VIEWS: 9 PAGES: 1 POSTED ON: 2/25/2012
ESR Operation and Development K. Beckert, P. Beller, W. Bourgeois, B. Franczak, B. Franzke, F. Nolden, U. Popp, A. Schwinn, M. Steck, GSI Darmstadt After a nine months’ shutdown period ESR operation started Decelerated heavy ion beams have been provided for vari- in June 2001. During the shutdown the internal target section ous atomic physics experiments. Bare uranium was decelerated has been modiﬁed and new pressurized air actuators have been from 300 to 43 MeV/u for the investigation of x-rays emitted installed which allow fast (within 1 s) positioning of charged during recombination of the ions with electrons in the cooling particle detectors. These detector drives have already been suc- section. The use of a decelerated beam allowed a considerable cessfully employed in physics experiments. Their motion is reduction of the background originating from Bremsstrahlung fully computer controlled and triggered by events from the ac- of electrons lost in the electron cooler. Up to ¿ ¢ ½¼ ions were celerator timing system thus avoiding any dead time between decelerated to the energy of 43 MeV/u and stored for several beam manipulations and data acquisition for experiments. A minutes. Later during this beam time the decelerated uranium new zero degree electron spectrometer has been installed about beam was also used for tests of the beam line to Cave A. 1 m downstream the interaction point with the gas jet. Mag- In the ﬁnal beam time of 2001 uranium ions decelerated to nets and vacuum system of the electron spectrometer section 20 MeV/u had to be transported to Cave A for channeling ex- are ready for operation, commissioning with beam is foreseen periments. Unfortunately the total beam time had to be cut short for the ﬁrst beam time period in 2002. All pick up electrodes due to problems in the Unilac accelerator section. Nevertheless of the stochastic cooling system have been equipped with indi- the machine cycle could be successfully established and also vidual low noise pre-ampliﬁers in order to improve the signal the beam line from the ESR to Cave A equipped with new di- to noise ratio and thus the cooling time. The improved perfor- agnostics for low energy beams could be commissioned. The mance has been tested and conﬁrmed with beam and is available decelerated uranium beam with an energy of 20 MeV/u was ex- for use in experiments. tracted with extraction times on the order of minutes by charge During June and July the ESR was operated together with the changing of the bare uranium beam. Electron capture in the SIS in the reinjection mode for ﬁve weeks providing bare gold electron cooler was chosen as the process providing hydrogen- ions in Cave C at nearly 1.5 GeV/u. A typical beam intensity like ions at a rate not exceeding ½¼ ions/s. The required ex- of ¾ ¿ ¢ ½¼ bare gold ions was injected into the ESR at an traction rate was accomplished by a reduction of the electron energy of 350 MeV/u. By application of an electron current of current to 100 mA with a total extraction times of 2 minutes 0.7 A the cooling time was 5 s only. After reinjection into the per cycle. Figure 1 shows some essential parameters of the ma- synchrotron SIS the ions were accelerated to 1.499 GeV/u and chine cycle in this operation mode. After a cooling time of slowly extracted over 5 s. Compared to previous beam times about 15 s the beam is decelerated in 8 s to an intermediate en- the cooling time in the ESR was considerably reduced thus the ergy of 30 MeV/u and after 6 s of cooling within 2 s to the ﬁnal processing time in the ESR contributed less than 30 % to the energy of 20 MeV/u. total cycle time. The efﬁciency of beam transportation from the ESR to the SIS could be continuously increased in the course of the beam time. More than 70 % of the ions circulating in the 0.7 ESR were transfered to the SIS resulting in ½ ¾ ¢ ½¼ bare gold magnetic rigidity [10 Tm] ions supplied to the experiment in one cycle of 20-30 s. Most 0.6 spill rate [104s-1] ion current [mA] of the losses between ESR and SIS can be attributed to down electron current [A] 0.5 charged ions which are produced during the cooling period in the ESR and which can not be transfered due to their difference 0.4 in magnetic rigidity. Bismuth ions in the hydrogen- and lithium-like charge state 0.3 were stored at energies of 415 and 393 MeV/u, respectively, for 0.2 measurements of the hyperﬁne splitting with a collinear laser. For efﬁcient production of incompletely stripped ions at this en- 0.1 ergy carbon foils recently installed in the beam line in front of the ESR were employed. Energies around 415 MeV/u corre- 0 0 25 50 75 100 125 150 175 200 spond to the highest electron energy in the cooler used for an time [s] experiment. Experience showed that reliable cooler operation near the current maximum energy requires careful conditioning of the electron cooling system over several days. Figure 1: Typical deceleration cycle for a bare uranium beam The transfer of ions through the fragment separator to the from 300 MeV/u injection energy to an energy of 20 MeV/u ESR was studied with a nickel beam at 390 MeV/u. Beyond with slow extraction. The curves show dipole ﬁeld, ion and general optimization of the optical setting of the transfer line electron current and spill rate at the experiment in the units in- the test revealed an unintentional transposition of a quadrupole dicated in the plot as a function of time. dublett which is, at least partially, accountable for beam losses.
Pages to are hidden for
"170"Please download to view full document