3D investigations of the 35 MV tandem Vivitron

Trying to understand the sparking on the metal bars which connect the discrete electrodes on the same radial position, the 3D program Antigone [1] has been used to model the Vivitron structure (which includes terminal electrode, tank, discrete electrodes (DE), and, of course, metal bars) and to find out the electric field distributions on the surface of the electrodes. 3D calculation shows that the maximum electric field happens on the metal bars instead of the edges of the discrete electrodes so in disagreement with the initial machine design expectation. According to the numerical simulations, 20 MV appears as the limit for the terminal voltage with the present machine structure. Improvements on the metal bars seem necessary if nominal voltage is still envisaged.