THE HITRAP DECELERATOR LINAC AT GSI* F. Herfurth # , W. Barth, G. Clemente, L. Dahl, P. Gerhard, M. Kaiser, H.-J. Kluge, S. Koszudowski, C. Kozhuharov, W. Quint, A. Sokolov, T. Stöhlker, G. Vorobjev, GSI Helmholtzcenter for Heavy Ion Research, Darmstadt, Germany J. Pfister, U. Ratzinger, A. Sauer, A. Schempp , Institute for Applied Physics, Goethe University, Frankfurt, Germany O. Kester, NSCL/MSU, East Lansing, MI 48824-1321, U.S.A. Abstract Highly-charged, heavy ions at rest are the perfect tools for cutting edge experiments in atomic and nuclear physics. The HITRAP facility at the GSI accelerator complex decelerates highly-charged ions as heavy as U 92+ produced by stripping of all electrons passing a 400 MeV/u beam through matter. In the experimental storage ring ESR the ions are decelerated from 400 to 4 MeV/u. An interdigital H-type (IH) structure and a radio- frequency quadrupole (RFQ) structure are operated in inverse to decelerate from 4 MeV/u to 0.5 MeV/u and to 6 keV/u, respectively. First deceleration down to 0.5 MeV/u has been demonstrated and the remaining parts of the linear decelerator for HITRAP are mostly installed and being tested. INTRODUCTION Heavy, highly-charged ions at very low, well defined energy are ideal systems for a number of precision experiments in different fields of physics. Using a single ion with just one electron left stored in an ion trap, the g- factor of the bound electron in extreme systems as for instance hydrogen-like uranium will be possible. This is not only a test of the most precise theory in physics, QED, but can be used to set new limits on fundamental constants like the electron mass. To measure the electron binding energy in an independent manner the stored ions mass will be measured for different electron configurations with a very high precision of 1 ppt. If many highly-charged ions are stored in a cryogenic environment and cooled accordingly, measurements of the hyperfine splitting can be performed with laser spectroscopy one thousand times more precise than presently possible based on the reduced Doppler broadening. Stored clouds of ions also give access to precision X-ray spectroscopy. Precise studies of processes like multiple charge exchange can be performed with complete kinematical analysis in ion – atom collision experiments. For that a well defined ion beam will be targeted to a cold sample of neutral atoms and the products are then investigated in a reaction microscope. If a highly-charged ion approaches a surface a huge amount of energy is deposited on a very small spot. This leads to nonlinear effects within the atomic structure of the surface. To disentangle those processes from the typical irradiation damage the kinetic energy of the projectile has to be lower than the potential energy deposited per ion and requires a beam of highly-charged ions below 100 keV total kinetic energy. At the GSI accelerator complex, using the universal linear accelerator UNILAC and the synchrotron SIS, highly-charged ions up to U 92+ are produced by stripping off all electrons passing a 400 MeV/u beam through a gold foil. The HITRAP facility is built to decelerate those ions to almost rest and to provide them to the experiments (see Fig. 1) [1]. In a first step, the ions are decelerated in the experimental storage ring ESR [2] from 400 to 4 MeV/u accompanied with stochastic and electron cooling to keep the emittance small. Then, in the HITRAP linear decelerator the deceleration is performed in two more steps. An interdigital H-type (IH) structure and a radio- frequency quadrupole (RFQ) structure are operated in inverse to decelerate first from 4 MeV/u to 0.5 MeV/u and then further to 6 keV/u. The major components have been installed and are taken into operation. A number of tests and simulations have been performed and decelerated ions at 0.5 MeV/u have been produced. THE LINEAR DECELERATOR After deceleration and cooling in the ESR the ion beam is extracted about every 30 to 40 seconds. To keep the cavities on constant temperature, the decelerator structures are still operated with a duty cycle of 0.5%. Due to the final electron cooling in the ring there is no internal time structure in the extracted ion pulse of about 1 μs length, which contains up to 10 5 uranium ions. To adapt this ion bunch to the limited longitudinal acceptance of the decelerator, a double drift buncher (DDB) operated at 108 and 216 MHz is used. The two coaxial-quarter wave resonators are separated by a 0.9 m SIS 400 MeV/u Cooler Penning Trap Linear Decelerator Stripper ESR FRS UNILAC 400 MeV/u 6 keV/u 4 MeV/u 4 Kelvin 0.3 meV Experiments Figure 1: Schematic view on the production of slow, highly-charged ions with HITRAP. ___________________________________________ * Work supported by the German Ministry of Education and Research (BMBF) # F.Herfurth@gsi.de Proceedings of PAC09, Vancouver, BC, Canada WE4PBC05 Low and Medium Energy Accelerators and Rings A08 - Linear Accelerators 1961