorbers was designed and commissioned. The production of the thin film getter was carried out in two cylindrical magnetron sputtering facilities described in details in Ref. [1]. The surface chemical composition and the good activation behaviour of the produced thin films (≈ 1μm thick) were proved at CERN and at the Magdeburg University by X-ray Photoelectron Spectroscopy (XPS). Considering that the vacuum chambers mounted in accelerators undergo several ventingactivation cycles, a deep investigation on the NEG ageing was performed last year at GSI by Elastic Recoil Detection Analysis (ERDA) [2]. In each activation cycle an identical quantity of oxygen is dissolved in the film bulk. However heating at temperatures lower than 250°C (as performed for the GSI coated chambers) does not allow a uniform oxygen concentration to be reached in the film and, as a consequence, oxygen atoms are settled in the film to form a concentration profile with the maximum close to the surface (see fig.1) which finally leads to accelerated performance degradation. The film degradation can be also explained in terms of C accumulation on the surface; in effect this element needs a higher heating temperature to be dissolved in the film bulk [3]. An abrupt reduction of the pumping speed was observed for thin coatings (0.25μm) after about 12 cycles when heating at 200°C [4]. Figure 1: The number of activation cycle is plotted from light to dark colour, carbon in red and oxygen in blue. * Work supported by EU design study, DIRAC-PHASE- 1 RP6 SIS18-2 contract No. 515876. # cee@aps.anl.gov In parallel to the production and characterisation of the thin film getters an ion catcher system was designed and commissioned to guarantee that the loss of the charge exchanged particles behind the dipole chambers of the SIS 18 occurs on a low desorbing material and in a high pumping environment (assured by a NEG thin film). Lowest desorption was observed for gold coated copper targets, where a thin nickel film between copper and gold avoid diffusion of the materials into each other during the standard UHV bakeout. For this purpose a Ni film thickness around 200nm is sufficient [2, 5]. Initially two geometries were realized: a wedge and a block shaped absorber. The first one, which separates the region where the losses occur from the circulating beam, has the disadvantage that the particles hit the absorber under grazing incident causing high desorption. The second one, which can be placed in the correct position by a linear motion feedthrough guarantees instead perpendicular incident with the disadvantage of an open geometry. Experiments have shown [5] that the block shaped absorber has a lower desorption yield on the level of 25 molecules per incident ion. Therefore it represents the favourite solution for the ion catcher system. At the end of 2008 two ion catcher prototypes, 14 coated dipole chambers and one quadrupole chamber were implemented in the SIS18. First experiments to evaluate the vacuum upgrade achievement showed a clear improvement in achievable U 28+ beam lifetime caused by the upgraded UHV system [6]. During the 2 upgrade shutdowns in 2009 all the remaining coated dipole and quadrupole (except for the one in S11) chambers and ten ion catcher were mounted in the accelerator. In summary, during the upgrade shutdowns from 2006 to 2009 24 dipole magnet chambers, 11 long quadrupole chambers, 5 short quadrupole chambers, 10 collimator chambers and 5 straight vacuum chambers were replaced by NEG-coated UHV chambers, which corresponds to app. 65% of the SIS 18 circumference. Commissioning of the UHV system upgraded will be performed in the beginning of 2010. Detailed measurements of the improved performance will follow in 2010. References [1] M.C.Bellachioma et al., Vacuum 82 (2008), p.435- 439. [2] M.C.Bellachioma et al., Proceedings of PAC09, Vancouver, BC, Canada. [3] P.Chiggiato and P.Costa Pinto, Thin Solid Film 515, (2006), p.382-388. [4] P.Chiggiato et al., unpublished, to be submitted to J.Vac.Sci.Technol. A. [5] Kollmus et al., this GSI Annual Report [5] G.Weber et al., Phys.Rev.STAB, 12, 084201 (2009).
