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GSI-ACCELERATORS-10 GSI SCIENTIFIC REPORT 2009
ESR Operation and Development
C. Dimopoulou, A. Dolinskii, O. Gorda, V. Gostishchev, R. Hettrich, C. M. Kleffner,
S. Litvinov, F. Nolden, P. Petri, U. Popp, I. Schurig, M. Steck
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 7 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 6 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
94 Ru 44+ ions decelerated from 100 to 10 and 9 MeV/u
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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 1+ 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 237 U 90+ at 186 MeV/u
was produced in a 10 mm thick beryllium target from a
381 MeV/u primary 238 U 73+ bunch of up to 2 × 10 9 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 5 helium-like ions had captured one more electron
circulating on an orbit radially further outside. These
comoving 237 U 89+ 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 236 U 89+ and 238 U 89+ 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.
References
[1] C. Dimopoulou et al., GSI Report 2009-1.