LINAC 2008
Beam transport effects for ECRIS
Experimental results from ion beams, extracted from an Electron Cyclotron Resonance Ion Source (ECRIS) are compared with the model used for simulation, which has to taken into account that the energy of ions within the magnetically confined plasma in a trap of several T is in the eV-range. Electrons do have a different energy distribution: there are hot electrons up to MeV range, but also low energy electrons, responsible for charge neutrality within the plasma. Because the gyration radius of ions is within the sub-mm range, ions can be extracted only if they are located on a magnetic field line which goes through the extraction aperture. Ion-ion collisions are not important for the path of the ion. Because of the gradient dBz/dz of the mirror field only these ions can be extracted, which have enough energy in direction of the field line. These conditions are fulfilled for ions which are going to be lost through the loss cones created by the hexapole. The extracted beam shows a typical behavior for any ECRIS: when the beam is focused by a lens (here a solenoid) directly behind extraction, the initial round and hollow beam develops wings with a 120-degree symmetry. Because of these considerations, the magnetic flux density in the plane of extraction is a good approximation for the minimum required flux density from which ions can be extracted. This surface is shown in 2D-cuts for two different ECRIS types. It is assumed that the plasma generator is able to produce particles in the required charge state at these locations. The model has been tested for different existing ion sources, and for ion sources which are still under design or under construction. more...
Author: P. Spädtke et al.