GSI SCIENTIFIC REPORT 2009 GSI-UPGRADE-ACC-10 Measurement of the acceptance of SIS-18 by means of a transverse RF beam excitation S. Sorge, G. Franchetti, and A. Parfenova, FAIR-AT, GSI, Darmstadt, Germany INTRODUCTION We present the results of a measurement of the acceptance of SIS-18. The need for such an experiment particularly arises from the usage of the GSI synchrotron SIS-18 as a booster within the FAIR project [1, 2] including regular high-current operation. In this regime, an uncontrolled beam loss in the pipe can lead to vacuum degradation and irradiation of the machine. A precise knowledge of the acceptance will help to optimise SIS-18 to reduce particle loss. Several experiments, which used diffusion, had been performed at different accelerators to measure the acceptance or the dynamic aperture. Here, the time required for the beam edge to reach the aperture was measured, see e.g. [3, 4]. Unfortunately, the estimation of this time is unprecise because a detailed knowledge of the initial beam size is necessary which translates in a large uncertainty in the derived acceptance. In order to avoid this uncertainty we study the evolution of the particle loss after the beam edge has reached the aperture. METHOD To determine the acceptance ǫlim of SIS-18, we excite a coasting heavy ion beam using the BTF exciter leading to an increase of the transverse beam width. When the beam width exceeds the limiting aperture, particle loss starts to occur. It can be shown that after a certain time the beam shape and the particle loss do no longer depend on the initial conditions. From this moment, the beam current will follow an exponential decrease entirely determined by the machine parameters and the excitation. This decrease can be modelled using a tracking algorithm. Here, we use the lattice parameters based on a MAD-X calculation and a realistic RF signal of the BTF exciter, U(t) = Ua sin(2πfCt + φ0(t)), (1) [5], where fC is the carrier frequency being adjusted to the fractional part of the betatron frequency. φ0 is a phase jumping between the values 0, π with a probability of 50 % and the time TS = 1/fS between two possible jump times. fS is the half width of the corresponding power spectrum. Within a trial-and-error procedure, we calculate the particle loss where ǫlim is varied until a computed curve is found which fits well the measured one. PARAMETERS USED AND RESULTS The experiments were done with a Ta 61+ beam initially consisting of 10 9 ions at the extraction energy E = 100 MeV/u. The working point was (νx, νy) = (4.17, 3.29). An RF signal with an amplitude voltage Ua = 28 V was applied. For technical reasons, only a vertical excitation and so, the measurement of the vertical acceptance, was possible. Measurements with fS = 0.01/T0 and fS = 0.005/T0, respectively, were performed. The resulting values for the vertical acceptance were ǫlim = 46 mm mrad and ǫlim = 45 mm mrad, respectively [6]. A measured curve and a calculated one for the larger width of the exciter spectrum are shown in figure 1. Within an I(t) / I max 1,2 1 0,8 0,6 0,4 0,2 measured calc. 0 0 0,5 1 t (s) 1,5 2 Figure 1: Normalised current measured and calculated for E = 100 MeV/u, Ua = 28 V, fS = 0.01/T0, and ǫlim = 46 mm mrad. error analysis, the maximum uncertainty for ǫlim has been estimated to 13 %. REFERENCES [1] P. Spiller and G. Franchetti, ”The FAIR accelerator project at GSI”, NIM A 561 (2006) 305-309. [2] ”An International Accelerator Facility for Beams of Ions and Antiprotons”, Conceptual design report, GSI Darmstadt, May 2006, Section 3, ”The Facility”, http://www.gsi.de/GSI-Future/cdr. [3] X. L. Zhang, V. Shiltsev, and C. Y. Tan, ”Tevatron admittance measurement”, Proc. of 2005 Particle Accelerator Conference, Knoxville, Tennessee [4] X. Altuna et al., ”The 1991 dynamic aperture experiment at the CERN SPS”, AIP Conf. Proc., No. 255, p. 355 (1991) [5] M. Kirk and P. Moritz, ”Noise excitation parameters for the SIS-18 RF-KO excitation”, GSI internal note, GSI 2006 [6] S. Sorge, G. Franchetti, and A. Parfenova, ”Measurement of the acceptance by means of a transverse beam excitation with RF noise”, submitted to Phys. Rev. ST-AB. 155
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