ESR Operation and Development

In the framework of machine developments at the ESR two new modes of operation were tested. After first unsuccessful attempts to demonstrate the slow extraction of a decelerated beam, the slow extraction test was performed at higher energy. The main advantage is a much shorter cycle time allowing faster variation of parameters and easier diagnostics in the HITRAP linac. With improved orbit corrections and after careful tuning of quadrupole and sextupole magnets the resonant extraction of a cooled bare argon beam at an energy of 100 MeV/u could be demonstrated. The extraction was performed by tuning the main quadrupoles such that the tune was close to the third order resonance Qx = 2.333, subsequent slow linear variation of two sextupoles shifted the beam tune across the resonance. Particles which are excited to large betatron amplitude enter the electrostatic septum which deflects them into the extraction channel. Extraction times up to 10 s could be achieved easily. The required magnet setting for the resonant extraction is thus known and can be applied, if slow extraction of bare decelerated ions is needed in the future. The second new mode followed a request to have shorter cycle times for HITRAP commissioning with a 4 MeV/u beam delivered to the HITRAP linac. It resulted in tests of the direct transfer of a Unilac beam using SIS and ESR as single pass beamlines. Although this transfer was tried out several times with different ion species, all tests resulted in beam loss after the first dipole magnet in the ESR. The tests were seriously hampered by the fact, that no diagnostics are available to detect the low energy beam in the ESR during a single pass, neither destructively nor non-destructively. Various problems of this mode could be identified. The power converters of the beam line magnets between SIS and ESR were not foreseen to operate at such low magnetic rigidity. The focussing of the beam through SIS and the beamline is different from normal operation and the beam could not be matched to the standard ESR optical setting. At the location of the main beam loss, the electrodes of the stochastic cooling system limit the acceptance, even further impeding the passage of the unmatched beam. The HITRAP commissioning was regularly continued in two blocks of about five days with bare nickel and xenon beams decelerated in the usual way from 400 to 4 MeV/u. Up to 2× 10 nickel ions could be decelerated to 4 MeV/u with an efficiency of 15 % for the deceleration in the complex deceleration cycle. For xenon, limited by the shorter lifetime in the residual gas, 2 × 10 ions could be decelerated, the total cycle time could be reduced to 45 s. Various experiments were performed at the internal target of the ESR. A nuclear physics experiment used Ru ions decelerated from 100 to 10 and 9 MeV/u and a dense hydrogen target. The reaction products of the (p, γ) reaction were studied with particle detectors installed in a section with large dispersion behind the target. Several high charge states (89+, 90+, 91+) of uranium and different energies in the range 120 to 400 MeV/u were used in an atomic physics experiment at the internal target. The experiment on time dilatation with precision laser spectroscopy of lithium ions was continued. A half life of the 59 MeV/u Li beam of 60 s confirmed that the problem with a tiny leak in the ultrahigh vacuum system of the ESR, which had hampered the experiment in previous years [1], has been solved. The mode for the production of rare isotope beams right in front of the ESR, tested before [1], was used for an experiment of dielectronic recombination of cooled lithium-like uranium. A beam of helium-like U at 186 MeV/u was produced in a 10 mm thick beryllium target from a 381 MeV/u primary U bunch of up to 2 × 10 ions. The helium-like charge state injected and stored close to the central orbit was used to breed lithium-like uranium ions by capture of electrons from the comoving high intensity electron beam (electron current 450 mA). After 2-5 minutes 1 − 2 × 10 helium-like ions had captured one more electron circulating on an orbit radially further outside. These comoving U ions were then moved to the central orbit by ramping all magnets of the ring to a higher field with a field increase of about 1 %. During this manipulation the electron cooling was continued at fixed energy. Finally, the selected particles circulated on the central orbit, whereas most other beam components, which were injected together with the wanted particles, were removed by inserting fast scrapers from inner and outer side into the ring acceptance. The lithium-like charge state was then used for measurements of dielectronic recombination spectra by scanning the energy of the comoving electron beam thus varying the relative velocity between ions and electrons. This measurement cycle was repeated for U and U in order to detect small shifts of resonant lines for different isotopes. For future experiments with single or few ions a new Schottky noise pick-up based on a pill box type cavity with a resonant frequency around 250 MHz has been designed and constructed. The stainless steel body was copper plated at GSI. A quality factor of around 1000 should considerably increase the signal to noise ratio compared to the existing broad band Schottky pick-up. The cavity will be tested and prepared for installation in the ESR early 2010.