Status of vacuum arc ion source development for injection of high current uranium ion beams into the GSI accelerator facility

摘要

To fill up the GSI heavy ion synchrotron (SIS) to its space charge limit with uranium ions, a new vacuum arc ion source (VARIS) based on the MEVVA IV ion source has been developed and implemented into operation. The ion source has proven its capability in several long period beam times at the high current injector at GSI. With the new ion source it was possible to exceed the space charge limit of 15 mA U~(4+) ions at the entrance of the linear accelerator (UNILAC) for the very first time. The reliability as well as the noise behaviour has been improved to such a degree, that this ion source can be used for injection into an accelerator without objection. In this article we present the improvements of the ion source with the most important operational data. As part of the ion source development the ion and electron energy distributions were measured with an electrostatic cylinder spectrometer device. The energy spectrometer discriminates charged particles with different energy to charge ratio which - in case of different ion species within an extracted beam - allows a charge sensitive evaluation of ion energy distributions. Energy distributions are measured for various discharge parameters, i.e. arc current, magnetic flux densities close to the discharge region, and cathode materials. Different plasma parameters can be derived from these measurements: the anode drop, charge resolved mean energy and the energy spread of ions, mean electron energy and electron temperature. The electron energy may support the development of a plasma model of a vacuum arc plasma consisting of high charge states. Observations of the vacuum arc with a digital video camera have been carried out as well as three-dimensional computer simulations for the multi-aperture accel-decel extraction system.