ISDEIV 2004
Influence of an Axial Magnetic Field on the Electron Temperature in a Vacuum Arc Plasma
The influence of an axial magnetic field on the electron temperature of a vacuum arc plasma has been studied experimentally and theoretically for moderate discharge currents of 400-600 A, magnetic flux densities of 0-50 mT, and different cathode materials such as uranium, titanium, and carbon. Experiments have been performed using the vacuum arc ion source VARIS . An 127° electrostatic cylinder spectrometer has been used to detect the electron energy spectra. The electron temperature has been derived by a Maxwellian approach applied to the experimental data demonstrating a significant temperature and mean energy rise with increasing magnetic flux density consistently with the model calculations. The electron temperature in the inter-electrode gap of a vacuum arc has been calculated from an energy balance equation that was supplemented by an equation for the cross-section of the plasma flow. The plasma flow is constricted by an external axial magnetic field instead of the free spherical plasma flow in its absence. This leads to an increase in the electron temperature. The constriction of the plasma jet is limited by the cathode group spot diameter at a flux density Bmax specifying an upper limit of the electron temperature. more...
Author M. Galonska et al.
Development of a vacuum arc ion source for injection of high current uranium ion beam into the UNILAC at GSI
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 U4+ 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. The emission current density of the new ion source has been increased from 60 mA/cm2 for the common used GSI-MEVVA to 170 mA/cm2. This results in a full beam ion current of 156 mA at 35 kV with a fraction of four fold charged uranium ions of 67 %. The analysed U4+ ion beam after dc post acceleration amounts to 25 mA at 131 kV which is 1.7 times higher than the requested ion beam current at the entrance of the RFQ. The reduced power density of the vacuum arc results in a higher efficiency and longer life time. Solenoids which are creating magnetic fields to enhance the charge state of the ions are no more placed inside the vacuum system. This ion source design results in a higher availability after ion source replacement at the injector, and longer life time. more...
Author: R. Hollinger et al.