To decelerate the ions that left the ESR from 4 MeV/u down to 6 keV/u a linear decelerator has been built. It consists of three main components, a double drift buncher (DDB), an interdigital H-type (IH) structure and a radiofrequency quadrupole (RFQ) as shown schematically above. The beam arrives in a one microsecond macro bunch and is first bunched to fit the 108 MHz time structure of the decelerator structures most efficiently. Then it is decelerated in the IH to about 0.5 MeV/u, rebunched and injected in the RFQ. The RFQ reduced the particles’ energy further to 6 keV/u, suitable to capture them electrically in flight in the Cooler Penning trap.
The ion beam is guided through the decelerator by magnetic dipoles D, quadrupole doublets QD, and triplets QT. As shown above the ions have to be bended first into the HITRAP beam line by two dipoles and then focused through a pumping barrier P that separates the vacua of ESR and HITRAP. Then it is injected into DDB and IH. To keep the beam together during deceleration there is one quadrupole triplet lens inside the IH. The matching into the transversal phase space of the RFQ is then done with two quadrupole doublets. The beam is steered with magnetic steerers and detected on Faraday cups, scinitillating YAG crystal screens, and wire grids.
Ion optical calculations have been performed to optimize the beam transport as well as the deceleration. Different codes where used and one example of a COSY infinity calculation is shown in the figure above from the ESR at the left to the RFQ input side on the right separately for the horizontal h and vertical v beam components. The IH has been optimized using LORASR and the RFQ with PARMTEQ and DYNAMION.
The Double Drift Buncher
To bunch the beam efficiently two consecutive drift tube bunchers at 108 and 216 MHz bunch the beam.