The Heavy Ion Storage Ring ESR
Storage rings are used to store and accumulate ions up to the highest possible currents. A high brilliance of the circulating ion beams, which means very small diameters and angular divergences and an extremely small velocity distribution is obtained and preserved by applying special techniques like electron- or laser-cooling.
The ESR is worldwide the only storage ring which fulfils these requirements for all ions from helium with nuclear charge Z = 2 up to even bare uranium with Z = 92 at ion velocities β ranging from below 10% to almost 90% of the speed of light. Therefore, the ESR provides unique possibilities especially for experiments with the heaviest available ions.
The ESR has a circumference of 108,36 m and a magnetic rigidity of 10 Tm. Thus, it is possible to store uranium ions at an energy of 560MeV/u. However, the experiments are carried out usually at energies about 300MeV/u. This is about 65% of the speed of light. The rotational frequency is then up to 2∗106 s-1. The high vacuum inside the ring of the order of 10-11 mbar is essential for keeping the number of collisions between the stored ions and the atoms of the rest gas as small as possible. Such collisions could change the charge state of the ions and lead to drastic intensity losses of the stored beams.
The six 60° dipole magnets together with the focusing quadrupole magnet systems take care that the ions move as close as possible along their ideal orbit.
The mutual repulsion of the ions due to the Coulomb interaction and to collisions between the stored ions and rest gas molecules lead to a blowing-up of the size and the angular divergence of the beam. This is prevented by electron cooling, a process in which heat is taken out of the ion beam in collisions between the hot ions and cold electrons travelling at the same velocity. This cooling not only compensates the blowing-up of the beam but even increases its quality to so-called “brilliant beams”. The picture shows a photograph of the electron cooler installed at the ESR.
These two figures illustrate the principle of very precise measurements of the masses of stored ions in the ESR. The closed orbits of four ions with different charge to mass ratios m/q along the lattice of the ring are plotted. Schottky Mass Spectrometry (left) employs the pick-up signals induced by cooled ions travelling at almost the same velocity. The masses of hot ions with different velocities which are produced for example in nuclear reactions, can be obtained from the time-of-flight signals in the Isochronous Mass Spectrometry mode (right). In this case, the average velocity of the stored ions approaches the “transition gamma γt” which is a characteristic quantity of the ring.