Experiments with viewing targets for ion beams from ECRIS
Electron Cyclotron Resonance Ion Sources (ECRIS) are increasingly used as ion source for different types of accelerators because of their high current densities for highly charged ions. To increase the available current densities further, the frequency for electron heating has to be increased as well as the magnetic flux density within the ion source to improve the plasma confinement. With increasing magnetic flux density the influence on the extracted ion beam becomes severe.
To investigate the ion beam quality, normally delivered to the RFQ of the high charge state injector at GSI, we had the chance to install a viewing target close to the position of ion beam injection into the RFQ. The profile visible on the viewing target could be recorded through a regular glass window by a video camera outside the vacuum.
At that beam line a CAPRICE type ECRIS with 14.5GHz and about 1 T flux density in the extraction was used to deliver different Argon charge states for the measurements. The beam line consists of a solenoid, a quadrupole singlet, a 135°-dipole, a quadrupole triplet, and another solenoid for ion beam matching to the RFQ. Ramping single beam line elements and simultaneous observation of the beam shape on the viewing target can contribute to a better understanding of the process of ion beam generation. We have found a highly structured ion beam distribution at that position. These structures caused by the hexapolar field within the ion source have already been observed directly behind extraction. They are transported through the beam line without becoming homogeneous, which indicates a high degree of space charge compensation for that cw-beam.
If the beam line is mastered by the dipole, all charge states show similar ion beam distribution on the target for a given extraction voltage. This is also a hint, that the structures already have been produced inside the source. more...
Author: P. Spädtke et al.