Voltage Holding Prediction in Multi Electrode-multi Voltage Systems Insulated in Vacuum

The voltage holding in the 1 MV ITER Neutral Beam Accelerator is recognized to be one of the most critical issues for long lasting beam operation, due to the complex electrostatic structure formed by differently shaped electrodes biased at different potentials. At present, no model is available to predict the breakdown probability of electrostatic system with a comparable complexity degree. This paper is aimed at proposing an innovative modelling for the voltage breakdown prediction of such complex systems, based on the implementation of the micro particle (clump) induced breakdown Cranberg-Slivkov theory into a statistical approach. After a detailed description, the model is applied to simple geometries and the results are compared to the experiments found in literature; finally, the model is applied to an electrostatic mock-up of the real ITER Neutral Beam accelerator, showing a good agreement with the experiment.